lapstrake seam compound?

[imported_ANDREW]

I'm helping a friend fix up an old (1920) cedar lapstrake sailing pram. We stripped off the old paint and the planking looks pretty good. My friend wants to spread some sort of goo along the laps prior to painting. I think 5200 is the WRONG thing to use, but I dont have a better suggestion. I need advice.

[ishmael]

Andrew,

Have a look at Walter Simmon's books on lapstrake boat building. He cuts a shallow narrow groove in the outside of the plank land (the meeting between planks) and fills it with a polysulfide.

I don't think 5200 would hurt much in this application, but isn't necessary. And check on the best way to go about it before smearing.

Jack

[Wayne Jeffers]

Andrew,

Before your friend resorts to caulking, it's worth asking the question, "Does the boat really need caulking?" It is possible that hardening up the plank fastenings will do the job.

If she is still weepy, you can use a veiner to cut a groove and use a polysulfide sealant, such as 3M-101 or BoatLife LifeCaulk, like Ishmael suggests.

5200 is a more permanent sealer, which may cause problems in the event you want to remove it at some point in the future. However, some reliable authorities recommend it for this purpose.

Wayne

[Mike Field]

Andrew, I think, as Wayne suggests, that you should try hardening up the fastenings as a first step.

Having done that, I wouldn't use any sealer at all in the laps until you've had the boat in the water for at least several days. You'll be surprised at how much the joints take up as the wood swells, and it may well be that you'll need no sealer at all between the laps because the newly-tightened joints keep the water out. (This, by the way, is exactly how the boat would have been originally built.)

Having done all that, then if you do decide that some sealer is still necessary, I would suggest you use a substance called (I think) Slickseam, which is very soft and can be squeezed out of the joints as necessary. To use any hard-setting substance is to invite permanent plank damage as the timber swells. I would class even a polysulphide as too hard for this situation, although others might disagree -- cedar is a very soft timber and it will be very easy to ruin the boat for good by using a hard sealer.

[ishmael]

Mike and Wayne are correct, hardening up on the fastenings, or some selective refastening, should be tried first, along with a good soak.

I'm not sure about the slickseam, I've never used it, but from what I've heard it seems more a temporary measure. I have seen old lapstrake hulls made water tight again with polysulfide, if the above measures fail. Finding and treating the offending seams might be the way to proceed, but Simmons uses it in all his new construction.

Best,

Jack

[landlocked sailor]

Walt Simmons uses Rule Marine adhesive for this step. It's sort of a "belt & Suspenders" approach for boats that are dry sailed. The stuff dries but remains flexible but so does 5200. It is much less "gooey" to work with though; spreads like acrylic latex caulk. I see no downside to using it. Rick

[mmd]

Not a suggested solution, but an instructive anecdote (true story) of a dialogue between a fine boatbuilder and a prospective client while inspecting the boatbuilder's latest jewel-like lapstrake Herreshoff Pram:

Client: "Do you make the plank seams watertight with modern, high-tech sealants or do you do it 'the old way'?"

BB: "I only use traditional, proven methods and materials."

Client: "So what DO you use to keep the planks tight?"

BB: "Good joinery."

[ishmael]

Landlocked sounds like he's more up on this that I. Thought it was polysulfide, but it's been awhile since I read his books.

You aren't putting it "in" the seam by the way. You are making a special "pocket" for it. Borrow one of his books from the library. A picture or two is worth a thousand words.