Prop slip formula ?

[Sam]

Can someone give me a different formula other than the one below for determining prop slip. I am not sure if this one is correct or not.

RPM divided by gear ratio X pitch divided by 1056 = theoretical speed, divide actual speed into theoretical speed to get % of slip.

THX
Sam

[Donzified]

Go to http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html
Is prop slip around 10% good?

[Sam]

I've been there Keith, using their formula I am at 3% avg slip. Using my posted formula I am at .9% avg slip.

Sam

[RickR]

What are your #s? I have a Merc slide rule.
Add 1" of pitch for a cupped prop.

[Voodoocanoe]

http://www.go-fast.com/Prop_Slip_Calculator.htm

[Rootsy]

Sam,

are you adding 1 inch of pitch to your prop for the cup?

[Sam]

In a nutshell my question is, could I possibly be running only 3% slip with my new Turbo prop. Here are just my top end numbers based on three props. The RPM difference dosent add up unless I am getting a very low slip factor with the Turbo.
1. Mirage Plus 14 5/8 X 21 62mph @ 4900 rpm
2. Mirage Plus 14 1/2 X 23 62 mph @4800 rpm
3. Turbo one 14 1/4 X 23 61-62 mph @ 4400 rpm

I will post the the entire performance ranges later, right now I have got to go to work, that sucks.

Sam

[Rootsy]

sam,

if you add an inch of pitch for the cup, putting 24 into the prop calculator it gives you a theoretical speed of 68 @ 4400 rpm with a 1.47 gear ratio... actual speed being 62 gives you 10% prop slip which is pretty reasonable.

the 21 mirage is at 11% with your numbers and the 23 mirage is just goofy @ 19% slip

[Sam]

James, we have to talk because I am not coming up with the same numbers as you.

Sam

[Rootsy]

i used the calculator at BAM marine www.go-fast.com (http://www.go-fast.com)

put in the prop pitch, engine rpm, gear ratio and it spits out theoretical top speed... put in actual top speed and click calculate prop slip... just remember to put in 22 for the 21 pitch prop and 24 for the 23 inch props

also on the horsepower calculator for speed i come up with a hull constant of approx. 180 - 181 for the 16 hull with V8 and alpha... your boat numbers and my boat numbers correlate well.. saying 280 hp for 59 which is what iwas probably at last year (250 HP 2V converted to 4V for 350 Mag numbers).. and you being say 350 hp @ crank or 320 @ prop gives 180 for a constant also @ 2600 lbs. just thought that was kinda interesting... and explains why the numbers we are getting do not correlate with our initial thoughts on the 210 hp deal, thinking we should be much faster than we actually are.. in reality it correlates well with the 250 hp motor @ 56 mph, 210 hp would give 51 mph and i was above that out of the box with a chewed up 19 vengence, somewhere in the 54 - 56 range...

also you need the following for slip calculations...

((theoretical speed - actual speed)/theoretical speed) X 100