The Real Boat Renaming Process

In Part 1 of our search for the correct or optimum propeller size for Indra, we discovered what size propeller we had and the many "issues" with it. In Part 2 of our search we increased our knowledge of propellers enough to understand what pieces of information we needed for a propeller size calculation. In this post, Part 3, we discover a online propeller calculator and evaluate its methodology and recommendations.

A internet search on boat propeller size calculator revealed a few potential candidates.

• Boatdiesel.com - this website has a basic and advanced propeller size calculator but access is restricted to paid membership. Both propeller calculators appear to be based on the concepts and formulas in the Propeller Handbook by Dave Gerr and uses the Crouch or slip method to determine propeller recommendations.
• Castle Marine Ltd - had a very outdated pitch calculator based on Windows XP/Vista software. Since this version of Windows is ancient history, we did not use this program.
• Marine Propeller Calculator - they had a very rudimentary propeller calculator which did not consider any boat characteristic information.
• PropExpert software - on website Hydrocomp, Inc. it offered for sale, PropExpert a propeller sizing and analysis software product. However, as it was intended for sale to commercial entities like propeller shops, distributors, and manufacturers it was not likely priced for purchase by the average boat owner.
• Prop Scan software - on website Prop Scan it offers Prop Tools Propeller Sizing Software however this website is in the business of franchising the Prop Scan Business Opportunity and not selling software to the general public.
• Recreational Boat Building Industry (RBBI) - they had a very rudimentary Boat Propeller Calculator which did not consider any boat characteristic information.
• Surfbaud Freeware Propeller Calculator - on website Surfbaud Marine Propeller they have a Microsoft Excel spreadsheet called Propking in Excel and HTML formats that accepts user input to determine propeller size. Another smaller version of this Excel spreadsheet called propcalc.xls can be found by a search on the filename on the internet. This propeller calculator spreadsheet appears to be based on the concepts and formulas in the Propeller Handbook by Dave Gerr and uses the Crouch or slip method to determine propeller recommendations.
• Victoria Propeller Calculator - on website Vic Prop they have a publicly available and free to us propeller size calculator for displacement/semi-displacement hulls. This propeller calculator appears to be based on the concepts and formulas in the Propeller Handbook by Dave Gerr and uses the Crouch or slip method to determine propeller recommendations.

We selected the publicly available and free to use the Victoria Propeller Calculator to enter Indra's data and review its recommendations.

We first ran Indra's current boat characteristics with the replaced old Yanmar 3QM30H 30 hp engine specifications. The calculation recommended a 17" diameter by 9" pitch propeller versus the existing 18" diameter by 13" pitch propeller. Next we reran the calculation with the same boat characteristics and the currently installed Yanmar 4JH5E 53.1 hp engine specifications. The calculation recommended a 20" diameter by 11" pitch propeller versus the existing 18" diameter by 13" pitch propeller. These differing propeller size results indicated that Indra's current propeller size was likely not the optimum size for either engine. These results required a more in-depth, detailed review and understanding of how these recommendations were derived with the goal of determining what propeller size is "best" for Indra. What follows below is our review and analysis of the propeller calculator data output and recommendations.

We relied extensively on the methodology outlined in the Propeller Handbook by Dave Gerr to investigate the propeller selection question; it is important to note that we have different opinions on certain recommended methods put forth in the book. There is a revised 2018 second edition of this book, however, for reasons unknown, it was out-of-stock or out-of-print on each vendor site we located - the author and public would likely benefit if it was available as a electronic downloadable version (Kindle, etc.) instead of the current paperback edition format - in our opinion paper media does not fare well in the moist boat environment; we avoid it in most cases. To facilitate multiple test configuration scenarios we used Microsoft Excel to store equations and boat variables instead of using a basic calculator to repetitively perform the tedious manual entries and calculations.

For Indra, we entered the following values into the propeller size calculator online form:

• Waterline length of vessel: 31.24 Feet - True North 34 specifications state waterline length is 30.5 feet. The Actual Loaded Waterline Length is 31 ft 2.85 inches due to added displacement weight resulting in Indra resting lower in the water.
• Beam at the waterline: 10.71 Feet - True North 34 specifications state beam is 11.0 feet. The Actual Beam Loaded Waterline Length is 10 ft 8.5 inches due to the curvature of the hull.
• Molded hull draft (excluding keel): 3.89 Feet - True North 34 specifications state draft is 5.5 feet. Loaded draft is 5 ft 10.8 in. Loaded draft without keel is 3 ft 10.67 inches - we calculated this value from scaled drawings. Do not see a feasible method to physically and accurately measure this value other than from a scaled drawing.
• Vessel displacement weight: 33500 Pounds - True North 34 specifications state displacement weight is 26,000 pounds. We used the actual indicated weight of a different, fully equipped True North 34 during its haul out as it hung in the travel lift's slings. This was about 32,500 lbs, however Indra has a larger engine, hardtop, wind vane, solar array, and likely more stuff aboard, so increased estimated weight to 33,500 pounds.
• Number of engines (Shaft lines): 1 - Only one diesel engine installed.
• Maximum rated horsepower of each engine: 53.1 hp - Obtained from Yanmar's 4JH5E engine specifications and installed engine identification plate - 39.6 kW = 53.1 hp.
• Maximum rated engine R.P.M.: 3000 RPM - Obtained from Yanmar's 4JH5E engine specifications and installed engine identification plate - the min⁻¹ term is the same as RPM.

• Gear ratio: 2.61 - Obtained from Yanmar's KM35P gearbox/transmission specifications and installed identification plate.

• Number of shaft bearings between the gear box and the propeller: 1 - Indra has only one cutless bearing installed on the propeller shaft; did not increase to 2 for the stuffing box shaft gland as instructions stated not to.
• Desired maximum speed: 6.5 Knots - As we already have a engine and Indra's speed is limited by her loaded waterline length this question was really not applicable.

With the information filled in we clicked the Calculate button and the results were displayed. The Data Input box listed the same input values and calculated the maximum shaft RPM. As I intended to verify (scrutinize) the calculated results, I researched the likely equations in the Propeller Handbook and manually calculated the results to see how close they were.

• Propeller Drive Shaft Revolutions Per Minute (RPM) = Maximum Engine Revolutions Per Minute (RPM) / Transmission Gear Reduction Ratio.
• Maximum Propeller Drive Shaft RPM = 3,000 RPM / 2.61 Gear Ratio = 1,149.43 RPM - result closely matches the displayed value of 1149 RPM.

The Horsepower Calculations box re-displayed the previously entered engine horsepower of 53.1 hp and calculated the total available torque ft/lbs at the engine.

