Ian Malcolm ) writes:

.... You
*can* get the same problems with center sheeting, but as the tackle is
typically half the length and it does not have to run to one
multi-sheave block but can be spaced out along the boom using a couple
of single blocks to keep the parts away from each other, its not a
common problem.

I think one advantage of centre sheeting is the blocks being one above the
other making the sheet less likely to hang in great loops and get tangled up.

Three 4mm wire stays, Forestay and two shrouds. You probably want a
fractional rig (the stays go typically 3/4 or 5/8 the way up the mast,
not to the top).

My understanding is a "fractional" rig is one where the jib only goes part
way up the mast, eg Albacore. When the jib goes to the top of the mast
it's called a "mast head" rig.

It might also be useful to point out that the forestay is only there to
keep the mast from falling down when the jib is not up. When the jib is in
up it's the jib haliard which takes the strain, and it's important that
the jib haliard be tight enough to stay straight or the jib won't work
well going up wind.

The big difference in stress between a stayed mast and and unstayed mast
is the stays direct the stress straight down the mast into the bottom of
the boat, called "mast compression". As the stays force the mast downward
they pull the gunwales upward, so the gunwales have to be strong. On an
unstayed mast the stress is off to the side. There is only 6 ft of 2"
solid spruce mast above the partners on the small 32 sq ft sail on my
Dogskiff but I can watch it bend off to the side as the wind pick ups.
Fore-and-aft it actually takes up a curved shape due ot the greater
pressure in the middle countered by the fixed position in the partners,
same as on a Laser.

On a small boat like a dingy the heeling force of the mast and sail puts
quite a torque or "twist" on the hull. If you look at the partners on the
thin plywood Dogskiff on my website you can see how I've used X-shaped
struts to redirect some of the sideways heeling force down to the chines.
This has worked quite well. On the Albacore the deck and floatation tank
underneath help stiffen the hull and spread the stress from the mast. I'd
think about accomodating these stresses in designing a home built dingy.

Do you mean the planes of the centreboards are tilted outwards at the
bottom?

Twin centreboards or keels are usually called bilge boards or bilge keels.

...Have you considered that the draft will increase as the boat
heels up to 20 deg.? This means that if you touch a shoal and the boat
heels a little more as the wind blows you sideways, the centreboard will
dig in.

Bilge keels are actually an advantage in shoal water. As soon as you feel
the lee one ground you tack away to lift it and keep it from digging in.

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