Prop slip factor

I've calculated my boat's prop slip factor, (the % difference between prop theoretical speed and actual speed), and it ranges from 33% at low rpm to 50% at WOT.
I'm just wondering how this compared to other full displacement boats.

I've read that many planing boats have a slip factor of around 10%, and it surprised me that mine was so much higher.

Have any other full displacement boat owners done these calcs for their boat?

To calculate theoretical speed in knots:

Prop pitch x rpm
Gear ratio x 1215

To calculate slip factor:

Theoretical speed - Actual Speed
Theoretical Speed

Except as an academic exercise, what possible difference could it make to anyone here? It is a moving target for large ships that operate in steady state conditions long enough that slip actually means something, it is another dancing angel to a featherweight boat that doesn't run constant power or speed for more than a few minutes at a time in any event.

Read the part that describes slip, page 15.

http://www.mandieselturbo.com/files/news/filesof11541/Ship Propulsion_Basic principles.htm.pdf

Rick
Thanks for the article.
Obviously the sea state and hull condition affect the propeller slip. I made my measurements with a relatively clean bottom and in fairly calm seas, and was wondering how it compares to other small displacement boats in similar conditions.
I'm thinking that it would be somewhat indicative of a boats hull efficiency.

I often run at constant power in steady state conditions for the entire day, so it might mean something to me. Also - with a motorsailer, when raising the sails while under power, the result is a decrease in prop slip which gives me increased speed and fuel economy. So I have an interest in this.

Yes, it is an academic exercise; but sometimes I like academic excercises.

AusCan said:

I'm thinking that it would be somewhat indicative of a boats hull efficiency.

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I fully understand the academic interest, but comparing your boat to others really is a fruit stand exercise. There are more than just apples and oranges involved.

Run your own numbers a hundred times and graph the results. I seriously doubt they would be even remotely similar to any other operators of the same or similar boats.

Don't forget to count shaft turns if you want to get accurate data. Just relying on a tachometer and a clock doesn't count - no pun intended.

AusCan, Yes I have figured my slip, at least at cruise speed. And my hull is full displacement like yours. I make 6.5 k at between 1200 and 1250 rpm, with 20" pitch & 2.57 gear, which puts my slip 15% & 19%. I wouldn't bother thinking about slip at OT because my hull (and maybe yours) can't use the 50hp I have available. On the prop hp chart in my engine manual, I am producing about 13-14hp at cruise.
I imagine that my slip would be way up there at WOT as well because I would be climbing out of a huge hole in the water.

Hmm, Interesting Brooksie,
A big difference. You have the extra waterline length, but I thought our hull shapes are very similar. Rick may have a good point; I should check the accuracy of my tach.

AusCan said:

Hmm, Interesting Brooksie,
A big difference. You have the extra waterline length, but I thought our hull shapes are very similar. Rick may have a good point; I should check the accuracy of my tach.

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I am quite comfortable with my tach as I check it against my hand tach any time I touch my alternator.
At 6.5 I am only moving Froude 1.1, I think if you were to do the same (Fr.1.1) based on your LWL, your slip would be much lower too. Also, my prop is the required diameter for my situation and 5 blade as well; yours may be a compromise diameter & area to minimize drag when sailing. Check your tach, try it again at a proper cruise speed for your LWL, and let us know what you come up with...
Remember, slip is not always inefficiency

Low slippage is a sign of efficiency but it's also a sing of inefficiency.

If you employ a very low gear ratio and a very large prop low slippage is assured ... but not high efficiency.

At some point the drag of the surface of the propeller blades sliding through the water will become greater than the loss of efficiency due to slippage. at that point increasing pitch and decreasing blade area and increasing slippage will be more efficient.

Also w a very large prop it's relative efficiency may or will be realized only at very high relative speeds and engine loads. At very low loadings as many here do even more slippage and a smaller prop may be more efficient. Back off to a good engine loading of 50 to 75% and the big prop unloads too fast as you reduce rpm. Another reason a big prop w low slippage may be less efficient at the speed you normally run. IMO.

Brooksie - I found this info on a marine engineering website, Propeller Frequently Asked Questions - Olds Engineering Marine Division which suggests I am in the right ballpark, but my tach accuracy is certainly worth checking anyway. Your low figure may be due to having a 5 blade prop.

