The loads on a fin keel on a sailboat are generally such that any fairing material is going to give. They flex. They have to given the loads involved.
Yes I see your point. It is just that I have seen fin keels with the same design with no crack and that is what I want. Any ideas on what product would you might use for fairing in a situation that needs this flexibility?
A rigid joint, using epoxy, is going to crack rather than stretch and contract with the movement. A lot of it depends on the hull-keel join design. I don't know your boat, and I am not a naval architect.
Yes for sure the design is a key factor I don't have any photos at present but I have attached a quick drawing done by computer (so it is a bit of a scrawl). The fin keel is the same as a Farr 38 (Cruising), which is very similar to many modern cruiser fin keels in shape and size.
It looks to me like it comes down to a choice. Taking nito account the design the choice is, either:
i) epoxy -> strong but brittle and no crack or
ii) 5200 -> strong and flexible but with a crack (and the risk of water accessing the bolts and causing corrosion eventually).
The nub of the problem as I see it is that if you have a crack along the hull/ keel join how do you know if the keel bolts are dry? It is not really practical to remove the keel every one or two years to check the bolts and rebed it.
The problem with fairing and glassing the hull-keel join is simple. If you damage the keel and need to remove it, you've vastly complicated the job. Also, that makes the hull and keel more rigid, but making it more rigid makes it far less forgiving in a hard grounding. IF it doesn't give, something else will.
Yes good point.
To get a handle on the different outcomes of strength vs fragility I have a thought experiement, I would be interested to know if you think it roughly describes the situation.
Consider a range of forces from 0 to 500 Netwons applied to the keel on imp.act. Then there are 3 possibilities: The force is low enough that nothing happens, the force is sufficient to cause damage in one form or another, and thirdly the keel is damaged to the point where the hull fails.
Epoxy oooooooooooooooo xxxxxxxxxxxxxxxx XXXXXXXXX
5200 ooooooooooooooooooooo xxxxxx XXXXXXXXXXXXXX
Perhaps others will have a different opinion.
BTW, if the bolts can't support the weight of the keel and the stresses it is under, even 5200 isn't going to do the job. All 5200 would do is help the keel tear off the outer layers of laminate when the keel falls off.
The design of the bolts is certainly sufficient for the size of the keel, in fact a bit heavier duty than used these days for similar sized and configured keels.
If the keel falls off, tearing the outer layers of the laminate is probably the least of my worries!
The dynamic forces on a sailboat keel aren't such that you can rely on a sealant alone to keep the keel in place. Thinking you can do so is both stupid and foolish.
Um... yes it would be foolish to rely on sealant only, and so I do not intend to do that. I want to make sure the keel bolts are OK and if not I will fix them.
It would probably be stupid to do nothing.
However when it comes down to it the main reason to check the keel bolts is to make sure they have not suffered corrosion over 20 years the boat has been in the water. I really want the confidence of knowing everything is OK.
Thanks for your comments "sailingdog" I appreciate your suggestions and ideas.