distance traveled per revolution

[Randy stoner]

So as the title of this thread implies I'm wondering if my math is sound. If a 17 pitch prop allows the motor to rev at 4000 rpm WOT and a 15 allows it to rev at 4650 WOT, then by doing the numbers the 15 should theoretically give me a faster speed. I come by this by calculating the theoretical distance the 17 travelled in one revolution. (17"), and the 15 (15") The 17 @4000 rpm should theoretically travel a distance of 68,000 inches or 5,666 feet in one minute. The 15@4650 rpm should theoretically travel 69,750 or 5812 feet in one minute. I know theoretical performance is just that but does this mean the 15 theoretically will move the boat faster at the higher achievable rpm? I know there are other factors of course based upon slip and design, diameter etc. but are my assumptions technically correct?

randy

 

[WesNewell]

yes. if you are only talking prop rpm, disregarding gear ratio and engine rpm., or if GR is 1:1. Formulas and calc for it are here.

Go-Fast.com > Knowledge base > What is Prop Slip? > Prop slip calculator

 

[Texasmark]

Pull this up: https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator

It's all right there. Zero slip is a screw turning in soft wood. No such thing as negative slip and a propeller in water will always have some slip....fast fishing boats at WOT setup properly can be as low as 5%. Tubs as high as 25% or more.

If your engine revolves twice for the prop to revolve once....you turning the flywheel and watching the prop rotate, your gear ration is 2....for 2:1 in Mercury terms and 0.5 for OMC terms...use the Mercury terms for the go-fast and forget the :1 part in the box.

All props have particular parameters and all are different and have different combinations for complex props doing complex jobs. Any time you change a mfgr. or model within a mfgrs. lineup, you are dealing with different combinations and should expect different results.

Remember, the engine has to provide the torque x rpm to get the HP to twist that prop and develop the thrust to drive the boat. So just playing with numbers isn't a linear equation.

Besides prop differences, there are setup and hull parameter particulars that change as boat speed and attitude change as a result of your diddling the prop, set back, height, trim angle, and engine resultant rpm change and all that.

Good luck and have deep pockets if you want to play the game.

 

[Randy stoner]

Going to try a 17 aluminum tomorrow as a posed to the stainless steel of the same numbers. Then I’ll try my 18 p aluminum and see what happens. The place I bought the 17 aluminum said they will let me trade it for a 15-15 if none of these give and better results

 

[Texasmark]

On prop loans to find what works, usually they take back what they gave you to test....that means you put it on, take your data, stop, take it off, dry it off, shine it back up, put it back in the box and reinstall your old prop and be on your way....remember there will be other customers such as yourself looking to buy the right prop and will be testing and possibly buying what you took back...it needs to be saleable!