how many lbs of thrust is equal to 1 hp?

Joakim
I can go better than this. How about a 6yo with 1HP-----

Just a matter of having the right prop and good power transmission.
Rick W

 

I have seen this picture before. Doesn't look very fast and fun and seems to be with two skis, but still a great achievement.

At ten years of age we skied a lot with 10 hp on one ski (9.8 hp Mercury with the default prop). It reached over 20 kn top speed with the skier and that was just fine for that age and skill.

Joakim

 

Brings back memories. A girl I knew in my mid teens had a 14ft run-about with a 25HP outboard (her father was wealthy and she had fun toys). It could get me out of the water on two skis. I would drop one and, while on single ski, I could turn the boat by pulling the stern around on the whip.

On the other hand I have had a 115HP Evinrude outboard bog down dragging a large man feet first through the water when trying a barefoot start off the beach. If I did not get enough speed by the time he hit the water he would stay submerged and just bog the boat down. He was a good skier but had lost condition and weighed around 90kg. The motor was no where near its power band.

Rick W

 

HA so the outboard proped for skiing produced a low enough thrust that the guy could hang onto it and keep it at what..5kts?
( or his nose filled up with water)
I thought I was trying to say that but it seems people think a 15 hp can be used as a tug boat?
I wasnt trying to say what could get you skiing, I was stating what you could hold back which means they produce way less than 200kg thrust at that rev/speed

 

The 115HP was not propped for skiing. The 17ft boat was set up for fishing primarily. It was quite heavy by today's standards and a deep "V". The trick was to get the right ballance between ripping his wet suit off as he was dragged down the beach or just not being fast enough when he hit the water. If the speed was too slow the motor just bogged down well below its power band. The same point I made back at post #25.

It all depends on the prop. I could make a single person pedal boat with a 1m diameter prop that you would find hard to hold against from a pier. Thrust in excess of 200lbf.

Rick W

 

Ok
got it.
Regards
Powerabout

 

Horse power and pounds of thrust

Hi,

I don't like formulas and overly complicated answers.

Horse power and thrust are directly related.

However 'thrust' is the more useful value because it is the force pushing the boat forward.

Horse power is usually the total force (over time) in the whole system.
Some is being lost in the gear box, inefficient prop, vibration and heat.
And a fraction of that total horse power ends up as thrust.

If the thrust figures are the force exerted on the boat when it is stationary at optimum revs..
Then:
I found the thrust figures for a range of 2.5 Horse power 2 strokes and the figures range between 70 and 100 pounds.

That means if you buy a 86 pound thrust electric, then it's giving you as much push as a 2.5 horse power 2 stroke (at low speeds at least).

There, don't need formulas for that.

 

so a 2hp could pull a water skier?
I'd like to see a graph of how this drops as the engine moves through the water.
It obviously drops very fast

 

Yes,

A 2 Hp could pull a water skier.
Very slowly.

 

Yes power and thrust are related, but still you can not get thrust from power without knowing many things.

Bollard pull thrust, which is the one you refer to, is not very useful, since it only tells the thrust at zero speed. Power is much more useful for selecting an engine.

Power is not force! Power is force * speed. Try how much can you pull with your car using 1st gear and then repeat that with 5th gear. You probably get about 50% of your cars weight at 1st gear and not much at 5th gear. It is just the same with an outboard. The ones capable of higher speeds will have less bollard pull at the same power.

Yes a typical 2.5 hp outboard will have a bollard pull of 30-40 kg (70-90 lbs), but buying an electrical motor of equal bollard pull will not give you the same performance.

Look at this test: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf

Note that electrical motors rated at 55 lbs only measured at 13-17 kg (29-37 lbs). Thus buying an electrical motor rated at 86 lbs, will likely give more like 60 lbs.

Torqeedo 801 (800 W = 1.1 hp) gave about the same bollard pull (31 kg) as the 2.5 hp motors, but it only had a top speed of 4 kn vs. 4.5-5.4 kn of 2.5 hp motors. Actually even Torqeedo 2.0 (2 kW = 2.7 hp) only reached 4.9 kn, but it delivered clearly more bollard pull (54 kg vs. 45 kg) than the 3.5 hp motor, which had a top speed of 8.8 kn.

