Feathering Prop - JF

[Don Alexander]

Hi Kevin,Last year I bought a 3 bladed feathering prop - JF make and 450 mm (17.71") diameter for my 376 with Yanmar 3JH non turbo 37 BHP engine. Like all featherers the blades are flat, i.e. they have no wash out towards the tips. The supplier said he would set it to 15" pitch as the original fixed 2 blade prop was 17" x 15" pitch. He said this equated to 25° of blade angle. I fitted the prop but the engine could not reach the recommended engine RPM and the boat reached hull speed far too early. The engine was overloaded.Then I did some calculations and realised the supplier had calculated this blade angle based upon a point at 57.8% of diameter. My appreciation of props is that most of the work is done by the outer portions of the blades so I would have set the pitch angle at a much larger diameter using either the geometric mean (root mean square) diameter of 70.7 % which gives 21° and I am tempted to work at 80% or 18.6°.Could you please advise what the pro's do in these circumstances.Regards,Don Alexander

 

[Kevin L. Woody]

reduce pitch

Hello Don,Thanks for your note. The pitch on the blade should be measured at apx. 70% from the center line of shaft towards the propeller tip. (17.71” / 2) x .7= 6.2” from center line out to tip. This point on the blade is the area to determine a propellers pitch. There is a section In Dave Gerr’s “Propeller Handbook” page 25 that explains the pitch analysis quite well. You are correct in assuming that the propeller needs a reduction in pitch in order to allow the engine to produce the recommended amount of Rpm’s. For each 1 degree of pitch reduced, or apx. .6” of pitch, the Rpm’s of the engine should increase apx. 5-6%. That % increase compounds upon it self as you decrease pitch and the engine increase’s in Rpm’s. I cannot comment on the apx. recommended amount of pitch reduction because I’m unaware of the current obtainable Rpm’s of the engine. In know that in the case of my Max-Props, a pitch between apx. 16-24 degrees is a very efficient range so I would have not problem in suggesting you run in that pitch range. Things to keep in mind are that there are other sources of load, which a manufacture like myself, find extremely difficult to factor into the complete package. A high output alternator could consume 2-3HP and an engine driven refer apx. the same. When all external things added up the little 3JH can become very taxed. Hope this helps. Kevin.