Offshoreonly.com - Avanti 22 speed vs.hp

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Quote:

Originally Posted by phragle (Post 4157846)

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?

Your plywood analogy is the premise behind cats. On a V, the hull is designed to push water to the side, and so it does it to the air quite easily. Certainly, there is some lift from a hull with a nose-high pitch, but at these speeds, it would not be significant. A flat bottom bass boat would gain more lift at a comparable speed than would a V.

Quote:

Originally Posted by Pilotpete (Post 4157842)

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.

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 :D

Quote:

Originally Posted by onesickpantera (Post 4158721)

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???

Quote:

Originally Posted by Smoking Joe (Post 4159020)

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???

I have found it extremely accurate when changing just the variables in the formula(weight, hp, speed). So, if you reduced the weight that would be calculated. If you added hp that would be calculated. Etc.

If you are converting from an inboard to outboard I have no clue.

Quote:

Originally Posted by onesickpantera (Post 4158721)

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

Well, I will respectfully disagree about the use on the formula in boats. The formula is just an algorithm for the laws of physics. It is accurate without regard to medium or speed. What makes it difficult is what we don't know. We don't know the Cd of the hull at a given speeds. Then you have to convert thrust to hp, and that requires knowing the curve of the prop(s) you are using.

I agree that for this application, your process looks like it would work very well. You have a very elegant and easy to calculate solution. All I was trying to show was the non-linear thrust(hp)/speed curve.

Comparing the 24 P&D 454 with the avanti Gil bracket outboard, totally different. The P&D was much better in the rough being all the weight low in the boat wasn't flying all over. The outboard in the rough with setback was out of the water a lot more. If I do it again be a 22 avanti or 24x7 with a hot small block in it.