You need the same power for the same speed regardless of engine type. Forget the bollard pull values unless you are only interested in acceleration at very low speeds.

 

Thrust to HP to Pounds to Cheesecake

While I have greatly appreciated the physics discussed in this overall thread, I find a lot of humor in the responses. While many aspects of what has been said is very accurate, I think the heart of the original question was barely responded to. I am old, and my undergrad and graduate degrees as an M.E. are quite dated, but luckily, physics is near-static at this point...so overall, the numbers are a moot point.

Guessing at what the original writer asked, as I wondered at it for a great time myself - having played in many rivers for decades...I always wondered, 'if I were to build a boat powertrain and wished to set it up to run on trolling motors, what numbers are given, if any, to compare gasoline HP engines to electric trollers? The answer is not directly given in physics, advertising, or the responses until you get down to one reply that references an old OEM statement that a 6-HP motor provides 160-lbs of thrust, and a 10-HP motor offers near 250-lbs.

For those struggling with newtons, force, and the quantum effects of cheesecake and fish snot on the reciprocal contributions of the Coefficient of drag on the waxed hull of the craft...please...stop.

The original question asks comparative thrust. Ok, envision, you have a 300-lb aluminum flat-bottomed skiff. It is tied to the pier, and you are in it trying to depart perpendicular to the anchor-cleat you are still lashed off on. First, you do this with your gas motor, and you have a linear scale inline with your tie-rope, so that you can take a force reading in pounds. Now, depart at WOT. What does it read with a 10, 20, and 30-HP motor? Good, you have your empirical data. Now, remove your gas motor and do this again with a (rated) 50, 70, 100 and 120-pound thrust trolling motor. Did the actual yield measured equal the rating? Ok. Now, compare. Is the 10-HP gas motor exerting a 50, 70, 120-lb force on the scale? While this measure is technically a measure of potential power, this is exactly what the user needs to know...because I know my 20-hp motor can push my boat upstream at 4-kts with 1000-lbs payload, I need to know if I change to electrics, what do I need to perfectly replace the previous thrust and maintain the 4-kts...(this is regardless to effects of voltage drops relative to Peukert's curve, which is compensatible via perpetual solar charging).

In other words...there are a lot of responses here, but you haven't answered the actual question...a few close guesses, but no 'exact' answers or offers of emperical testing data. Again, the closest I have seen offered is the data of 160-lbs from a 6-HP motor, and 250-lbs from a 10-hp motor. I actually doubt these values for a greater reason...because this implies that a 10-HP motor could yield enough force to pull someone up out of the water as a skiier...and I just don't envision that happening...as it normally requires a 70-hp or greater motor to effectively ski behind...else, you would beat the skiier to death pulling them out of the water.

In the end, the only value that truly matters is the force of the thrust, or push, or pull (pick your vector). If I take a ski-rope and tie it to a scale tied to a pier pylon, and I go WOT, what is the greatest force reading I can exert on the scale meter? Ok...now what can a 70-lb trolling motor really exert on this same empirical test? Ok, what can a 10, 20, 50, 100-hp motor exert? Yes, prop shape has a lot to do with this...just like hull shape and drag have a lot to do with speed...but in the end, when you are trying to just compare motors...you really just need to know in the general configuration as you buy the motor, what is true force yield in the water, where it counts...

 

It seems to me you've overlooked accelerating the boat the motor is attached to...

-Tom

 

You would need a 20-hp electrical motor to replace your 4-kts upstream performance. Equal power is needed to produce the same "pull" at the same speed regardless of the source of power.

I have been pulled out of the water with a 20-hp motor on one ski many times. I weigh 82 kg (180 lbs). I have also done it with a 10-hp motor, but then I was a child and only 50 kg (110 lbs). Probably could still do it with two skis and a 10-hp motor, but haven't tried.

My sailboat has a 10-hp motor as well. It can pull about 1000 N (450 lbs), but you can not ski with it, since its top speed is only 6.5 kts.

