In our recent test of waterproof epoxies for making underwater repairs, we also looked into making underwater patches. Obviously, your success in this effort depends a lot on the situation and the size of the repair. And luck.
A common scenario is that only part of the damaged area is underwater. A boat that has been holed just below the waterline can sometimes be heeled on the opposite side of the hole, so that only part of it is submerged. Another common situation is that you are making repair in a flooded bilge inside the boat. With fortune on your side and the right tricks, you can make a pretty serviceable patch in either of these scenarios.
Here are some of the things we learned during our recent test of underwater epoxies.
- Laminating resin. It must be relatively thin and capable of both bonding and curing underwater. West Systems G-Flex will do that.
- Cloth. You want to do this in one layer, so you need something that both conforms well and builds some strength. Two layers of 1708 biaxial fiberglass is a good choice.
- Prepare the surface. Bottom paint needs to be sanded off to bare fiberglass. Use course wet/dry sandpaper.
- Pre-wet the cloth on dry land. This is the key. Pour the resin onto the cloth, spread, and work through the cloth using a laminating roller. Ideally, this is done on top of peel ply, but a plastic trash bag will work perfectly well in a bind.
- Apply. With the peel ply still in place, dive under the boat and press firmly over the desired area. Working from the center, slowly work your way outwards to make a good bond. Leave the peel ply in place.
There are two reasons for using peel ply. One is for ease of handling in the water. Although we usually work pre-laminated cloth without peel ply on dry land, it makes handling it in the water much easier. Because the cloth will initially stick to your hands more than the boat, the peel ply makes the patch much easier to apply. Finally, if there is still some inward leakage, the peel ply prevents the water flow from blowing the resin through the cloth and creating leaks.
Once resin is cured, you can remove the peel ply if you want to, but there’s really no need to for an emergency repair, other than to inspect your work. Just leave it.
You can buy a kit for this (compositepatch.com). It comes prepackaged, with resin and fabric in a bag. At over $250 for a few square feet it’s rather expensive, but it makes for neat work in awful conditions.