• Torque (T) in Ft/Lbs = (5,252 x Horsepower (hp)) / Revolutions Per Minute (RPM)
• Torque (T) = (5252 x 53.1 hp) / 3000 RPM = 92.96 ft/lbs - result closely matches the displayed value of 93 ft/lbs.
• The number 5,252 in the equation is a constant value derived from 1.0 hp = 550 Foot-Pounds per Second, which is then divided by the circumference of a circle 2π divided by 60 seconds per minute. The 5252 constant = 550 ft lbs/sec / (2π / 60 sec) = 5252.113.

The total available torque of 92.96 ft/lbs at 3,000 rpm from the engine was compared to the Yanmar 4JH5E torque curve and it appeared to be reasonably close.

The Horsepower Calculations box displayed 3% hp loss at gearbox and 1.5% hp loss at shaft bearing. The Propeller Handbook by Dave Gerr stated the same horsepower loss percentages for the gearbox and shaft bearing but did not elaborate on the rationale that determined these horsepower loss percentages.

• Gearbox/Transmission Horsepower Loss = Fuel Stop Power Brake Horsepower x 3% = 53.1 hp x 0.03 = 1.593 hp - result closely matches the displayed value of 1.6 HP.
• Shaft Bearing Horsepower Loss = Fuel Stop Power Brake Horsepower x 1.5% = 53.1 hp x 0.015 = 0.7965 hp - result closely matches the displayed value of 0.8 HP.

So the total horsepower loss is 1.593 hp plus 0.7965 hp which equals 2.3895 hp.

A review of the Yanmar engine specifications power curve (see below) revealed a dashed line that represents power loss and available power output at the propeller shaft. These power curves were likely based on the Yanmar 4JH5E engine as originally configured and tested with the originally equipped 80 Amp alternator. We replaced the original 80 Amp alternator with a 150 Amp alternator. The alternator manufacture estimated that for each 25 Amps it would require 1 hp of engine power. Therefore, 150 Amp minus 80 Amp then divided by 25 Amps per 1 hp yielded an additional horsepower loss of 2.8 hp not accounted for or asked for in the online calculation. Replacing a low-amperage alternator with a higher output amperage alternator is commonplace today amongst boat owners; propeller calculations should account for this item as the horsepower loss is significant the larger the alternator amperage size is. The online calculation also does not request input for other potential engine driven ancillary equipment that would reduce the total available horsepower to the propeller/shaft even further.

The Yanmar engine specifications state the maximum horsepower output at the propeller when the engine is at its maximum rating is 50.96 hp indicating a loss of 2.14 hp by the gearbox. This gearbox loss of 2.14 hp is higher than the propeller calculation estimate at 3% of maximum rated engine horsepower which equals 1.593 hp. The gearbox loss of 2.14 hp is equivalent to 4.03% of maximum rated engine horsepower.

So Indra's actual total horsepower loss is 5.74 hp based on 2.14 hp due to the gearbox, 2.8 hp due to the alternator, and 0.80 hp due to the cutless bearing. The actual total horsepower loss is 3.3505‬ hp higher than the propeller calculator estimates.

The Horsepower Calculations box displayed the total horsepower available at the propeller.

• Propeller Shaft Horsepower = Fuel Stop Power Brake Horsepower - Total Horsepower Losses (gearbox, bearing, alternator, etc.).
• Propeller Shaft Horsepower = 53.1 hp - 1.593 hp - 0.7965 hp = 50.17 hp. The Horsepower Calculations box displayed 50.7 HP versus the manual calculation of 50.17 hp which indicates the calculation has induced rounding errors.
• The corrected Propeller Shaft Horsepower with Indra's actual total horsepower losses is 53.1 hp - 5.74 hp = 47.36 hp - a notable difference from result of 50.7 HP.

The Horsepower Calculations box displayed the total torque in ft/lbs available at the propeller.

• Torque (T) in ft/lbs = (5,252 x Horsepower (hp)) / Maximum Propeller Drive Shaft RPM
• Torque (T) in ft/lbs = (5252 x 50.17 hp) / 1149.43 RPM = 229.24 ft/lbs. The Horsepower Calculations box displayed 232 ft/lbs versus the manual calculation of 229.24 ft/lbs; the difference is likely due to induced rounding errors.
• The corrected Torque with Indra's actual total horsepower losses is - Torque (T) in Ft/Lbs = (5252 x 47.36 hp) / 1149.43 RPM = 216.40 ft/lbs - a notable difference from result of 232 ft/lbs.

The Speed & Power Calculations box displayed the estimated displacement hull speed.

• Theoretical Maximum Hull Speed in Knots = Speed to Length Ratio x (Loaded Waterline Length in Feet)^0.5
• Theoretical Maximum Hull Speed = 1.34 x (31.24 ft)^0.5 = 7.489 Knots - result closely matches the displayed value of 7.49 Knots.

The Speed & Power Calculations box displayed the minimum horsepower required at the propeller for the theoretical maximum hull speed of 7.49 knots which has a Speed to Length Ratio of 1.34.

• Theoretical Maximum Hull Speed Minimum Required Propeller Shaft Horsepower = Displacement in Pounds / (10.665 / Speed to Length Ratio)³
• Minimum Required Propeller Shaft Horsepower = 33500 lbs / (10.665 / 1.34)^3 = 66.45 HP - a notable difference from result of 73.1 HP. The reason for the difference in values was not determined. As Indra's 53.1 hp engine will not support a speed of 7.49 knots, this difference is of no concern.

The Speed & Power Calculations box displayed the HP required at the propeller for desired 6.5 knots speed.

• Propeller Shaft Horsepower = Displacement in Pounds / (10.665 / (Hull Speed in Knots / (Loaded Waterline Length in Feet)^0.5))³
• Propeller Shaft Horsepower = 33500 lbs / (10.665 / (6.5 kts / (31.24 ft)^0.5))^3 = 43.43 hp - result closely matches the displayed value of 43 HP.

The Speed & Power Calculations box displayed the maximum speed in Knots with existing 53.1 horsepower.

• Knots = (10.665 / (Displacement in Pounds / (Fuel Stop Power Brake Horsepower - Total Horsepower Losses)) ^1/3) x (Loaded Waterline Length in Feet)^0.5
• Knots = (10.665 / (33500 / (53.1 hp - 2.3895 hp))^1/3) x (31.24 ft)^0.5 = 6.8445 - result closely matches the displayed value of 6.81 Knots.
• The corrected Knots with Indra's actual total horsepower losses is = (10.665 / (33500 / (53.1 HP - 5.74 hp))^1/3) x (31.24 ft)^0.5 = 6.6902 Knots.

