#### Laminar Flow

##### Well-known member

Irregardless of any other factors, such as stern shape or hull form, the biggest single factor governing resistance is displacement.I'm interested in fuel consumption for sure, also just general.

I'm not talking about waves in particular but meant by "wave drag" that part of resistance that comes from climbing up your own bow wave. When you start planing, it's no longer accurate, right? So if I can get one equation that can match my dinghy and my big boat both, up to, say F# 7, I'll call it good. I've noticed vicprop in Canada do seem tto get the right anwers, but they don't show their work! : )

In fact, to get a rough estimate of resistance, it can and often is expressed as a percentage of displacement.

Dave Gerr has published a table in "The Nature of Boats" that correlates shaft HP required fora given speed and displacement.

For example: to reach hull speed you need 1 HP per 500lbs. This corresponds well with results from Vicprop which bases their calculations on available SHP, displacement, DWL, waterline beam and hull depth. With these data it is possible to fairly accurately estimate frontal projection and using a drag coefficient of about 0.4, specific gravity of water and velocity, make a decent stab at resistance. The Delft series has shown that hulls are more similar than not.

Resistance by waves (not what is also known as form drag, but sea waves proper) is indeed of consideration when specking HP or a corresponding prop. Larsson & Eliasson offer some calculation models for that and in their example of a 40', 8.5t sloop and it, fairly consistently, amounts to about 200kg more resistance for speeds of 7,8,and 9 kts in rough seas, or, in terms of overall resistance, an increase of between 60% to 30%. It should also be noted that while a heavier boat has more resistance in waves, it also has more momentum to push trough.