Speed to Length Ratio
Steel is absolutly is a heavy construction material when compared to fiberglass and composite construction methods. However, did you know that the potential speed of a displacement sailboat is not governed by its weight, but the length of its waterline. Let me explain!
When a displacement sailboat begins to move thru the water, water being incompressible, must be moved aside to accommodate the passage of the hull. At low speeds the distance between the crests, along the length of the waterline are short and numerous as shown below.
As speed increases the distance between wave crests becomes longer and fewer along the waterline length as shown below.
Continued increases in speed increase the wave length until the second crest of the bow wave coincides or is the crest of the stern wave.
When this convergence take place, the length of the wave matches the length of the waterline. The hull is now suspended between its bow and stern wave crests.
This bow/stern wave coincidence determines the upper potential speed limit of a displacement hull, and occurs at a ‘Speed to Length Ratio’ of 1.34.
Any attempts to increase speed is futile. Increased horsepower will only cause the hulls to sink lower into the stern wave therefore raising the height of the bow wave that the hull is attempting to climb over!
Knowing that a ‘Speed to Length Ratio’ of 1.34 marks this coincidence of bow and stern waves for any given waterline length for a Displacement hull we can calculate the potential hull speed for any waterline length.
1.34 x Square Root of (Waterline Length in Feet)
= Hull speed in Knots
However, thus far we have not mentioned the Depth of the hollow between the bow and stern waves.
Displacement to Length Ratio:
The deeper of the hollow between the bow and stern wave the heavier the boat therefore more resistance to forward motion. Here the ‘Displacement to Length Ratio’ (DLR) is used to compare the relative mass of displacement boats no matter what their length.
A DLR less than 200 is indicative of a racing sailboat. Here the depth of the hollow between the bow and stern wave would be shallow, whereas the depth of the hollow of a hull with a DLR greater than 300 would be deeper indicating a heavy offshore boat.
In other words, the higher the DLR more water has to moved as the hull moves forward. Moving more water requires more energy from either the Sailplan or Engine.
The following ‘Displacement to Length’ chart indicates the type of sailboat design that would result according to it weight alone.
Ultra-Light Under 90
Light 90 to 180
Moderate 180 to 270
Heavy 270 to 360
Ultra-Heavy Over 360
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