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This form allows you to calculate the theoretical maximum speed of a displacement hull. This is an approximation, and boats may occasionally exceed their theoretical hull speed for brief periods, as when surfing down large waves, or when temporarily planning in unusual conditions. Also, this limitation generally does not apply to multihulls, though they are technically displacement vessels.

Where:    LWL is water line length.

This is a useful rule of thumb for determining the relative "heaviness" of a given design; in other words, is the boat heavy for its length? A boat of a given length may fall into one of a number of categories, from extreme ultra light racer to armor-plated crab crusher.

Where:    Dl is displacement in long tons (2240#) and LWL is waterline length.

The sail area/displacement ratio is a measure of power to weight. Though the speediness of a given boat is determined by a host of other factors in addition to this one, it's still a useful comparison tool. Other factors being equal, the boat with the higher ratio will be the faster boat overall. Boats with sail area/displacement ratios below 16 will generally be fairly slow. Boats with ratios over 22 will be very fast, and some extreme multihulls will have ratios over 30.

Where:    SA is total sail area and D is displacement in cubic feet. D can be approximated by dividing the displacement in pounds by 64 (i.e. 1 cubic foot of saltwater = 64 pounds).

The Capsize Screening Formula was developed by the U.S. Sailing Association as a rough rule of thumb, helpful in comparing the initial resistance to capsize of various boats. Boats with a Capsize Screen factor of 2 or less will have a normal initial resistance to capsize and will in general be safer offshore boats than boats which exceed this value. The lower the value, the better, all other things being equal.

This formula is used to determine the vertical load in pounds that a chain plate or shroud can be subjected to as a function of the stability characteristics of a particular vessel.

Where:

PT   = vertical chain plate load.

RM₃₀ = Righting moment at 30° of heel. RM₃₀ can be found by multiplying RM₁ by 28.65.  RM₃₀ or RM₁ can be found in the vessels rating certificate or by contacting U. S. Sailing or the U.S. Yacht Racing Union.         

1/2 Beam = Transverse distance from the chain plate to the vessel centerline in feet.

The resultant PT is normally multiplied by a safety factor of 3. This is then multiplied by the factors listed in the table below to arrive at the minimum breaking strength required for the various stays and fittings on a mast head rigged single spreader sloop with double lowers.

 

Head and Back Stays	0.45
Upper Shrouds	0.45
Lower Shrouds	0.325

For further reading on the subject of rigging we highly recommend "Understanding Rigs and Rigging" by Richard Henderson, which can be found in The Surveyors Library.

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