The Speed & Power Calculations box Notes stated the propeller sizing calculations are based on 90% of full RPM to provide engine reserve power for variable loading in the vessel. It was recommended in the Propeller Handbook by Dave Gerr that propeller diameter be calculated at 100% of engine maximum RPM and propeller pitch be calculated at 90% of engine maximum RPM.

The Propeller Size Calculations box calculated a recommended size in diameter and pitch for a 2, 3, and 4 blade propeller.

Propeller Size Diameter Calculations based on 100% Engine Horsepower and RPMs:

• Three Bladed Propeller Diameter in Inches = (632.7 x (Propeller Shaft Horsepower)^0.2) / Propeller Shaft Revolutions Per Minute (RPM)^0.6
• Three Bladed Propeller Diameter = (632.7 x 50.17^0.2) / 1149.43^0.6 = 20.1835 Inches - result closely matches the displayed value of 20.4.
• Corrected Three Bladed Propeller Diameter with Indra's actual total horsepower losses is = (632.7 x 47.36 hp^0.2) / 1149.43^0.6 = 19.9522 inches or standard propeller diameter size 20 Inch.

• Two-Bladed Propeller Diameter = Three-Bladed Propeller Diameter x Two-Bladed Propeller Diameter Conversion Factor
• Two-Bladed Propeller Diameter Conversion Factor is 1.05 - reference the Propeller Handbook by Dave Gerr, Table 5-2.
• Two-Bladed Propeller Diameter = 20.1835 x 1.05 = 21.1927 Inches or standard propeller diameter size 21 Inch.

• Four-Bladed Propeller Diameter = Three-Bladed Propeller Diameter x Four-Bladed Propeller Diameter Conversion Factor
• Four-Bladed Propeller Diameter Conversion Factor is 0.94 - reference the Propeller Handbook by Dave Gerr, Table 5-2.
• Four-Bladed Propeller Diameter = 20.1835 x 0.94 = 18.9725 Inches or standard propeller diameter size 19 Inch.

Propeller Size Pitch Calculations based on 90% Engine Horsepower:

1. 90% Engine Horsepower = 0.90 x (Fuel Stop Power Brake Horsepower - Total Horsepower Losses)
   1. 90% Engine Horsepower = 0.90 x (53.1 hp - 2.3895 hp) = 45.639 hp
2. 90% Maximum Propeller Drive Shaft RPM = 0.90 x Maximum Propeller Drive Shaft RPM
   1. 90% Maximum Propeller Drive Shaft RPM = 0.90 x 1149.43 RPM = 1034.487‬ RPM
3. Knots @ 90% = (10.665 / (Displacement in Pounds / (90% Engine Horsepower)) ^1/3) x (Loaded Waterline Length in Feet)^0.5
   1. Knots @ 90% = (10.665 / (33500 lbs / 45.639 hp)^1/3) x (31.24 ft)^0.5 = 6.608 Knots
4. Propeller Slip = 1.4 / (Knots @ 90%)^0.57
   1. Propeller Slip = 1.4 / (6.608 kts)^0.57 =0.4772
5. Three Bladed Propeller Pitch in Inches = (12 in/ft x (Knots @ 90%) x 101.3 kts/ft/min) / 90% Maximum Propeller Drive Shaft RPM) x (1.0 + Propeller Slip)
   1. Three Bladed Propeller Pitch in Inches = (12 in/ft x (6.608 kts) x 101.3 kts/ft/min) / 1034.487 RPM) x (1.0 + 0.4772) = 11.4703 Inches or standard propeller pitch size of 11 Inch.
   2. Pitch values should be rounded down unless the decimal is 0.7 or greater.

• Two-Bladed Propeller Pitch = Three-Bladed Propeller Pitch x Two-Bladed Propeller Pitch Conversion Factor
• Two-Bladed Propeller Pitch Conversion Factor is 1.01 - reference the Propeller Handbook by Dave Gerr, Table 5-2.
• Two-Bladed Propeller Pitch = 11.4703 Inches x 1.01 = 11.585 Inches or standard propeller pitch size of 11 Inch.

• Four-Bladed Propeller Pitch = Three-Bladed Propeller Pitch x Four-Bladed Propeller Pitch Conversion Factor
• Four-Bladed Propeller Pitch Conversion Factor is 0.98 - reference the Propeller Handbook by Dave Gerr, Table 5-2.
• Four-Bladed Propeller Pitch = 11.4703 Inches x 0.98 = 11.2409 Inches or standard propeller pitch size of 11 Inch.

Propeller Size Pitch Calculations based on 90% Engine Horsepower with Indra's actual total horsepower losses:

1. 90% Engine Horsepower = 0.90 x (Fuel Stop Power Brake Horsepower - Total Horsepower Losses)
   1. 90% Engine Horsepower = 0.90 x (53.1 hp - 5.74 hp) = 42.624 hp
2. 90% Maximum Propeller Drive Shaft RPM = 0.90 x Maximum Propeller Drive Shaft RPM
   1. 90% Maximum Propeller Drive Shaft RPM = 0.90 x 1149.43 RPM = 1034.487‬ RPM
3. Knots @ 90% = (10.665 / (Displacement in Pounds / (90% Engine Horsepower)) ^1/3) x (Loaded Waterline Length in Feet)^0.5
   1. Knots @ 90% = (10.665 / (33500 lbs / 42.624 hp)^1/3) x (31.24 ft)^0.5 = 6.4593 Knots
4. Propeller Slip = 1.4 / (Knots @ 90%)^0.57
   1. Propeller Slip = 1.4 / (6.4593 kts)^0.57 =0.4834
5. Three Bladed Propeller Pitch in Inches = (12 in/ft x (Knots @ 90%) x 101.3 kts/ft/min) / 90% Maximum Propeller Drive Shaft RPM) x (1.0 + Propeller Slip)
   1. Three Bladed Propeller Pitch in Inches = (12 in/ft x (6.4593 kts) x 101.3 kts/ft/min) / 1034.487 RPM) x (1.0 + 0.4834) = 11.2592 Inches or standard propeller pitch size of 11 Inch.
   2. Pitch values should be rounded down unless the decimal is 0.7 or greater.

So to summarize the results, both the Propeller Handbook by Dave Geer and the Victoria Propeller Ltd online propeller size calculator recommended for Indra a 20" diameter by 11" pitch three blade propeller. Despite the difference in total horsepower calculation and some minor numerical rounding errors, the end result was the same size propeller recommendation. The important item to realize is the method of pitch calculation is based on 90% of maximum horsepower/RPM and this methodology favors a slightly over pitched propeller. The other contention we have with this method, besides total horsepower calculation, is the overall basis for calculations is based on 100% of the engine's maximum ratings which is limited to 5% of engine operational time. We are of the opinion that calculations should be based on the other 95% factor which equates to the normal engine range for everyday use. Whether this difference in opinion results in a different size propeller is what the next post, Part 4, will try to determine.

