Michaels article about “Die Fledermaus” brought back an old Idea in my head that I had seen in the old book of H.J Sass “Katamarane, Trimarane und andere Mehrrumpfboote” from 1973. Sass was quite fit in boatbuilding and had the idea to build a hull in “Quarters”. He calls it “der ideale Rumpf” (the optimum hull).  Perhaps its time to pull this old idea back into the future.

The basic idea is that you have a negative building mold in which you laminate 4 totally identical panels for one hull. Starting with a full circle bulkhead in the middle of the hull it becomes a oval in the front and then the bow ...  that is also rounded in profile to allow the “flow” of the projection. Join the two quarters and you have a 1/2 hull. The same is with the back of the boat. For a tacking proa you just turn the “back” for 90° and join it with the front end. The result is that you have a planing bottom at the aft end of the boat.

If you want to have a shunting proa, assemble the two halves without the turn and you get a symetrical proa with two bows.

Material: The best material for this is glass/epoxy sandwich, but it should be possible to laminate wood also. I think of 3 - 4 mm veneers that are preshaped and -laminated on one side with a light fiberglass. 2 sheets of 2 mm Plywood should work too. I think in a true conical projection the panels are 100% devellopable.

The Mold itself is as simple as can be and from osb- oder partical board. And you need only a few stations, lets say a station every three feet. Thinking of a 8m (26’) proa the mold would be 4m (13’) long and have 5 or 6 stations.

Building: From my experiance with laminating surfboards I think that it’s possible to laminate one quarter panel in one day, incl. preparation. If the negative mold is built to a high standart there will be only minor fairing and sanding. Ready to assemble the hull on day 5????  Ok, a little too optimistic.

Advantages of a hull like this:
- minimum wetted area because of the true semicircle underwater sections
- minimum weight because of the smallest skin area possible for the given volume, again the full circle main bulkhead.
- only very thin materail needed because of the stiffening effect of the round form
- minimum windage because of the rounded deck.

Disadvantages of a hull like this:
-you cannot walk on the deck of the hull because it’s full rounded
-it will be difficult to build a cabin in this hull.

Let me read what you think! Best Regards, Michel

Here are the scans:

Picture 1 Lines and Building method
Picture 2 Tacking Proa with planing hull

I think this shape could be achieved using large diameter PVC or HDPE pipe for an ama. If a cylindrical plug was pushed to the middle of the pipe, then each end could be heated and formed into a bow. Or, cut two slits in from each end and then progressively apply pressure to deform the pipe. If the difficulty of forming could be overcome, it would yield a very durable ama with the desired shape.

Had not heard of this particular approach but it is elegant!

For a tacker it’s hard to ignore.

For a shunter I think I might go so far as to ‘flip’ the mold in order to fairly easily make my ultimate hull based on sliding a parabola along sheer and keel curves.

Thought I had a shot at actually doing same for a one way boat (3 man marathon racing outrigger) but haven’t heard back from the prospective client for a while.

cheers,
Skip

That reminds me of the constant camber method.

Cheers
Alexander

No doubt many variations on this idea, I gave it some thought a while back.

For a proa:
- Make your 1/4 moulds split vertically. i.e. along keel line and halfway across the deck.
- The moulds are taller than required, the deck can be formed by ‘cutting in’ a flat.  (you would not actually build more than required)
- The outrigger hull is formed from the centre part vertically, so it is less high, more narrow and not so long. It would also be sharper along the keel.

(hope this makes sense without a drawing)

As a method it would produce a sophisticated looking craft with minimum effort.  Probably best suited to that nast smelly GRP stuff though.

Cheers
Mark