Sea Nymph 16M - Benches Cracking

16Nymph · Aug 21, 2021

Picked up an early 90s Sea Nymph 16M which is a semi-V hull. Apparently has a bit thicker gauge aluminum than their other models. Anyway, I've been really pushing it to its limits in rough waters. Resultingly, its been flexing quite a bit and now the benches are cracking where they're riveted to the mounts at the hull. The cracks have gotten progressively worse and some rivets have broken off. It probably wouldn't happen if the support bracket extended the full width of the bench.

Any ideas on how one might repair this without somehow fabricating entirely new benches?

The best I could come up with is 'sandwiching' the width of the bench between 2 pieces of 1 or 2"wide aluminum flat bar and replacing rivets with bolts. Hopefully that'd stop it from getting worse and restore the structural support to some degree. I've put this off because the support bracket I'd need to remove for access uses solid rivets (not easily replaceable pop rivets) at the hull. I wonder if pop rivets would be ok to use here if I drill those solid rivets out.

I appreciate any feedback.

Weldorthemagnificent · Aug 21, 2021

Jeez that's a tough one. The aluminum is obviously fatigued and further stress will cause more cracking. This may cost a few bucks but what if you fabricated some aluminum rod storage boxes? 14gauge perhaps. The boxes would replace the gussets and bolt or rivet to the benches and hull at the gunwale. If you could get a flat bar stiffener underneath the bench (if accessable) even better. My thinking here is the boxes would lend some rigidity and share the load along the length.

If this isn't an option, maybe some gusset sides that overlap the existing seat brace and fasten also to the bench sides. A left and right piece for each bench end.

Sent from my CLT-L04 using Tapatalk

16Nymph · Aug 21, 2021

Great ideas, much appreciated!

While I love the idea of storage boxes, I'm hesitant to go down that road because of complexity and more likely the cost involved. This was supposed to be a keep it simple boat...I know I know :lol:

I'm trying to imagine the 2nd idea. The 'overlap existing brace' is kinda throwing me off. Are either of the ideas in this photo close to what you're suggesting?

Some sort of side brace or gusset is a great idea. The downside though going down this path is a bunch of new holes in the hull under the waterline with amateur riveting experience. I'm just trying to think through the downsides.

Weldorthemagnificent · Aug 22, 2021

B. But only against the hull above the seat, keeping any rivets or bolts above the waterline.

Sent from my CLT-L04 using Tapatalk

gogittum · Aug 22, 2021

A would prob'ly work quite well but B would require some fancy and precise bending to conform to the contour of the hull. Apparently, the existing braces have held up for a long time. How about simply making new ones ??

I've recently finished refurbing my 16 ft Starcraft Seafarer and learned a few things. Also done a lot of pop riveting in the past.

What my own experience has shown is: 1. Aluminum pop rivets are almost useless. Too soft. On my boat, I pulled a seat to install a folding grab bar and used 3/16" steel pop rivets to put it back and to repair some broken solid rivets. Like any pop rivet, they don't hold well in tension but they're extremely strong in shear - just about un-breakable.

I used steel back-up washers on mine and they hold "very" well in tension now. The washers are on the same rack in the store as the pop rivets. Peening them after popping "really" adds strength....if you can reach the back.

Using 3/16" steel rivets, you'll need a heavy duty puller. I found a dandy at Harbor Freight for $20 that makes it easy:

https://www.harborfreight.com/heavy-duty-17-1-2-half-inch-hand-riveter-with-collection-bottle-66422.html

Pulling steel rivets with a standard kit hand puller is extremely difficult, no matter how strong you are.

2. For the ones that go thru the hull, I used 3/16" sealed end, waterproof steel/aluminum rivets and they did very well and don't leak. I did bed each one in silicone, just to be sure.