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

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

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### MuerteJunior Member

Thrust to HP to Pounds to Cheesecake

Well, I just managed to stumble across some 'mosquito' motors and their related BOLLARD PULL test values...though they are reading far above what I would have thought they could do...for it seems that the 3-HP range are getting close to 75-lbs of thrust...then again, the thrust-force-curve may be quite curved instead of linear...Again, I don't know, but am very interested to find out.

I did 'MS Excel' the following values, and came up with an equation of y=15.7x + 35.3, where x is horsepower and y is gross thrust.

Extrapolated to a 20-HP motor, that would be 349.3-lbs thrust...and as my first 'rough guess' of my 20-hp motor making 250-300 lbs seems about right...then it appears feasible that there is some form of equation for thrust, though there may be some loss factor I am not seeing as the motors get larger.

Overall, it looks like for my application, a couple of high-thrust trolling motors and a rechargeable battery bank would work quite well as a replacement. Again, this is what I was looking for...and I believe what the original poster was looking for. If the above formula is correct, then his answer would be 1-HP is equal to 51-lbs thrust, but this is not a direct amount. It is by using the linear slope of the actual Bollard Tests given below...(again, sure would be nice to have 20-30 samples of real-world data to verify the formula above is correct)

http://ezinearticles.com/?Choosing-an-Outboard-Motor-For-Your-Dinghy&id=2008077

"To judge one engine against the another several tests were completed. A Bollard pull test showed that the Mercury 3.5hp and Tohatsu 3.5hp were the most powerful at 90lbs of thrust (These two engines along with the Mariner are virtually identical). The least effective was the Honda 2.3hp at 66lbs of thrust. In between were the Suzuki 2.5hp at 83lbs of thrust, the Yamaha 2.5hp at 78lbs of thrust and the Parsun 2.6hp at 70 lbs of thrust."

2. ### Submarine TomPrevious Member

I don't believe I have missed your point. What I have missed is "The" question you refer to.

So, it must be the original one. Thrust vs. Hp depends largely on your propeller efficiency and RPM. There is no direct correlation between Hp and thrust.

I should probably point out that as a Master Limited Captain with a towing certificate I am well familiar with the term BOLLARD PULL.

-Tom

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### JoakimSenior Member

Muerte, I do know very well what bollard pull is and what it is used for. The original question did not state that bollard pull was the main issue, he just wanted to compare gas and trolling motors. For most purposes this comparison should be made using power, not bollard pull.

Please read this: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf

You can find the measured bollard pull and top speeds for several outboards and trolling motors. Note that Torqeedo 2.0 had a very good bollard pull of 54 kg (120 lbf), but still the top speed was only 4.9 kts. Compare this to say Suzuki 2.5, which is about equal in power. It has only bollard pull of 35 kg (80 lbf), but still it has a top speed of 5.9 kts. So which one of these has more thrust at 4.9 kts? Even Torqeedo 801 had almost as much bollard pull, 31 kg (70 lbf), but only 4.0 kts top speed. How much thrust do you think it has at 4 kts?

So you are running 4 kts upstream. What is your speed through water? Which of the above would provide the biggest thrust at this situation?

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### poweraboutSenior Member

the fastest one

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### MuerteJunior Member

You gentlemen miss the point, entirely. If I have a boat, and wish to change the powerplant from one motor to another, different one, then that change is SOLELY effected by the BOLLARD PULL test. Not speed, not power, and not propeller efficiency. (This is in regards to my pontoon vessel, and NOT speed-craft, which function under a few different rules, including issues related to prop designs, cavitation, hull cavitations, etc.) Just a slow-moving riverboat...saavy?

Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right? I mean, is their a fail to my logic on this? If so, please enlighten me...why would speed even be considered in this event...I just need to know what a new motor pulls/pushes against the transom mount of ANY vessel, so as to replicate 100% of my current motor's mechanical effect to my transom...anything else considered is superfluous to the engineering design or boat effects.

Ok...so if I have a BOLLARD PULL table of actual empirical yields of motors, say from 2-50 horsepower, with common OEM props on each (say 72% efficient), then I should be able to ideally convert outboard (gas motor) thrust to electric motor thrust, if the actual push or thrust of such electric motors is being reported truthfully as a bollard value.

Now, that said, can you unequivocally state that a 55-lb thrust trolling motor is actually yielding a 55-lb BOLLARD PULL value, or is that a base-mounted value for the work-head, or exactly where is that 55-lbs being read from? Is that torque yield from the rotational event itself? Where is "55-lbs" being reported from? My first knee-jerk response to this is that the "55-lbs" is coming from a static head-mount in a lab somewhere, and is tested against ideal conditions and power input closer approaching 13.2 Volts, not 12.2 as is the nominal in the 'field'.

Ok, so let's assume (you know what they say about assuming, right?) that my current 20-HP motor is making Y=15.7x + 35 in thrust, and that converts at .9x to a BOLLARD PULL. Therefore, the motor is making 349-lbs of thrust, and for-which, it is yielding 314-lbs of BOLLARD PULL event. Ok, lovely. Now, I want to either EXACTLY MATCH this force (as work = force x distance, remember?) or add more...I don't want to lose work-potential, when I CHANGE MY GAS MOTOR OVER TO ELECTRIC MOTORS! This is the point that I was trying to get to...what force-potential do I need in electric motors to 100% replace my gas outboard! The original question asked more-simply, "how much force-thrust is equal to 1-HP...to which at the work head, it appears to be generally answered as Y(thrust)=15.7x(x=HP) + 35. AND...it further appears, that conversion, at least in my-case (not sure where it is coming from, unless it has to do with less-than-perfect angles of approach relative to my outboard's trim) that to take pure thrust and convert to a BOLLARD PULL value comes to (.9Y), or 90% of the calculated thrust value.