Q. What are normal slip percentages for various craft?
A. With propellers correctly selected for the operating conditions, the slip percentages would be as follows:
racing hulls 10 to 15 percent
planing runabouts 15 to 25
planing cruisers 25 to 35
displacement cruisers 30 to 40
sailing auxiliaries 35 to 40
work boats 40 to 80.

Your right Eric, prop slippage is not really important in comparison to engine efficiency and proper load.

AusCan said:

Brooksie - I found this info on a marine engineering website, Propeller Frequently Asked Questions - Olds Engineering Marine Division which suggests I am in the right ballpark, but my tach accuracy is certainly worth checking anyway. Your low figure may be due to having a 5 blade prop.

Q. What are normal slip percentages for various craft?
A. With propellers correctly selected for the operating conditions, the slip percentages would be as follows:
racing hulls 10 to 15 percent
planing runabouts 15 to 25
planing cruisers 25 to 35
displacement cruisers 30 to 40
sailing auxiliaries 35 to 40
work boats 40 to 80.

Your right Eric, prop slippage is not really important in comparison to engine efficiency and proper load.

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Yes, I am aware of those general guidelines. But as I said, if I were trying to cruise at a higher speed to length ratio, I'm sure my slip would go up higher. So even those guidelines which may or may not be for WOT are not very precise are they.

Al said:

Rick/Eric

Okay, you two seemingly have engineering thinking. Maybe you can break it down to See Dick, see Dick run! for this old high school boy.

WL length 25' WL beam 9.6' engine HP 58@3000 RPM. Reduction gear 2:9.1 wheel 22X16 RPM 2400=6.7- 7knot. According to Michigan and my calculator this is the proper wheel for the application. Michigan recommends not over propping. Perhaps this is a protective measure by them to avoid potential damage by operators who do not understand or ignore the potential of damage.

Desire: To obtain the same 7 knot at 18-1900 RPM
This requires over propping. Yes, I understand that running at WOT or near that with over prop will result in short engine life. The rule becomes running at or very near the 1900 as the new maximum WOT setting.
Knowing that one inch in diameter equates to approximately 350-400 RPM drop or each inch increase in pitch equals approximately 200 RPM drop, a increase of either one inch diameter or 2 inch in pitch would achieve on paper the goal.
What say you to this solution? encourage or discourage and why if you would.
Regards,
Al

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Without even running you through a prop calculator: I would say 1) your 58hp engine is well loaded as it is, trying to push a 25' LWL to 7K. 2) you will not save any fuel by overpropping and lugging down your engine & you may damage it. 3) if it is the noise and vibration associated with the 2400 cruise rpm, I would concentrate on that using modern soundproofing materials and mounts. All this assumes you are currently correctly propped for full rpm @ WOT w/ full weight aboard.

The simple way to contemplate how you are doing is to log fuel burn.

58 HP would be 3 , 3.5 at least GPG , but I would guess you probably burn closer to 1 GPH than 3.

Although 6.5K or 7K is high for a 25 fr LWL looking to economise , a bit over 5 to 5.5 might be ideal,

When you have a rough idea of the fuel burn at your cruise you can decide where the RPM should be to generate that HP + about 20% for incidental loads . The engine MFG table will be your guide.

IF the burn is low you could contemplate as low as 1600 for cruise , but of course all the overproping restrictions would always apply.

First take a run and simply listen to the engine and feel the boats vibration.

You may find a smooth quiet sweet spot , that could be the LRC RPM cruise goal.

Al said:

Okay, you two seemingly have engineering thinking.

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I don't just think about it. I am not a soothsayer either.

AusCan said:

I often run at constant power in steady state conditions for the entire day,

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You might often run at a constant rpm but if you put a torquemeter on your shaft you would see that the power varied widely.

A small light boat is greatly effected by wind, waves, steering drag, changing trim as even a single person moves fore or aft, etc. so what appears on a tach as "steady state" is anything but.

Which is one of the reasons I think this thread is as meaningless as soothsaying performance based on a photo of the rudder and propeller or a brokerage pic of the boat at a dock. I guess it is possible that there are people who can look at a photo on the internet and develop performance charts along with a stability book, but I ain't one and I'm not about to pretend either ... I will leave that to the usual suspects.

Yes Rick is right. A photo of the after plane of the bottom would be helpful.