In the first part of our search for the correct or optimum propeller size for Indra, we discovered Indra had a right hand rotation, 18" diameter by 13" pitch propeller sized for a 1.25 inch propeller shaft. We also discovered a lot of issues with the current propeller and propeller shaft, but still had no idea of what was the right size propeller, nor did we know how to determine it.

As we tinkered around on Indra located on a patch of concrete at Holiday Oceanview Marina, Samal Island, Davao, Philippines, a few of the other boat owners would stop by and chat. On the subject of propeller size, most really did not have a clue what size propeller was on their own boat or how to determine the correct size, and the best advice given was to ask an expert, like a propeller manufacture. A few had feathering props but based on their comments, they purchased it not on practicality, but as a "ego" item to boast about and increase their prestige perception. A significant number of boats there had engine issues and most opted to replace the engine; only one pulled his engine for a rebuild which took over 2 years to complete. The reason for most folks engine replacement was pretty much the same: after hard use and the passage of time the engine was worn out, corroded, lost power, was smoking excessively, consuming or leaking oil, and unreliable. Not one person that changed an engine out changed their propeller. I was informed that the engine they selected as a replacement was chosen to "drop right in" without the need to change the propeller, the propeller shaft, shaft couplings, exhaust hose, or other impacts. Most engine replacements had a higher horsepower rating. I was often told that there is very little reliable wind in South East Asia and you'll use your engine more than your sails - so it better work reliably.

As time availed, we researched the internet and found an abundance of generalized information, many unique terms, and facets related to propeller design and selection. It became very clear that selection of a properly sized propeller was dependent upon a lot of variables - most, if not all, manufactures/sellers used a computerized tool to narrow the selection process down. A few manufactures of propellers offered a free analysis by inputting your boats information into an online form, click submit, and wait for a response. We did this on multiple websites and many never responded back, even after follow-up inquiries. A few examples follow:

• On website Accutech Marine we filled in and submitted their "Application Engineering Sizing Form" and received a quick response. They stated their sizing analysis and recommendation costs $250.00 and is refundable upon the purchase of a propeller. This sticker shock drove us away from even considering their services. In the spirit of public transparency and disclosure they should have stated their exuberant fee on their propeller analysis form.
• On website Michigan Wheel we tried their Prop-It-Now Inboard Propeller Size Calculator and it recommended a 3 blade, 23" diameter by 15" pitch propeller - this was way too large to fit on Indra. Their Prop-It-Now Inboard Propeller Size Calculator did not ask for any boat characteristic data which means their recommendation is really nothing more than a WAG (Wild Ass Guess) - very disappointing as they should have put up a disclaimer notice as to the results. We submitted their "Request a Propeller Sizing Analysis" on their Prop-It-Right Analysis web page with a maximum diameter limit of 19-inches. In a few minutes we received a computer generated email confirming the submittal of our request and that one of their representatives would contact us shortly - we never heard anything back from them. The Michigan Wheel manufacture has franchised out their Propeller Sizing Analysis and it can be found on the Propeller Depot website, Deep Blue Yacht Supply website, and many others.
• On website VEEM Propellers we filled in and submitted their propeller quote form. It did not ask for displacement weight, beam, or draft and the smallest shaft size was 1.5-inch. Had the general impression their propellers were for planning hulls verses displacement hulls. A computer generated email response was received and recommended a VEEMLoadstar 4-blade, 20" diameter by 14" pitch propeller with a blade area ratio of 55%. The quoted cost of over $2K was well over what we anticipated. In many cases, the same diameter propeller for a 4-blade propeller pitch is reduced by 1-inch compared to the 3 blade propeller - so for a 3 blade propeller this would likely equate to 20" diameter by 15" pitch propeller or a 19" diameter by 18" pitch propeller.
• On website West By North they offered a propeller called the Campbell Sailer, a more efficient, airfoil shaped, narrow fixed blade propeller with substantially reduced drag under sail due to a slim blade design. We filled in and submitted their Propeller Recommendation Form and received a response within a week. They recommended a 3-blade RH 18.0" diameter x 12.0" pitch propeller at a cost of US $850.00.

Reviewed many articles, research papers, and books found via the internet. We discovered a very good informative book about propellers, especially for those who know nothing or very little about propellers like us, called the Propeller Handbook, The Complete Reference for Choosing, Installing and Understanding Boat Propellers, Dave Gerr, 1989, 2001.

The general consensus by "experts" was that all propeller size recommendations were scientific estimates, or best guesses, due to variables that could not be accounted for or changed under many varying circumstances. However, the common sense advice to achieve the optimum propeller size estimate was to determine as realistically and accurately the variables or boat/engine characteristics that a propeller size estimate was to be based on. Equally important was to determine what propeller performance goal was most important. As Indra was a True North 34, a heavy and (very) slow cruiser, our basic performance criteria was a propeller design that maximized a slow speed with fuel efficiency. The characteristics of a True North 34 ruled out the use of modern feathering or pitch adjusting propellers as the return on cost expended would not achieve any significant improvements in performance; besides these type of propellers would not fit Indra's enclosed propeller aperture area.

As a result of our newly found basic propeller knowledge we knew what information we needed. The minimum boat characteristic information needed for propeller size calculations was Loaded Waterline Length in Feet, Beam Loaded Waterline Length in Feet, Loaded Draft Without Keel in Feet, and Actual Displacement in Pounds. We determined all the pertinent boat characteristic information, both design and actual, as we had other uses for this information.

The design specifications for a True North 34 were obtained from website Sailboatdata.com. The design specification values are not the values to be used to determine propeller size, the values needed are actual current measured values. We intended to use the design specifications as a baseline to compare with actual valves to reveal differences in Indra's performance measures.

As we have scaled boat drawings, we imported them into Microsoft Visio and calculated hull length/beam and draft measurements by use of numerical ratios that should be very close. We compared our Visio measured values to design values and they were dead on. As we had taken exterior pictures of Indra, in the water the first time we visited her, it was very easy to determine how far down she was on her 8-inch waterline boot stripe - the water level was just above the middle of the boot stripe. Indra's displacement weight was estimated by using the actual travel lift weight measurement of another fully equipped and loaded True North 34 and adding additional weight based on observable differences - a 7,500 pound increase over the original 26,000 pound design specification. Recording the weight of your boat during a haul out should be on everyone's list to do if you’re lucky enough to be lifted by a machine with weight scales. Indra was not so lucky on our current haul out, it was by rail system so she was pulled out, not lifted.

