Joining plywood to cast iron keel

[Hercules]

I am repairing an old Thunderbird plywood yacht. The hull around the keel was rotted and needed replacement. As plywood is particularly vulnerable on the edges for water damage, I am trying to come up with a better way to join the ply and the keel. I will seal the edges of the ply with a special epoxy as best I could, but want to go one step further and would appreciate any comments on the following.

I am thinking of inserting a hardwood strip between the ply and the keel and perhaps also in between the plywood planks. I am hoping that should any water find its way in, the hardwood will expand and stop further water entry.

The alternative is to use a special waterproof glue that bonds to steel and wood, and expands to fill cavities.

[bloggs1968]

Hi Hercules,

welcome to the forum.

Do you have any pictures of tthe area you can post?

regards,


AD

[Hercules]

Thanks for the reply. I will get some pictures and post them over the weekend.

[malb]

I would be inclined to seal the ply and the rest of timber in contact with the CI keel independantly, and use something to form a gasket type seal between the sealed timber and the metal keel.

My reasoning is that anything that can bond the two dissimilar materials and seal them completely would most likely be rigid and moderately brittle. In the event of grounding, the impact would possibly be sufficient to damage the bond and seal, allowing water to wick into the woodwork and restart the rotting that you had to clear.

If the base of the hull were completely sealed, the keel bolted on, and a flexible sealant placed between the two, an impact could mouve the keel a small amount without breaking the seal, and thus keep the permanent seal for timber intact.

[Hercules]

MALB,

Thanks for your response.

The way the ply and the keel join makes it virtually impossible to seal the ply separately as I do not want to remove the keel. The idea of a hardwood strip is to form the gasket you are talking about. If I am correct in my thinking, it should expand if ever exposed to moisture, preventing further ingress of water.

I have a water proof marine glue (one part polyurethane) that maintain a high degree of elasticity after drying (does not become hard or brittle), fills gaps when applied (if any) and forms a permanent bond between steel and wood. The hardwood gasket would be a failsafe should water ingress take place.

Regards

Hercules

[Boatmik]

Howdy, the purpose of a gasket is generally to provide some sort of mating surface so that the whole surface is waterproof.

Also it is perhaps more likely (but not completely so) that the rot started from inside the hull - usually between the ply skin and some internal timberwork with a space underneath.

But if rotted from the outside it could be that the bottom of the boat was glassed or glass taped using polyester resin (common at the time) - the result is that water could have been trapped between the glass and the timber - leading to the problem. Redoing glass with an epoxy would resolve that problem.

Some pics would be interesting and useful as I am just throwing things around at this time.

From memory the Thunderbox (! - nickname - but the boats deserve huge respect - very good sailers providing you don't let them heel too much (maybe 15deg or a bit more) has a cast iron keel that butts directly to the hull. Is this correct - about the keel?

Best wishes
Michael