Preview: Caulking a Wooden Boat, Part 2 – A Deeper Look at the Craft

10 Responses So Far to “Caulking a Wooden Boat, Part 2 – A Deeper Look at the Craft”

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  John Gandy says:

  I am re-caulking the white pine deck of my 46′ Newfoundland schooner. These videos are very helpful but I am still undecided about what compound to use. The yard where I winterover likes the SIS 440 teak deck compound for everything. One problem is that it will not take any coating over it. Any suggestions?

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  Suhitha Edirisinghe says:

  Do boats ever get caulked from the inside?

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    Maynard Bray says:

    Hi Suhitha,
    The idea of caulking from the outside is that sea pressure (or rain) helps push the caulking material deeper into the wedge-shaped caulking seam, not dislodge it as would likely take place were it caulked from the inside. So it’s a rare instance when it makes sense to caulk from the inside out. Rare, but if the boat is afloat, has swelled up, and water still pours in through a seam, you have to do something to stop the flow—and that may mean caulking from the inside, then fastening some kind of retainer over the seam to hold in the caulking.
    Hope this answers your question.
    Regards,
    Maynard

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  Dennis Dunn says:

  (NOTE from OCH: A really great exchange happened in the comments section here on this video between a member of OCH, Dennis Dunn, and OCH co-founder Maynard Bray. We’ve included that exchange as one question/answer session below:

  QUESTION / DENNIS: These videos on caulking raise an interesting question … What are the pros and cons of Carvel planking with caulked seams vs. tight seam construction?

  REPLY/MAYNARD: With carvel planking, the seams between the planks have had a long and successful tradition of being caulked to make them watertight. Fitting the planks, one against the other for caulking, is a lot less fussy than making a perfect tight seam, and when the boat ages, the seams open, and the boat begins to leak, caulked seams are easily made watertight once more through recaulking. Tight seams are not only tricky to fit in the first place, but, unless glued, are very prone to leak after repeated shrinking and swelling.

  QUESTION / DENNIS: I’ve read that the caulking serves to keep the water out and to stiffen the boat. In tight-seam construction, one relies on the swelling of the planks to achieve the same result. But even with caulked seams, the goal is to achieve a light-tight fit at the inside edge of the plank, so even without the caulking won’t the planks swell tight just like in tight-seam construction? Certainly, a seam that is open to the inside of the boat would leak when first put in the water if it weren’t caulked, so caulking has merit in this regard.

  REPLY/MAYNARD: If you examine a few carvel-planked boats, I bet you’ll find some seams that aren’t wood-to-wood on the inside—either because they never were or because of repeated shrinking and swelling. Hopefully, most seams are still wedge-shaped which compresses the caulking as it is driven in from the outside.

  QUESTION / DENNIS: Relying on the swelling of planks to stiffen a boat and make it water tight is not without problems. When two planks swell against each other the edges of the planks are crushed. Later, if the boat is allowed to dry out, the seams will open up because the crushed edges can’t completely recover (they suffer from what is called compression set). Is it possible that a caulked seam would not open up because only the inside edge is crushed, and the cotton caulking and seam compound would preserve the integrity of the joint?

  REPLY/MAYNARD: Compression set is a tough one, for sure, and it occurs frequently. The cure is 1) to drive in more caulking or 2) to hope there’s enough elasticity remaining in the planking that the seams, through swelling, will eventually come wood-to-wood and keep the water out. The moral is don’t let the planking dry out. The caveat is that over-caulking (option 1) can stress out the frames and lead to their failure.

  QUESTION / DENNIS: There is another potential problem with tight-seam construction. When planks swell against each other along the full width of the edges, tremendous forces are generated. These compression forces can’t be good for the plank and may be worse for planks that are not quarter-sawn. What about the frames? I suspect that in most cases the compressive forces on the top of a plank are balanced by the forces on the bottom, but maybe not. Can this over stress the frames?

  REPLY/MAYNARD: Swelling planks surely do stress the frames. Mahogany planking (either caulked or tight-seamed), because it’s so dense and incompressible, usually leads to frame failure in lightly-built smaller boats. It and other too-firm woods have proven disastrous in small boats like the Yankee One-designs and Atlantic Class sloops. Cedar is much more forgiving, and Douglas-fir seems OK.

