Friday, September 20, 2013

Proa commenced

The time hath come, saith the walrus, to talk of many things; principally whether I'm going to pull my finger out and actually build a proa or just talk about it.

To recap. - Last summer I sailed an Optimist, a tiny boat designed for tiny people to learn to sail. I am not tiny, but I am learning to sail. Optimists are safe and stable and easy to launch but marvellous slow, so you need a summer onshore breeze which we get every morning from ten thirty onwards. And since the Tasman Bay is shallow, I need a high tide. High tides and ten o'clock coincide for three days a fortnight. So Optimist sailing is limited. But that isn't why I want a different boat. It's because as an elderly person I feel foolish in an Optimist. I discover myself to be a Fashion Queen.  Shallow, like Tasman Bay.

 I cannot get anything bigger than eight feet into the van, and I don't want a trailer with its concomitant cost, registration, Warrant of Fitness, and general garden clutter. I don't want anything so bulky as to require a launch team. A slightly larger dinghy was the concept, until the idea of a proa returned along with Paul Candlish's observation the other year when I bumped into him again at the Mot Marina, that a nesting dinghy is dead easy to make and I should get on with it. Mr. Candlish is a Canadian who sailed here in a sailing boat, and lives on one, and rides a recumbent which is how I know him.

John vetoed a larger dinghy. "We've already got an Optimist. Make something fast." The proa is a sailing outrigger canoe that you don't tack: you move the mast from one end to the other. I promptly bought, read, and studied Mr. Dierking's book, and then the late Mr. Best's book and then Mr. Grill's blog. Mr. Grill observed that all his capsizing was done while shunting, and Mr. Best observed that not all pacific canoes shunt. Mr. Dierking observed that actually shunting and tacking canoes aren't much different in speed. And provided the ends are the same, you can make a tacking canoe into a shunting one afterwards. And it doesn't need an eccentric hull. Mr. Dierking's simplest plywood sailing canoe, easy to build and very functional, was a hideous box. Wandering along the seafront I chanced on another hideous box, only this one was a catamaran tender.

It occurred to me that sliced in half and butted end-to-end, it would make a nice shape, so I applied some paper to it with masking tape and took measurements. Matt at the bike shop gave me some bike boxes, and duly flattened they became rough templates which taped together revealed what one end of the boat would look like.

Bike box templates: sides and bottom

I worried only a little that the shape wouldn't be glorious. A boat is a box that keeps the ocean out. Boats come in all sorts of shapes so logic suggests that fine detail of shapes is less important than I as a layman imagine. My guess is that we are looking at an 80/20 rule, where you get 80% of the benefit with 20% of the worry over shapes, and to get the remaining 20% of benefit from a refined shape, you would need to do 80% more work. We shall see if I'm right.

So, to the four identical bottom panels, to be of 7mm exterior 5-ply which is cheap and, the timber merchant said, glued with marine-ply grade glue. To test the glue I took some scraps and put them in a pan of simmering water and left them for an hour and attended to emails and then, distracted, went for a bike ride, and suddenly realised ten miles from home the pan had been simmering for two and a half hours and you never saw anyone ride home so fast in your life. Boiling water remained in the pan, and no sign of delamination.

To check the displacement I used the floor area of the Optimist, which doesn't sink unduly under my ten-and-a-half stone. The back of the Optimist is 34" wide, the front 20" wide, and the middle, 40" from the back, is also 40" wide. Overall floor length is 70". So I calculate it's about 2380 square inches.

My canoe floor is going to be twice 87.5 inches long, and sixteen inches wide for the middle 110 inches, and at each end it will taper to nothing, so that's probably 2520 square inches. - I bet I've got the maths wrong. -

First I cut out a template for the end curvy part, fairing the curve on a disc sander. Then I cut four eight-inch wide plywood planks and found that they weren't eight inches and they weren't especially straight even though I used a circular saw and a straightedge and took infinite pains. So I called in Graeme, who lives next door and has been sailing these seventy years and made several boats. He was reassuring.
"It's a boat. Not engineering. The Tahiti canoes are made with an adze and a log. You planning on entering the Americas Cup or something?"
"What about the Vee where the two curved planks meet?"
"Filler and glass tape."

 Wooden template, redrawn from corrugated card template, redrawn in turn from template made with stuck-together lengths of newspaper

With a sharp pencil I outlined a plank and this will be the template for the others. From the one straight edge of the original ply sheet, here are the measurements, to the nearest half millimetre. The first measurement is the point, so there's only one of it of course. As the curves flatten out, I did fewer measurements. This may be what they call offsets. I don't really know what offsets are. I'm not a proper boatmaker.

The first column is millimetres from the end; the second column is the centreline of the boat measured in millimetres from the edge of the plank; and the third column is the outer edge of the boat, again in millimetres measured from the same edge of the plank. These are just for the shallow floor-pan-thingy, which they probably call garboard strakes if I've got that right.

0        116
25      97      120
50      82.5   124.5
75      70.5   128.5
100    59.5   132.5
125    51      134.5
150    45      140.5
175    39      144
200    34.5   148
250    27.5   155
300    21.5   162
350    17      168
400     14     173.5
450      6.5   178.5
500     10     183.5
550     8       187
600      7.5   190.5
650     6       193
700     5.5    194.5
750     5       197
800     4       198.5
850     3       200
900     3       201
950     2.5    202
1000   1.5    202.5
1050   1       203
1100   0.5    203.5
1150   0       204
Thereafter the plank is eight inches wide if 204mm is eight inches, which it isn't.

 We shall have to see whether they work when the thing's sewn together.

And if there are any arithmetickal glitches it's because either the pencil line was too thick or I'm stupid, both being likely.

This is what one half will eventually look like. Perhaps.

It will be objected by the proa specialists that my hull is the same on both sides, and not as deep, and doesn't have the dramatic Vee bottom of proper proas. Well, I don't know if the thing will work, so I'm being conservative.


Post a Comment

Subscribe to Post Comments [Atom]

<< Home