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Avanti 22 speed vs.hp

Old 07-24-2014, 06:46 AM
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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
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???
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Old 07-24-2014, 10:41 AM
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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???
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.
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Old 07-24-2014, 12:00 PM
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Originally Posted by onesickpantera
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.

Last edited by Pilotpete; 07-24-2014 at 12:41 PM.
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Old 07-24-2014, 12:23 PM
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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.
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