Choosing a New Propeller

Part of the re powering project for  White Pepper involves choosing a new propeller to match the new engine.  Such a task is not easy.  A poor selection can lead to engine failure as well as poor performance.  Indeed I now suspect that some of  White Pepper's current problems stem from "under pitching" the prop when it was changed out in 2007.

The universally acknowledged expert in selecting a prop is David Gerr and his book, "Propeller Handbook."I ordered one from Amazon and set to work.  I must say that I have never spent better money on a nautical book. I wish I had learned this stuff years ago.

Mr. Gerr's book is for those who want to know why and not just the answer.  The book is well written but dense.  I had to read some sections several times.  The math is about the level of a good Algebra II class in high school.  I used the scientific calculator on my smart phone.  My chief complaint about the book is the poor reproductions of the  graphs.  I really had to squint at times. There are propeller calculators on the internet. Indeed I used one from Max  Prop and got the same answer, but I felt more comfortable having worked it out myself.

The process starts with consulting the power curve of your engine.

Power Curve for Yanmar 3JH Series

Where the two lines meet is the only spot where the propeller is optimum, that is at 3000 RPM and 38 mhp. MPH is metric horsepower which is almost identical to an English horsepower.  Then chose a reduction gear.  Yanmar has two--2.4 and 2.6.  The 2.6 gear will turn the prop at 1150 RPM.  This slower speed allows for a larger blade and less chance of cavitation. Torque is slightly higher for lower RPMs since torque is power divided by RPM. Variable pitch propellers are available which will allow the two curves to match up.  These are so expensive they are only for tug boats and mega yachts.

The "Propeller Handbook" provides two methods of estimating propeller diameter and pitch from horsepower, shaft RPM and boat dimensions.  There is the old traditional slip method of Crouch and a newer, more numerical, but not necessarily more accurate method called Bp-delta. For those who do not know, the pitch of the propeller is how many inches it would advance in one revolution if it did not slip.  A typical sailboat slips about 45%, so a boat with a propeller pitched at 13 inches would advance about 7 inches each revolution.  That works out to 9,250 inches per minute (at 1250 RPM)  or about 7 nautical miles per hour.

The slip method yielded a suggested propeller dimension of 18" in diameter by 14" of pitch for the 2.6 gear.  This was close to the propeller suggested by the Max Prop calculator on the internet.  Max prop suggested 18x13. 

The Bp-delta method gave 20 inches for the optimum diameter.  I thought that this was too large for the aperture.  Choosing an 18" blade,  gave a pitch of 15"  and an efficiency of 46% (55% is the best you can do.) This choice was confirmed by a call to Michigan Wheel, the premier manufacturers of bronze propellers.

Bp-delta diagram for a 4 blade prop

By comparison the current prop is 18" x 10".  The Yanmar 3HM was rated at 3600 RPM and 27 horses max.  The last time the old girl saw 3600 RPM was the day she raced for a bridge opening in 2009.  The transmission failed the next day.  Based on calculations of diesel consumption, I estimate that the power output was down to about 8 HP by 2013.

The Bp-delta method also allow you to calculate expected speed, 7 knots, and thrust, 880 pounds.  I was surprised at how low the trust was.  A full press of sail will generate thousands of pounds of force on the rig and anchor loads in a brisk wind are in the thousands of pounds.

David Gerr speaks glowingly of the advantages of a feathering prop and calls it ideal for the long distance cruiser who need to sail and power.  He reports that the drag of a fixed three blade prop is equal to the drag of the entire under body of a well designed yacht.  Compare that to the drag profile presented by a fully feathered propeller.

3 Bladed Max Prop looking forward when fully feathered.

I exchanged e-mails with Max Prop.  The price of the 3 blade classic is "only" $3300.  which is a bargain compared to the price I paid for my old two bladed prop 15 years ago.  I lost that one to electrolysis, but nowadays the Max Props are fitted with a large zinc bolted directly on to the hub.  That is what you are looking at above.  I asked about the new heavily promoted 4 bladed Eccowind Max Prop that has a spring which automatically adjusts the pitch to the load.  Unfortunately it will not work at the RPMs generated by the Yanmars.

The blades are obviously flat and efficiency suffers without the familiar twist we all see in traditional propellers.  However, the degradation is only 5%.  Furthermore, according to Dave Gerr  the efficiency enters the velocity and thrust calculations as the cube root, i.e. 2%.  So the expected speed for the re powered  White Pepper will still be 6.8 knots and thrust 836 pounds.  I can live with that if she will sail at 7.4 knots.

On the other hand a fixed 3 blade prop from Michigan Wheel Co would cost about $600.  The yacht would likely motor at 7.4 and sail at 6.8.  The fixed prop would likely be slightly more powerful facing a headwind and chop.  Choices!