Prop Slip Explanation

Cricket Too · Dec 10, 2003

Hey all, can somebody explain the basic theory of prop slip to me, I have read some things but can't get a clear description of exactly what it is and how it affects performance. Thanks. Mike.

Dhadley · Dec 10, 2003

Re: Prop Slip Explanation

Its theory vs. actual. Pitch is measured in inches of forward motion. A 19" pitch prop will -- theoretically -- travel forward 19" in one revolution. In reality it slips some and does not make it to 19". The difference is expressed in %. Hope that helps!

JB · Dec 11, 2003

Re: Prop Slip Explanation

Howdy, MMCPeck. If you have a 20" pitch prop and you are turning it at 3,000rpm, you can calculate that it "should" be moving the boat at 56.8mph. It doesn't. It probably moves the boat at about 50mph. That missing 6.8mph is because the prop is in water and it is moving water back as well as the boat forward. The 6.8mph is "slip". It is usually expressed as a percentage of the theoretical speed. In this case the prop has 6.8/56.8, or 11.97% (call it 12%) slip. Hope that helps.

JB · Dec 11, 2003

Re: Prop Slip Explanation

Apologies, DHadley. You posted while I was typing.

Dhadley · Dec 11, 2003

Re: Prop Slip Explanation

Its a tie! Does the tie still go to the runner? Or are we going on a time vs distance thing? Or do we have to re-run it? Or was it a photo finish? Never mind -- see what happens when theres no NASCAR?

steam_mill · Dec 11, 2003

Re: Prop Slip Explanation

Moderator: This is an excellent explanation of Prop Slip. I think this should be moved to the General Outboard discussions...... Just a suggestion...

JB · Dec 11, 2003

Re: Prop Slip Explanation

Good idea, Steam_Mill. Consider it done.

Cricket Too · Dec 11, 2003

Re: Prop Slip Explanation

Thanks guys for the explanation, but i still don't understand how you calculate it. I see that you used the rpm and the pitch, but doesn't the weight of the boat or gear ratio have anything to do with this? Thanks.

Dhadley · Dec 11, 2003

Re: Prop Slip Explanation

Yes the gear ratio has a great deal to do with it. Thats why JB used the prop rpm of 3000. A typical 100 hp outboard uses a 2:1 ratio. So JB's prop rpm of 3000 meant the motor is turning 6000. The weight, hull design, X dimension, prop design and more are all ingredients of the percentage. No matter how much a boat weighs, no matter what hull design you are working with, the path to efficiency is that percentage. No matter what rig you have if you reduce that percentage, that particular rig is more efficient. We use many factors to reduce that number like prop design (cleaver, round ear, over hub, thru hub, bow lift, stern lift, total lift, progressive pitch, progressive cup, total cup, cup length, rake, number of blades etc), hull design (V, deep V, pad bottom, notched transom, full tunnel, tunnel with a center pod, slotted V etc) gearcase shape and location, set backs, jack plates, trim angle and on and on. True, reducing weight will probably reduce the slip %. What if you cannot reduce the weight? Momma doesnt want to stay home or its a race boat that has to weigh a certian ammount. Many times just relocating the existing weight (change the center of gravity) will improve that %. Are we having fun yet?

Forktail · Dec 11, 2003

Re: Prop Slip Explanation

The weight of the boat and gear ratio do have a lot to do with slip. Weight will generally have a drastic effect on speed. And the exact boat speed must be present to calculate slip. Added weight makes it harder for the prop to push the boat, and thus the prop slips more. Gear ratio is directly related to prop rpm, another factor effecting speed (and generally prop size). If you know what RPM your outboard is operating at, and you know the gear ratio, you can find the rpm at the propeller.... An outboard operating at 6000 rpm with a gear ratio of 2.0 means that the engine turns twice for each revolution of the prop. Or, the prop is going 3000 rpm. Once you know the rpm of the propeller you can multiply it by the pitch of the prop to determine how far the propeller can theoretically move in one minute. Again, pitch represents the distance (inches) the propeller moves forward in one revolution, under perfect conditions with zero slip. Lets assume the prop had a pitch of 15. So 3000 rpm X 15 inches per revolution equals 450,000 inches per minute. Now we need to convert in/min to MPH, so we need to know that there are 5,280 ft in a mile, 60 minutes in an hour, and 12 inches in 1 foot. So 450,000 in/min divided by 5,280 ft/mile X 60 min/hr X 1 ft/12 inches equals....42.6 MPH. 42.6 MPH would be our perfect, zero slip top speed. We'll call this our theoretical speed. Now we need to compare this theoretical to our actual radar speed (most boat speedometers have a 5 to 10% error) So lets say we measured our actual boat speed at 36.5 MPH. This indicates about a 14.3% drop in speed from theoretical because 42.6-36.5/42.6 = 14.3%. This %speed drop can directly be related to %slip. We can also work backwards to find the actual slip by trying to determine a new pitch. 36.5 MPH (the actual speed) converted back to inches per minute equals 38,544 in/min. Because 36.5 X 12 divided by 60 X 5,280 = 38,544. We must assume the same engine rpm of 6000, and the same prop speed of 3000 via the gear ratio of 2.0. Dividing 38,544 inches per minute by 3000 revolutions per minute results in 12.85 inches per revolution....or a pitch of 12.85. The difference in 15 in/revolution and 12.85 in/revolution is 2.15 inches per revoltion, or about 14.3%. Notice that the %slip and the %speed difference are one in the same, as they are directly related. Hope this wasn't too confusing. I did the best I could over the keyboard.

