On sailing a Folksong – Speed formula

A calculation I have never made before:

The speed that a yacht’s hull can be made to travel through water is related to waterline length.

The formula for an average sea-going yacht of conventional shape is:

Speed in knots = 1.4 x Square root of the L.W.L. in feet

The multiplier is altered according to the type of hull. It may range from 1.25 for a tubby hull to 1.5 for a large racing yacht.

Therefore Blue Mistress’ theoretical maximum speed at L.W.L of 19′ 8″:

= 1.4 x Square root of 19.66 ft

= 1.4 x 4.434

= 6.2 knots (Always remembering that speed through water is not the same as speed over the ground).

We were doing a little less in the clip below

One thought on “On sailing a Folksong – Speed formula

  1. Hmmm, that is a 14 hour trip to Guernsey (~80nm) and 19 days straight 24/7 sailing to Greece (~2800nm)…Not bad at all 😀

    The complete hull is measured in some racing types and a coefficient is worked out from the set of measurements even for boats in the same class (!!!). This used to done (and i think it still is in some places) with a very simple instrument composed of two spring pulleys with a common thread winded between them. The instrument is situated on a fixed point and a pointer attached to the thread is used to mark points off the hull. The difference in the thread length can give the 3d coordinates of a point (along ONE plane of course) within some range limits.

    There is of course the laser triangulation system solution today which one would think that it is much faster in acquiring points and more accurate but unfortunately we are yet to find a way to bend light around places (an inexpensive one, preferably). So….not so high tech after all 😀


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