Anchoring in a blow?

[jdc]

My figure for load on anchor cable comes from the standard table in my naval architecture text book.
Everything depends on how you calculate the block coefficient below the waterline and the effective wind resistance of masts and rigging as well as the hull block above the waterline. Above waterline, the envelope is used.
The following table is based on tests (I cannot say exactly what the tests consisted of) and is known as Martin's formula.
Wind force Beauf Pounds force per sq ft.
2 0.25
3 0.5
4 1.0
5. 2.0
6. 3.25
7. 5.0
8. 7.4
9. 10.4
10. 14-4
11. 19.6
12 25.6
The equivalent tractive effort to achieve a speed of seven knots in still water would have been just a trifle under the force 11 wind effect on an 80 ton three masted schooner.
The formula is standard for sailing ships.

I think these numbers are probably spot on. If one converts the 19.6lb/sqft to SI units it's about 940 Newtons/sq metre or 96kg/sq m. Area of a 42' boat is probably 3m x 4m x 1.3 = 15.6 sqm (the 1.3 is frankly a v rough guess for the extra due to the rigging and mast).

15.6 x 96 = 1500kg. This is pretty close to my figure of 1400kg, so I think we agree what force on a yacht is due to a steady wind.

The bit I don't agree with is the force required to propel a yacht at 7kts in still water is anywhere near 1400kg.

It's true I don't have an 80 ton, 3-masted schooner, but the force needed to go at 7kts simply can't be 1400kg. Really it can't; such a speed is hardly unusual for the modern AWB, and is quite easily achieved in quite moderate wind strengths or with engine. They really are not pulling that hard - the plot in my previous post is both that from the design data and measured by me: it's less than 190kg to go 7kts.

 

[[2574]]

In case you've missed it there is a very interesting continuation of this discussion in the dreaded lounge, see here: http://www.ybw.com/forums/showthread.php?t=341996

Dylan's attachment of an old article on rode geometry is particularly appealing to those who didn't listen in maths lessons while in school (that's me!)

Rob