Displacement

The weight of water displaced by the bottom and the sides of a vessel below the water line. The displacement is a predetermined quantity and time and care are taken to figure it correctly.

One way is to set off at the station of cross section the area in linear feet, on a small scale for convenience; then make a curve of versed sines for the forward body. The after body being a trochoidal curve, based upon a radius of one-tenth of the length of the after body. A curve is drawn to take in these points and the L W L divided up by ordinates and a calculation made of the contents. As the linear feet at cross section represents the area of that section, so does every point in the curve represent the area of a section at that point, and the result of the calculation is the half displacement. It has been found that the displacement, as shown on this curve, comes very close to that of the best boats and may be accepted as a law.

This method was outlined by Mr. John Hyslop, of New York, about the time that it was presented to the public by Mr. Colin Archer, of Norway, and we are indebted to the labors of these gentlemen for a displacement shown on one line.

The theory is that the line of least resistance is a curve of versed sines for the fore body and a trochoidal curve for the after body, and this idea is applied to the cross sections and not to the water lines. For, though there have been large vessels that had a true versed since curve below the water line no rule could be given as to how much or how little should be taken off the fine end of the curve for a water line for a small boat, and moreover this line could only be applied to a boat of great beam and shallow body, and this type of model is a poor sea boat and unsafe.

If the displacement should be too small a larger cross section must be made and the curve projected anew and a new calculation made. The draught of water power to carry sail and general behavior of the boat depends upon the displacement.