Q. I have determined that the displacement of my yacht is heavier than that shown in the designer/builder specifications. How does this affect the other CRF data declarations that I need to make?
A. If you are declaring a displacement that is heavier than the designer/builder specification, it follows that your declaration for the LWL that corresponds to that heavier displacement should be longer, and that your declaration for Draft should be deeper, than the designer/builder specified values. One way to quantify the differences in LWL and Draft (DM) would be to estimate the sinkage resulting from the difference between the designer/builder spec and the declared displacement (DSPS). The ‘Pounds per Inch Immersion’ (sink) for most boats can be approximated by: Lbs/in Immer = 1.1*LWL^2. It follows that actual sinkage (in inches) = delta DSPS/Lbs per In Immer. This estimated sinkage would equal to the amount added to the designer/builder specified Draft (DM) in inches , and for most boats multiplying this sinkage by 6 approximates the amount added to the designer/builder specified LWL, also in inches.
For example: Suppose that the designer/builder spec for the displacement of a boat is 40,000 lbs, but the actual displacement has been determined to be 46,000 lbs. If the published value for LWL = 38.0 at the lighter floatation, an estimate for ‘pounds per inch immersion’ would be 1.1 * LWL^2, or 1588 lbs. This implies that the boat would float deeper than the original spec by (46,000 – 40,000) / 1588 = 3.8 inches, or 3.8 / 12 = 0.31 ft. This, in turn, would imply that the actual LWL would be 38.0 +6 * 0.31 = 39.9 ft. Similarly, if the published draft were 5.33 ft, the implied actual draft would be 5.33 + 0.31 = 5.61 ft.