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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.

George.

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