Plywood Hull Crack Repair

[SilhouetteII]

Hello everyone,

I own a 1958 Silhouette II made from plywood and most likely originally kit built. A crack has formed in the hull adjacent and parallel to one of the bilge keels which I think has been caused by a rough ride on the trailer over the years, flexing the keel outboard (laterally) and stressing the hull. The crack is fairly obvious from the inside (not so obvious from the outside) and does let some water in. A perfect repair would involve taking a the keel off which I don't want to do as I want to sail this year!

The crack can be seen above the keel bolts in the interior photos where the paint has been ground away. The rust marks were from the old keel bolts which have now been replaced with stainless. The last photo is of the affected bilge keel looking forward from the outside, some cracking is faintly visible to the rear of the keel on the left hand side.

I don't have any experience in working with plywood or boat building but can do a bit of DIY. To keep costs down I will need to do the work myself. Having done a bit of reading, I would be very grateful if people could let me know if the following plan would be suitable:

1. Epoxy new ply patch to the inside of the hull
2. Sand paint away from hull exterior and bond some fibreglass matting over the crack, repaint.

Am I on the right lines here? Are both steps necessary or can I just patch the inside of the hull?

I have attached some photos which hopefully make the above text a little clearer. Advice is gratefully received, thanks for reading.
PXL_20230314_152241292.jpgPXL_20230314_152256028.jpgPXL_20230314_153058849.jpgPXL_20210716_155323978.jpg

[spirit]

Re: Plywood Hull Crack Repaibr />
I think it's time to take the keel off and save what's left...

[Peerie Maa]

Re: Plywood Hull Crack Repaibr />
Take off the bilge keel and the internal and external doublers. Then, at the bare minimum, fit a much wider bilge stringer. If you are up to it, scarf in a dutchman to replace the crumbly ply with at least 1:6 or 8 slopes all round.
The crack is too close to the doublers for any internal patch to be strong enough without taking out the bilge stringer.

[Woxbox]

Re: Plywood Hull Crack Repaibr />
What you describe doing is a patch that may last some years, or it may not. It's possible the damage came from a single impact at some point, but it may well be what you suggest -- years of riding on the trailer. If that's the case, then the other bilgeboard is going to be suspect as well.

So you can patch it and go sailing this year -- keeping a careful eye on both keels. But sooner or later the boat is going to need more care than that.

If it were my boat, I'd remove both keels and fit a strip of plywood inside that would extend a few inches out either side of the keel stringer and bond to the ply hull. This would have to be tied in carefully with epoxy, filler, etc., of course. Then put the keels back on. It looks like the bolts are long enough to take an extra 3/8" thickness. This would spread the torque and impact loads from the keels over a wider area of the hull. Right now, all the loads are concentrated along the edge of the stringer, which is why the crack formed there, of course.

But a quick patch as you describe wouldn't prevent a more complete fix in the future.

BTW -- examine the ply around the crack carefully. If it's been there any length of time, rot may be setting in. Tap the wood with the handle end of a screwdriver or a small plastic mallet. The wood should resonate nicely. If it sounds dead anywhere, poke at the wood with an awl. If you find any soft spots, you really don't want to put the boat in the water until they have been addressed. (Unless you want to write an exciting story about the day the keel fell off!)

[neil.henderson]

Re: Plywood Hull Crack Repaibr />
My Dad used to have one of these - moored at N.Queensferry near Edinburgh. Fun boats & great for a day out.

Hope your repairs go well :-)

Regards Neil

[SilhouetteII]

Re: Plywood Hull Crack Repaibr />
Thanks everyone for your replies - understand that the best course of action would be to take the keels off and get a saw out. Unfortunately there is not currently the budget nor the skill to take on that work myself so I'll have to save up for a boatyard to take it on.

Woxbox, do you think an internal patch on its own would do the job as a temporary fix or would I need to patch one the outside with fibre glass as well? Either way, I'm glad that a temporary fix wouldn't prevent a more complete repair in future. Fortunately the boat lives on a trailer undercover so I hope it won't be rotting but I will take a close look.

[Hugh MacD]

Re: Plywood Hull Crack Repaibr />
Keep in mind that an internal patch may keep water out of the boat, but it won't keep water out of the surrounding wood. If you want to save as much as you can, patch from the outside.

[Woxbox]

Re: Plywood Hull Crack Repaibr />

Woxbox, do you think an internal patch on its own would do the job as a temporary fix or would I need to patch one the outside with fibre glass as well? Either way, I'm glad that a temporary fix wouldn't prevent a more complete repair in future. Fortunately the boat lives on a trailer undercover so I hope it won't be rotting but I will take a close look.

As Hugh says, you can't ignore the outside. The ply planking is flexing next to the keel, and the only way to stop this is to make it mechanically sound once again. Also, as Hugh points out, water is getting into the ply, a very bad thing. Most ply is not rot-resistant, and the fibers in the layups tend to wick water deep into the planking.

What I've seen repeatedly in plywood hulls is that once you have water ingress into the ply, the damage spreads beyond the ingress point unseen. Sometimes surprisingly far. This is why it's important to make sure that the wood is solid in the area of the obvious problem. If it is, then patch and reinforce as best you can. If you've caught this problem early, it shouldn't be hard to set right, but be aware that there could be more to it that you've spotted so far.

[SilhouetteII]

Re: Plywood Hull Crack Repaibr />
Very helpful indeed, thanks everyone