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Interesting scenario, what exactly is prop slip?

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Interesting scenario, what exactly is prop slip?

Old 06-17-2007, 07:43 AM
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Default Interesting scenario, what exactly is prop slip?

I saw a similar thread on this from a guy with opti's....but it wasn't getting much in the form of reply, so I thought I'd start a new one:

We have a Grady Marlin that we bought with Yamaha F225's....the starboard engine would only occasionally jump from around 4000-4300, then run normal again....do it a few times then normal....but not all the time.., other than that they ran fine..so it really wasn't a huge deal to me and it was only a couple minutes out of hours run..the port engine never did it at all, then we had problems with our service dealer on many other unrelated issues, the engines were nearing their warranty end date...so we traded for twin 250 etecs...., new props etc.....but I noticed the same exact occurence..., same engine...again, port never does it....and the etec does it almost identical to the yammie...., and only every once in a great while....once they get up over 4400 it goes away....with the etec in seas yesterday, I also noticed that trimmed all the way down...it never does it....., and in smooth seas only rarely does it even trimmed up....so, like the opti's mentioned before in another post...could this be water turbulence from a thru hull, or prop slip due to that...exactly what is prop slip? or do you think they are mounted slightly too high or low ( I wouldn't think so because port never does it.
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Old 06-17-2007, 07:52 AM
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Default Re: Interesting scenario, what exactly is prop slip?

you can run the calculators but to feel it was difficult at first. I could'nt tell I had it unitl I got a pair of props that did not have it. I had a set of mirage 3 blades on my boat and in the ocean it seems ok, but I was inexpereinced with the boat. . Then I switched to rev 4 - 4 blades and it felt much more "hooked up" - especailly in the slop when the motors just seemed sometimes to lose forward thrust, like a car on ice. I also felt it a bit when I switched to 3 blade enterias...
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Old 06-17-2007, 07:58 AM
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Default Re: Interesting scenario, what exactly is prop slip?

Sounds like a slight case of ventilation. When I had my old motors that were mounted farther apart, my port would do it if I made a fairly hard right turn at speed, enough to lean the boat, dig one prop in and lift the other. But those motors were mounted way too far apart, distance apart won't be your cause on the Grady, just illustrating losing bite, I happened to know what the cause was n my boat before I repowered and moved the engines to where they belong.

On your boat there may be something mounted in front of the prop as you suspect. Doesn't appear to be mounting height if your boat doesn't lose bite even tilted up in calm water.
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Old 06-17-2007, 08:03 AM
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Default Re: Interesting scenario, what exactly is prop slip?

True prop slip is inherant in all propellers to some degree. It is simply the measure of efficiency of a particular wheel or type of wheel as has been mentioned, 3 Vs 4 blade etc and is best left to Naval architects..it is a constant not a variable unless you have controllable pitch props and we don't.

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Old 06-17-2007, 09:01 AM
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Default Re: Interesting scenario, what exactly is prop slip?

You said it NEVER does it when trimmed all the way down, and very rarely in calm water. I vote for the ventilation. Your prop's grabbing air once in a while, allowing the motor to rev.
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Old 06-17-2007, 09:03 AM
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Default Re: Interesting scenario, what exactly is prop slip?

Just a guess here but here it goes. With twin outboards your motors are not aligned with the keel of the boat so the traction of the motors is sensitive to listing or rocking. Your boat probably lists just slightly to starboard while underway likely because there is a little more weight on the starboard side (water tank, holding tank, driver, etc.) So the port motor is running higher in the water than the starboard motor and therefore not getting the same grip on the water as the starboard motor does.

You may also have a thru hull or something else effecting water flow to that motor.

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Old 06-17-2007, 03:06 PM
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Default Re: Interesting scenario, what exactly is prop slip?

Trim of the boat doesn't affect it so much...I've intentionally dropped that side of the boat with the tabs when it'sdoing it...., only when the engines are trimmed down, no matter where the tabs are does it completely go away 100 percent when it does it....I'm going with through hull fittings as my first guess....and I'm thinking since it's happened for 3 years, on 2 sets of engines, and it is a rare occurence, I'm probably going to just live with it like I've been doing...., thanks for the input though....I now understand prop slip better.
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Old 06-17-2007, 03:53 PM
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Default Re: Interesting scenario, what exactly is prop slip?

mikie - 6/17/2007 8:01 AM

You said it NEVER does it when trimmed all the way down, and very rarely in calm water. I vote for the ventilation. Your prop's grabbing air once in a while, allowing the motor to rev.
I agree, this is what is happening. The stbd wheel is having instances where there is less load on it because air is contacting the blade(s) surface instead of water. Less resistance on the prop, rpms increase. Turbulence is causing it. Finding the source of that turbulence is the key to remedy. Measure the bracket height on each engine, you might find that the stbd one is slightly higher, meaning that the stbd wheel is going to break loose sooner than the port as you trim the motors up. Inordinately distributed weight in the boat can cause this, too; but it has to be fairly extreme.

Do you have to set the trim tabs at different angles to achieve level running in a flat sea?

And yes, prop slip is the measure of efficiency loss of a propeller in water. Ventilation is what you are experiencing.



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Old 06-18-2007, 07:03 PM
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Default Re: Interesting scenario, what exactly is prop slip?

http://sites.mercurymarine.com/porta..._schema=PORTAL

Diagram of what prop slip is.
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Old 06-18-2007, 10:49 PM
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Default Re: Interesting scenario, what exactly is prop slip?

If the cav plate is just barley skimming the surface it can occasionally grab some air and direct it around the prop causing slip. The cav plate should be out of the water. Your motor may actually be a little to low causing slip. My Concpoet did this and it went away when we raised the motor. Kind of contrary to what you would think.
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Old 06-19-2007, 01:32 AM
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Default Re: Interesting scenario, what exactly is prop slip?

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Old 06-19-2007, 03:09 AM
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Default Re: Interesting scenario, what exactly is prop slip?

Ok then all of these replys are correct ,assuming that there is a skin fitting sitting in front of the enging some where it will be affected by whats called "Dirty water". this is simply water and air mixed and happens every time you drop off a wave some times when every thing lines up this will be trapped or disturbed buy the skin fitting,you will find that some thing simple like running a silicon bead around the front of the skin fitting making a ramp like cove will change the water flow around the skin fitting and spread out the air as it travels around the fitting itself,
Also depending on what the fitting is for you could also buy a shield it fit over it thus changing the water flow even further.
These are simple fixes and if there is a skin fitting there it will fix it and cost you next to nothing.bussing a engine is never a good idea even if its not alot you are still straining the motor every time it happens.
Good luck with it
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