3 blade prop size??

[glcalahan]

I need help determining the size prop for a 2GM20F Yanmar to power my 1972 Pearson 33. Water line 25.80', Disp. 10,930#, Beam 10'. I bought a prop and it was made one off and believe it is not the correct prop. 14"RH12 is what I was told I needed. I appreciate any advice/suggestions. I paid $400 and waited 7 weeks. I am not going install an incorrect size.

[DougR]

One more thing necessary to calculate the prop size is the transmission reduction ratio, and you didn't give it to us....

There were three ratios available on your engine: 2.21:1 , 2.62:1 & 3.22:1. For the prop size which you stated, 14 X 12, I suspect you have the 2.62:1 ratio because that prop is pretty close to what my program comes up with.

For that ratio, the ideal prop would be something like a 16 X 9, but because you have an aperture you might be limited to a 14" prop. As you reduce diameter you have to increase pitch to absorb the power available, so other options would look like this:
16 X 9
15 X 10.2
14.5 X 10.7
14 X 11.2 .....But a 14 inch prop is getting a bit light on blade area and will be operating at the edge of cavitation at high throttle settings.

I would guess a 14 X 12 is a reasonable place to start, and it may be that your prop shop has done this boat/engine combination before. Probably it's best to give it a try and adjust accordingly.

DougR

[glcalahan]

Thank you Doug! You certainly are on point...my ratio is 2.62:1. My bad as it appears that it maybe correct and the previous prop was 14RH10 so difference will 2° more pitch make?? (14RH10 to the new prop 14RH12?)

[DougR]

If you had a 14 X 10 prop before, I would guess that this was slightly undersized and would have let the engine over rev slightly, perhaps giving something like 3700 or 3750 RPM at full throttle.

Moving to a 14 X 12 with the additional two inches of pitch will pull the RPM down perhaps 200 at full throttle from where it was with the old prop. This might give a slightly higher mid range cruise speed at a given RPM.

I suggest you install the new prop and do some sea trials with a clean prop and a clean bottom. Record speed and RPMs and see where the max. RPMs end up.
If you are 100 or 150 revs below maximum ( 3600) you could have a bit of pitch removed from the prop, but as long as you don't push it hard above say 90 % of attainable RPM you should be OK as is.

DougR

[glcalahan]

Thank you Doug for your help. I am doing a complete refit of this boat....everything is new but the engine. I will be keeping records and certainly take your advice!

[Caliber40]

I'd call Michigan Propeller. They are pros and will tell you what you need.

Greg