Sailing Boat Design The theoretical hull speed for a displacement hull (like most sail boats) is a function of the length of the wave created by the boat as it moves through the water. Wave speed is a function of wavelength; longer wavelength is faster. Longer boats make longer waves. Since longer waves are faster, boats that make longer waves are faster. The hull speed, by the way, can loosely be thought of as the speed at which the boat, in order to go faster, has to start ``climbing up'' over its bow wave, which takes a lot more power. The formula for hull speed (HSPD) is where HSPD is given in knots and the length of water line, LWL, is given in feet.   The displacement length ratio is a measure of a boat's speed potential. For displacement boats (most sailboats), speed potential is a function of waterline length (unless your planing or surfing down a wave). Longer waterline boats can go faster. Lighter boats accelerate faster and reach hull speed with less wind. All else being equal, the boat with the lower D/L will be a better light air performer. Lower displacement will also make a boat more sensitive to loading. 2000 lbs of gear will have a larger effect on performance for a 10,000 lb boat than for a 20,000 lb boat. The formula for the displacement length ratio (D/L) is where the displacement (disp) is given in pounds (lb) and LWL is given in feet.   The Capsize Screening Formula is   Here, the beam (the greatest width) is given in feet, and the displacement in pounds (lb). This formula is a somewhat controversial figure. It came into being after the 1979 Fastnet race in England where a storm shredded the race fleet. The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. A lower value is supposed to indicate a boat is less likely to capsize. a value of 2 is taken as a cut off for acceptable to certain race committees. However this is an arbitrary cut-off based on the performance of boats in the '79 Fastnet. The CSF takes no account of hull shape or ballast location. The CCA characterizes the formula as "rough". They go on to say that "While the capsize screening formula places a limit on excess beam, which is important for good stability range, it does not control for another main determinant, ballasting. With only simple data, this is as far as we can go." Naval Architect Robert Perry calls it, "...far too simplistic to be always accurate, but it is one of the currently popular ways of looking at a boat's offshore suitability." (Sailing Magazine, Nov. 2001, p.44). Any two boats will have the same CSF value if their displacement and beam are the same. One could have all it's ballast in a bulb at the bottom of an eight foot fin, the other could have it in a 2 foot deep full length keel. The stability characteristics of the two boats will be drastically different despite the identical CSF value.   In this exercise you are asked to find suitable design values for the length of water line (LWL), the beam, and the displacement of a sailing boat. The conditions are: The displacement is set to a fixed value of 10'000 lb A displacement length ratio of exactly 165 Your aim as a designer is to wind up with a CSF value of 1,5. This value will give you high safety without compromising performance. You try to reach this aim by varying the LWL and beam values. 1. Create a spreadsheet in which you write up those formulas. Use the formula editor for this purpose (Sett inn -> Objekt... -> Formelredigering). Explain what the various symbols mean, and which unit is used. If you label some of the cells, your sheet will be more transparent.     2. Use the problem solver to find suitable values for LWL and beam that give you the desired CSF value and that comply with the remaining conditions.

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Oppdatert 26/10 - 2004