Strength of Fiberglass Transom

Discussion in 'Boat Design' started by rrrwel, Jan 26, 2010.

  1. rrrwel
    Joined: Jan 2010
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    rrrwel Junior Member

    I'm mounting a swim platform to a 1988 37' sportfish bridge. The transom is not cored, solid fiberglass. My concern is the fiberglass is just over 1/4 " thick.

    The platform is about 11 1/2 feet wide and will mount on 5 brackets thru bolted to the transom. I plan on bolting thru to some good sized back plates for some added stiffness and load area. They will not be glass in.

    The transom does have some other structual attributes with the floor decking galssed to it along with the flush trim tabs having a it's enclosure glassed to the transom, but still is a 1/4 inch strong enough for this? I'm not worried about the weight of the platform I'm worried about having a couple people standing on it.

  2. mark775

    mark775 Guest

    I've noticed the same thing on Bertrams. I was charged with building a FRP swim platform for a Bertram and when I looked at the transom it was miniscule compared to the sides of the boat - I guess they figure that there's never much force acting there. I could literally put my shoulder against the transom and buckle it in - like a Japanese car body. It was very clean in the lazarette, so I lightly sanded, wiped with MEK and layed two plys of 3208 BTI from the stringers, up the transom, and about six inches under the deck. In all, it took about an hour and I felt much better about it. Of course, large backing plates are de rigueur. Don't worry about splashing from two people standing on it. It's just that it wasn't designed to have a platform there and all that flexing will eventually loosen something. More force than standing on it is often waves from astern slapping under it when anchored. Huge stresses there and I've seen seemingly pretty add-ons crack at the joint from this force. Nobody dies from this flimsy transom but you should build it up IF it flexes much.
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The platform won't fall off if you use backing plates, but the transom will look better if you use one long strip of marine plywood and a few layers of glass instead of separate pieces.
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If it starts flexing, you may end up with gelcoat cracks. A large backing plate would be good.

  5. TollyWally
    Joined: Mar 2005
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    TollyWally Senior Member

    While potential rot is a very real downside plywood is a very traditional cheap material for gaining both stiffness and strength. If I didn't want to go to the effort of stiffening the actual transom I'd use as big a piece of prepainted treated plywood as I could fit as an inner backing plate with some stout metal plates over the top of that.
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