Another pertinent measurement is the size of the propeller aperture area. It is important to know the largest diameter propeller size that fits within the recommended clearance of 15-20% of the propeller diameter size. Indra's propeller aperture vertically is about 2-feet; so the largest diameter size propeller is about 19 inches with 20% clearance. ( 24 inches x 0.80 = 19.2 inches )

Next, we determined Indra's engine characteristics. The minimum engine characteristic information needed for propeller size calculations was Maximum Propeller Shaft Horsepower and Maximum Propeller Shaft Revolutions Per Minute. Just like the boat information, we determined and recorded all the unique engine characteristic data - some of the information was needed to determine the values of propeller horsepower and RPM.

STOP!!! Can you read critically and recognize the inconsistencies as you review the engine specifications below?

We downloaded the most current manual for the Yanmar 4JH5E engine from Yanmar's website. We then verified that the information contained on the engine identification plate matched the Yanmar manual we were using - if it did not match we had older Yanmar manual versions we could have used. The min⁻¹ term in the picture below has the same meaning as the term RPM (Revolutions Per Minute).

The pertinent engine information from the Yanmar 4JH5E Operators Manual is shown below. The Yanmar manual had multiple Warnings and Cautions about not overloading or over speeding the engine as it could reduce vessel performance, lead to increased smoke levels, or cause permanent damage to the engine. The emission of black exhaust smoke was stated as a symptom of overloading which shortens engine life. A cause of overloading was stated to be improper propeller matching - a over pitched propeller.

The engine specifications stated in the Yanmar manual needed to be converted from metric to imperial values for use in the propeller calculations.

• Item #1 is the maximum horsepower the engine is rated at: 39.6 kW (53.8 hp metric)/3000 min⁻¹ converts to 53.1 hp at 3,000 RPM. In item #6, operation at the maximum rating is limited to less than 5% of engine operation time.
• Item #2 identifies the gearbox/transmission; the KM35P model is installed aboard Indra.
• Item #3 identifies the maximum horsepower output at the propeller when the engine is at its maximum rating. This propeller horsepower value is less than the maximum engine horsepower rating due to horsepower losses thru the gearbox. The 38.0 kW (51.7 hp metric)/3000 min⁻¹ converts to 50.96 hp at 3,000 RPM indicating a loss of 2.14 hp by the gearbox. The 50.96 hp value is the intersection point of the engine performance power curves shown below. In item #6, operation at the this maximum rating is limited to less than 5% of engine operation time.
• Item #4 identifies the continuous rating the engine should be operated within and per Item #6 this is for 90% of operating time at 2,800 RPMs or less. The 36.0 kW (48.9 hp metric) /2907 min⁻¹ converts to 48.28 hp at 2,907 RPM. This continuous rating is considered the normal top operating hp/RPM range and should normally not be exceeded for any extended time period. The horsepower available to the propeller is 48.28 hp minus the gearbox horsepower loss of 2.14 hp which equals 46.14 hp at 2,907 RPM. This means only 87% of the 53.1 hp maximum rating of the engine is actually available to the propeller to generate thrust under normal operating conditions. Note: The 46.14 horsepower available to the propeller is optimistic, in reality there are additional horsepower losses that reduce the total available horsepower to the propeller further.
• Item #5 is Yanmar's guidance on the criteria a propeller should be propped or sized for: 200 RPM above max RPM or 3,200 RPM. This minor increase in RPM is considered a engine over-speed condition and for a propeller to match this criteria it would need to be slightly under pitched.

The pertinent KM35P gearbox information from the Yanmar 4JH5E Operators Manual is shown below.

• Item #1 identifies the gear reduction ratio of the KM35P gearbox - the 2.61 number is the most significant. The red arrow points at the 2.61 gear ratio number on the gearbox identification plate - the only real way to identify what ratio the gearbox has.
• Item #2 identifies the forward speed of the propeller and shaft rotation at the maximum continuous rated horsepower. This number is determined by dividing the maximum continuous engine RPM of 2,907 by the 2.61 gear ratio which equals 1,113.79 RPM.

The Yanmar 4JH5E engine performance curves provided a visual representation of these engine specifications. The power curve shows the maximum horsepower output at the propeller shaft (black dashed line) intersecting the propeller power curve (lower blue line) at 3,000 RPM. This intersect point is the value 50.96 hp described in the engine specifications above, Item #3 - maximum rated horsepower minus inherent gearbox horsepower loss. The propeller power curve (lower blue line) represents the theoretical optimum propeller size for the amount of horsepower available.

So, what use are all these horsepower values at a stated RPM good for? The end intent is to form the basis for an "informed and educated" propeller size selection. Common sense dictates that engine horsepower is not the most important factor; what is, is the amount of usable horsepower at the propeller that can be effectively converted into thrust and make the boat move forward. Another common sense factor is the heavier a boat is the more horsepower is needed and the lighter a boat is the less horsepower is needed to achieve the same result. This is also true for the boat's resistance in the water and why it's advocated to have a "clean" verses "fouled" hull. All the engine specifications and power curves shown above DO NOT take into account the boat's displacement/water resistance; you are the one that's supposed to know what your boat's weight and other characteristics are and make that informed determination. Be extremely cautious of online or in-person propeller "experts" that do not considered the boat's displacement/water resistance in their "knowledgeable", "expert" and "experienced" propeller size recommendations.

The two primary indications on any boat underway on engine power is speed in knots and engine RPMs. Other observable indications are feel-vibrations, sounds, sight and smell of engine exhaust. It has been stated by many that unusual vibrations, unusual sounds, and exhaust smoke are indicators of engine problems which again is common sense. Another subtle clue is if the engine's throttle is fully advanced at its maximum setting and the indicated engine's RPMs are below the engine's maximum rated RPMs, it is likely the engine is overloaded - the propeller is likely over pitched. The opposite is equally significant, if the engine's throttle is fully advanced at its maximum setting and the indicated engine's RPM are above the engine's maximum rated RPMs, it is likely the engine is under loaded - the propeller is likely under pitched. Then a desirable goal is to achieve the optimum propeller pitch that results in the engine's maximum rated RPMs being closely displayed with the throttle at maximum - this goal is beneficial to engine longevity and is in line with Yanmar's recommendations. A propeller that is pitched for maximum thrust at a reduced engine RPM in an attempt to achieve a faster speed with reduced fuel consumption might have increased fuel economy, but it is gained at the expense of a overloaded engine that likely shortens engine longevity.

