Rebuilding transom - how many layers of fiberglass?

[ants]

Hi,

I am rebuilding the transom on a 17' speedboat which is unusally shaped, and am concerned to ensure it is strong enough as obviously it takes the weight and force of the drive leg and half the engine.

You can see a pic of the inner transom HERE before I glassed it over, as you can see it has a flat section in the middle and curved either side so cant just use a large sheet of ply. The curved sections had 4mm balsa core (rotten), which I have replaced with hardwood strips bedded onto thickened epoxy, then sanded to the curve (same thickness as previous balsa core). The flat section has 45mm of marine ply (originally 36mm thick which was also rotten) again epoxied to the original outer skin. The outer skin itself it approx 4mm thick.

The central square flat section has now had 2 layers of 300gsm woven roving layed onto it, overlapped onto sides, top, and bottom, plus extra strips around the edges. The curved side sections have had 1 layer 300gsm woven roving, and 3 layers of 300gsm CSM, again overlapping onto the sides of the hull and back onto the square flat section, plus a couple of extra strips at the outsides and bottom. All layers have been laminated with polyester resin, and all the wood was primed with polyester thinned with styrene before being laminated over.

Do you think this is suffiecent or do I need lots more layers, or more additional strips at any edges? From memory I think what I have done is at least as thick as the GRP that was there before, but I was stupid in throwing the old grp away long before I relaminated, so cant now compare the two. It seems pretty solid to me, but I imagine the forces of an 140hp outdrive pushing into it to be quite large!

Someone has suggested that all the edges should have a curved fillet about the radious of a jam jar, and then laminate that a few times. It never had it to begin with, is it worth it or shall I add more layers over everything?

Many thanks,

Anthony

[dereksireci]

Just a couple of observations as I am not an expert on outdrive boat transoms.

Try to make the transition from the solid core in the center to the curved outer part less abrupt. Any time a laminate changes thickness quickly you set up a stress point.

The hull structure (stringers) are removed in the photo. Make sure when you install the stringers, they tie into the solid part of the transom.

djs

[ants]

Hi,

Thanks for the observations, I did wonder about that center section myself, but thats how it was originally built, I have only cut away the inner skin over the wood and replaced the wood, then reskinned it. The sides of the square center section , i.e. where they run aft from the edges of the center square to join the curved sections seem pretty solid, and I actually suspect that they support the outboard drive more than the curved side panals.

No probs re the stringers, they will be replaced and laminated into the transom, as will the sides of the boxes further out to the sides, that contain large volumes of foam. Both of those will be made from 9mm ply which is how it was originally.

Still anyone got any idea on how much to laminate the inner skin? I am thinking of putting another 3x 450g csm over the 3 layers it has already, that will prob make it thinker then the outer skin so should be plenty I figure?

Anthony