Trim tabs and cavitation

Installed a set of home-made trim tabs and they perform better than expected. Was able to get rid of the hydrofoil, reduce planing speeds to about 15-16MPH and eliminate porpoising at high speeds. Now for the problem . I think I am getting some cavitation now when trimmed up and running at WOT. There is a "hollow grumbling sound" I didn't have before and occassionally the prop will loose its bite and the rpms will increase. Can you hear cavitation ? The transom is only 58" wide at the bottom and I'm thinking the tabs are introducing air or excess turbulance into the prop area. There is about 16" from inside edge of the tab to the centerline of the boat (or 32" between inside edges of tabs). The tabs are 10" wide x 7 1/2" deep flat 1/8" SS plate. I was thinking of adding a downward facing angle to the inside edges of the tabs to try and minimize the effect of the tab wash into the prop area. Anyone experience similar situations with tabs ?

Re: Trim tabs and cavitation

The tabs are lifting the back of the boat more than it used to which is also lifting the prop more out of the water. Also they are probably not allowing the front of the boat to come up as much when trimmed which also changes the angle on the prop making it sit higher and need to be trimmed more. Result- easier cavitation and blowout.This is one reason why they make power adjustable trim tabs.

Re: Trim tabs and cavitation

RodC<br /><br />If you recall on the last posts we discussed the compromises that you would need to deal with when adding fixed trim tabs. You will need to reduce the angle of deployment. ddaigle is correct the trim tabs are lifting the stern too much at cruising speeds. This will get worse as the speed goes up. You are running bow down and stern up.

Re: Trim tabs and cavitation

Yup to above. The trim and tabs are actually fighting each other at high trim levels.<br /><br />Just to clarify, what you are hearing is actually ventilation not cavitation. Ventilation is the prop grabbing air, cavitation is actually extreme low pressure in front of the blades literally causing the water in the immediate vicinity to boil. I don’t know if you can hear that.

Re: Trim tabs and cavitation

glad you pointed that out, quietcat. drives me nuts when people refer to their engine having a cavitation plate, when it's actually an anti-ventilation plate.

Re: Trim tabs and cavitation

There is actually a decent explanation here on Iboats. Hard to find through the site, but I Googled cavitation vs. ventilation and got this . . .<br /><br /> http://www.boat-props.com/prop_info_2.cgi?page=ventilation

Re: Trim tabs and cavitation

Yes, cavitation can cause wear, usually starts as little pinholes but can get real ugly.

Re: Trim tabs and cavitation

Thanks for the responses. Makes sense the way you describe it. Nautijohn, I basically built a set of home-made self adjusting tabs much like the Smarttabs. The only pieces I needed to buy were the gas springs and mounts. Everything else was constructed from materials I had around the shop. Will changing the force or the angle of deployment have a more positive affect ?<br />Quietcat thanks for the clarification on cavitation vs ventilation. I was never sure of the terminologies. Is cavitation what can cause wear on the prop blades ?<br />Any thoughts on the addition of the angle pieces to the tabs ?

Re: Trim tabs and cavitation

RodC;<br /><br />The plate brackets allow you to change the leverage point without discontecting parts or remounting. This would allow you to reduce the prssure which is causing your bow down issue. These brackets could be mounted to your eisting plates. Both the actuators and the brackets are available as replacement parts.

Re: Trim tabs and cavitation

Any thoughts on the addition of the angle pieces to the tabs ?

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I Can't see this as a Good Idea...... It Could have Alot of Undesirable Effects..... <br /><br />They would at as a Pair of Rudders.........<br /><br />I'd try changing the angle/pressure on Your Tabs,+ Trimming Out Less.........

Re: Trim tabs and cavitation

Id lighten up on the amount of angle you have. Keep changing it untill you reach a happy meadium which is all you can expect.

Re: Trim tabs and cavitation

Guess I'd better try the angle and pressure adjustments first before adding anything else into the equation. Never thought about the rudder affect, thanks for pointing that out. I'll go play for awhile and report back.