Propeller sizing

[R.J. Choinski]

I have recently removed the old atomic 4 from my 1971 32' Irwin sloop. I am replacing with a M3-26 Universal diesel. The old atomic 4 turned a 12" prop with 3/4" shaft , and that is all the clearance allowable for the new prop, with out making major changes! I was told the new diesel with its 26 hp 3000 Rpm, 2:1 reduction, will be best suited with a much larger diameter prop, i.e. 16"x10". The size I will need to go with is 12"x14" to load the engine properly and have the proper clearance. I was told I would have terrible performance sizing down to 12"? Could it be worse than the 12" prop with atomic, as before? Is there any special prop configuratrion to get more "push" out of the 12" to make up for lost performance over the 16" prop? What are the general thoughts about this type of problem in propeller sizing and performance?

 

[ed]

reproping

I would call general propellor in bradenton and talk to them about ratios. but i think you are making a major mistake not reproping to the right size prop.I have done a 32 irwin conversion to a yanmar with great sucess. we modified to hull over the prop. it is not nearly as bad as it sounds and we can attest to a wonderfuly sucessful job. try me at edsboat1@aol.com and ill give you a phone number in tampa.

 

[RonD]

Props

In general, the diameter is fixed by the shaft-to-hull geometry. Short of doing a bit of hull modifications, you're stuck with what you have.The other parameters are pitch and number of blades. More blades typically give more thrust, but present greater drag when not being driven (i.e., you are under sail).For a given diameter, the pitch must be matched to the engine/transmission and the blade count. Generally, it is determined to produce hull-speed at about the max cruise RPM. Best to discuss the options with your local engine dealer rep. They usually have the proper diameter/blade/pitch parameters worked out for most production boats.--RonD