# how many lbs of thrust is equal to 1 hp?

Discussion in 'Outboards' started by deermaster, Nov 15, 2008.

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### brucebSenior Member

Lots of gallons

Crossing the Atlantic changes the size of a gallon, (I guess the colonies got the small ones) and then you can fix your car with a whitworth wrench. Well maybe.

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### sailfish25Sailing the Great Lakes!

All this math is hurting my brain!

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### brucebSenior Member

I feel the strain- or is it thrust

My brain hurts too, but I guess it needed the exercise- I have learned something. Thanks everyone. Bruce - 2,4,6,8, I'm too old to metricate

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### Manie BSenior Member

thanks i learned a lot from this thread as well

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### mikelinmonmike l inmon

When we (Roger Macgregor and I, Mike Inmon) were testing the righting moment or self righting of the 26M ( you can see the movie on the macgregorsailboats.com) we had a scale and blocks, etc to measure the force needed to hold the Mac on its side; he said "wonder how much thrust the 50hp Merc bigfoot has"? We hooked it up to the dock and to a scale
using one block so we would still be on scale. Result, exactly 750 lbs on the dial at wide-open. 25lbs per hp by measure includes all the prop loses and one ball bearing block thrown in. Cool huh! All that math was just about the same as the totally unrelated testing by totally uninvolved folks. I could have done that math, yes I could have!

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### brucebSenior Member

lots of thrust

Very cool and! we now know about what it takes to plane a 26' sailboat in real numbers. Great! Thanks Mike.

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### jehardimanSenior Member

My money says you were not getting full power and the number is actually closer to ~30 lbs/hp ....but who really wants to pay for that test.

Ahhhh...no. there are two problems with trying to corrolate bollard thrust of an OB to eph at planning speed. First, I'll bet bollard was reduced by tip and face caviation. And second, thrust falls off with J so there is actually less thrust a planing speed than at bollard assuming constant sigma (cavitation). Without Kt and Kq curves, it would be a pretty broad guess and only if you knew max rpm and pitch of the wheel.

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### brucebSenior Member

real world performance

Of course you are correct- but, I think it is still nice to have some measured numbers. I have run a M-26 with about the same set up, and it is performance that I can relate to. Bruce

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I wonder if you scaled down tug /AHTS vessels hp and bollard pull you get a meaniful number at the bottom end of the scale?
re the low numbers, I;ll bet if you give me a ski handle tied to a say a 15 hp I could hold it on the dock...
Think about say an outboard of say 100hp, put both feet in a single ski and no way will it pull you up so the skier is holding all that force less the fact it will be pulling you through the water at about 4-5 kts...what did I just prove?

10. ### Guest625101138Previous Member

If you do the calculations on these tugs:
http://www.hitzler-werft.de/tugs.html
They consistently achieve 170N/kW. This is about twice as good as the 50HP outboard per unit of power given earlier. Not unexpected given that a tug is designed to give high thrust at low speed. This is quite the opposite objective for most outboards.

The absolute thrust limit is:
F = rho^(1/3) * pi^(1/3) * (1.5 * Power * Prop Diameter) ^ (2/3)

So each engine in the "Santantonio Primo" has an absolute limit of 737kN assuming prop is 4m diameter while the actual is 320kN. So slightly less than half of the absolute value.

The 50HP (37kW) has an absolute limit of 9.6kN with a 0.3m (12") prop. The 750lbf (3.3kN) measured is hence about one third of the absolute limit. This outboard will not be propped to get the best bollard pull.

The bollard pull gets down to prop design. The 15HP outboard with a low pitch, large diameter prop would not have much trouble pulling you off a wharf. Absolute limit with a 14" prop is 5.8kN (1300lbf). So with a well designed prop you might get 600lbf. I doubt that you could hold that on the pier.

Likewise the 100HP outboard correctly propped for skiing would pull your arms out of their sockets if the rope was up to the task.

If you have been able to hold a 100HP outboard from planing then the motor is not propped for skiing and will not be getting up into its power band. It will be developing much less than 100HP. If the prop diameter is 16" (0.4m) then the absolute limit is 18.7kN (4197lbf). So with the best possible prop and the motor in its power band the bollard pull would be up around 1 tonne.

