Prop slip?

[Mac]

I am trying to help a friend figure out what props to try on his boat. What is a good number to use to for prop slip to calculate top speed? I can figure out top speed based on drive ratio, engine RPM and inches of prop but I do not know how much slip is normal. The boat I am calculating is 34' long 8' beam 24 degrees deadrise, has two 600 HP big blocks running through 1.5 to 1 TRS drives, the engines should be able to spin to 5800 RPM. Using these numbers if I wanted a top speed of 85 MPH I would use a 23 pitch prop. I am assuming the engine can pull that speed. That is not taking into account any slip, how much slip should I guess I will have?

[Greg K]

My guess is 10-12 percent would be a good place to start. Think you'll find that 25 or 26 pitch props may work best. Then again this for single engine application. If I remember correctly, you may have to go down a couple inches for twins.

[rickrsbro]

here's a plug and pray guide if you want to play around.

prop slip calc (http://www.go-fast.com/Prop_Slip_Calculator.htm)

there are some typical slip percentages for different hull configurations right here (http://www.go-fast.com/boat_speed_predictions.htm) .

[Mac]

Thanks, looks like about 9% slip for the style of hull. So 25 should do the trick, or at least be very close. Funny my friend wanted to start with 32"s !!, even with the slip that's 106 MPH, never gonna happen with 1200 HP.