Kevin, question on prop speed.

[Ernie Rodriguez]

If one has two choices of transmission gear ratios, which is the more desireable from the point of view of the prop and its effectiveness in providing max thrust for the boat, the gear ratio that gives the lowest prop speed or the highest prop speed? For the typical sail boat trying to cruise at 6 or 7 knots, is there some optimum prop shaft speed? In my particular case, the gear ratio is 2.61 in forward for a Yanmar 3GMD, which for a max rpm engine speed of 3400, the prop speed would be 1302 rpm. For the same engine I could also have a gear ration of 3.2, for which the max prop shaft speed would be 1062. Would the prop sized for a 1062 rpm perform significantly better than the one for 1302 rpm?Thanks,Ernie

 

[Kevin L. Woody]

Prop speed

Dear Ernie:Thanks for your letter. Larger propellers are often more efficient then smaller units when looking at the distribution of torque. Lower propellers speeds are almost always more desirable installations. You do however need to keep in mind that a propeller also operates at an optimum diameter to pitch ratio. (0.8) Often problems arise when installing a larger reduction gear, like your 2.62 to a 3.3, when it comes to installing the new propeller. If the larger reduction gear is used on your 3GM, your new propeller would increase in pitch and diameter to apx. 19” x 12” pitch three blade. However the hull may only be capable of supporting a 16” diameter prop. The pitch would then have to be increased to apx. 17”when using the 16” diameter. The increase of efficiency of the new reduction gear would be completely lost and possibly more efficiency would be lost by now having a pitch that exceeds its diameter. Designers often design the running gear placement using the available reduction gears. We see the biggest problems with this space issue when a conversion of a gas motor with a 1:1 reduction is converted to a diesel engine and the customer is forced to use a 2:+ reduction. Most likely the vessel in questions was designed to use a 12” diameter prop and now they need a 16”. Most times the boat owner has no efficient ability of using the new power available to them. I would be very careful when looking into the new reduction gear. Sometimes the hopeful benefits become a very rude real world awakening. Hope this helps. You have asked a very technical question, which is a very important question. There are people, which have dedicated years to this very question. If you are interested in this and other propulsion topics a book by Dave Gerr, PROPELLER HANDBOOK, is a great source of information. Sincerely, Kevin L. Woody