Ventilation/Cavitation Problems

[Sea&Air]

My Boat, a 30' Searay, has two Mercrusier 260hp V8s coupled to Alpha One I/Os. I have hydraulic trim tabs but still seem to get a lot of ventilation/cavitation. I think the boat was just designed poorly because at 75% power I can see the top of the ventilation plates sticking out of the water. Does anyone have any suggestions? I've considered installing either a Stingray, Whale Tail, or some other type of hydrofoil but on these pages I see people say that they are junk half the time, the other half swear by them. I had our local prop shop change the props because of this issue. Any ideas?

 

[TilliamWe]

Re: Cavitation Problems

Re: Cavitation Problems

I suggest looking at the props. I would avoid a whale tail. I doubt the boat's design is bad.

And I also suggest that you call your problem "ventilation" and those plates on the drive "ventilation plates," because that is most likely the problem you are having, and that is what they are called.

 

[Sea&Air]

Re: Cavitation Problems

Re: Cavitation Problems

"Ventilation". Thanks Captain.

 

[Grand Larsony]

Re: Cavitation Problems

Re: Cavitation Problems

I have a 1989 SR 300 Sundancer with 260 merc Alpha1's and don't have any issues at all. First ensure you have clean props, outdrives, hull, and tabs.

Start with drives all the way down (trim) and trim tabs full down. This minimizes wear on the drives and gets you up quickly, avoiding bow rise.

Throttle up to 3000 rpm. When boat comes up, rpm will increase to about 3,400.

Raise trim tabs all the way up (about 8 seconds of switch activation). Raise outdrives 1-2 seconds each. Reduce throttle to 3,000 rpm.

This gives me best operation and about 30 mph\kph speed. Boat feels right, running free and easy with lots prop bite. Usually well under 15 gph fuel consumption, too, although that's only a very rough estimate and I don't really care that much.

I then apply very light trim tab to level out if conditions or boat loading warrant such (usually not required). For large parties of 6-8 standing in the cockpit I usually need to put them both down a little to hold plane at 3k RPM.

What year and configuration are you working with here? Sundancer, weekender, etc. ?

 

[tashasdaddy]

Re: Cavitation Problems

Re: Cavitation Problems

agree proper use of the trim and hydrolic tabs, tab and Hydrofoils are not to be used together. also hydrofoils, apply lifting pressure on the outdrive, not where it belongs, the lifting pressure belongs on the bottom (lower) transom. lift the boat.

 

[Philster]

Re: Ventilation/Cavitation Problems

"...at 75% power..." is helpful, but not complete information.

Is this at 75% under cruise and while on plane? I.E., planing means you've already overtaken your bow wave and are planing.

Or is this under load/accel, when trying to come on plane and climb over your bow wave?

Under accell, your outdrives need to be trimmed down (not to be confused with trim tabs). This raises the bow and lets the props bite. When on plane, you then can raise the outdrives to lift more hull out of the water. Lift them too much and you have issues, so you have to go as far as it helps performance, not risk stability or not have the props lose too much.

Don't confuse trim tab trimming with outdrive trimming. Accelerate with outdrive trim low, and raise as speed builds to get more hull out of water and stress off engines.

In calm waters, you should find the trim tabs mostly unnecessary. I'd just keep them up and out of the water unless specific sea and cruise issues required they be lowered (one or both).