How much slip does your prop have ?

[jimmbo]

This was with a RAKER, a vented Prop. The Enertia(has Adjustable Vents) is quicker out of the hole and has better top endView attachment MVI_0362.m4v

 

[cyclops222]

I like the, Bow Light does go above the horizon logic. Ny Chaparral has 0 bow lift. When floored from a dead stop. I use no drive trim at any time. The drive is VERTICALE to the water surface. It feels like a car from a dead stop.

 

[Texasmark]

Unless its excessive slip, it really doesn't matter in the way people think it would... You can have a 23p with 25% slip that'll run near a 19p with single digits and have the same speed at any given RPM. I've tested props that behave that way and the only thing I've noticed is that they tend to be slower to plane and blow out easier. In the same breath, props with really low slip are more sensitive to blowing out too and be slow to plane in my experience.... so it works both ways lol.

And the other thing... calculations of slip based on theoretical speed to actual speed are at best a guess... between blade surface area, cupping, pitch variations... what you calculate and what it actually is may very well be completely different. There's lines of calculations that would need to be done to tell you what the actual slip would be. It's more like CFD. And I've never cared enough to learn.

I use Go-Fast.com slip calculator. It has a zero slip column and then using the inputs to the chart, with your actual speed vs zero slip speed, you get your % of slip. Big heavy loads slip a lot as do cruising speeds where a lot of the boat is in the water.

Easy planing boats with the right prop can get into the single digits. My avatar is a picture of my 17' alum. BB with a stock 90 Merc and 24P Ballistic SS prop at 5500-5600 RPM...kissing 50 with slip in the mid single digits per the go fast calculator.

 

[Lpgc]

I've been interested in why manufacturers use different gear ratios with different diameter and different pitch props and what the compromises are but it's not something I'm going to be concerned about for my own boat as long as WOT rpm and speed are close to spec.

Seems to me prop slip could be defined in 2 ways, (1) Speed of boat versus pitch of prop x prop rpm, (2) Speed of the column of water the prop is shifting versus prop pitch x prop rpm. I expect the 2nd definition is always going to be a higher figure than the 1st definition.

All props have some slip by the 1st definition - If you're tied to a mooring but drop into gear you can see the prop turning but the boat isn't moving, that's 100% slip by the 1st definition, difficult to test or calculate slip by the 2nd definition? If there was no slip (and since there's no torque convertor or clutch to 'feather') the engine would just stall if there was no slip because the boat wasn't moving when you dropped into gear. The column of water the prop is moving (even if the boat is tied to the mooring and not moving) gives some thrust, the mooring line is going to be tight and if you untie from the mooring the thrust will move the boat.

I think delving much deeper into it will soon get complicated, probably easier to do by experimentation than try to calculate what would be the ideal prop over a range of conditions. I might expect a bigger prop to have more grip in the water because it's gripping in a wider column of water but the bigger prop is also going to have more drag in the water than a smaller prop, unless fitting a smaller prop would mean having to turn it faster to achieve the same thrust or more pitch, in which case the smaller prop might have the same or more drag than the slower turning bigger prop?

Seems you need some definition (1) slip or acceleration would be slow, without it it would be like trying to set off in top gear in a manual car without a torque convertor. Still definition (2) slip difficult to measure or calculate, might expect bigger pitch to be more efficient at higher boat speeds and vice/versa because the angle of the prop blades points more toward the direction of the blades travel through the water (more pitch = less drag at higher speeds but more drag at lower speeds)?

My boat has a 14.5" diameter by 19" pitch prop, I wonder how a smaller diameter prop with more pitch or bigger diameter prop with less pitch would compare... I expect it would be possible to get the same RPM at WOT but I think it would be difficult to calculate how they would affect top speed, acceleration and part throttle fuel economy. Taking it to the extreme, if it were possible to fit a Titanic sized prop to my boat and change the gear ratio so the engine could reach correct WOT rpm I don't expect it would move my boat as quick as the prop I have fitted even if it had less slip (and weighed the same as my prop). But at the other extreme if I fitted a 2" diameter prop and geared it up, that wouldn't work either. But how would 13" diameter with more pitch or 16" diameter with less pitch, if they both allowed the engine to reach the same rpm at WOT, compare at full throttle and at part throttle?

 

[jlh3rd]

I've posted this before. It is 3 different props all giving the same performance #'s . Plugging in the #'s give different slip percents.
wonder what the % slip is at different rpms.
I ran a merc 3-16x12 for a few years to pull tubes full of teen agers. I wanted higher rpms under load. Plugged it into the chart @ 6000 rpm and got 22-23% slip.
Interesting that the chart doesn't ask for diameter or blade #'s.

 

[Lpgc]

All other things being the same (such as boat trim affecting boat drag) it seems to me it comes down to prop thrust versus engine power needed to turn the prop to make that thrust, for a given trim the boat getting more thrust will be going faster. I might expect various prop diameters / pitches to be better than others in certain conditions - not just about top speed but also acceleration, trim and mpg. The Titanic prop fitted to my boat (and if geared for it) would have less slip than my prop but would also have a lot more drag, the drag would mean a lot less thrust at higher speed than my prop. But if I could gear it so my engine was doing 4500rpm with the boat not moving tied to a jetty I expect the Titanic prop would more easily pull the jetty than my prop geared to do 4500rpm tied to the jetty lol.

 

[Texasmark]

I've been interested in why manufacturers use different gear ratios with different diameter and different pitch props and what the compromises are but it's not something I'm going to be concerned about for my own boat as long as WOT rpm and speed are close to spec.

Seems to me prop slip could be defined in 2 ways, (1) Speed of boat versus pitch of prop x prop rpm, (2) Speed of the column of water the prop is shifting versus prop pitch x prop rpm. I expect the 2nd definition is always going to be a higher figure than the 1st definition.

