3% slip?

[Tacklewasher]

Testing props today.

18' Starcraft SS with an 1981 Evin 115. Motor is raised as high as possile.

13X15 Stilletto SS prop.

37.9 mph (gps) at 5500 rpm. Never had a question on the tach. Motor trimmed all the way up.

I calculate that as 3% slip.

Same setup but with a 13X17 aluminum OMC prop gave me 38 mph at 5100 rpm. That calcs at about 7.4% slip.

So the 400 rpm difference makes sense but I sure seemed to pick up efficiency with the stainless prop. Hole shot is great and runs well throughout the whole band. Had some venting in turns, but no venting while running straight. Bit choppy so I would not be surprised if I can get more speed/rpm on calm water, but unless I've done something wrong with the math, that is pretty freaking good.

I've done a full decarb (man that makes a big cloud and I forgot the back door to the house was open) but the carbs could probably use a rebuild as I did it last 4 years ago. But for now it's time to buy a prop lock and figure out how to go fishing.

 

[craze1cars]

Re: 3% slip?

I went through all that too, except I got you beat as I get a true zero percent slip based on GPS and calibrated shop tach readings. Yay! But obviously wrong.

I was told here by several that it's because my prop has a lot of rake (as does your Stilletto). High rake on a prop is similar to adding about an inch of pitch, so when you do the math you need to consider your 15 pitch prop closer to a 16 pitch prop. That will probably net you a more realistic number.

I don't fully understand it myself, but that's what many seem to say here. Slip calculators are nothing but a rough guide for comparing different types of props to each other. So if you're getting a lower number out of a given prop, that's generally better for higher performance applications. But the calculated number in and of itself is pretty meaningless. Obvoiusly I can't possibly have 0% slip, and I believe you can't possibly have 3% slip...there's always going to be a bit more than that

But I'm glad you posted...seems the Stiletto works well for your boat. And I'm considering buying one for mine! Thanks for another tidbit of info to help me make a decision...

 

[towtruck1]

Re: 3% slip?

Hmmm..

Based just on speed, the OMC prop seems better (more speed at lower RPM). At higher RPM, your engine should theoretically be generating more power, but you are actually losing .1 mph.

You mentioned good hole shot & low cavitaion for the new prop, but how did the OMC prop do in those areas?

 

[Tacklewasher]

Re: 3% slip?

Oooh.

Don't like using 16 pitch. Gives me 10% slip

towtruck1, my goal is to manage my RPM at WOT. Final speed isn't that critical to me, I want to run 5500+rpm to keep the motor in good shape (as others here suggest). No intention of running this fast unless there's a storm coming or something. The 17 just doesn't let the motor run fast enough, but will be good for a river prop.

Funny thing is, my 19 OMC Stainless lets me run ~4,800 rpm at 38 mph. Seems the boat doesn't want to go any faster

 

[Randybeall]

Re: 3% slip?

Actually it seems you have found the speed that 115 horsepower will make you go. Put a 140 hp engine on the 19 inch prop and you will continue to gain speed. No matter how perfect we get the prop it takes a certain amount of horsepower to push a given hull combination through the water and air.

 

[Tacklewasher]

Re: 3% slip?

Yeah. Figured something like that.

And in all honesty, except for the fact that it came with a 115 cowl, I have no real idea as to the HP as the label is gone and the freeze plug has been sanded down.

Still think I can get 40 out of it though. Maybe if I run my electric at the same time