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Thank you George

First, is "draught" the same as US "draft?  (She draws 3 feet, her draft is
3 feet?) Somehow (perhaps erroneously) I equated the downward pressure wave
with speed and length at waterline.  The longer the hull (at waterline) the
bigger the bow wave and bigger waves can move faster than smaller waves (1.3
sqrt).  So I *assumed* a larger vessel (draft being equal) would also create
a greater "downward* pressure wave.

If I understand the below only the draught/draft (probably ignoring thin
"blades") and speed is of importance relative to the depth.

Thinking it through, we are trying to maximize our speed--sail at hull
speed.  A 34'craft might have a hull speed of 7.6 knots, while a similar 50'
craft may have a hull speed of 9.2 knots.  If both had the same draft and
were moving at hull speed, and depths were equal, the larger craft would
reach the critical number before the smaller craft because of her higher
speed.  In the real world, the larger craft may well have a deeper draft
than the smaller craft, further hampering her speed at the same depth the
34' boat experiences.

So the length-at-waterline issue may be folded into the equation you
provided?

Bill

> Earlier today I posted about message on this thread, in which I wrote-
>
> "Marchaj puts the critical speed, in ft/sec, as 5.7 x (square root of depth
> in ft.). And he provides a graph which shows that around that speed,
> shallow water effects increase the drag by 220 % if the draught is 3 x
> depth, by 100% at 4 x depth, by 70 % at 5x, 50% at 6x, 25% for 8x."
>
> That was wrong. Where I wrote "draught is 3 x depth" (and so on), it should
> of course have been "Depth is 3 x draught". Otherwise, the vessel's speed
> would certainly have been zero! Sorry about that.
>
> So that paragraph should have read-
>
> Marchaj puts the critical speed, in ft/sec, as 5.7 x (square root of depth
> in ft.). And he provides a graph which shows that around that speed,
> shallow water effects increase the drag by 220 % if the depth is 3 x
> draught, by 100% at 4 x draught, by 70 % at 5x, 50% at 6x, 25% for 8x.
>
> Presumably, with a thin-keel or centreboard, the relevant draught would be
> that of the hull only, without appendages. As for the relevant draught of a
> planing craft, who knows?
>
> ===================
>
> To save some arithmetic, Marchaj gives the critical speed for a depth of 10
> ft as 10.6 knots, that speed being proportional to the square-root of
> depth.

   

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