The plywood backing plates for our Oday 28's seacock thru-hulls have dry-rot and are going to be replaced as a DIY project. Currently, the the screws securing the thru-hull and seacocks are on the outside of the hull. My intent is to repair/fill the screw holes which go through the hull and build up a fiberglass backing plate on the inside of the hull upon which the flat surface of the seacock will be attached. Here's my question. I have been advised to "glass in" bolts during the build up. These bolts will hold the thru-hull/seacock in place. My concern is that the bolts may break free of the fiberglass "backing plate" when torque/pressure is put on the nuts securing the thru-hull. Has anyone done this before and can you offer any advise/tips or simply allay my concerns? Thanks.
DIY New Thru Hull Backing Plates...advise needed
- Thread starter flyhop
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Leave well enough alone
I'd be inclined to leave the mounting bolts as is. This is the best way to secure a thru hull. As far as the backing plate goes, marine ply, or fiberglass laminate will work equally well. Consider how long the current backer plates lasted.
I'd be inclined to leave the mounting bolts as is. This is the best way to secure a thru hull. As far as the backing plate goes, marine ply, or fiberglass laminate will work equally well. Consider how long the current backer plates lasted.
wood backing plate
We didn't use glass to build up the inside when we took out the rotted backing plate, we just belt-sanded a new piece of plywood to match the curve of the hull. It was either pressure treated or a teak/sapile mahogany ply. Then putting good torque into the seacock while bedding it was able to grip the wood without potentially cracking anything. We didn't use screws despite Casey's book description of that. Glassing in bolts somehow doesn't appeal to me either, perhaps because of not being able to easily take it apart again without grinding and surgery. So far so good with our 4month old repair.
We didn't use glass to build up the inside when we took out the rotted backing plate, we just belt-sanded a new piece of plywood to match the curve of the hull. It was either pressure treated or a teak/sapile mahogany ply. Then putting good torque into the seacock while bedding it was able to grip the wood without potentially cracking anything. We didn't use screws despite Casey's book description of that. Glassing in bolts somehow doesn't appeal to me either, perhaps because of not being able to easily take it apart again without grinding and surgery. So far so good with our 4month old repair.
Fly hop, why not use a seacock without bolts?
The seacock has a mechanical connection of threads thru the hull to a thru-hull fitting. Why not allow that to be the connection? The chance of the thru-hull breaking off the seacock is minimal. I think your idea of building up the area is OK if you do not make a hard spot. I would taper the buildup to allow for natural diffusion of the loads on the thruhull. The hoses should allow enough relief of stress. The factory did want to go to the expense of building up individual thru-hulls so they used wood. r.w.landau
The seacock has a mechanical connection of threads thru the hull to a thru-hull fitting. Why not allow that to be the connection? The chance of the thru-hull breaking off the seacock is minimal. I think your idea of building up the area is OK if you do not make a hard spot. I would taper the buildup to allow for natural diffusion of the loads on the thruhull. The hoses should allow enough relief of stress. The factory did want to go to the expense of building up individual thru-hulls so they used wood. r.w.landau
Instructions
Valves & Thru-Hulls Installation at: groco.net/SVC-MAN-06/Seacocks-06/PDF/IBV-FBV-TB.pdf http://www.groco.net/SVC-MAN-06/Seacocks-06/PDF/IBV-FBV-TB.pdf
Valves & Thru-Hulls Installation at: groco.net/SVC-MAN-06/Seacocks-06/PDF/IBV-FBV-TB.pdf http://www.groco.net/SVC-MAN-06/Seacocks-06/PDF/IBV-FBV-TB.pdf
Thru hulls
Cruising World had a good article on this a few years ago. I followed it when replacing all mine thru-hulls last winter. 3/4" marine plywood disks, sanded/routered with rounded edges, glassed to the hull with 3 layers of cloth. Then lag screws through the seacock flange and into pre-drilled holes (after the seacock was in place) in the plywood. The plywood was sealed with a few coats of epoxy first. If you did want to embed threaded rods or bolts into the backing plate first, doing a good epoxy job should be enough to make sure they won't come out while torquing the nuts on (see link).
Cruising World had a good article on this a few years ago. I followed it when replacing all mine thru-hulls last winter. 3/4" marine plywood disks, sanded/routered with rounded edges, glassed to the hull with 3 layers of cloth. Then lag screws through the seacock flange and into pre-drilled holes (after the seacock was in place) in the plywood. The plywood was sealed with a few coats of epoxy first. If you did want to embed threaded rods or bolts into the backing plate first, doing a good epoxy job should be enough to make sure they won't come out while torquing the nuts on (see link).
wood has a virtue
I don't think there are too many thru-hulls that use bolts through the hull any more: if you're going to do this job, given how much oxidation/corrosion there may me inside the valves, why not buy a new fitting and reset the clock on problems? When I replaced a rotted wood flange on an earlier boat I had considered using Starboard, which would never rot, but decided to go with wood again because it had the virtue of allowing itself to be glassed in and epoxied. Starboard doesn't stick to epoxy products very well. Glass it in or coat it with epoxy and you will never do this job again...
I don't think there are too many thru-hulls that use bolts through the hull any more: if you're going to do this job, given how much oxidation/corrosion there may me inside the valves, why not buy a new fitting and reset the clock on problems? When I replaced a rotted wood flange on an earlier boat I had considered using Starboard, which would never rot, but decided to go with wood again because it had the virtue of allowing itself to be glassed in and epoxied. Starboard doesn't stick to epoxy products very well. Glass it in or coat it with epoxy and you will never do this job again...
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