Planking the Port Ama

This past weekend, we started planking the Starwind 860's port ama. With her first section of hull skin in place, the Starwind is looking more and more like a boat!

Each side of the ama is a single piece of (scarph-jointed) plywood. We've stitched them together along the keel with chain-link fence ties.

The two hullside panels are then spread apart and placed over the bulkheads.

One of the rules of boatbuilding is to always accept help when it's available. Dennis – civil projects expert, father-in-law, and creative carpenter – was eager to lend a hand.

With the flat panels draped in place, it's time to assemble the A-frames that we'll push against to force them into their final curved shapes.

This simple clamping rig applies the necessary pressure to bend the panels into the desired curvature near the keel.

With everything wedged approximately in place, the shape of the hull emerges.

In most sheet plywood designs, individual panels are conically developable – at any given point on the surface, they are curved in only one direction. That's not the case here; the Starwind 860 has a small but significant compound curvature in the hull surfaces. Making this work means that we have to force the extra curvature into the plywood, carefully and symmetrically. Which means a LOT of clamps.

The details of exactly how we get this compound curvature, without distorting or twisting the hull, will be in my next post.




developed plywood

Hello, I'm following the 8.6 metre trimaran as I am ready to build my own ( 6.6 sailing one).
For what I have read, developed plywood follows cylinder, not cone surfaces curves...
One author (D'Alì) in Italy that drew many boats using this technique suggests to pre-bend the ply just by "painting" it with warm water with a large brush, then inducing a "C" shaped curve by putting two rectangular panels together and then arching them with spanish windlasses or just ratchet belt tensioners. Once dry, you have a roughly pre-curved surface that will take much more easily curves like those on your amas. Great work and great blog alltogether !
PS I would like to know on what software you have developed the sketch and done the hydrostats for the trimaran please. I read "Godzilla" is that correct ? Could not find it on the web.

Developed plywood

Matthew's picture

Sheet materials (plywood, aluminum, etc.) will naturally bend to fit conic surfaces (or, perhaps more accurately, piecewise conic surfaces). A cylinder is just a special case of a conic, in the limit where apex angle goes to zero.
Plywood can, with some coaxing, be forced into compound curvature as long as it isn't too tight. It fights you every step of the way, but is very stiff and strong once the shape is locked in. Hot water (on the concave side) does make this a lot easier!

Michlet/Godzilla is a resistance calculation code by Leo Lazauskas ( ). The interface is not very friendly, but under the hood is a very sophisticated combination of wave system analysis code and genetic optimization algorithms. For hull design, I use Rhinoceros ( ) and for hydrostatics, ArchimedesMB ( ).

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