Rick W

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Rick

yes thanks
the outboard cannot get to max torque thats why you can hold them back.
A 100hp will only have 14 1/2 max diameter and a 2:1 box if your lucky.

But saying that a good slalom skiier can pull back any V6 even on the plane
Try pulling 3 to 4 skiiers out with a deep water start and they can certainly hold a v6 back regardless of the prop so when you divide the theory up it would say it would pull your arms off. I still have my arms although long..
I think we are talking theoretical maximum versus what you get as a dock buster.
As you might know, Moomba was the first international waterski tournement to use outboards and the first 200+ foot jump was recorded there.
The secret was the Mercs had 15" props so they came in at about 7000 and the skier could pull them back to under 6k so they pulled against peak torque and couldnt slow the boat very much at all after that. (The boat being split timed twice in the course to ensure speeds in an allowable range)
The speeds kept were better than any inboard hence the skier had better boat speed when he hit the ramp.
We even tried a 2:1 box ( from a 150) on the 200 ( for more torque) and it only lasted a few runs.
Merc told us the pinion is too small a diameter (for the power) and would break..they were right
The same engine on the same boat would have a 21" or more prop to keep it in its correct operating range for a consumer
all that was 20 years ago..
as for a 4 stroke outboards...gutless
regards
PS I was a torny skiier but not good enough to represent oz or even Vic

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### JoakimSenior Member

A 100 hp engine propped for skiing (= for about 30-35 kn maximum speed) shoud have about 2000 N (=200 kg) of thrust. That is quite much to work against and you are not going to be able to hold for more than a couple or tens of seconds (measure how long you can even carry your own weight, I quess about 2 minutes). A 15 hp motor propped for 7 kn speed will have about the same thrust!

Joakim

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When you say 200kg do you mean at max thrust i.e. right on peak torque or do you mean at the rpm it will attain while trying to pull a skier out?
What I can say is just a simple change in the gearbox ratio makes an enourmous difference to pulling skiers out.
Thats why a boat with a 135 or 150 V6 can pull out as many skiers as the same boat with a 200 or 225.

Inboards crap all over outboards in ski boats when it comes to pulling out skiers as they usually get to peak torque, ( just over 2k) even with a 1:1 shaft and a 12 or 13" diameter prop, when pulling them out

Outboard just cant get to peak torque as it will be well over 4000 rpm.

I think someone needs to go water skiing....

BTW you are now able to get torque curves from the BRP site as they are comparing them to 4 stroke Yamahas.

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"A 15 hp motor propped for 7 kn speed will have about the same thrust!
Jaokim"

Is that regardless of the gear ratio and the prop diameter?

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### JoakimSenior Member

I have done plenty of water skiing. No problems getting out of water with one ski (only one leg in the bindings though) with a 40 hp outboard and have done it even with a 20 hp outboard and actually also with a 10 hp outboard, but back then I was 10 years old.

Even a 40 hp outboard propped for skiing and on a light boat is a tough "opponent". If you don't know how to get out of water, you are not going to be able to hold the rope for more than a half a minute or so unless you are considerably stronger than average.

The approximate 200 kg thrust is valid from start to top speed, IF the propeller is well sized for that 30-35 kn top speed. Of course it is not absolutely constant, but in order to calculate the thrust curve you need to know much more details. On a heavier boat most of this thrust is "wasted" to the drag of the boat while starting to get to plane.

The problem for a bigger motor in the same boat is that the bigger motor is typically propped for a bigger speed. Thus it is driving on "a bigger gear" and will have less thrust for the same torque and less rpm, thus not able to get to good torque range. It also weighs more, thus the drag of the boat increases considerably, especially at lower speed. Most evident this fact is on some very fast boats, which have problems getting into plane even without a skier despite huge power/weight ratio. If you would prop it for the same top speed, it would be a much better puller.

Of course the thrust of a 15 hp motor propped for 7 kn speed is not regardless of gear ratio and prop diameter. A typical displacement vessel with a 15 hp motor will have a thrust of about 200 kg. If you put a typical 15 hp outboard on that displacement boat, it will have much worse performance, since a 15 hp outboard is designed for 15-20 kn speeds.

The key to all this is the top speed which is used to select the gearing/pitch/diameter combination. Power = Thrust * Speed. When you increase the (top)speed, you reduce thrust with the same power.

Joakim

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