View Full Version : lapstrake planking

thomas cashman
07-17-2001, 01:25 PM
I am planking a Catspaw dingy in lapstrake. greg Rossel's book and Leathermanmake it a point that the overlaps must be flat. The importance of this as I understand is the if the overlaps are concave the boat will leak. However in "Building the Hershoff Dingy" they emphasize that the overlaps should be convex to ensure the boat will not leak. Does anyone have an opinion on this and what is the best way to cut a convex overlap? i have been using a 1/2" gouge and a rounded sander. i get the convexity but sometimes it is not exaxtly straight.

07-17-2001, 01:33 PM
Oppinion only.Some minor experience speaking here.Flat is going to give the most surface to surface contact so I would opt for that.
Concave will act like a ring gasket as the wood is drawn together by fastener and/or swelling.

07-17-2001, 01:40 PM
I've left mine flat. I have heard of dishing out the bevels with a disk sander to make the surface somewhat concave. The rationale for this is that all of the epoxy will not squeeze out of the joint when its clamped, giving a better bond. I can't remember where I read about the disk sander trick - maybe someone else knows.


Andy Farquhar

Tom Dugan
07-17-2001, 02:04 PM
Refresh my pitiful memory here. The Catspaw Dinghy is lapped plywood strakes as I recall. You're using thickened epoxy as filler/adhesive. In that case you want them matching as closely as possible, then be sure you don't clamp so tightly as to squeeze all the epoxy out.

Now is the Herreshoff dingy also plywood? If it's (ferinstance) cedar planking, I can maybe see the rationale for convex surfaces, which would be riveted, and swell once the boat's in the water.

Of course, if it's plywood, then ya got me. http://media5.hypernet.com/~dick/ubb/smile.gif


07-17-2001, 04:59 PM
I like Charlie's rationale for the concave lap, but you have to be careful and have the cavity extremely shallow. I think that hand sharpening a block plane so it is slightly!! convex is plenty. The danger is that in pulling the lap tight you cause splits along the rivet line. Flat works. In any case you want the bevel to give the fair and needed angle for the next plank land.

Phil Young
07-17-2001, 06:47 PM
I think it would be a good idea to find out whether we are talking glued ply or rivetted/clenched timber here. I think it makes a difference.

Jamie Hascall
07-17-2001, 08:36 PM
Do you have time to make a couple of test pieces and try each type out for strength? In general gluing theory, flat would usually give you the most consistent bond and greatest overall strength. I'm not sure how much the concave style would help and it sounds like a lot more work.


thomas cashman
07-18-2001, 11:46 AM
thanks for your response I am planking with 3/8" Alaskan Yellow cedar and riviting the planks. No epoxy between planks

07-18-2001, 12:52 PM
Sounds good to me. As the ease, when adjusting the lands of the laps; that is, with the plank in place and you are preparing for the next plank spileing, you will have the width of the next plank marked on your ribs or molds and you will go along with a plane (block plane is perfect) and a straight edge making sure that the bevel lines up with the upper line of the plank at all the mold stations and is fair all along the run. If you are working with a block plane sharpened as I described above, with the bare hint of a convex arc, the concavity happens automatically.

Tom Dugan
07-18-2001, 12:54 PM
I was taught to plane matching flat bevels on the planks for all the boats I've worked on with this construction (admittedly only 3 or 4). This is the traditional method. Making the planks convex guarantees a matching surface, but only along a narrow band of a probably much wider lap.

I'd stick with flat laps in the traditional method and be careful enough that you're assured of a close match. Don't forget that the rivets will pull them together some, but don't count on them to fix obviously non-parallel lap faces.


Phil Young
07-18-2001, 08:34 PM
This thread has to be one of the greatest examples on the board of mixed/dangerous/bad advice largely borne out of a poorly posed question misunderstood by those answering. You get what you pay for round here. I have no experience with traditional clinker building, only glued lap. What you've read about traditional clinker sounds right. If your faying surface is concave, you will get very little wood to wood contact, there will be a tendency for the edge to split, and with any flexing you will get water coming into the joint, and into the boat, as either side of the joint opens and closes minutely. With flat surfaces you get maximum contact if its all perfectly done, and if there is no flexing. With a slight convex on your bevel, and a flat inside plank sitting on that, you will get good contact all along, even if your carpentry is imperfect, and contact will be maintained even if there is some flexing. I don't think you would need much convex, just a final rub down with some sandpaper would probably do it.