Sizing of Propellers
By Tom Assenmacher
(SHEARWATER 1975 MK-II Yawl)
June 28, 2011

Hit Counter Visits since 6/28/2011

We recently received the following correspondence with Jim Larkin of Mt. Kisko, NY(AERIE, 1987 Yawl, #247 – next to the last A-37 built):

“…..I am replacing the prop on my ALBERG 37 hull # 247.  It had a 13x14 machine pitch 3 blade prop which may have been the wrong prop for my 2003 Volvo Diesel engine, possibly leading to the failure of my transmission . I have replaced the transmission and would appreciate any information which would correctly guide me in selecting the correct replacement  prop?...”.

(Ed. Note: Having gone through this process several times during our re-engining/transmission failures/transmission replacements, the following are our comments and discussion regarding Jim’s question. – the following discussion is provided for information only, and should NOT be construed as the ONLY solution for Jim’s question .)

“…..To make a long story short, we've been through a lot of transmission problems over the past years - but hopefully, we have finally found the solution (in a different brand of transmission).. Have you read our write-up of our latest saga of trans. woes on the A-37 website?  If not, go to the 'home page', and check it out - specifically:

http://www.alberg37.org/Project%20DB/SW-Twin_Disc-Trans-Install-Web/SHEARWATER-Twin_Disc_Installation_9-2010.htm 

 For all our re-engining/re-transmissioning/re-proping, we used the prop calculator on the Boat Diesel.com  - unfortunately, it will cost you $25 to get into the Prop Calculator program (our subscription recently 'ran out', otherwise I would send our user name and pass word).  In any event, it might be worth your time and a few $$ to subscribe to the prop formula.   I feel it's very accurate and has been 'right on' for the 2 different props we've used.  It allows one to enter the displacement, hp, rpm, reduction gear ratio, etc. as entering arguments.  If constrained by prop aperture size, it allows you to  enter the max diameter of the prop to be used, and figures out the pitch.  There are several other prop formulas out there, but BoatDiesel seems to work best for us.  Also, you might be able to find a prop shop in your area who can make size (diameter and pitch) recommendations.

  The criteria that we used to determining whether we had indeed the correctly pitched prop, was to run the boat (with a clean bottom and prop): 

1) To see if we could get max rated engine RPM at full throttle while underway without overheating, and without any 'black smoke' coming from the exhaust.  This test shows that the prop is not too large (Pitch or Diameter) for the engine/trans combination.  On our Kubota based 37.5 HP @ 3000 RPM, we can just achieve 3000 rpm at full throttle with no signs of overheating or engine 'loading'.

 And -

 2) To see if you can get 'hull speed' at the RPM at which the max torque curve intersects with the fuel consumption curve (sort of an indication of max 'miles per gallon').  On our engine/trans combination, we get hull speed (about 6.4 kts in flat water, no heel) at about 2200 rpm, which is about the max MPG for this engine.  Your engine literature should have the performance curves listed somewhere . 

Also, there are a couple of good discussion forums (free) out there:  Cruisers Forum and the Seven Seas Cruising Association .

 There are a number of posts about props/transmissions, etc. on each of these forums. Some of them by us!