If Skenes is a little too deep I'd suggest David Gerr's "the nature of boats" - it's a pretty thorough book that covers this and much more at a level that is easily understood.

As JonB said above it's about mechanical similitude - but also relativity.

For example, although wetted area and surface friction are critical to speed, so in fact is hull shape (not just a fine bow as opposed to a barge, but also length to beam and the overall under body shape).

ergo - You can't calculate the HP required to achieve hull speed without taking into account the prismatic coefficient (shape) of the boat.

Just going by displacement and HP (forgive me if I use sail power vs HP, this is SailNet not MotorNet) -

I've got a 20,000 pound Irwin 38 CC - Displacement to the 2/3 = roughly 48, and Sail Area (771 sq feet) to displacement (SA/D) = 16.7

Per D. Gerr, you multiply the two (48 * 16.7) and get 801 - so I'm slightly under - canvassed for 'good' performance.

For the die hard crowd:

We can also calculate the HP as a factor of sail power, there are numerous charts out there to do so, I'll just use the 'standard' of .02 HP per square foot at force four wind (13-15)

My Irwin 38 CC has 771 square feet of sail (100 percent fore triangle). So in a force four breeze I can expect the sails to generate 15.4 HP.

Thankfully force five winds produce twice the power

You can do the math backwards - 48 *16.7 = 801 sq feet of sail for good performance - 801 * .020 = 16hp

Here's the real life - at 2200 RPM she makes 6.3kts in 1 foot chop - and according to the RPM - HP charts about 20 HP - meaning about 16HP delivered to the prop (altenator loss and transmission loss take the rest).

Funny how math just seems to work out isn't it..