Cavitation

[Pbakk]

Hello. 2019 23’ GSR pontoon with Yamaha F115 and Ultra Legs (Sea Legs). Stock white Yamaha 3 blade prop. Wide open throttle exactly right RPM’s (5,800ish) but never able to trim motor as prop catches air / cavitates. Will check motor mount position this week, but thinking that is the only option (see if motor can be lowered). It has been like this since purchased new. Top speed is 21 to 23 mph. Am I missing anything. Should I be able to trim motor. Maybe it is the sea leg weight? Thanks.

 

[Pbakk]

Looks like the motor is mounted as low as possible. Other than chasing prop out, which would change RPM’s, not sure if anything can be done to avoid cavitation when trying to trim up

 

[Michiman]

My uneducated guess would be that the extra weight of the sea legs is contributing to the issue. From your picture, it also appears to me that the engine height was previously adjusted, not that that matters. And, as an aside, I think your problem is actually ventilation rather than cavitation. My recommendation would be to reach out to the folks at Propgods.com to see if they have any suggestions.

Good luck!

 

[Pbakk]

Thanks. Much appreciated.

 

[Potomacbassin’]

What is your engine model and prop size? A poorly matched prop will absolutely slip too much in turns if encountering any kind of resistance or ventilate if too much diameter. A short shaft outboard (20" vs 25") will be more susceptible to both of these.

 

[Vikingstaff]

The extra weight and distribution of that weight after adding my sea legs in 2019 caused this to happen a bit at certain trim points when tubing where it had not done so prior.

I can trim out of it, but it is different now. I have sometimes thought about experimenting with going 1p smaller in the same prop to see if that dials everything back in, but since its a minor issue, and correctable with trim adjustments for us, we have not sprung for it given prop costs.

Maybe there is some wiggle room for improvement for you as Steve and Poto mentioned above???

 

[Pbakk]

What is your engine model and prop size? A poorly matched prop will absolutely slip too much in turns if encountering any kind of resistance or ventilate if too much diameter. A short shaft outboard (20" vs 25") will be more susceptible to both of these.

Engine is standard Yamaha F115, non short shaft, turning standard aluminum 13.5 x 12-k Yamaha 3 blade prop installed by dealer with no damage. Interesting that I noticed neighbor with sea legs, lifting strakes, Premier pontoon with Yamaha 115 Vmax turning a 4 blade black prop. Always thought that the rule was anything under a 150hp should use a 3 blade.

 

[Potomacbassin’]

I would suggest your neighbor is using a 4-blade to get more surface area and reduce the slip. That's probably your issue is the combined weight of your hull and sea legs are creating so much resistance that the 13.5" diameter prop can't move enough water to overcome it.

If you really believe your prop is ingesting air (ventilating) maybe show a picture of where your lower unit sits level in the water. But my guess is the prop is slipping so going larger diameter might be the cure at the expense of losing a little speed. Props are all about tradeoffs.

 

[Bill N]

I am not well versed in props, but the reason there is so many options is that they excel in different areas. Seek out one that resists ventilation better than others.

I'm guessing you have a short shaft which would be the correct engine for a 2 tube boat.
F115LA or F115 LB = 20" shaft (short shaft)
F115XA or F115 XB = 25" shaft (long shaft)