Thread: Restoring a clinker built Huon Pine dinghy

A few years back I posted (long winded thing if memory serves) a piece about restoring lap strakes.

You can scab in something on that plank, but it's probably not going to hold long, especially if it's a solid timber plank and not plywood. Planking on a traditional build (carvel, clinker, whatever) is a consumable item, just like an air filter on your car. Once it done, it's done and that particular plank is done and should be replaced. It's not as hard as you might think, once you get your head around the issues you'll face. The hard part is getting the old plank out fairly cleanly, so you can use it as a template for it's replacement. This usually boils down to how bad the frames and fasteners are. Cutting the fasteners is easy, just hacksaw through the roves or clenches. Wedge open the seam, typically in the middle of the boat and using a fine tooth blade (no handle) insert it into the seam and slide it to a clench or rove. A few quick hand strokes will cut right through it and you move on the the next one. Working out from the center is best, getting to the ends about at the same time, the plank will be all but falling out by itself, when you release the hood end fasteners (usually screws). Now you have a plank that can be placed on a workbench, repaired or copied and replaced.

I'm not sure what the story is with that particular plank, but most often these failures (other than impacts) are from fasteners getting loose, letting in moisture, which causes havoc with the frames, planking holes and the fasteners themselves. Replacing the plank isn't good enough, you have to fix the structure (frames, floors, stringers, fasteners, etc.) too. This is because lap strakes like to "move", which means every contact point (clench to plank, plank to frame, stringer to frame, etc.) wiggles just a bit, as the boat transmits loads underway. In time, the fastener holes open a wee amount and this lets in moisture and a vicious spiral begins, the holes get bigger, the fasteners lose their grip, move movement grinds the faying surfaces within the laps, etc.

With luck, the rest of the planking just needs some new fasteners and a Dutchman or plank replacement on that beat plank. The well built traditional lap strake will use polysulfide as a sealant on the laps. All the major manufactures started using this stuff in the early 60's and it helped tremendously increase the lifespan of a lap strake. Lastly, you'll want the hull dry as a bone for repairs and if the seams have sealant prolonged soaking shouldn't be necessary. If it does need to "soak up" then you have issues elsewhere and should address them too. If there isn't sealant used in the laps, then yeah, she'll need to soak and it doesn't matter if fresh or salt. This is the reason the manufacturer's started using polysulfide in the laps, to remove this need to let 'em soak up come spring commissioning time. Simply put, a tight, seal lap build doesn't need to soak and if it does, there's a problem. On the other hand, if there's no sealant in the laps, yeah it sure will need to suck up for a day or two.

I'm unable to source Polysulfide as yet. Sikaflex recommend Sikaflex 291 for marine application, apparently it's a polyurethan product, flexible and able to be sanded. Think that will do?

Polyurethane will release if continuously immersed, but is fine on a trailer boat, that's allowed to dry out regularly, so you'll be fine. A trick with polyurethane is to make sure it's under pressure, during the full cure (2 - 3 weeks). With a lapstrake this means it's not applied "topically", but is applied inside the laps and the laps screwed, clenched or roved tight while it's still gooey. If it's applied topically, like a bead of caulk around a bathtub, it just pull right off the substrate (planking in this case), like a big rubber string. I've seen lots of carvels ruined by using this approach, to seal the seams. A few years ago I inspected a fairly famous universal rule racer, that had sunk at its berth. When I arrived I saw several long polyurethane "strings" dangling from the seams and it was pretty obvious what had happened. You'll be fine, so long as you don't plan on storing the boat in the water for more than a week at a time.

I would recommend Silaflex 292 instead of 291, because it's a higher modulus product with more aggressive adhesive qualities. Silaflex 292i is another option and it's just like 292, except it's thicker, so less likely to sag while being applied. I also think it's slight stronger than regular 292.