When doing calculations of surface area, I found that with the displacement of Harrys, from about 12:1 to about 16:1 for the ww hulll there was not a great difference in surface area. It doesn't need much power to start getting into displacement speeds for the ww hull and so an 8:1 would give a fair bit of drag at those intermediate speeds before there was significant weight transfer. It is hard to set up a shunting sailing boat for planing and not go to too much form drag at lower speeds without having significant moveable ballast, though I am looking at having a slight flare back from the bows a bit to get a little lift when going through chop. This allows a bit more room in the hulls while keeping the waterline skinny. I would not go below 12:1 myself.
I can't see any advantage of rocker with a shunting vessel with two rudders. It means a fatter or deeper hull overall, more susceptibility to hobbyhorsing, and greater bow depression when pushing hard. It may be worth rounding the forefoot slightly to reduce damage and I am looking at a v for the last 8% to handle coming off the top of a wave and slamming down in fairly extreme conditions and reverse stem to make working through heavy chop that little bit easier--- In harryproa@yahoogrou
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> Rob, I have read somewhere that cats and tris aim for a minimum of 8:1 to prevent stern squatting in the manner of a mono-hull. For the ww hull an 8:1 would have lower surface area for a given displacement than an 11:1 so less resistance at low speeds. At higher speeds some of the displacement of the ww hull would be carried by the lw hull and therefore the greater wave making resistance of 8:1 would be less of a factor. Are there other more important considerations such as drag on the steering?
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> Longer and skinnier is still better for speed probably.
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> > The longer and skinnier the better, but as long as it does not drop
> > below 11:1 l:b, you will be fine.
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