Originally Posted by JonJo
A 'constant' 25 knot wind (is imaginary) presumably means averaging 25 knot which means gusting to 35 knot. ...
Actually no - Prof Knox gave a figure for the peak cable tension on a 40' boat
at 30 knots of around 250 kgf. I took this figure as a starting point, except that I applied his formula, not to this peak figure, but to one half of this figure, which he gave as the average
By inference, the average cable load is not applicable either to gusts or to snubbing load *
I worked backwards from the holding force Prof Knox measured for the 15kg Bruce, namely 80kgf.
I applied his inverse square formula to work
out the windspeed which he would predict would provide a steady-state average load of 80kgf.
The answer was less than 24 knots, from which I inferred the conclusion stated in my earlier post:
From the observations and formulae in his article, a 40ft cruising boat setting a 15kg Bruce in medium-hard sand would drag continuously at a steady 24 knots of wind with no fetch and hence no wave loading.
If this were true, such an anchor would not be useful except as a "picnic pick" on such a boat in any moderately windy part of the world.
Bruce anchors (when they were still available) were almost standard equipment
in several of the windiest waters on earth, and their knockoffs are still popular in such venues.
- - -
* in answer to DumnMad: you certainly can have flat water at 30 knots, if you have no fetch to windward.
In any case we're talking about a theoretical figure which does not include allowance for wave induced loads
If it did, it could not follow the inverse square relationship he subscribes to, which accounts only for windage load.