Prop slip question

[clemsonteg]

I was thinking about something this week that I wanted to ask. The scenario is this:

Let's say you have 40% slip on a prop at wot and a set rpm. If you were to change props and everything as far as size stayed the same but the slip was cut to 20% would your rpms drop or would you pick up speed or some other combination?

Edit:
Since the topic ended up being about my prop setup I figured I would post a few pics of it

 

[Maclin]

Re: Prop slip question

First off, 40% is a big number for slip.

The answer is hard to pinpoint not knowing the weight,size, etc of the boat being pushed. Reducing slip would gain MPH assuming the engine has enough HP left to get to the same rpm as before.

At any rate, a 40% slip indicates a mechanical component needs to be repaired or replaced.

 

[clemsonteg]

Re: Prop slip question

First off, 40% is a big number for slip.

The answer is hard to pinpoint not knowing the weight,size, etc of the boat being pushed. Reducing slip would gain MPH assuming the engine has enough HP left to get to the same rpm as before.

At any rate, a 40% slip indicates a mechanical component needs to be repaired or replaced.

Well part of this is due to my current setup. I have a prop that is oversized and kind of beat up on the edges which according to the mercury racing prop slip calculator I currently have 39%. This is of course on a pontoon but I am ordering a new prop today if I can decide on one and it will probably be a 4 blade. I am thinking my prop slip will drop quite a bit between the new prop and being a 4 blade vs my current 3 blade.

 

[steelespike]

Re: Prop slip question

15% or more is considered high slip.
A pontoon or other hard to push boat might have higher numbers.
10% slip is considered ok but on the high end.
I think you can see that a typical improvement of 5 to 10% would likely require an accurate gps and a tach to tell much.

 

[steelespike]

Re: Prop slip question

If you give us the details of your setup we can probably help decide what prop.
My limited experience slip calculation is only as accurate as the details put in
and many times wildly inaccurate.
I don't think a racing prop selector is the best for a pontoon setup.

 

[clemsonteg]

Re: Prop slip question

1978 85 hp Johnson on a 24 ft pontoon. Current setup with a 13.75x15 3 blade with rough edges does 4600 rpm and 20mph. Gearing is 2:1. Speed and tach are a garmin gps and a tiny tach. Considering the 13.25x13 solas amita 4 blade prop. Hopefully it keeps me in range. If not I plan to have the prop modified down to an 11 pitch since they don't make one. Most of that was figured out from a previous thread I was just trying to get a handle on prop slip and how it affects other parameters.

 

[Texasmark]

Re: Prop slip question

Your prop is not in all that bad a shape. Thing about rpm's is load on the engine and engine torque at the desired rpm. 4600 rpm's is on the low side and with that, I'd say that you could unload the engine, get your rpm's up to 5000 or 5500 and do your engine a favor. Changing your prop a couple of inches in pitch is a larger percentage than increasing your rpm's 500 rpm, but if you can unload the engine, allowing the engine to run up the torque curve you could gain a higher increase in percentage than the change you made. So if you dropped your pitch a couple of inches and you expected to gain 500 rpm and 2 mph (current setup) you might increase rpm's 700 and gain 3 mph. Don't know till you try it.

Purring along at 20 mph with an aluminum tank toon seems to me to be moving right along. I think you would be hard pressed to get much more out of that.....too much drag.

Mark

 

[clemsonteg]

Re: Prop slip question

Yeah it seems to run pretty well. My goal is just to make sure it's running the best it can.

 

[Maclin]

Re: Prop slip question

Those burned edges to me look like they are from cavitation, super-sonic air bubbles being created while running that can actually melt and reshape the aluminum. Is your prop far enough into the water?

 

[clemsonteg]

Re: Prop slip question

Those burned edges to me look like they are from cavitation, super-sonic air bubbles being created while running that can actually melt and reshape the aluminum. Is your prop far enough into the water?

Yeah it is. This is after I spent some time with a dremel smoothing it out

 

[Maclin]

Re: Prop slip question

I see. Looks just like some burned edges I have seen before from cavitation, thanks for the feedback.

 

[Texasmark]

Re: Prop slip question

Well, cavitation can certainly add to slip. Maybe a cupped prop is in order with a look at pitch reduction.

Mark

 

[clemsonteg]

Re: Prop slip question

New prop did 21 mph and 4600 rpms so I guess that answered my question. My slip dropped to 26% but due to the better bite I still can't get the 5500 rpms. I'm on the very bottom of the recommended range so maybe I will re-pitch to 11 to get me in the middle of the range