Trim Tab Cylinder Failure

On a 78' we kept on shearing the plastic pins rather than the whole cylinder. In the end we fitted 2 pairs of rams to each plate to spread the load and had a pump for each plate.

We also fitted stainless cables of the exact length so that they could not be over extended and strain the rams.

Lenco make replacement rams in stainless. Electric rather than hydraulic.

Bennett Hydraulic to Lenco Electric

Are your tabs bolted to the stern and extend out, or built in underneath? If extended, are you backing up fast while fighting fish? The pressure of the water while backing up maybe pushing down so hard it is snapping the rams? This may be the cause but hard to stop it from happening. Replacing with metal makes sense then.

If you're backing down with them halfway down it could do that. They could be in a poor location and the stern thrust just sucks them right down. But it sounds like the rams either binding, or possibly a hose is getting pinched and when you bring them up, the one ram still has fluid in it and running or something else causes it to snap. Another possibility would be the temperature where you are using the boat up there, might be making the plastic brittle. Have you tried calling Bennett and then Bob Roscioli to see what they think?

Contact Bennett. They have great customer service. As does Bob Roscioli.

It looks like the cylinder is being extended too far by the other cylinder. Can you run the tab all the way down and see if the cable is stopping the tab or is that an old picture? Are the tabs mounted wider at the top than on the tabs or is it the angle of the picture?

I would be tempted to unhook the cylinder from the tab and cycle it through its up and down and see if it follows the range of motion of the tab. I've never seen a bennett cylinder bolted anyway but flat on the transom. I would have to think the builder checked all that out when he made the tab recesses. It looks like its tearing the bottom out over time but it's hard to say from a picture. Interesting problem though

Were the failing rams on both trim tabs or only the stbd side?

Bennett makes a safety bracket that protects the cylinder from overextension much better than a cable would, they don't show it in their catalogs but if you call they will be happy to send you some.

Until recently when I worked with them on a really large set of tabs (72" wide!)I was completely unaware they offer a much more robust system designed for sportfishing and larger craft specifically addressing problems such as you are experiencing. Their SXT and BXT series use stainless rams and are a true hydraulic cylinder driven in both directions not just down with a spring return as the plastic ones are. Of course they are more expensive and require a different pump setup but in the grand scheme of things it is a drop in the bucket. I can't say I've ever seen a set on any builders larger boats but I will be using them on my next one.

That is really good to know, thanks. That info must have passed me by. I will be checking with my supplier here in the UK.

Bennett Marine: The world leader in trim tab innovation, quality and service

Bennett Marine: The world leader in trim tab innovation, quality and service

I think one is higher also, but it also looks like the geometry on the 2 rams is not parallel. It almost looks as if the outboard ram is angled a little more to the outside of the boat.

Looking closer at the pictures it also appears the safety cable is not tight when the rams are fully extended (if they indeed are in the pics).