Repairing H-18 Stern Damage/Transom Damage

mrkurtz

Rank: Lubber

Registered: Jul 27, 2008

Last visit: Oct 09, 2011

Posts: 8
Posted: Jul 27, 2009 - 06:04 PM

I am finally getting around to fixing my H-18 transom.
I have the photo in the beachcats technical section titled Stern Damage. With the broken part cut out, the stern looks like the photo before it titled Transom Damage.

I put in an access port, and I have my materials ready to go.

My question is: Should I try and emulate what was there before, with fiberglass/thin foam/fiberglass? or should I just build it completley with fiberglass?

I realize with solid glass there will be a slight weight factor. I am also wondering if it will be stronger, or perhaps, without the foam, more brittle and weaker?

My plan was to put in a thin piece of foam, like builder's blue-board, to match the core, and fiberglass on both sides, inner and outer.

I am not racing this boat, just having fun, but I would like to have my repair stay in place.

Am taking photos. Will post as repair continues.

Thanks.
DaC

Rank: Lubber

Registered: Jun 26, 2009

Last visit: Dec 29, 2009

Posts: 27
Posted: Jul 27, 2009 - 06:55 PM

Foam sandwich construction adds excellent strength with less cost (because foam is cheaper). But the other side is that it's also much LIGHTER, as you have already pointed out. If done right, it's also incredibly strong.

Flexing is the problem with a core design, as it leads to the skins delaminating over time. Making it strong enough to avoid excessive flexing is how you overcome that problem. The dilemma is how strong it needs to be to stay light enough to justify using it in the first place.

It's the same principle as torsion box vs. solid beam construction (wood). A 2 inch thick torsion box beam that is 10"x72" with only 1/8" skins will deflect less than 1/4" under 250lb load. The solid version will deflect less, but at 4 times the weight.

A guy at my boat yard was experimenting with foam (vacuum bagging) and balsa core construction and he made up a set of dorade vents with just skins and a balsa core. He was testing to see how strong they were and drove his car over them!

The guys building boats have gotten pretty good at the technique of using cored construction. Having a boat with a sandwiched core no longer means your bound to see delamination down the road. But if you are not careful about getting out the air pockets between the skin and core, you might find yourself having to rebuild them down the road.

If it were me I would forget the foam and layup with solid glass. It's only going to be brittle if you have too much resin (70% glass to 30% resin is ideal). So when you layup you want to make sure that your rolling excessive resin out of your cloth because it's only going to make it brittle and add weight.

When I use Knytex (18 or 20 oz) cloth I put the cloth down and pour resin over it and manually spread it around. Then I roll the cloth tight like a cigar which squeezes the resin well into the cloth. By rolling it tight a few times any excess resin gets squeezed out of the cloth. Your gonna need to use something as a backer board to help you lay the new cloth in place.

Dan