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Trim Tab Cylinder Failure

Discussion in 'Stabs, Tabs & Gyros' started by DOCKMASTER, Jan 28, 2014.

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  1. DOCKMASTER

    DOCKMASTER Senior Member

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    Last year when hauling the boat for the first since purchase I found one of my trim cylinders failed. The ram broke right in the middle of the plastic cylinder body. We replaced the cylinder and put the boat back in the water. After a month or so another cylinder broke. Again, it was a similar failure where the plastic cylinder body broke. Thinking the cylinders were just old we replaced all four and went on with the season. We recently hauled the boat for winter storage and upgrades and found one of the new cylinders installed had failed some time during the season. Any idea why these would continue to fail? They are Bennett cylinders and each trim tab has two cylinders. The boat is a 54' Donzi Sportfish. I'm not aware of hitting anything and the tab itself shows no sign of impact.

    Does anyone know if you can get cylinders that are not plastic? Perhaps aluminum or stainless bodies?
  2. Fishtigua

    Fishtigua Senior Member

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    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
  3. mwwhit1

    mwwhit1 Senior Member

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    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.
  4. DOCKMASTER

    DOCKMASTER Senior Member

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    The tabs do not extend past the transom. There is a recess they fit into. And where I fish there is no backing down on the fish so the only stern way is backing into my slip which is done in low idle at very slow speed.

    My tabs already have two rams per tab and also have stainless cables to prevent overextending.

    I'm really puzzled why these keep breaking.
  5. Capt Ralph

    Capt Ralph Senior Member

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    What king of break? Bend, compression or tension.
    Bennett Rams are pretty tough. It does take allot to break them.

    I have witnessed the below;
    Fork lift operator snapping them?
    Beds on your trailer hitting them when retrieving the boat?
    To fast of a launch from the trailer.
    Is your diver using them as a step?

    If you install a Lenco and whatever is happening continues, You could have more extreme Damage.
  6. DOCKMASTER

    DOCKMASTER Senior Member

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    The cylinder body (O.D.) is literally broken in half but the push/pull rod remains intact. I don't know what you mean about forklift operator snapping them and the boat is not trailerable so there is no launch and retrieval.
  7. Capt Ralph

    Capt Ralph Senior Member

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    I messed up & missed the 54Donzi Part. My examples were for smaller boats.

    We have a 58' Bert with the same stuff. Heavy tab use and never had a failure.

    When the cylinder failed, does it look like a failure at the molding seams?
    I can not imagine the stock pump(?) blowing up the cylinders.
  8. Capt J

    Capt J Senior Member

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    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?
  9. DOCKMASTER

    DOCKMASTER Senior Member

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    Those are good possibilities I need to check on. I have not contacted Bennett or Donzi yet but plan to.
    I don't think it is temperature related as our summer water temps are low 60's and lots of boats up here with these systems and no problems. My last boat had them and never an issue.
  10. Capt Bill11

    Capt Bill11 Senior Member

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

    DOCKMASTER Senior Member

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    Thanks. Will do. Here's a few pictures as well. You can see the cyliner on the right is broken and the spring has fallen out of the cylinder.

    IMG_0603.jpeg

    IMG_0605.jpeg
  12. dsharp

    dsharp Senior Member

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    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?
  13. Capt Ralph

    Capt Ralph Senior Member

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    Maybe not from backing down but a stern wake from chopping the throttles quickly (in forward motion) could be a culprit. That's a serious down force popping the bottom out.
  14. dsharp

    dsharp Senior Member

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    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
  15. Capt J

    Capt J Senior Member

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    Were the failing rams on both trim tabs or only the stbd side?
  16. Bill106

    Bill106 Senior Member

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    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.
  17. Fishtigua

    Fishtigua Senior Member

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    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
  18. RER

    RER Senior Member

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    I agree with dsharp ...it looks to me like the outboard ram is mounted a good 1/2" higher than the inboard when indexing from the top of the units. It would bottom out before the inboard ram. Same one keeps breaking?
  19. Capt J

    Capt J Senior Member

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    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.
  20. Bill106

    Bill106 Senior Member

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    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).