  QUESTION / DENNIS: Ok, let’s say that we agree that caulking the seams is a good idea.
Why caulk the garboards and the butts? It would seem the seams that are backed with butt blocks or backbone timbers (properly bedded) would not need caulking – at least not to prevent leaks. Even if water can find its way past the seam, the bedding stops it from going any farther.

  REPLY/MAYNARD: Even though garboards and butts are backed up (i.e. have a back rabbet), they are not watertight and require caulking to make them so. Bedding compounds or glue may work at first to keep out the water, but as wood shrinks and swells and the bedding dries out and maybe the glue lets go, you get leaks. Caulking cotton quickly swells when it gets wet and pretty much keeps out the water.

  Other questions related to planking include …

  QUESTION / DENNIS: What is the best bedding compound for planking? Traditional, polysulfide, polyurethane?

  REPLY/MAYNARD: There’s not really a need to bed planking to the frames or to each other. If you have time and opportunity, painting where planks land on frames might increase durability.

  QUESTION / DENNIS: Where should it be used? Between planking and the backbone, planking and frames? Planking and butt blocks? Only below the waterline or everywhere?

  REPLY/MAYNARD: See above. In backbone scarfs, it’s best to make good fits instead of relying on bedding compounds. Thick paint will then be all you need.

  QUESTION / DENNIS: Should all the faying surfaces be painted before bedding?

  REPLY/MAYNARD: Yes, as it helps keep the oils from leaching out of the bedding compound and into the wood. Despite the claims, most bedding compounds dry hard eventually, but painting the surfaces delays this. Roofing tar is one exception, staying forever flexible, and is great for underwater uses where its color (black) isn’t a problem. In other places, Dolphinite fungicidal bedding compound works well.

  QUESTION / DENNIS: What is the best seam compound? One well known builder recommends 3M5200! Is this really a good idea?

  REPLY / MAYNARD: To fill over caulked plank seams, I think you’ll find that 3M5200 is too flexible to sand or hold paint. It’s also a mess to use and takes forever to cure. It’s great stuff elsewhere because of its tenacity and flexibility, but just not right for plank seams. Oil-based topside seam compounds like Interlux #31, which dry enough to sand after a day or so, work better.

  QUESTION/DENNIS: Should butt blocks be used in new construction and restorations or is it better practice to scarf the planks, say using resorcinol or epoxy?

  REPLY/MAYNARD: I’d generally go for scarfs put together with thickened epoxy.

  Responses by Maynard Bray

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    Dennis Dunn says:

    Thanks for the comprehensive reply. By the way, I’ve heard that Dolphinite no longer contains a fungicide (health issues I guess). Have you heard otherwise?

    -Denny

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  Richard Gill says:

  These videos on caulking are very good. For good order, in connection to painting and seams do you paint the seams (if unpainted), caulk, and then paint over the caulking cotton in the seam (in the case of bottom seams w for ex Red Lead)? This then followed by seam compound and bottom paint? Please amplify.

  -Rg

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  Dennis Dunn says:

  I’m restoring a 50 year old 40′ yawl and I’m faced with having to replace some of the planking. The hull is double planked H. Mahog. over Spanish Cedar (I think), but the lower 8 strakes are single planked tight seamed. Some of these lower planks have suffered what I believe is compression failure from cyclic swelling and drying out over the years. Consequently, I’m considering using caulked seams on the replacements rather than the original tight-seam construction. Is this a good idea? Thanks.

  -Denny

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    Geno Scalzo says:

    any caulked planking should be replaced as originally built. any double planked sections renewed as original. mahogany is a fine replacement as long as the wood is sound tight grained. moisture content should be consistent with yr haulout location. no cement floors and no dry indoor storage. paint with oil based primer before final install.

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      Dennis Dunn says:

      Geno:

      Thanks for the prompt reply. I understand that I should replace double planking as original, but this boat has no caulked seams (except for the hood ends). The bottom 8 planks are not double planked but single planked tight seamed. What I’d like to do is replace these 8 planks with caulked seams, because it’s easier and less stressful on the planks and frames. Do you agree with this proposal? Thanks.

      -Denny

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  Doug Crews says:

  Wonderful series, great look into what many consider to be a lost art. Thank you.