Prop speed theory...please help
 

Theoretical speed for a b1 26p at 4200 (prop rpm) is the same as a a 30p 3633 rpm = 103 mph

Thats theory. The actual speed is NOT the same. This can ONLY be related to slip. There are 3 factors involved 1) what are the effects of prop speed 2) what are the effects of pitch 3) what the boat likes.

forget the boat for a minute... I first want to understand what the prop is doing before i try to figure out if the boat likes it.

ALL else being equal....will a b1 have less slip at higher rpm or lower rpm.

All else being equal...will a b1 have less slip at higher pitch or lower pitch?

I have been told the exact opposite from two experts...So which of these is correct?

Thanks

Theoretical speed with a 1.5 ratio drive
26p at 4200 rpm 68 mph
30p at 3500 rpm 66 mph

Thero speed with 1.36 ratio drive
26p at 4200 rpm 76 mph
30p at 3500 rpm 73 mph

Your not taking the reduction ratio of the outdrives into consideration

here ya go



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

My numbers are based on prop rpm...NOT engine rpm. I have used the prop slip calc. Thanks

My question is how does rpm effect slip...how does pitch effect slip. Different gearing (prop rpm) and different pitch (to match gearing). Should mean that speed stays the same.....but it does NOT. Theoretical is the same but the slip changes so top speed changes. What is going on with the prop?

I missed you "prop rpm" in your original question

I've wondered the same thing ( how does prop rpm affect the dynamics of the design?)

No sweat. I am just trying to take the boat and drive out of the equation.

I would imagine that higher rpm AND increased pitch INCREASE slip and you must have one or the other for speed...I THINK that pitch increases slip more since the prop must do more over a shorter distance. So do you increase diameter? Any experts?

IMO, you can't take the boat and drive out of the equation.

Different boats will react differently to the same changes.

My thoughts would be that less rpm and increased pitch would create less slip. It seems like higher rpm and less pitch would cause more aeration/turbulence in the water.

You have to take the boat into concideration.
As speeds change so does drag, prop depth, possble arriated water, etc., etc.

In a perfect situation, screwing the propeller into the ground/dirt, there is no slip. So, rpm and pitch don't matter. What matters in the boat bottom, the X dimention the geometry of the propeller, water conditions, etc.

Potentially higher rpm with lower pitch "could" cause more slip. But, that's only if it is the boats fault do to set up.

Brett

ok i get it the boat is reacting to the prop and vice versa...



here is another way to ask the question....what "typically" happens to top speed when someone goes from a 1.5 ratio to a 1.36 ratio bravo drive. Prop rpm increases so pitch is reduced to get back to the same theoretical speed. same boat so that is a constant.

Should be the same speed...is it? why?