Speed calculations: A Formula that works.

[modvp]

Guys, I've been using this formula probably for over 12 years to calculate speed and it's pretty accurate on V-hulls. You can find it in an early 90's issue of Hotboat or Powerboat magazine.

Speed calculation formula:
[Prop pitch (inches) x rpm/1056 x gear ratio] – ~10% (for V-hulls)


For instance: a 24” prop at 6000 rpm with a 1.87:1 gear ratio.

24 x 6000/1056 x 1.87 => 144000/1974.72 => 72.9 – 10% (~7.3 in this case) = 65.6 mph.

Try it and see if it works for you.

[pyro]

Or just use this:
http://www.rbbi.com/folders/prop/propcalc.htm
:D

[FrenchPhil]

Frankly I'd prefer a formula which takes into account boat weight. I found one on a UK site:

Speed in mph= C (constant) X SQ. root of H (hp) / W (weight in metric tons)
The constant will depend on the boat but 5-6 works for most light tunnels.
Now on my Argo 18 400 kg w/motor -> .4 tons , 90 hp :
SQR 90/.4 = sqr 225 = 15 so
Speed = 5 x 15 = 75 mph

Now with a 3 seater STV 800 kg w/motor then .8 , 300 HP
SQR 300/.8 = SQR 375 = 19
Speed = 6 x 19 = 114 mph higher C, hull more efficient

1 kg = 2.2 lb

[dc96819]

If theres more pitch is there more slip ?
Is slip 10 or 15%.

[drunabout]

You calculate pure speed at zero slip.

You then take your wonderful speed guessing machine (radar, gps or speedometer, or the number of tears in your eyes) and compare it to the 100% calculated speed to determine the slip.

You do not calculate speed and guess slip to determine actual speed. This is called wishful guessing.

This is the formula for mph.

Formula for MPH at zero slip is:
MPH = RPM X PITCH X GEAR RATIO divided by 1056

Example:
106.352 MPH = 7500 X 28 X 0.5348 / 1056
0.5348 is 1.87 to 1 gear ratio

If your wonderful speed guessing machine reads 95 mph.
Your slip is 11.352 divided by 106.352 or 10.67%.
And that's only as accurate as your wonderful speed guessing machine.

One thing for sure, if you have 2 28's and one is showing 10.67% slip and the other is showing 15% slip at the same rpm, you have one good prop and one not so good prop. But, of course you already know that because your wonderful speed guessing machine has already told you.

I guess the whole thing boils down to: What's the objective? And in reality slip just goes along for the ride. If I'm running a kilo, I would surely rather have a prop that ran 101 at 12 % slip than a prop that ran 100 at 10% slip. And if I'm dragging, I just want the prop that gets me to end of the quarter first. And I don't care what the % slip is.

And, of course boat size and weight and number of passengers and hull shape and on and on and on has everything in the world to do with it, but who cares? All you care about is what works best on your boat under the given conditions at that given point in time. Yesterday and tomorrow make no difference and slip is just an interesting number.

As an old buddy of mine, Ron Hill, one of the owners of www.******************** (http://www.********************/) once told me, back in the days of Mod VP. Well ****, I'm a boat racer, am I'm a prop builder, but if I'm at a race and someone hands me a prop that's better than the one I'm running and I can win with it, I'll run it, and I don't care who built it. And slip just went along for the ride.

[CDave]

You calculate pure speed at zero slip.

You then take your wonderful speed guessing machine (radar, gps or speedometer, or the number of tears in your eyes) and compare it to the 100% calculated speed to determine the slip.


That's what I do but I use this program. http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

Enter pitch, gear ratio, speed and then click the calculate slip button. I would think to get dead on you would need to get the pitch checked on your prop first and not go by what is stamped on it.

[Reese]

Great points...


I agree....if you had three props with the same pitch and you recorded speeds of 81, 82, 83....what difference does it make if the prop that went 83 had the highest slip numbers. Most of us use slip (usually 10%) as an indication of how close we should be to the theoretical top speed....if we can get it close to the 10% barrier we feel we've done a good job....but I agree, why not just use the theoretical top speed and just forget about slip altogether.