Converting a Carval Plank Hull to Glued Lapstrake Ply

[Stephen Hopkins]

I know its possible to convert a round bottom carval planked hull to Lapstrake plywood, I still have a few questions that maybe people on this forum can address.
The boat in question is a 18 ft long catboat, which specifies 3/4 inch planking.
1)What would be the scantlings for glued plywood lapstrake hull?
2)Should the boat be framed as per the orginal plans?
3) Finally should Copper rivets and roves be used as secondary fasterners?
Thanks very much for your time
Steve H

[PAR]

A carvel hull shape can be lapped, but the construction methods and scantlings would differ considerably. You'd basically end up having to redesign the structure for the specific lap technique (there are several) you desire, in the shape of the original carvel craft.

Typically, the frame spacing (and frame configuration) is dependant on planking thickness and construction method, which is worked out in the reasonably early stages of the design process. A glued lapstrake hull has next to no frames, but a lightly planked craft (conventional lap) will have closely spaced steam bent frames, though this could be increased (frame spacing) if sawn frames are used every so often (like every third frame bay for example).

Lap fasteners (I'm assuming this what you mean with secondary fasteners) seem to fall into two different camps. What the designer prefers and what the builders prefers. I've seen several different types of fastener schedules used in lapped hulls, particularly small ones. Every thing from all screws, through all clenches or rivets, with every possible combination tossed in, to add to the confusion. Some builders just like one method better then the other (I know I do) and the same is true of designers (ditto for my lap designs).

I've not really helped much, in regard to your specific questions, but much depends on the individual boat. Since you have 3/4" carvel planking, I'm assuming she has a pretty healthy displacement. Carvel needs a fair amount of planking thickness to accept the caulking seam. In smaller craft (like yours) this means light weight planking and reasonably close frame spacing. I'll guess she's got cedar or white pine planking over hardwood (likely oak or mahogany) frames, which could be bent (oak), double sawn or built up (or combination). Another guess would be your current frame spacing (10"?) if steam bent, which would probably be around a 1 1/4" square. Sawn frames would be 5 to 6 times as deep as they are thick. As a very general guide, lapstrake planking will be approximately 80 to 85% the thickness of carvel on the same framing structure, which gets you a 5/8" thick lapped plank.