AN INVESTIGATION INTO THE EFFECT OF RUNNING WETTED SURFACE AREA ON THE RESISTANCE COMPONENTS OF A CATAMARAN

An investigation has been made to measure the variation of the actual or running wetted surface area (RWSA) of a round bilge light displacement monohull and a catamaran for different separation ratios to determine the effect on the calculation of the resistance component of the total resistance and hence on the predicted power requirement of a full scale vessel.

The results of the investigation showed that the RWSA for this type of round bilge light displacement hull was larger than the static wetted surface area (SWSA) for the monohull by up to 10% for a Froude number of 1.0 and as much as 20% for a Froude number of 1.2. The change in the proportion of frictional resistance to residuary resistance was shown to reduce the estimated power requirement for a sample 25m vessel by approximately 10%.

No systematic variation of the RWSA with hull separation (S/L) was found for the catamaran configuration, except for the narrowest separation (S/L =0.2) over the Froude number range 0.6 < F _n < 0.7, so that the variation of RWSA was found to be essentially the same as for the monohull. Consequently the interference factor between the hulls was only found to be affected at the higher Froude numbers.