Pontoon prop selection 3 or 4 blade??

[hoopstoon]

I have the same situation as another guy with a similar prop/boat. Specifics of boat: 22 foot bentley pontoon (2) with a optimax 115 and 15 degree pitch. my max rpm is about 5300 on a calm lake with no cavitation and probably 24.8 MPH top speed. Mercury guy advised me to go to a 13 aluminium so as to get up to 5700 which he says is WOT optimum. Their prop caculator gave me the idea my speed should be a lot faster, as they didnt have an option below 27 to select on the speed prop selector....

I did the prop selector on the Merc site and was advised to try a 13.75" 13 degree pitch spitfire which is a 4 blade. their discussion says it rund up to speed faster, holds better in turns and is just as fast. going down to a 13 should give me 400 RPM???? that is a question I guess--which puts me in the range of max wot, so would appreciate thoughts on the setup from anyone familiar with the 4 on a pontoon. ALSO, someone said to drop the motor down a notch, but when I look at it at top speed, I cant see the plate on the top of the prop at water height, it is about 2 inches or so under water. I had been told that plate (where the torque compensator is bolted) should be just under the crest of the water.. seems to make sense that with the axial rotation of a pontoon on a rough water that deeper would be better??? thoughts??

thanks
lake champlain is my territory.
Bob

 

[steelespike]

Re: Pontoon prop selection 3 or 4 blade??

I can't speak spicifically to the Spitfire but the hype does sound very good.
Pontoons produce high slip numbers.Any 4 blade will rersist slip better than a typical 3 blade.
A 4 blade will run smoother,resist venting and slip and handle better both underway and at the dock.I haven't seen any optimax tests but 115 Etecs on various toons speeds range from about 22 to about 34with 27 to 30 being typical.Have seen props from 11" to 15".
The 11 logically produced 22 at about 5600.The Etec has a 2.00 gear ratio.
As far as lowering the motor if your not ventilating a lot may not be necessarty.
But if your looking for improvements it won't hurt to try different heights.
Raising the motor is a possibility as well.Usually a 4 blade will allow maximum height.

 

[SparkieBoat]

Re: Pontoon prop selection 3 or 4 blade??

lowering the motor will hurt your preformance if you are not venting, raising it can help a lot. I have run my pontoons with a 3 blade 13P and gotten good performance. I would drop a pitch or 2 if going to a 4 blade, maybe a 12 or 11.