Need prop advice - low WOT

[gibsog]

Hello,
I have been impressed with all the knowledge exchanged on this site, and have a problem I am hoping someone can help with. I purchased a 22' pontoon with a 50hp Johnson 4-stroke (read Suzuki) on it. The guy I got it from was running a 11 5/8 X 12p 3 blade prop, which got me to 5000 RPM WOT with no ventilation, and a speed of about 15 MPH. The specs for this motor are around 6000 RPM WOT, so I tried to make a prop change, hoping to increase my WOT as well as reducing the 40% slippage I was seeing (2.27 gear ratio, BTW). I did a bunch of research and ended up with a 12 X 9p 4-blade Vortex prop. This new prop ventilates terribly. My top speed is now 13 MPH at half throttle - anything more and it ventilates and revs to over 6000 RPM.

Clearly I have made a mistake. I had high hopes for getting my WOT RPM up, and get a few MPH more, but this was not the right thing to do. HELP!

 

[Dakota47]

Re: Need prop advice - low WOT

put the old one on.. it aint no jet boat..

 

[steelespike]

Re: Need prop advice - low WOT

Boy that has to be dissapointing.I can't give much techical info but it sure woud seem to me a 4 blade would do better.you could try saving it by lowering the motor one hole.(vertical height)The slight difference in diameter
may have introduced the venting problem(bringing the tip a little too close to the surface)I would think if you can get the 4 blade to bite it would get you where you were hoping to be.I think with an ideal setup you could approach 18-20mph.I bet slip figures are higher on a pontoon.

 

[gibsog]

Re: Need prop advice - low WOT

Hmmm, not a jet boat. I guess that is true enough. My main reason for a prop changes was to get the WOT RPMs up into the right range (~6000). Everything I have read on this site, and everywhere else for that matter, says that is THE most important feature when choosing a prop, or you will trash your motor. I was not expecting a lot more speed (especially going DOWN 3 inches in pitch), but was hoping the additional thrust of the 4-blade, larger prop may decrease the slippage to something a bit more reasonable even for a toon, like 25-30%. Between the decreased slippage and increased WOT RPM, I was thinking 1-2 MPH gain may be realistic.

As far as lowering the motor, that is a fine idea. Unfortuantely it is already at the bottom, resting on transom.

Disappointed - yup. Surprised - nope. I know props are a bit of a black magic thing. Any other thoughts appreciated.

 

[Silvertip]

Re: Need prop advice - low WOT

I have a 40 Suzy (same as your Johnson) on my pontoon and it was fitted with a 11-1/4 x 11 three blade. Seems to dig fairly well. I have no WOT data yet since we are still in break-in mode but there is no evidence of ventilation. I would expect the four-blade would work fairly well. You might check weight distribution to keep the stern down a little if you can't lower the motor. You might also check "under" the toon to see if there isn't something hanging down that disturbs water flow to the prop.

 

[gibsog]

Re: Need prop advice - low WOT

Thanks Silvertip. The only thing hanging down is the motorwell, or whatever that big basket in front of the transom is called. It does disturb the water flow, but only at "higher" speeds (like where it starts to vent). The motor's anti-ventilation plate is about even with the bottom of the motorwell. I could see where lowering the motor about 2" would help. Any ideas how to do that??