AUGIDAWG · Dec 15, 2003

Re: Prop Slip Explanation

The amount of slip, is directly related to the efficiency of the prop, boat weight, and drag on the hull. For example: If you took a brand new 19" pitch prop and removed 1/2" of material from the leading edge, and 1/2" from the trailing edge, it would still be a 19" pitch prop, but would definately have more slip. If you add 4 more people to your boat, you would have more slip. If you glued a 2x4 to the bottom of your transom, you would have more slip.

jim dozier · Dec 15, 2003

Re: Prop Slip Explanation

I guess if you are in salt water, which is more dense than fresh water, you would have slightly less slip than in fresh water. Lets do all our speed runs on Great Salt Lake. (The boat would ride higher too).

AUGIDAWG · Dec 15, 2003

Re: Prop Slip Explanation

Don't forget to unstrap your trailor before the high speed run.

Cricket Too · Dec 15, 2003

Re: Prop Slip Explanation

Wow, thanks alot guys, the explanation of slip and the formulas to calculate it are great. Now I understand what it is and I know how to go about re-propping my boat next season. Right now my props are all wrong. I have 21" props on twin 200 Johnson's, and I'm only getting 5000 out of both engines, I know it should be higher. I am assuming that the gear ratio is around 2:1. So right now I have a theoretical speed of 49.7, but according to GPS my actual speed is only 41 mph. This is with normal load of 2 people and half tank(102 gallons) about 5400lbs. So it looks like my slip is 17.5% and I deffinetely want it lower and I have to get my rpm's higher so I don't beat on these motors. I have a pair 17" props that I'm gonna throw on and see what I get, RPM wise. I would really like to get these things running at at least 5500 at WOT. By the way can anybody tell me the operating range of these motors they are 1999's. Thanks again guys. Mike

seahorse5 · Dec 15, 2003

Re: Prop Slip Explanation

Your Johnson motors have a 1.86:1 gear ratio, not 2:1 They should be propped in the upper half of the operating range with full fuel and a normal load, that means between 5500-6000. In warm muggy weather the rpm will be less so prop on the high side when its cool and dry weather. since you are running below the motor's powerband, you may pick up a mph or two with 17's and get better acceleration too.

Trent · Dec 15, 2003

Re: Prop Slip Explanation

Here we go again......

Lock the thread now JB.....

JB · Dec 15, 2003

Re: Prop Slip Explanation

Mike got his answer, in triplicate, at three levels of complexity, and he understands the principles, which is what he asked for. Please do not confuse the subject with "yabuts" or "whatifs". Thanks.

walleyehed · Dec 15, 2003

Re: Prop Slip Explanation

mmcpeck, I think you will really see those 200's come to life at 5500-5700, and yes, 6000 would be best! Good luck....

kalteri · Dec 15, 2003

Re: Prop Slip Explanation

interestin conversation, i'm working with my prop problem right now too, i made an excel sheet which calculates theoretical speed and i can also vary slippage %. I measured boat speed last summer with GPS. if i trust omc tacho there is no slippage in PRACTICE with wot. so is this possible? or do i have something wrong i use very low pitch prop

walleyehed · Dec 15, 2003

Re: Prop Slip Explanation

Kalteri, Water is fluid, not solid, so 0% slip is impossible....

Prop Slip Explanation

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