The Official Magazine of the Bluewater Cruising Association

A New Way Of Caulking

Barb Peck & Bjarne Hansen

Hoku Pa'a
Niagara 35
December 30th, 2020

Why open this article with a photo of a crocodile? Two reasons – first, I wanted your attention. Launching right into a discussion of our leaky chainplates doesn’t capture the eyes nearly as well. I promise to be brief, and not to digress further.

We used several select sailor words earlier this year during a tropical rainstorm. The rain itself, though unusual in Bahia Banderas in January, didn’t irritate us. Rather, it was the devilish drips landing on books perched on our library shelf. That sneaky water made its way inside where our chainplates penetrate the deck. A few years ago we had removed and inspected those chainplates, and rebedded them with butyl caulking (a grayish-white sticky substance with the consistency of putty). The advantage of using butyl caulking for such a job is that it can tolerate a lot more joint movement than caulkings that cure (e.g. 3M4200), and the chainplates can be pulled out for future inspections without using sharp knives and block-and-tackle. No water got past them for the first couple years of sporty sailing and rainstorms, and we figured they ought to have remained watertight for another 5 or 10. What went wrong?

Leaky chainplate due to failed caulking

We pulled the deck cover off one chainplate. In the photo you can see dark dust stuck to the surface of the butyl caulking, but there is also a large gap between the deck and the chainplate. This gap used to be filled with caulking; where did it vanish to?

Chainplate with Butyl caulking drip

This interior photo solves the mystery of the missing caulking. But what caused such a putty-like material to run? I think two reasons: high temperatures, and a wide gap. Summers in the Guaymas boatyard can exceed 40 C, and sun hitting aluminum will further boost the temperature. Butyl caulking, because it doesn’t ‘set’, thins out as it gets hotter. The fairly wide 5 mm gap between the chainplate and the deck then allowed the butyl to flow down and out of the gap.

How to keep the leak from happening again? I didn’t want to fill the entire gap with a curing caulk, as then future removal becomes arduous. But if a bead of cured caulking were applied at the bottom of the hole, it would keep the butyl from running out the next time it softened in the sun. However, our butyl supply was low so I pondered alternatives. We carry plumbers’ wax, used for sealing toilets to the floor flange, for use in emergencies; we have in the past applied it to portlights leaking on passage, when one doesn’t have the opportunity for a more thorough fix. It’s very inexpensive, sticks well to everything yet cleans off with turpentine, applies easily, flexes with the joint, and is of course waterproof.

I decided to make a barrier at the bottom of the deck hole with 3M4200, and then fill the remainder of the hole with plumbers’ wax. By melting it in a salsa tin and pouring it into the hole, all crevices were filled fully and quickly. I then reinstalled the deck plate, reattached the shroud, and was done. In the future if it gets hot enough to melt the wax, the 3M4200 should keep it contained. Time will reveal how well this caulking technique works.

The second reason I started with a crocodile? Perhaps it’s a metaphor for boat problems that creep up on you slowly, and then suddenly bite you in the bum 🙂 This particular crocodile is one we saw in Bahia Banderas, Mexico, while walking a trail at an estuary reserve in Puerto Vallarta. Cruising isn’t *always* work.


  1. Jim Bowen says:

    Brilliant. If this stands the test of time, you will become a rock star of the boat maintenance world. So creative and so logical. Cheers, guys.

  2. Sally Holland says:

    Thanks for this excellent article!
    I have a roof leak in the skylight in my floating home, and this idea might just do the trick!

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