Cabin Beam arcs

[Peter Jacobs]

Working on a Folk Boat, I'm in the lofting stage and have a question about cabin roof beams: Apart from the method outlined in March/April Wooden Boat for deck beams, is there a proper way to develop the cabin roof curve so that plywood can be used? My construction plan shows, in cross section view, an initial smaller radius curve up from the cabin side, then a gentle curve across the cabin beam, ending in the same smaller radius curve on the other cabin side.This profile gives a little more headroom closer to the cabin sides. The plans call for 150mm (6") of rise in the center at station 8, which is about level with the forward end of the main hatch opening. Width between the cabin sides at this point is about 1500mm (5').

Lofting has been a lot of fun and a real eye-opener as to the work that goes into a set of plans. It seems everthing I need to know is there if I look hard enough ... the areas I'm having difficulty with are mainly due to inexperience.

-Peter-

[Dave Fleming]

Hint, they the deck and cabin beams, are not true arcs of a circle.
;)

[Alan D. Hyde]

Dave, can you draw them as an ellipse, with two nails, a string, and a pencil?

Alan

[Bob Cleek]

Develop your reference points and spring a batten across it. If it's fair, and you're happy with the shape... done deal! No problems should be encountered using plywood for cabin tops or decks, so long as there are no compound curves. You can easily bend plywood in one direction, such as over the top of a rounded cabin top. What it won't do adequately is bend in more than one direction, developing compound curves. You can make a cylinder out of it, but you can't make a sphere.

[Dave R]

Peter, any idea what the radius of the bend is out toward the cabin sides? How thick is the plywood? What sort of plywood are you using for the roof? You might get away with starting along the side and working toward the middle as you are fastening the plywood down. You can get more leverage that way. That would complicate shaping the edge, though since you would likely need to shape that edge before putting it on.

Could you use very thin plywood such as door skins and make several layers? Or maybe strips of plywood along the edges transitioning to larger pieces as you get into the flatter protion of the curve?

Just brainstorming.