posted October 22, 2005 05:33 AM
The traditional hull speed calculation is 1.34 x square root of Waterline Length in feet for a speed in knots.I once saw how that formula was derived. It was the speed that a perfectly cycloid (kinda like sinusoidal) wave, with a peak to peak length the same as the length of a boat, progressed through water. The 1.34 was the resultant of pi divided by some constants for water, etc. This formula assumes that the boat is perfectly nested in the wave.
However, Sail magazine once had an article for calculating the recommended horsepower for an auxilliary engine. The article used a slightly modified version of the formula. Instead of 1.34, it used the calculated "Speed-Length Ratio":
8.26 / ( "Displacement-Length Ratio" ^0.311) where
The Displacement-Lenght Ratio is:
Total Displacement / 2240 / (( 0.01 x Waterline Length )^3)
For example, if the all-up weight of a 'Piper with 2 crew is 1945 pounds, the Displacement-Lenght Ratio is 257.28 (medium displacement cruiser) and the Speed-Length Ratio is 1.470 (as compared to the traditional 1.34).
So, the hull speed of a Sandpiper is:
1.470 x sq.rt(15 ft) = 5.694 knots!
These little boats can rip!
These calculations have 1 sad side effect. Any extra gear and paraphernalia will add to the displacement which will trickle through the calculations to result in a miniscule reduction in speed.
Now you understand some skippers' fanaticism with weight reduction.
[This message has been edited by SuperPiper (edited October 22, 2005).]
posted October 20, 2005 04:21 PM
posted October 26, 2005 01:35 PM
