What Causes Prop Slip

[Drago]

Ours has lots of slip. I think I calculated 25% or so years back. Does raising the motor reduce slip or is it simply the shape of the prop. I know Bryan has spent $$$$$ playing with props. I would spend a bit more if I thought that another brand of the same size and pitch would improve results.

Clearly loading the hell out of the boat would increase slip. Funny thing is, no matter the load in ours, the max rpm is always 6200. Speed drops.

[ralph]

I think the rubber hub inside the housing of the prop starts separating. Not sure of a cure other than prop replacement.

[Bryden24shp]

Well Ken, the best way to explain prop slip is to imagine a dragster on the track. Zero % slip would be like having full on traction, where the tires do not spin at all. Slip would come in when the tires spin on the track. Since water is not solid, you'll never get zero slip, the prop always "slips" in the water because it can never get 100% traction. If it did, your boat would spin around the prop. Just kidding, you would be breaking all kinds of stuff, because a boat can not be solid enough to take the punishment. We diminish slip by running as close to the best prop we can, to minimize the slip co-efficient. A higher rake, bigger diameter prop will grab more water than a small rake, small diameter prop. So that SHOULD be what we are looking for. Now take in gearcase diameter and ratio's, weight, engine height (where your cavitation plate is riding) and the recommended RPM's we want to run to make the motor efficient and that blows our predesigned ideas to hell. Having your lower unit too deep in the water, causes drag on the gearcase, and increases slip, because you are pushing too much, having the lower unit out too far, will cause slip by causing the prop to suck air and ventilating. Too small of a prop will spin (slip) too fast in the water and not bite, that's cavitation. Its hard to get the right combination of engine height and prop to get the best slip figures. 25% isn't bad. Race boats get down to 8-5% slip. That's about the best you can get. Ya, it can get costly! I helped replace props on an Offshore boat a few years ago, the props we were playing with were 6 bladed Hering's at $12,000 per prop. And they had at least 5 sets in the van. I wish there was a magic prop wand to wave! I've been at it since 1982 and I still get baffled. But, I enjoy the crap out of playing with them until I find that sweet spot. I'm down to 13% with the new Manitou and still playing with it.

[Drago]

Thanks, Bryan. Maybe I just need a bigger prop. We just got back from a cruise on Liberty of the Seas. Got a chance to tour the bridge and was shown some pix of the construction. Man, is that ship high tech! I'll think about getting one of these for our boat...
https://www.flickr.com/photos/royalcari ... otostream/

[pond tuuunes]

Bryan, if I could get my speed from 20% down to 10%, for that shoot out. That would be nice. I think I need to try a 27-29p 3 blade.

[Bryden24shp]

Thanks, Bryan. Maybe I just need a bigger prop. We just got back from a cruise on Liberty of the Seas. Got a chance to tour the bridge and was shown some pix of the construction. Man, is that ship high tech! I'll think about getting one of these for our boat...
https://www.flickr.com/photos/royalcari ... otostream/

I think that will do the trick, Kenny!

[Bryden24shp]

Bryan, if I could get my speed from 20% down to 10%, for that shoot out. That would be nice. I think I need to try a 27-29p 3 blade.

Are you running a Bravo I, 4 blade, now or a Mirage Plus?

[rancherlee]

Ours has lots of slip. I think I calculated 25% or so years back. Does raising the motor reduce slip or is it simply the shape of the prop. I know Bryan has spent $$$$$ playing with props. I would spend a bit more if I thought that another brand of the same size and pitch would improve results.

Clearly loading the hell out of the boat would increase slip. Funny thing is, no matter the load in ours, the max rpm is always 6200. Speed drops.

I'm running more blade area on a 140hp then you are on a 225hp. I have 13.75x17 4 blade on mine to keep my slip down, I only approach 25% slip with a FULL load on my pontoon with this prop. I have a 3 blade 14x18 that slips quite a bit more with a load and has trouble staying hooked up while turning and pulling tubes, very much what your are seeing, hardly loose any RPM as the speed drops on that prop.

[Bryden24shp]

I raised the motor 2 holes yesterday and ran a Mercury Revolution 4 blade 14 3/4 x 19 I dropped 500 RPM's and lost 7 MPH. I didn't figure the slip, because its not gonna do the trick. I have another Salt Water Series II in a 15 1/4 X 19 to bolt on and try. If it grabs like 17, it should be the one. My cav plate was 3" under the surface before I raised it. I was by myself yesterday when I was out and wasn't brave enough to lock it WOT and run back to see where it was. But from the looks of the wake, I think its close. That Rev 4 sure raised the bow, though! I would love to find a 16X17 ECO to run. I have my eyes peeled for one.

Just ran the slip-co on the Rev 4--->27% Not as bad as I thought it would be. But, still going the wrong direction... Still going with my instinct and saying the big eared 3 blades are best. I really think that 4th blade may be creating too much turbulence and increases the slip. The first Manitou, the Premier and now this Manitou, just don't seem to like 4 blades. I'm off Monday and will run that 19 SWS II... Keep ya posted!