 

power

I supplied those HP-lbs numbers, and I think they were correct, but have to be used in context. The manufactures put them in their sales guidelines and I suspect they had some test results to base them on. The models were intended for aux power for sailboats, and were propped for the job. Top speed was not important, but good thrust at 0-6kts was. The 6hp engines would push a 5000 lbs sailboat at 5-6 kts in calm conditions and usually manage about 3-4 kts in wind and waves. A 10hp would do about the same up to 7500lbs, and anything larger got inboards, often not as powerful, but the prop stayed in the water much better. I am sure one of the new hi-thrust electric trolling motors would replace my current 5hp engine for manuvering around the dock, but would be useless for a trip down the lake, unless I carried a lot larger battery. There are several electric launches on our lake, around 16'-18' that certainly carry several people easily and they seem to be using about 3hp electric motors. B

 

Thrust to HP to Pounds to Cheesecake

Well, Joakim and Submarine Tom are kinda missing my point...which was an echo, hollow that it was, of what the first poster said.

Let's restate this kinda, with a much larger ship test, known as a BOLLARD PULL. This is done for tug boats. Since HORSEPOWER isn't really HORSEPOWER in the water, you have to measure true pull in a tug boat, so you do a BOLLARD PULL to determine if your little tuggy is as strong as your older tuggy...

<<<at this point, I ask all in contention to the idea of PULL versus HORSEPOWER, please wikipedia the term BOLLARD PULL>>>

Now, welcome back...

Since I can make a propeller that is 85% efficient, and I can make one 72% efficient, and I can form it to do many different things (i.e.: high rotational speed, low noise, high thrust, minimal cavitation pull-side, minimal cavitation push-side, etc.) two 6-HP motors with differing props can thrust totally different. Saavy? Ok, so since trolling motors are meant to effect lowest noise and disturbance...while outboards, well, outboards are for power-thrust...and hence, you cannot easily compare gas outboards to electric trolling motors based upon the conventional transition of 1 Horsepower is equal to 745.7 watts (or 550 ft-lbs/sec)...because you are talking about the work-head, not the work done. Vast difference. The actual work done, for purposes of barges, tugs, and even pontoon boats is more equated to the measurement given in a BOLLARD PULL test...and hence, you now are fighting the same battle that automotive manufacturers have dealt with for years.

Which is faster, a 500-horsepower Corvette, or a 500-horsepower Mustang? If you say they are equal because of power, I say enter your wager for the Mustang, I shall take the Corvette, and I will also take your money. They are NOT created equal...for there are finite details in the distribution of power to real-world work, that measures the actual winner. To such a point, you can reciprocally ask, 'ok then, what is the measure of the application and the full equation, to where I can calculate the actual horsepower the Mustang will require to be speed-time-performance equivalent to the Corvette. By the way, this can actually be done...ask any race car crew chief.

Now, back to real world...applying a BOLLARD PULL test to my Pontoon Boat, which has a 20-HP outboard motor, what total 'thrust pounds of trolling motor' do I need, to match the 'PULL' exerted by the outboard? If the actual 'PULL/THRUST' is the published value given on the trolling motors per advertising...then we come back to the question...how much 'PULL', in Newtons, KN, Pounds-force, etc. units, by the conventional 20-HP outboard?

I've still not heard the answer...but the answer will be given in LBS-FORCE (my preferred anti-SI units) against the BOLLARD PULL test standard...or you have NOT answered the question.

By the way...I have NOT done this, for I do not have a sufficient scale to do so...but I would estimate my 20-HP motor would exert around 250-300 pounds-force on the linear scale while it is tied-off to a pier cleat. I have not tried this with a trolling motor, to compare it's pull...but the pull from either, whether mounted to a canoe, a flat-bottom boat, a V-hull, or a pontoon boat...really should be within 3-5%, and therefore negligible to whatever hull-form it is attached to. Hence, it is my supposition that the lbs-force yield from a BOLLARD PULL test is the purest form of testing for actual motor comparisons...

Now...all that long-windedness aside...doesn't this seem correct? Does anyone have any data on such testing? That would be most-valuable, indeed.