Let’s take the drawing of the IOM CD1 .
Apparently is not far from the specified characteristics, the PC is 0.57
the DSPL is 4043cm3, the appendix volume is about 6.4% of SA (6000cm²)
the Draft is 54mm against the max allowable 60mm.
The LWL is 980mm.
The Water plan is 1172cm² - That’s means that for 1mm variation close to the LWL level, the volume of the hull will change of about +117.2cm3.
I would like to continue with this last point because in my opinion is rather important.
When the wind is blowing into the sails, a vertical component force is also exercised on the Hull.
Short calculations made by an expert, friend of mines, proved that at 10° of heel the wind force may push down the hull of about 1.5%, at 20° of heel by 5.3% and at 30° of hell up to 12.5%.
i.e. the 5.3% at 20° of heel is equal for an IOM class boat to 4000x5.3% = 212g pushing down force and 500g at 30° of heel.
This condition will alter the water plan form and hull volumes and the IOM LWL will not change too much since already close to the max length of 1000mm.
Assume the same conditions for a hull with overhangs. The sinking will produce an important variation also of the LWL length. It will turn out that in spite of the increased volume and wet area, there is a compensation derived by the LWL increase that will favours the increase of the Critical Speed. It is obvious that if the boat is heeling at 30° the wind velocity should be elevated.
Such a thing like Critical Velocity increase will not occur with an IOM type hull that exploits the maximum LWL length.
I wonder how many models designers take this aspect into consideration.
This is my first thinking !