Hull speed, sometimes referred to as displacement speed, was a common rule of thumb used to provide the approximate maximum speed possible of the hull. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation It is generally described as the speed corresponding to a Speed-Length Ratio of between 1. 34 and 1. 51 depending on which of the limited sources one refers to.
In English units, it is expressed as:
where: "LWL" is the length of the waterline in feet, and "v" is the speed of the vessel in knots
It should be noted that Speed Length Ratio is not usually used with metric (SI) units.
Hull speed is typically not a term used by naval architects (they use, instead, a specific speed/length ratio or Froude number for the hull in question) but is often used by amateur builders of displacement hulls, such as small sailboats and rowboats. Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles The Froude number is a Dimensionless number comparing inertial and gravitational forces An amateur is generally considered a person attached to a particular pursuit study or science without formal training or pay In some cases less is more The purpose of this article is to give an overview With regard to Watercraft, rowing is the act of propelling a boat using the motion of Oars in the water
Background
Wave making resistance begins to increase dramatically in full-formed hulls at a Froude number of about 0. Wave making resistance is a form of drag that affects surface Watercraft, such as Boats and Ships and reflects the energy required to push 35 which corresponds to a Speed Length ratio of slightly less than 1. 20. This is due to a rapid increase of Wave making resistance due to the transverse wave train. Wave making resistance is a form of drag that affects surface Watercraft, such as Boats and Ships and reflects the energy required to push At a Froude Number of 0. 40 (Speed-Length Ratio about 1. 35) the Wave making resistance increases further due to the increase of the resistance caused by the divergent wave train which is added to the transverse wave train resistance. Wave making resistance is a form of drag that affects surface Watercraft, such as Boats and Ships and reflects the energy required to push This rapid increase in wave making resistance continues up to a Froude Number of about 0. 45 (Speed-Length Ratio about 1. 50) and does not reach its maxima until a Froude Number of about 0. 50 (Speed-Length Ratio about 1. 70).
This very sharp rise in resistance at around a Speed-Length Ratio of 1. 3 to 1. 50 was probably insurmountable in early sailing ships and became an apparent barrier.
It must be pointed out that these values will change dramatically as the general proportions and shape of the hull is changed. Modern displacement vessels can easily exceed their Hull Speed.
References
- A simple explanation of hull speed as it relates to heavy and light displacement hulls
- Hull speed chart for use with rowed boats
- On the subject of high speed monohulls, Daniel Savitsky, Professor Emeritus, Davidson Laboratory, Stevens Institute of Technology
- Low Drag Racing Shells
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