Prop slippage

[60hpomc]

Checked my prop slippage and comes out to about 31% is that good or bad seems like a lot to me. It's a 60hp top speed is 26 and the engine turns 5800 with a 11-3/4X17 prop. This was with 3 people in a 1,000 LBS boat. About 600 LBS.


Thanks,
Chris

 

[Silvertip]

Re: Prop slippage

Give us some more information. Is this a two-stroke or four stroke motor. Standard gear case or BigFoot. It would also help to know what type boat you have. It makes a difference if the motor is on a barge, pontoon, or planing hull.

 

[60hpomc]

Re: Prop slippage

evinrude, small gear case, 16 ft tri hull, glasstron

 

[60hpomc]

Re: Prop slippage

2-stroke

 

[Texasmark]

Re: Prop slippage

Back in the '70's I had kin and friends run pretty much the same boat (60 hp OMC's, 15' tri hulls) you are running with a 17P prop. If they wanted 3 folks in the boat and go water skiing, then they dropped to a 15P. Both were OEM alum and nothing fancy. Back then we didn't know about prop slip and all that so I have no data for you. Don't remember their mph but I had another friend with a 65 Rude on a 16' Kingfisher Bass Boat (flat bottom back then), ss prop, 19P I believe and he ran about 38....totally different setup than yours but this is just a piece of data. Again no tach or anything other than speedometer.

Your rpm's do sound a tad high for an OMC 3 cyl looper.

What you didn't do is show us/tell us about your current prop.

HTH

Mark

 

[Texasmark]

Re: Prop slippage

It's raining outside so I came back in and decided to dig into your question.

I went to old-omc.de site which is a super site a guy put up for us with old catalogs from OMC over the years.

Anyhoo I picked a 1970 60 hp Rude and it's specs are:

RPM: 4500 to 5500
Gear ratio : 12-29 which translates to 2.41:1
Weight 202#
Standard prop 13 1/4 x 17
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You said 11 1/4 x 17 in your text; I'm thinking you meant 13 1/4....need to clarify as it could help in the answer.
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The BAM slip calculator said that you were right on at 26 mph with 31% slip (It actually calculated 33%). It said that your non slip speed should be 39 with the specs you gave and I dug up. A reasonable slip of say 15% with a light load on that boat would yield a speed of 33 mph which is very reasonable and do-able.

Either you have a bad prop, more boat load than you think, a poor efficiency hull (which that Glastron is not), barnacles or other crud on the hull, or your speedometer or tach is/are in error.

You are 300 rpm over max first off. Don't know how that engine handles that. Since you seem to be there with a full load, when sporting about by yourself you would be way over the upper WOT rpm limit.
--------------

On the boat: Looks like '67 is the first year Glastron made the trihull and the closest to 16' is the 15' 6" Sportster. The weight per the spec sheet it either 500 or 600# empty(can't read the data very well)....pretty light boat, even for glass.

See: http://www.classicglastron.com/67gl-specs-web.html

So your guns are loaded with info; time for some more feedback from you.

Mark

 

[60hpomc]

Re: Prop slippage

Thanks for the info,
-My motor is a 83 2cyl 60hp (operating range 5000-6000 rpm)
-No power trim
-boat is a 78 sportster XL
-Dry hull weight (no Motor) is 750 lbs
-Motor 190 lbs
-fuel and gear about 100-150 lbs
-three people 550 lbs
-Hull is fairly clean one repair spot and a few blisters though
-the water was kind of rough that day but the speed was about the same as a nice day
- I would be thrilled to get 35 mph out of it
- I feel the same way you do about over revving with just me
- the prop 11-3/4X17 it is stamped right on it
-17P seems to be the largest affordable prop produced for it.
Thanks,
Chris

 

[Texasmark]

Re: Prop slippage

Sorry for the delay. Dried up around here and was cuttin hay.

I noticed in the brochures that the 2 cyl engines are rated at a higher rpm than the 3's, even back where I checked. Today I got to a '77 55 hp and it has the following specs:

RPM's: 5000 to 6000 rated 55 hp at 5500
Gear box: 12:29 (2.41 as before)
Prop in sales brochure (0nly one listed): 12 1/4 x 15, 3 blade alum

Just a note. With your engine running 6000 rpm's it tells me that the engine is not handling the current load adequately if it too is rated in the middle of the rpm range.....says you could pull 500 rpm worth of more load....higher pitched prop, in short you are not using your 60 hp. If you change props you will load the engine more and SHOULD pull the rpm's down.

Butttttt that's not always the case. If changing props causes you to get the boat up and out of the water, reducing hull-water resistance, then you will increase rpm's (rather than experience the drop you would expect) and increase speed by reducing your slippage.

I guess we need another data point and that is with you in there alone without the normal loadup of fishing things.

I would say that you might have your tilt pin in too close to the transom causing the boat to plow, which increases hull drag and reduces speed but it also reduces rpm's which you aren't having a problem with.

Where did you shop for props?
I just went to MWC and found a dealer in Washington state (tacomapropeller.com). Iboats may have this info also but the site is down for scheduled maint. and I couldn't check them.

Seems your current prop showed up under "Vortex" props which is said to be a high perf but meets OEM specs (confusing)....does have a removable plastic hub. Then it has a listing for regular alum props with a rubber hub and they go to 11x19; price $94. Don't know how yours would compare to this one (would you get more speed out of it and drop your rpm's a tad) as all props are different to some degree. Their pics look pretty much alike....on the Tacoma site.

Will wait for your light load speed/rpm numbers. Lets get this data first.

Mark

 

[60hpomc]

Re: Prop slippage

With just me,
5850 rpm @ 27mph
with trim pin out one hole
5930 rpm @ 28mph but not very good handling but better in rough water
Still High amount of slippage 33%
What to do?
RPM did not change with 350 lbs less in boat. Why?
Thanks,
Chris

 

[Texasmark]

Re: Prop slippage

Let's hit the easy first. My boat also seems to be insistive to total weight when at WOT. I guess it's in the hull design. Since mine has 3 steps, I must be on the last step pretty good with the loads I have run. Probably if I loaded it more at some point it would drop down and the 2nd step would contact the water and I would have reduced speed.

Notice that when you went out one hole the rpm's and the speed increased? That says that you got more of your hull out of the water, decreasing drag which allowed the engine to pick up speed. Also in reducing the drag you helped to reduce slippage.

So, that says to me that IT IS POSSIBLE that you could increase pitch and increase your speed and if you added cupping you could reduce your slippage (if it's real). A 17P SS prop (cupped of course) would be a good place to start seems to me.

SS will reduce blade thickness which will improve performance (efficiency). Cupping (usually comes with a SS) will reduce slippage, reduce tendency to ventilate, especially on turns, and act as an increase of about 1" in pitch.

With improved efficiency the boat ought to accept the increase by coming out of the water more (reducing drag) and generating higher speed. The cupping will probably propel the boat better and even though you now have essentially more pitch, speed and rpms will probably increase which is what you want to happen......you still have ~150 rpm to get to your max recommended WOT rpm.

Still think you need to check your instruments. A gps will get your speed accurate. Don't know what to tell you about a tach.

Don't know where I got the idea you were currently running 6000 rpms at WOT. Must have looked back and picked up the rated rpm number, not your actual in haste.

Mark