prop cavitation and top speed ? updated with more info

[k5guy38]

So I recently purchased a sanpan 25ft with a 4.3 I/0. When at wide open which seem to be around 4700 rpm and trimmed all the way down I keep cavitating. I have tried adjusting the load balance and it does not seem to affect it. Do you guy have any ideas ?? Also it seems to top out at 27 mph according to my phone gps it does have lifting strakes and underskinned. I was thinking it should be faster than this ?? I know the prop diameter is 16" but not sure of the pitch yet.

[Bryden24shp]

Is it actually cavitating? When you cavitate a prop, you usually loose all bite and have to stop, and re-plane. Or is it slipping? Is the boat slipped or trailered, a huge amount of crap on the toons can cause performance problems like this. You said when trimmed all the way down, how is it in mid to upper trim angles? Lots of stuff to look at with this proplem. (ya....I know)
Keep us posted!

[k5guy38]

The whole trim thing is new to me. But that is a good question it sounds as though it is cavitaing and it will continue forward motion but at a lot less speed. The boat is slipped and the marina that I just bought it from didn't clean the toons to my likeing. I am going to pull it soon and redo them myself.

[Scott1]

Also make sure your trim gauge is correct. Did you look at the motor when it said it was trimmed all the way down? My buddy's trim guage was completely backwards when he got his new pontoon boat.

[RcgTexas]

I say it would be better to not trim it all the way down but not to high either . The prop shaft being parallel with the bottom of the logs would be a good place to start. If you don't have a center log or motor pod, you may be indeed having cavitation, but if so the rpm will fluctuate considerable, and the speed will drop off.

Let us know how its doing.

[brumbyvet]

It sounds like it could be prop slip also. Check the hub and make sure the prop is not slipping on it.

[teecro]

My boat did this my last time off ship. The wife & I were taking a ride running along at WOT and trimmed up no higher than normal when the prop out of the blue just blows out. We stop and check things out and find nothing. Try to get going again and the prop again blows out at WOT even trimmed down to which it mattered not if we weighted the bow or stern. So we turned around and stopped at the closest marina for dinner and gas as I was indeed low on gas as the tank took almost 28 gallons on a 35 gallon tank. On leaving the marina we could no longer get the prop to blow out at WOT with or without trim...

So while this may not be your problem it does sound close to the same. I'm going to try both a 3 blade prop with one inch less pitch but with extra cup as well as a new 4 blade Spitfire as I did chowder my prop on the rocks a little last year trying to run a shallow hourglass shaped shortcut...

[k5guy38]

Ok guys I have been running my pontoon for a couple weeks now and have some better info. The prop is a 3 blade aluminum 16 diam x 14 pitch. The prop does seem to be blowing out to the point where I have to stop and start over. I have tried different trim situation while keeping the weight on the boat the same along as fuel. Last night I had 5 adults 3 kids a fuel 37 gallons of fuel and a cooler and on clam water or cutting wake it was fine but if the water was choppy it would blow out?? Also when the water was smooth I could max it out at 5200 rpm which I know is to high and she seems to really like 4500 What should I do for a prop change to try ?? Also do you think a whale tale would help the blow out ??

[garygve]

I would try a SS prop they don't seem to blow out as easy and they grab the water quicker after blow out. I have a Mercury Enertia that will blow out on a sharp turn when pulling a tube with the engine trimmed slightly but if I tuck it back in while moving it bites back in. When I try the same thing with a aluminum prop it blows out sooner and i have to really slow down for it to grab the water again. There are other SS props that would probably work see if you can find a dealer or prop shop that has some used props that you can try.

[rancherlee]

I'd suggest 4 blade, Aluminum or stainless. My pontoon doesn't like 3 blades much at all, even 3 blade stainless will blow out fairly easy. I REALLY have to work hard to get the 4 blade to aerate. The problem on mine its my DIY negative angle lifing strakes, I installed them the FULL length and they trap air and bring it all the way back to the prop.

[k5guy38]

Ya i have been thinking about tryin a 4 blade aluminum. I am affraid of a stainless because our river always has crap it and the level changes alot. The lifting strakes are factory along woth the under skinning.. Is there a certain brand 4 blade i should look for ???