Sealing outside of keel / hull joint

[John_Clarke]

My attempts to seal this joint with Sikaflex (despite careful preparation and priming) have resulted in the sealant peeling away from either the keel or the hull. I have noticed that Starlight 35s have a band of sealant about 50mm wide covering this joint which seems to work well. I understand that this is the sealant that is used to bed the keel to the hull. It has been spread out on the surfaces next to the joint by the boatbuilder.

I am thinking of trying Thioflex 600 which is a 2 part polysulphide sealant made by Fosroc for sealing engineering structures such as dams. A 50mm wide strip about 5mm thick seems about right over a 20mm wide bond breaking strip of polythene tape. This bond breaker is standard engineering practice to stop the sealant sticking over a 20mm wide band along the joint area. It should provide a wide enough strip that will stretch as the joint opens and closes while sailing, and reduce the pull on the area where the sealant is stuck to either the hull or the keel. That is the theory!

Has anyone tried something like this or have any successful solutions to this problem?

 

[vyv_cox]

May be a silly question...

but why are you trying to seal it? The critical part of the hull/keel joint is immediately around the bolts, to prevent ingress of water and subsequent corrosion, plus leakage into the boat, of course. This is a relatively easy joint to make, being under considerable compression from the bolts.

By contrast, the external edges of the joint are extremely difficult to seal. The width of the visible joint varies according to whether the boat is standing on the keel ashore, hanging in the slings, or afloat. Dependent upon the flexibility of the hull this width can be a couple of millimetres, even on a heavily built construction. Very few sealants have the strength or adhesion to withstand variations of this magnitude.

If you were really determined to make the joint solid, I suggest that you need to slacken the keel bolts and introduce a single layer of sealant over the whole keel surface. Surfaces of the hull and keel would need to be very well prepared to ensure adhesion of the sealant. Bolt up and leave the boat hanging in slings to ensure that the widest possible joint gap is filled with sealant.

I doubt very much whether all this effort is worthwhile. Virtually every iron-keeled boat will have this gap, but is it a problem?

 

[iangrant]

Re: May be a silly question...

John, Vyv is right - I was worried about the seal on a boat I once owned, like yours I suspect you could see bits of the top of the keel. Inside the boat was some rusty water (sometimes) - which meant the keel bolts were shot, allowing the ingression of water. The fact is no amount of sticky stuff/goo will stick on top of boat bits like hull/keel joints that will move. Check yer bolts!!

Good luck

Ian

 

[tonyleigh]

Re: May be a silly question...

Concur with above. After several seasons of failure with Sikaflex I now clean back, treat with anti-rust, and apply several coats of Keel Prime covering an inch or so either side. Only lasts the season but much more satisfactory than using sealant.

 

[John_Clarke]

A good question of yours

You are quite right to question the need to seal the external edges. Westerly let the sealant harden before tightening up the bolts fully so I expect no water is getting near them.

I suppose when I saw the 2 Starlights at the yard with nicely sealed joints I rather fancied the same on my boat to make the ragged joint tidy and well cared for.

I am sure you are right that the movement will be +- perhaps 3mm which is why gunned in sealant cannot cope. My plan is (maybe was?) to apply a wide band of sealant over a 20mm strip of bond breaker tape (like the type used in the bottom of teak deck joints). The +-3mm will then be absorbed over the 20mm width of the tape. A good quality sealant like Thioflex 600 should be capable of a 15% stretch of this sort without pulling away from the bonded areas on either side of the bond breaker strip.

If I go ahead with it I will let you know if it is successful, but it will need several seasons to give it a good test.

I am concerned that rust will slowly creep in under the edges of the sealant despite good preparation and priming. The Starlights have lead keels and therefore do not have this problem.

Thank you all for your helpful advice and comments

 

[david_e]

Re: A good question of yours

John, all the responses seem spot on. Having said that I saw a First 31.7 that had faired/prepared for racing which included the addition of a sealing type fillet around the hull/keel joint to reduce drag. This boat, campaigned by the dealer Dickies of Bangor, has had a fair amount of use and the joint still looks good so it might be worth asking them how they did it and what they used. Good luck.