Slip Figure Change?

[Boilermaker]

The way my boat is set up now, I get a .171 slip figure when I run the numbers on a prop calculator. <br />I am going to raise the engine 2 holes on its mount. Will this just give me more rpm & speed from less drag, or will it also reduce the slip percentage figures???<br />Is the slip figure static>>>more to do with the prop design??? or does it change with the other variables??? Is there an aprox. figure in RPM increase per hole changed??? <br /> I am running a 1978 Johnson 115Hp with a Stiletto 17 pitch prop on a 18ft Modified Vee hull, with the engine at lowest setting (A.V. plate aprox 1/2" below transome) <br />Last outing boat ran 5100RPM @ 36.4mph WOT, with a heavy load.<br />Thanks<br />Jim

 

[Dhadley]

Re: Slip Figure Change?

Slip percentage should, in theory, come down as you lose drag. <br /><br />Think of it this way -- youre running the boat at wot but at full negative trim. Theres lots of boat in the water. As you trim out you lose drag from the wetted surface of the hull. You gain rpm & speed so your slip % goes down.<br /><br />That being said -- as we work on losing gearcase drag by increasing the X dimension, there will be a point where any given prop design will lose "bite" and start to vent, therefore increasing the slip. At that piont the rpm goes up and the speed comes down.

 

[Boilermaker]

Re: Slip Figure Change?

Thanks D,<br />Am I correct in assuming that with a higher engine mounting (X-dimension)I will not be able to trim the engine out as far, before lossing prop "bite"???<br /> I notice you seem to recommend raising the engine as high as possable without venting on hole shot exceleration>>>is there a trade off of less engine drag to more hull drag (because of less out trim)???

 

[Dhadley]

Re: Slip Figure Change?

Each prop will have different run traits. As a prop loses cup at the tip the trim range will become less. Some props will indeed trim out further than others. Naturally you'd expect a $900 Mazco to hold bite higher and with more positive trim than a $125 aluminum prop.<br /><br />Props are indeed one area that, generally speaking, you get what you pay for.