# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Revisiting hull pressure wave
From: George Huxtable
Date: 2005 Jan 26, 11:13 +0000

```Bill asked-

>A while back the topic of not being able to achieve hull speed due to a
>downward pressure wave interacting with a relatively shallow bottom.
>
>My interest still runs high in finding a formula to relate
>length-at-waterline and velocity to the depth that it comes into play, and
>the affect as depth decreases past that threshold.
>
>Any help would be appreciated.

=================

Response from George.

Two of the books on my shelves touch touch this topic.

Try "Sailing theory and practice", by C A Marchaj (Adlard Coles 1964),

Or "The science of yachts, wind, and water", by H F Kay, (pub. G T Foulis,
1971).

These authors agree that at a certain critical boat-speed, which depends on
the depth, there is a considerable increase in drag due to shallow-water
effects.However, if the speed can be considerably exceeded beyond that
limit (possible by a planing craft, but not a displacement vessel) then the
drag in shallow water can actually become slightly less than the
corresponding drag in deep water would be.

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.

Marchaj gives no references for what work this graph is based on, and I
would take the details with a pinch of salt. But it may well give a flavour
of what to expect.

For an authoritative naval architect's account of shallow-water effects,
probably Kenneth C Barnaby's "Basic Naval Architecture" (Hutchinson, my
edition is 1953) is as good as any, but like most naval architects, he is
concerned with quite a different type of vessel (merchant and naval) so
will not be very relevant to Bill's quest.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

```
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