The goal of engine longevity versus fuel efficiency was a controversial discussion point on a few website discussions - individual decisions were different, often based on perceptions and not the true impact of all the facts. Look at this issue from a different perspective. Consider two identical boats with the same proper maintenance routines - the only difference is the propeller on boat #1 is pitched for optimum engine longevity and the propeller on boat #2 is pitched at 70% RPM for maximum thrust at that lower RPM to achieve fuel efficiency. The owner of boat #2 boasts he saved $2.00 in fuel costs every hour of engine operation at his cruising speed of 70% of maximum RPM. Both boats have 5,000 hours of engine operation use. Which boat would you buy knowing this information?

It is very likely the engine on boat #2 will need to be replaced, sooner rather than later compared to boat #1, due to the increased wear on the engine operating in a overloaded state due to the propeller being over pitched. If the cost of a replacement engine and its installation costs $15,000.00 and it occurs at 6,000 hour of engine use the total cost per hour of use is increased by the cost of engine replacement - $15,000/6,000 hours = $2.50 per hour. So boat #2 might have saved $2.00 in fuel cost per hour, but the added cost of engine wear cost $2.50 per hour; the difference is minus $0.50 per hour compared to boat #1. So over time, boat #2's operational cost for 6,000 hours totals $3,000.00 more than boat #1. While this is a simple example to emphasize a point, reality is most engine replacement costs are significantly higher and the boat owners that replaced engines at our location had significantly less than 6,000 hours on their worn out engines. Another factor at our location in the Philippines is shipping costs and import duties - figure about $1,500.00 in shipping costs and import tariff at 100% of value. That $15,000.00 replacement engine now costs $31,500.00. I know why Indra's previous engine was replaced at Kudat, Malaysia - zero import duties for a yacht in transit, unlike the Philippines. The internet abounds in stories of boats having engine problems in very remote places with repair expenses beyond comprehension. As to the issue of engine longevity versus fuel efficiency in propeller size selection - decide for yourself, but choose wisely.

Oh, our answer to the question of: Which boat would you buy knowing this information? We would select boat #1. If we were to put an offer on boat #2, we would subtract a high percentage of the cost of a engine replacement from boat #2's asking price. Next time you see a boat for sale listing or a marine surveyor's pre-purchase survey, don't be surprised if this type of information is missing - ask yourself why?

Did you recognize any inconsistencies as you read the engine specifications?

• Item #6 in engine specifications states engine use at 5% of maximum hp/RPMs and 90% at continuous hp/RPMs. Where is the remaining 5% of engine use time? Believe the 90% value should likely be 95%.
• Item #5 in engine specifications recommends the propeller be propped at 100-200 RPM above the maximum engine rating of 3,000 RPM - an engine over speed condition. Yet the manual has multiple Warnings and Cautions about damaging the engine if over speeding the engine occurs. If this is the optimum recommendation for propeller sizing why does the Power Curve diagram not show the 3,200 RPM range and the intersection of the propeller power curve at that 3,200 RPM range?
• In engine specifications it states maximum continuous hp/RPM as 2,907 RPM; and then in Item #6 it states 90% use at 2,800 RPM or less. Why was the continuous rating reduced from 2,907 to 2,800 RPM? Is 2,907 RPM not really a continuous rating? Or is the RPM range above 2,800 and below 3,000 RPM the missing 5% number?
• In engine specifications it states maximum power values for engine hp/RPM and the maximum power value for output at the propeller shaft, then it states the propeller rotational speed in the gearbox section based on continuous hp/RPM without informing of the rating it was derived from.
• All the engine's Performance Curves are based on maximum engine hp/RPM which is restricted for use to 5% of engine operation. Common sense dictates that you don't select the optimum propeller size for a 5% use condition, but the condition it will be used the most in - the 95% condition is what the propeller size selection should be based on for optimum performance.

We now know realistic boat characteristics and engine performance values for propeller size calculations, but there is another value needed - total engine horsepower loss. It is common in the boating community to add high output alternators, a second alternator, generators, hydraulic pumps, air conditioning pumps, or water maker pumps driven by the existing engine. These additions should be accounted for in total horsepower loss as this directly reduces available horsepower output to the propeller. We have reviewed many websites of "experience" cruisers that boasted of their engine driven modifications for increased self-reliance and comfort, but not one emphasized the potential impacts of horsepower loss on propeller performance and the need to re-evaluate the propeller size to optimize their speed, fuel economy, or engine longevity. Additionally, many cruisers have elaborated on their social media about their deeper waterlines as over time they accumulated more stuff and added modifications to their boat - again these changes impact the amount of power needed to move the boat, but none mentioned that propeller size should be re-evaluated. The point - everyone should evaluate their propeller size for their boat's current characteristics - it is very likely this has been over looked by many due to this subject being seldom mentioned on social media. Strangely, not one website, propeller calculator, or propeller "expert" asked about additional engine horsepower losses in their propeller sizing estimates - they all advocate that total propeller shaft horsepower is the most significant factor in estimates, but don't ask the common sense question about additional horsepower losses, why?

Indra's Yanmar 4JH5E engine came stock with a 80 amp alternator that we replaced with a 150 amp alternator. The Yanmar engine specifications accounted for the horsepower loss of the 80 amp alternator in their engine power curve and specifications. We included the additional horsepower loss due to the increase of 70 amps caused by the new 150 amp alternator at 1 hp for each 25 amp increase as this ratio was specified by the alternator manufacture. The increase of 70 amps equates to an additional loss of 2.8 hp. It is important to consider that the alternator has its own power curve and the highest output of the alternator occurs close to the Yanmar's engine maximum rated 3,000 RPM.

An additional horsepower loss item is the propeller shaft cutless bearing. The standard for bearing loss is 1.5% of the 53.1 maximum engine rated horsepower which equals 0.7965 or rounded 0.80 hp.

So Indra's total horsepower loss is 5.74 hp - 2.14 hp due to the gearbox, - 2.8 hp due to alternator, and - 0.80 hp due to the cutless bearing.

Some would advocate to subtract the 5.74 hp total horsepower loss from the 53.1 maximum engine rated horsepower for propeller size calculation, however the use of the engine at its maximum rating is limited to 5% of operational use - as 5% of use is not realistic for engine operation we don't subscribe to this advice. Instead we subtracted the 5.74 hp total horsepower loss from the 48.28 continuous maximum engine rated horsepower for propeller size calculation as it represents the specified normal operational range of the engine. This results in the more accurate value of 42.54 total horsepower available at the propeller shaft and propeller to make the boat move forward. This means that only 80.1% of the 53.1 maximum engine rated horsepower is actually available to the propeller during normal day-to-day engine use.

The following power curve chart illustrates the differences in points of horsepower selections for propeller calculations.