However the photo would only help identify (to a degree) how fast and/or efficient the hull is and to what degree the boat is a semi-planing hull. This would indicate how much power to install, how big the prop and rudder should be. If Al's boat is FD then 58hp is way overpowered. But I don't think it is FD. And I don't recall seeing one of Al's boats (can't recall the name) out of the water. In a way it dosn't really matter if your'e going to do it right and prop to 3000rpm.

However there's one thing to gain and that's a significant reduction in noise. At your usual cruising speed if you over propped, your engine would be very slightly under more load than if you propped to rated rpm .. 3000. And you would burn very slightly less fuel .. perhaps 3 to 5%. Like most anyone else on this forum. So whether it's FD or SD dosn't matter.

So if I;ve read this post properly you want to cruise at 1900rpm and go the same speed as you now go at 2400rpm. If I were to guess (here I go) it would take a lot of over propping to reduce your rpm by 500. Brooksie said it well .... prop it correctly and do some (or a lot) of sound insulation. And of course you could do both .. over prop and sound insulation.

Another thing to consider is that your boat probably has a reduction gear that is too low (numerically). Changing to a 2-1 gear would probably make you smile. You're not getting any benefits from the 3-1 gear. It is more efficient of course but only at WOT or near there. So w your 35% engine load a smaller prop could be more efficient. Big props and deep gear reductions w a slow turning wheel unloads more quickly as rpm is reduced. Say your boat had a 30" dia prop w a 4-1 gear. And I had the same boat w a 1.5-1 gear and an 18" dia prop. Both propped to 3000 rated rpm. Then of course the prop would be loaded w 58hp at 3000 engine rpm on both engines/gears/boats. Now lets say we reduced rpm to 2000. The smaller prop would have more load. Look at prop loading curves. I'm saying the smaller prop curve will be flatter and the curve for the bigger prop will be .. well .. more curved or steeper at it's upper end. The idea is to push water wether all the numbers are socially correct or not.

But the main thing here is the question of what you wish to achieve Al.
And I really like your "see dick see dick run" But I think Jane is supposed to get in there too.

Eric- Good response. Going to a 2/1 reduction is not going to happen voluntarily. Sound insulation is not an option. The cabin floor is covered with some sort of "Lead " cloth, like a sub mattress to wall to wall carpet which is the case with our cabin. The engine compartment is otherwise wide open to the extent that I would not attempt to clutter up the working area about the engine/batteries/pumps.
That leaves the prop. You mention " a lot of over propping" to reduce 500 RPM" One inch diameter increase will do that. So- with a per-conceved mind, I am watching for a used wheel 23X16 LH or a 22X18 LH wheel. Hold it till next yard pull and change. May be a while.There is one other option. Run at 2000 RPM as compromise and be satisfied with 6 knots if a down hill fair wind run. Start an hour earlier and run an hour later. Accept that the voyage is more than half the adventure.

Al

Do you have a link for the eng. mfg and his theoretical prop curve ?

When I was involved with the off shore race cats 3 to 5 percent was not an unusual slip percentage. the V bottom boats were closer to 10. To15 on the well set up hull. . but as it turned out a low slip percentage doesn't always guarantee the fast the setup. and we would spend thousands of dollars to gain 1 or 2 miles an hour. it seems silly now but it was fun

Most people think bigger is better as far as efficiency and diameter of the prop. Then there's the question of is the reduction gear deep enough for a big prop.

But propeller loading is perhaps more important than most else. However there's no doubt a larger slower turning propeller is generally more efficient.

Motion,
3 to 5% slippage is really small. Must have been a very unusual setup.

Al,
the "prop curve" FF is talking about is from horizontal = prop speed (higher to the right) and vertical = power applied. I almost said thrust developed. Max engine speed is to the right. Max power is low or toward the bottom. A 100rpm increase on the left side increases the power applied but not much. A 100rpm increase on the far right increases power a lot. Perhaps four times as much as on the left.

This curve is not accurate but fairly close. It's for a typical hull and a specific engine usually that of the graph and engine maker to aid the customer or yard where they are setting up the boat or refitting. The engine manufacturer wouldn't be inclined to make a graph for a specific boat unless hundreds of their engines were to be installed in the same boat like a NT 32. Or they'd have to make thousands of graphs.

The graph is meant to get real close re prop dia and pitch but there are other differences as well like boat weight and prop type or style.

Al go to an engine manufacturers web site and see if you can find a theoretical prop curve.