All props have some slip by the 1st definition - If you're tied to a mooring but drop into gear you can see the prop turning but the boat isn't moving, that's 100% slip by the 1st definition, difficult to test or calculate slip by the 2nd definition? If there was no slip (and since there's no torque convertor or clutch to 'feather') the engine would just stall if there was no slip because the boat wasn't moving when you dropped into gear. The column of water the prop is moving (even if the boat is tied to the mooring and not moving) gives some thrust, the mooring line is going to be tight and if you untie from the mooring the thrust will move the boat.

I think delving much deeper into it will soon get complicated, probably easier to do by experimentation than try to calculate what would be the ideal prop over a range of conditions. I might expect a bigger prop to have more grip in the water because it's gripping in a wider column of water but the bigger prop is also going to have more drag in the water than a smaller prop, unless fitting a smaller prop would mean having to turn it faster to achieve the same thrust or more pitch, in which case the smaller prop might have the same or more drag than the slower turning bigger prop?

Seems you need some definition (1) slip or acceleration would be slow, without it it would be like trying to set off in top gear in a manual car without a torque convertor. Still definition (2) slip difficult to measure or calculate, might expect bigger pitch to be more efficient at higher boat speeds and vice/versa because the angle of the prop blades points more toward the direction of the blades travel through the water (more pitch = less drag at higher speeds but more drag at lower speeds)?

My boat has a 14.5" diameter by 19" pitch prop, I wonder how a smaller diameter prop with more pitch or bigger diameter prop with less pitch would compare... I expect it would be possible to get the same RPM at WOT but I think it would be difficult to calculate how they would affect top speed, acceleration and part throttle fuel economy. Taking it to the extreme, if it were possible to fit a Titanic sized prop to my boat and change the gear ratio so the engine could reach correct WOT rpm I don't expect it would move my boat as quick as the prop I have fitted even if it had less slip (and weighed the same as my prop). But at the other extreme if I fitted a 2" diameter prop and geared it up, that wouldn't work either. But how would 13" diameter with more pitch or 16" diameter with less pitch, if they both allowed the engine to reach the same rpm at WOT, compare at full throttle and at part throttle?

I had both the Mercury 2 stroke 90 HP (I3) and 115 HP...both the I6 and the I4 versions manufactured later with loop charging. They share the same mid section and lower unit. The gear ratio on the 90 is 2.33:1 while the 115 is 2.0:1. Remembering a Merc. sales brochure back in the 1990's when the Tower of Power I6 was in production, rather than the current 3 and 4 cylinder loop charged engines, it classed the 90 as a work engine and the 115 a higher performance engine. Having had both, that extra 25 HP does make a difference.

 

[Texasmark]

I've posted this before. It is 3 different props all giving the same performance #'s . Plugging in the #'s give different slip percents.
wonder what the % slip is at different rpms.
I ran a merc 3-16x12 for a few years to pull tubes full of teen agers. I wanted higher rpms under load. Plugged it into the chart @ 6000 rpm and got 22-23% slip.
Interesting that the chart doesn't ask for diameter or blade #'s.

I think blade numbers and diameter would be a constant for a comparison...in short, you have a rig that has a certain engine that uses a certain range of diameters......small range and diameter change is a function of the prop design....the more "Rake" on the blades, the smaller (slightly) the diameter for a given range of props. Higher pitched props would have more rake to help them function better at higher speeds and higher transom mounting.

Slip is just that...what would a screw do in a piece of wood vs a bowl of Jello.....there's zero slip in a piece of wood......once enough threads have entered to level out the playing field.

 

[Texasmark]

This was with a RAKER, a vented Prop. The Enertia(has Adjustable Vents) is quicker out of the hole and has better top endView attachment 408220

Like other props from other suppliers, like a couple of Mercury Laser (I and II) the ports are there to allow air (engine exhaust) bubbles to flow across the blades reducing the density of the medium allowing the engine to rev. faster like would happen with a shallow pitched prop.

Once up and running, the "slip stream", new water coming across the prop, has enough pressure at some point in the acceleration to seal off the leaking exhaust and the actual (usually higher pitched) blades have solid water and can develop full thrust and propel the boat to higher speeds.

On the original SS Laser I had on a Ranger bass boat, the hole shot was a vertical trim and a firewalled throttle. The engine jumped up in RPMs and within a couple of seconds was "on" the water (bow had come back down) with high revs. Once the critical speed was reached the prop "locked up" and the boat shot forward......you could feel the abrupt change....Pushing out the trim and it easily climbed on the "pad" and away it went. I had a lot of fun with that boat.

The neat thing about ported props is that you have 2 props in one.....essentially a low pitched prop for things like fast hole shots and pulling water toys (at lower speeds where the prop hasn't locked up) and the high pitched prop for light running at high speeds. Hustler brand is the only mfgr. of which I am aware, that has aluminum props with all the goodies of the high performance SS props except for material...which is a special formula to limit blade deflections. I have half a dozen of them too in 3 and 4 blade designs....preferring the 3.

 

[dingbat]

Interesting that the chart doesn't ask for diameter or blade #'s.

Diameter and # of blades have an effect on thrust, but play no role in the “pitch” of the prop.

 

[jlh3rd]

Diameter and # of blades have an effect on thrust, but play no role in the “pitch” of the prop.

well, no...not directly.
but if propping a boat, blade # and diameter still affect the max rpm @WOT, just like blade pitch does.
If a 3 blade 14x14 is at 18% slip, then a 3 blade 15x14 ....or a 3-16x14 would give different slip % ?....the chart says no?...
you're saying the "thrust" changes but "slip" stays the same .....

still, gets back to you prop for rpm.....let slip take care of itself.....
go have fun.