The upper red line is the original engine power curve for the maximum rating at 53.1 hp at 3,000 RPM - it is extended further to intersect the 3,200 RPM line and its intersection point is estimated at 53.6 hp.

The red arrow points at the original intersection point of 50.96 hp at 3,000 RPM. The black dashed line represents the 53.1 maximum engine hp rating minus the 2.14 hp horsepower loss by the gearbox - it is the same shape and parallel to the red line just lower by 2.14 hp. The dark blue solid line that intersects the black dashed line at the 50.96 hp point is the propeller power curve. These lines and the intersection point at 50.96 hp are unrealistic representations for a propeller calculation as 50.96 hp is not realistically available to the propeller under any condition.

The yellow arrow points at the intersection of the maximum engine rating estimated at 53.6 hp at 3,200 RPM, minus Indra's calculated 5.74 total horsepower loss, making the intersection point at 47.86 hp at 3,200 RPM. The 3,200 RPM value is based on Yanmar's recommended criteria for propeller size selection - see above engine specifications Item #5. The solid yellow line is the propeller power curve that intersects at the 47.86 hp point. As this is the lowest angled propeller power curve, it reflects the lowest pitched propeller when compared to the other propeller power curves. As Yanmar restricts engine operation at 3,000 RPM for 5% of engine operation, see above engine specifications Item #6, and 3,200 RPM is essentially a minor engine over speed condition that should be avoided, this intersection point is not realistic for propeller calculation. This intersect point can be considered Yanmar's subtle preference to operate the engine under slightly less loaded conditions than the maximum capability of the engine, maybe in the interest of engine longevity - the propeller for this condition would be considered slightly under pitched.

The green arrow points at the intersection of the green dashed line that represents the maximum continuous rating of the engine of 48.28 hp (line not shown) minus Indra's calculated 5.74 total horsepower loss, making the intersection point at 42.54 hp at 2,907 RPM. The 42.54 hp value more accurately represents the horsepower available to the propeller for propeller calculations. The solid green line that intersects the green dashed line at 42.54 hp represents the propeller power curve. A propeller sized to emulate this power curve has slightly more pitch than the Yanmar's recommendation represented by the solid yellow propeller curve. The 42.54 hp value is what we will use for propeller size calculations as it is more accurate and reflects normal engine day-to-day operations.

The blue arrow points at the intersection of the green dashed line that represents the maximum continuous rating of the engine of 48.28 hp (line not shown) minus Indra's calculated 5.74 total horsepower loss, making the intersection point at estimated 34 hp at 2,100 RPM. The 2,100 RPM value is derived by taking 70% of the maximum rating of 3,000 RPM and this represents a desired cruising speed of 70% of the engine's rating. The solid blue line represents the propeller power curve where the propeller would be over pitched to achieve the maximum potential speed as if it intersected the total available continuous horsepower of 42.54 hp - this represents the position of advocates to over pitch their propeller to increase fuel efficiency and lower fuel costs.

The actual cruising speed of a boat is determined by the total horsepower available to the propeller not the maximum horsepower rating of the engine. The average cruising speed selection varies from about 70% to 85% of the engine's rating determined by the decision of the boat owner. In engine specifications Item #6, Yanmar states cruising speed as 2,800 RPM or less - this is 93.3% of maximum rated RPM or less. Using the information pointed to by the blue arrow, what might Indra's cursing speed be?

Some assert that the term "hull speed" is the real speed of the boat, it is not. The equation for "hull speed" is: Theoretical Maximum Hull Speed in Knots = 1.34 x (Loaded Waterline Length in Feet)^0.5. Inserting Indra's values into this equation results in a hull speed of 7.49 knots (= 1.34 x (31.24 feet)^0.5). For Indra to achieve a speed of 7.49 knots the minimum horsepower available to the propeller would need to be 66.46 hp which is determined by the equation, followed by Indra's values: Propeller Shaft Horsepower = Displacement in Pounds / (10.665 / (Knots / (Loaded Waterline Length in Feet)^1/2))^3 = 33,500 Pounds / (10.665 / (7.49 knots / (31.24 feet)^1/2))^3 = 66.4570 hp. A potential engine size with a ~10% increase above the minimum horsepower available to the propeller of 66.46 hp would be close to 80 hp - Indra's 53.1 hp engine does not have enough horsepower to achieve the theoretical hull speed of 7.49 knots.

Indra's 53.1 hp engine can provide a continuous maximum horsepower rating of 42.54 hp to the propeller. The 42.54 hp equates to a maximum speed of 6.46 knots which is determined by the equation, followed by Indra's values: Knots = (Loaded Waterline Length in Feet)^1/2 x (10.665 / (Displacement in Pounds / Propeller Shaft Horsepower)^1/3) = (31.24 feet)^1/2 x (10.665 / (33,500 Pounds / 42.54 hp)^1/3) = 6.4551 knots. As we selected the cruising speed based on 70% of the engines maximum RPM rating which is 2,100 RPM the speed in knots at this RPM is approximately 5.6 knots for a non-over-pitched propeller, but the intent is to over pitch the propeller to achieve the same thrust needed for 6.46 knot at the 2,100 RPM setting. What is the difference in diesel cost for these two conditions? With an over pitched propeller, if 6.46 knots is achieved at 2,100 RPM the fuel consumption curve indicates 0.7 gallons/hour at 2,100 RPM - if a gallon of diesel cost $4.00 the cost of fuel consumed is $2.80. With a non-over pitched propeller, to travel the same distance of 6.46 knots at a speed of 5.6 knots at 2,100 RPM requires 1.15 hours at 0.7 gallons/hour, the cost of fuel consumed is $4.60. So fuel cost for the over pitched condition is $2.80 versus $4.60 for non-over pitched condition - a savings of $1.80 per engine hour is realized. The potential cost savings is what appeals to advocates of propeller over pitch but this puts the engine into a over loaded condition that causes eventual internal wear of the engine - as this cost to the engine's longevity is paid at a future date, the attitude of out-of-sight, out-of-mind denial is prevalent.

So the original point of this post was to gather the information needed for propeller size calculations, we actually collected more than we needed. We entered this information into an Excel spreadsheet and in Part 3, we evaluate an online propeller size calculation tool.

Indra’s propeller had “issues”. We discovered it had galvanic corrosion and dezincification caused by lack of zinc anode protection and no protective paint coating. While we cleaned it up, sanded it, applied protective paint, and installed a zinc anode on the shaft, the lingering question remained – was this propeller the proper size or "optimum" for Indra?  We wanted to know the answer to this question as a replacement or spare propeller was warranted due to the "minor" degradation of the original propeller.

