Speed calculating?

[Itsahobby]

I know this has been discussed before, but how do you estimate speed for a Vdrive?
I guess what I am asking for is a formula.
Thanks Guys,
Jerry

[Carnivalride]

I know this has been discussed before, but how do you estimate speed for a Vdrive?
I guess what I am asking for is a formula.
Thanks Guys,
Jerry
Not sure if this is what you wanted but heres one formula.
Engine rpm x gear ratio x (propeller pitch/1056) = theoretical mph x efficiency = estimated mph.
Example: 7200 rpm x 1.12 x (16/1056) = 122.18mph x .83 = 101.4mph
Here’s where all the controversy comes in, no two boats, props or combinations are exactly the same. So you’ll see people claim prop slips from 10%-25%. If you’re an average true flat with a decent prop 20% may be about right and then again maybe not.
JMO,
Loren

[Itsahobby]

Thanks, this is what I was looking for. I have a 21 Rayson Craft, 15% gears.
I'm just trying to get a rough estimate to what it would take to get 70MPH.
I know I am right round 55 MPH at 5500 rpm and I can get 6000 rpm, so I will work backwards to get a estimated prop slip.
Thanks again,
Jerry
Not sure if this is what you wanted but heres one formula.
Engine rpm x gear ratio x (propeller pitch/1056) = theoretical mph x efficiency = estimated mph.
Example: 7200 rpm x 1.12 x (16/1056) = 122.18mph x .83 = 101.4mph
Here’s where all the controversy comes in, no two boats, props or combinations are exactly the same. So you’ll see people claim prop slips from 10%-25%. If you’re an average true flat with a decent prop 20% may be about right and then again maybe not.
JMO,
Loren

[Morg]

I set up an excel spread sheet with the formula in it.
Send me a pm with an e-mail & I can forward it.

[LeE ss13]

Not sure if this is what you wanted but heres one formula.
Engine rpm x gear ratio x (propeller pitch/1056) = theoretical mph x efficiency = estimated mph.
Example: 7200 rpm x 1.12 x (16/1056) = 122.18mph x .83 = 101.4mph
Here’s where all the controversy comes in, no two boats, props or combinations are exactly the same. So you’ll see people claim prop slips from 10%-25%. If you’re an average true flat with a decent prop 20% may be about right and then again maybe not.
JMO,
Loren
That is the formula I have used for years and compaired it to MPHs run at the drags and now with GPS and 80% is about standard and about 85% on a good runner bottom. However, that usually only works up to about 100 MPH. That is because it is not about prop slip so much as it is about the coefficient of friction of dragging parts thru the water and air, that increases exponetially as the speed go up. That is why twice the HP does not make you go twice as fast. At 120 mph, the efficiency is about only 70%.

[hairysituation]

It is money that makes you go faster. :umm:

[Carnivalride]

That is the formula I have used for years and compaired it to MPHs run at the drags and now with GPS and 80% is about standard and about 85% on a good runner bottom. However, that usually only works up to about 100 MPH. That is because it is not about prop slip so much as it is about the coefficient of friction of dragging parts thru the water and air, that increases exponetially as the speed go up. That is why twice the HP does not make you go twice as fast. At 120 mph, the efficiency is about only 70%.
LeE ss13,
I was only trying to give him an example. I use time slips from the drags and/or a GPS to figure my efficiency. I have no idea what happens at 120mph because I haven't been able to get there yet. But I think everyone agrees it takes exponentially more power to go faster. :cry:
So is that 70% at 120mph of yours in a runner or a true flat?
Loren

[GofastRacer]

I set up an excel spread sheet with the formula in it.
I have one of those, works pretty well!.. :)

[GofastRacer]

It is money that makes you go faster. :umm:
You got that right!.. :D :D

[72Hallett]

Does this formula also apply to larger boats, like a 21' Howard. I assume it would, but the efficiency number would be lower. Any idea what that efficiency number should be on a day cruiser?
Thanks, Jeff

[vdriver4ever]

I always used the tear method. If your tears fly off within one inch of your eyes then your only going 60 MPH. If they fly off within two inches then your going 75 MPH. If they fly off between your sideburns and ears then your going 85 MPH. But the good part is you can tell your rider your going well over 100 MPH... :)
This method takes all the slippage, prop size, and gear out of the equation!