Originally Posted by phragle

All kinds of variables in you really think about it. At higher speeds, less boat in the water = more boat weight on a smaller area of water, but then again, decks are pretty flat, the bottom has a big curve under the bow, at 70 is the air not giving the boat a bunch of lift by pushing on the curved underside? ever try and hold a peice of plywood on the roof of the car at 70?

Originally Posted by Pilotpete

Well, first off, I can't take credit for the formula. I went to one of my books on the shelf and copied it all over. I got through some of my math classes by dating the professor's assistant.

In answer to your question, all else being equal, yes. But that being said, everything else isn't equal. For cars and planes the basic rule says that when speed is doubled, drag is squared. But for a planing hull, the area of wetted surface is reduced as you go faster. Additionally, water is incompressible, so there are a few other issues that come to play. Now dereknkathy is correct that when you reduce the wetted area, you increase the aerodynamic drag, but that is nothing compared to the hydrodynamic drag. You'll see it when you change the trim on your boat at speed. Push the bow down and the boat slows. The more you lift out the water, the faster you go. And we're only talking about Vees here. Cats bring a whole set of additional calculations and considerations to the dance.

Please also note that I just picked some random numbers for fluid density and Cd. However, the principle remains the same.

So there are numerous variables that come into play here. In addition to the change of the wetted surface area, you have a difference in the prop efficiency as well. most props create thrust on a non-linear curve as well. Just as your hull has an efficiency "sweet spot", so will your prop. So "8x" thrust may take 8x hp, or more, or less.

Originally Posted by onesickpantera

I agree many variables with boats. That formula doesn't carry over to boats very well. On a decent performing hull, going from 40 mph to 80 mph would take about 4x the power.

I have found that the formula used for speed calculators to be very accurate. Divide current speed by the sq root of (HP/weight) to get your constant

If a boat does 60 mph with 350 hp and weighs 4000 lbs

350/4000 = .0875
sq root of .0875 = .29580
60/.29580 = 202.84 constant

Once you have your constant, you can use square root of (HP/weight ) x constant = speed

Say you add 150hp

500/4000 = .125
sq root of .125 = .35355
.35355 x 202.84 = 71.71 mph

So, in this case adding 150hp would add 11.7 mph

Using these formulas I predicted two engine transplants within 1 mph

Originally Posted by Smoking Joe

So if other variables were to change, like more hp, a reduction in weight, conversion from inboard to outboard, do you think the formula is still relevant???

Originally Posted by onesickpantera

I agree many variables with boats. That formula doesn't carry over to boats very well.