To answer this question we first had to determine what size propeller was installed and the potential rationale as to why it was chosen.

What size propeller was installed? We initially discovered the size by accident as a result of cleaning and sanding the propeller. Visible on the bare metal surface was a series of numbers. Note the reddish-pinkish spots - that's indications of dezincification.

The numbers displayed were:

• D 18 - Diameter 18 inches.
• P 13 - Pitch 13 inches.
• SN 00052 - Serial Number 00052
• 32 - 22 - Internal metric taper of the propeller shaft bore hole. Forward side 32 mm tapered down to 22 mm on the aft side.

We later discovered an old receipt aboard - the propeller was second hand, of brass construction, size 18-inch diameter by 13-inch pitch, Right Hand (RH) rotation, for a 1 1/4-inch propeller shaft. The shipment address and date of the receipt - 7/12/2010 - coincided with the same time frame that Indra's diesel engine was being replaced by the previous owners while located at Kudat Industrial Estate, Kudat, Malaysia.

STOP!!! Reread the above information critically - did you notice the discrepancy?

A review of the then, newly installed 2010 Yanmar Model 4JH5E inboard diesel engine manufacture's manuals did not reveal specific propeller size recommendations. However, review of the previously installed and replaced Yanmar Model 3QM30H inboard diesel engine manufacture's manuals and specification sheets stated a specific propeller size - 18" diameter by 13" pitch.

Also discovered aboard Indra was a year 2004 stern gear assembly drawing that identified the same Yanmar Model 3QM30H inboard diesel engine and stated propeller size of 18.1" diameter by 13" pitch. The gear assembly drawing initially "appeared" to have been drafted by a "knowledgeable" individual.

So Indra's current propeller size, 18" diameter by 13" pitch, selection was based on the old Yanmar Model 3QM30H inboard diesel engine rated at 30 horsepower and was NOT reevaluated for the newly installed 2010 Yanmar Model 4JH5E inboard diesel engine rated at 53.1 horsepower. It also meant that Indra's physical boat characteristics were not considered in the propeller size selection but based solely on information contained in the Yanmar manuals. This meant that the currently installed 18" diameter by 13" pitch propeller was most likely the wrong, or not the optimum size in diameter and/or pitch for either diesel engines.

So, did you notice the discrepancy? The propeller was stamped in its brass metal surface 32 - 22, a metric measurement, but the receipt stated it was for a 1 1/4-inch propeller shaft, an imperial (US) measurement. Basic common sense dictates you don't mix and match items of different measurement systems.

To understand and evaluate the impact of this discrepancy we first confirmed the propeller shaft size using a caliper - it measured 1.251 inch. Additionally, the Stern Tube Drawing above stipulates a shaft size of 1.25 inch made of type 2205 stainless steel. Indra's propeller shaft was definitely 1.25 inch in diameter. If you reviewed the Stern Tube Drawing you should have noticed the drafter mixed both metric and imperial measurements - a very inconsistent and amateurish practice. We later discovered that the currently installed 18" diameter by 13" pitch propeller was undoubtedly the incorrect size, which infers the supposedly "knowledgeable" individual that drafted the Stern Tube Drawing above was incompetent in the methodology of proper propeller selection. 

Research on the internet revealed that propeller shafts and the bore hole in propellers are stipulated by standards, and there are different specification standards for the metric and imperial systems - no surprise here. The imperial (US) standard is stipulated by the Society of Automotive Engineers (SAE) standard J755, Marine Propeller-Shaft Ends and Hubs. Review of this standard indicated that the small end bore hole size in the propeller hub should be in the range of 1.015-inch minimum to 1.017-inch maximum for a 1.25-inch shaft. The 22 mm number on Indra's propeller converts to 0.866 inch - definitely outside the required tolerances for a 1.25-inch propeller shaft. If you were observant, you likely noticed the Stern Tube Drawing above stipulates a 30.73 mm maximum size for the large end of the propeller hub bore hole, instead of the 32 mm stamped on the propeller.

We now know the propeller bore hole is the incorrect size for a 1.25-inch shaft, but did it cause any problems? Pragmatically the answer is no - it didn't come loose or fall off; and it was firmly attached to the shaft, as the propeller was extremely difficult to remove from the shaft. However, after removing the propeller from the shaft and cleaning the surfaces up, there was something strange looking - see picture below.

The picture reveals concentric circles etched in the metal of the shaft. The green arrows point to an area where the concentric circles are lighter and the orange arrows point to an area where the concentric circles are darker. The red arrow points a "dent" in the machined surface. These concentric circles imply the propeller rotated around the shaft but that seemed not very likely as the metal key would prevent that.

A search on the internet revealed the concentric circles are likely caused by a process called lap-fitting the propeller to the shaft in an attempt to achieve a "perfect" fit between the tapered shaft and the tapered propeller bore hole. The differences in color of the concentric circles reveals high and low spots indicating a perfect fit was not achieved. Notice the color gets darker toward the end of the shaft. This is a reasonable explanation as the taper of a metric sized propeller bore hole is slightly different than one based on imperial standards. The propeller lap-fitting process uses a grinding compound that removes metal from the mating surface areas as the propeller is rotated on the shaft - this rotational "sanding or grinding" of the propeller against the shaft caused the concentric circles. The impact of lap-fitting a metric sized propeller bore hole to the 1.25-inch shaft is more material is needed to be removed to fit the smaller metric size propeller bore hole - the small hole on the metric propeller is 0.866-inch compared to the larger imperial minimum size of 1.015-inch. The red area in the following picture illustrates the material likely removed by the lap-fitting process - whether more material was removed from the internal side of the propeller hub or the exterior surface of the propeller shaft was not determined due to our inability to obtain a reliable measurement on the angled surfaces.

The likely implication of this is a new propeller of imperial standards will not fit properly on this shaft due to the excessive amount of material remove to accommodate the metric propeller. When a new propeller is ordered, this means it would be prudent to order a new propeller shaft. This minor error is going to significantly increase the cost for what was originally envisioned as just a "simple" propeller replacement.

Lesson: Don't mix and match items of different measurement systems.

So now we know that Indra's propeller size is right hand rotation, 18" diameter by 13" pitch, and the propeller shaft size is 1.25 inch. We also know that the current propeller bore hole is the incorrect size for the 1.25 inch propeller shaft. We know the propeller diameter and pitch size was not evaluated for the increase of available horsepower for the current engine. We know that Indra's displacement and waterline lengths were not considered for proper propeller size calculations. We know if we replace the propeller, we should replace the propeller shaft at the same time.

We still do not know what size propeller is correct or optimum for Indra.  The quest for the answer continues............in Part 2.