Does propslip make you go faster?
 

Looking at the new Powerboat with the Performance Trials #5.
Some of those fast V's they tested are slipping in the 20-24% range at WOT.
Does this help for speed?
Those X dim. have to be pretty high for that kind of slip, or is that typical of vented bottom boats?
With my conventional V bottom we get 11% at WOT.
I don't think I understand this slippage/X dim. thing as much as i thought.
:confused:

Lower slip numbers mean the prop is hooked-up with the water better and is not slipping. Therefore, the boat goes forward further per revolution of the prop than one with higher slip numbers. This means the boat is faster with lower slip numbers if it turns the same rpm.

As the X dim goes up you will reach a point were the prop starts to loose its grip on the water and the slip will increase. There is a trade off between speed increase caused by the lower drag of the drive being higher and the speed lose because of the prop slip increasing. Prop selection/labbing with higher X dim can help in this situation. You need to find the sweet spot between drive height and prop design/tweeks.

Good luck,
Rick

wouldnt running a higher slip number make loose speed.....inturn rev highr rpms.....and inturn make you go up in prop size to compensate? So what i'm saying is...the more prop slip and higher X dim make a boater compensate by running a bigger prop? I know there has to be a happy medium somewhere in the melting pot or we'd all be running swamp boats! :D

Isnt that the same as spinning your tires in a car?

Not spinning your tires. More like having a clutch or trans slip. You are not putting the power the ground.

I agree with HyperBaja, the prop is spinning but is not hooking up with the water. A clutch slipping would mean the prop is not turning as fast as it should.

I was comparing the spinning tire analogy to having the prop blow out or cavitate. I was hoping the slippage would be compared to a efficency (power) loss, not a rpm loss.

Come to think of it, I was thinking of prop slip as not really robbing power, but that xx% of water that is being displaced not resulting in forward motion, is still displaced, and it took power to make that happen. I am of the opinion that a low slip number should be pursued. I read an article in PB by Sandler that said you could not have thrust without slip. I don't quite swallow that.

As long as the rpm is raised more than the prop slip, it will go faster, for example:

5000rpm, 24", 1.5, 5%slip = 71.96mph
5700rpm, 24", 1.5, 15%slip = 73.41mph

So increase the rpm by more than the 10% increase slip = faster.
All this is a big it.

Prop slip is a good tool.
 

But the numbers on the GPS or radar gun are more important than the slip percentage. My fastest prop is a 14.5" x 29 Spinelli that I turn 6200 RPM through 1.5 gears. At 97.8 MPH on the GPS this translates into around 14% slip. Now my 15" x 27 Hydromotive at 6100 RPM and 96.5 MPH translates into about 7% slip. For cruising the Hydromotive wins hands down. I can cruise at 4500 RPM and 72 MPH compared to mid 60's with the Spinelli at the same RPM. As for acceleration the Spinelli wins hands down. It really RPM's quick and as shown earlier produces a little more top end. I like both props and they both have their place on my boat. If I want to cruise I go with the prop that hooks and if I know there will be some racing I go with the Spinelli despite the high slip factor. Slip is a good tool, but don't be surprised if the higher slipping prop is the fastest.:D

Im not fast and only slip 8.9% :D

Hmmm...rv I got it.
The higher the x dim. the less drag/drive in the water and then the more your prop vents itself out or slips.
Thus loosing speed because of the slip...so to compensate, pitch is increased and its all good with the top speed thing again.
But i would guess having more of that pitch and just being at a cruise speed you would loose throttle responce and the boat will seem sluggish.
With my 10-11% slip my boat has tremedous throttle responce and feels very fast, or very powerful in the seat of the pants.

Where is Tomcat when we need him......

we were trying to find efficiency curves for props ( thrust vs slip)
don't seem to be in the public domain.

I wonder. in the extreme case...... what do the big cats with surface drives have for prop slip. that should tell you about how to set a surface piercing prop up. and a high x dimension conventional drive might be somewhere between the approximate 15% optimal for a submerged prop and what they run on surface piercing setups.

Good point...what is the magic number or sweet spot for speed and efficiency in % of slip.

exactly back....
Tomcat and I were trying to apply BEA (basic engineering analysis) to save time and $$$ and determine the optimal prop for a given boat and power configuration.

we can't seem to find any curves on particular props, but it seems the few generic curves by mercury racing show the peak thrust around 15% slip for submersed props

so who has a dialed in surface drive boat, and what speed, rpm drive ratio and pitch do you have.....

An excellent question. I think it is worthy of following up on. Maybe one of you could call Team Archer, Arneson, or Pulse Drive and get the answer.

As for the ventilated hull problem, I can imagine a situation where a ventilated hull might let the prop slip more, but (due to less resistance in the water) have the boat go faster.

I think conventional hulls vs ventilated hulls vs surface drives might be apples vs oranges vs melons. Dunno.

I like MELONS, myself.:D

um.......kaamascarab, do you not have kaama surface drives??????:eureka:

what props, drive ratios. rpm, and speed you runnin:D

My #3 Speedy drives aren't true "surface drives" , but the prop shafts are about 1" above the bottom.

At the engines turning 5,900 (they will turn more) and drive ratio of 1.5-1 + props 17.5 x 35 4 blade Merc Labs with a lot of cup (have to calculate them as 36"). Speed is 121.9 on GPS.

Every hull has different requirements and needs. With my setup the slip is about 9%.

Diameter is the key on my boat. But you can't swing that diameter on either a Bravo or Outboard.

Most of the posts here are thoughtful. There is no "magic" slip number. It is not a coincidence that there is a close correlation of of diameter/pitch ratio. If you use one of the examples above, and just increase the diameter, the "efficiency" (lack of slip) will increase like crazy, but the speed and rpm will surely go down. I like Wette Vette's analogy.

The main thing to remember is that you can't apply one setup to all hulls. Some of the Skaters (& other new cats) are running only 4-5% slip (maybe less). The diameter was recommended for my hull by Dennis C. at Merc Lab; he was correct.

Do any of you guys remeber that thread by Steve David on his 43' Black Thunder? He had triple whippled (that just sounds cool!)hp500s that he experimented with props and kept accurate data with each setup. I know that boats are all different, and that you can even show gains from either end in some cases, I just thought it was real interesting how he progressed though different props and slip. I think he ended up with 5 blade hydros and a very low slip number for that size of boat. I wish someone could explain the details between a result like that and one like Wette Vette's. Why is it that some rigs do better with a higher slip setup than others. I think Audacity mentioned some time back that he got 3-4% slip on his factory class boat..

Those low slip #'s are only possible if you don't count the cup as pitch.