Ok...so if I have 349-lbs thrust, and 314-lbs of Bollard Pull, then I need trolling motors which can do the same thing...so I can either fit up three quantity 115-lb thrust trolling motors (115 x 3 = 345-lbs thrust x .9 = 310.5-lbs Bollard Value). <This is a snarl-factor...I hate mated linkages>

Now, taking that into power demand against three 115-lb thrust trolling motors, I am looking at (after consideration of Peukert's Curve for storage batteries) around 4-hours load-time against a 6-battery charge bank, or 6-7 hours of cruise time against the same battery bank, with 12-hours charge required at 20-amps for such battery bank...and now, I have resolved how to replace a 20-HP outboard with 3-DC trolling motors and 6-deep cycles, and remove \$30 a day of fuel requirements from my boat use.

You see, there is ingenuity and method-to-madness from all of this...the problem is, there are too many intents of seeking "power", "thrust", "pull", etc.; when it all really comes down to one simple fact, and that is whether I have a team of horses pulling 349 constant pounds of force on my vessel, or an outboard or three trolling motors are doing it...doesn't matter. This is the true purpose of a BOLLARD TEST...to compare apples to apples over FORCE POTENTIAL-TO-KINETIC. Believe it or not, Evinrude did just this thing against Yamaha with the eTec motor, by connecting two boats stern-to-stern, and 'PULLING' against each other...while fitted with the OEM recommended propeller...
Yamaha got swamped, Evinrude drug it into the blimey deep, and Yamaha cried foul...that a "BOLLARD PULL" wasn't a 'fair test' to do...that it mean nothing.

Well...what can you say, it is all about the media, hype, and lies. In the end, any engineer will tell you, same boat, same hull, then bollard 350-lbs beats bollard 320-lbs, every single time...and the 350-lbs is stronger AND faster, when you are testing them on the same boat...and is why if you use BOLLARD of a motor to compare motor-to-motor-to-motor, you can find what you need to upgrade, or to change over to other systems...

Sorry...didn't mean to cause an uproar...just tough being retired and no engineering group to sound off to...kinda had to talk myself into the right answer on this one, and now I have found what I required.

Looks like to replace my 20-hp outboard, I will need 3-quantity 115-lb thrust trolling motors, tied with mated linkages...connected to a 6-cell deep cycle bank and 20-amps of solar cell charging overhead on the fixed canopy, with inline charging regulator that can handle up to 600-watts . <wow...gonna look like the African Queen, when I am done!> LOL

I hope SOMEONE other than myself found this 'enlightening'...

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### DCockeySenior Member

Different engine/motor, gear ratio and propeller combinations will have different curves of thrust vs boat speed. In general if two combinations happen to have the same thrust at zero speed (bollard pull) then the thrust at non-zero speeds will be different. That's why speed matters.

At sufficiently slow boat speeds the curves will be close to each other if the bollard pull is the same at zero boat speed. How slow is sufficiently slow depends on the particular combinations involved and how close is close enough.

So if your are only interested in slow enough speeds your logic is okay, otherwise it's wrong.

If you're convinced you are right then go ahead and replace the outboard with the electric motors. As long as you're satisfied with going slowly then it may work out.

7. ### Submarine TomPrevious Member

Well, it sounds to me like you've answered your question.

Personally I think you are mistaken but I don't believe you are really interested in what I think, and that's okay, it may be me that is mistaken.

All the best to you in your efforts and, please, let us know how it turns out.

-Tom

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### DCockeySenior Member

The Yachting Montly comparison test from March 2009 on the link which Joakim posted provides some interesting data. His points are correct.

I extracted the bollard pull and top speed for each motor.

Gasoline Bollard Pull Top Speed
Honda BF 2.3 13 4.5
Suzuki 2.5 35 5.9
Tohatsu S3.5 45 8.8
Mercury 17 5.4
Parsun 27 4.4

Electric Bollard Pull Top Speed
Minni Kota Riptide 55 17 2.5
Flover 55TGS 13 2.5
Torqeedo Cruise 2.0 54 4.9
Torqeedo Travel 801 31 4

At the same speed the thrust of the engine on the boat will be the same. If the speed is higher the thrust will be higher. It's clear the relationship between bollard pull and thrust at speed is different for the electric outboards compared to the gasoline outboards.

9. ### Submarine TomPrevious Member

Ah, well, if it's in print in Yachting Monthly then it just has to be true!

-Tom

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### DCockeySenior Member

Plot of speed vs Bollard Pull from the Yachting Monthly comparison test.

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### DCockeySenior Member

I'm an engineer and I won't tell you that. 350 lbs of bollard pull will beat 320 lbs of bollard pull in a tug-of-war, but it doesn't indicate which one will push the boat faster.

See the chart posted I posted above which is a test of a variety of engines/motors on a single boat.

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### Mr EfficiencySenior Member

In any event, running a petrol outboard at its max will kill any economy, so assuming the electric can operate to the max without a similar penalty, you have to compare them with that in mind. The figures quoted about 1 hp = 80 lbs or whatever are clearly ridiculous, That would mean even allowing 50% conversion of HP to prop thrust giving about 8000 lbs of thrust with a 200 hp outboard when gunned hard........don't think it would be 25% of that, with change.

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### JoakimSenior Member

It is of course true one shouldn't trust everything printed on a magazine, but it shouldn't really be hard to measure bollard pull and top speed. All you need is a force scale and a GPS. Do you think this is too complicated for Yachting Monthly crew?

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### JoakimSenior Member

No! That is not the way it works. Trolling and most other electrical motors are especially good at bollard pull, that is ZERO speed NOT 4 kts. Probably their propellers are designed for that and also an electrical motor can deliver its max torque at low (even zero) rpm. If you read the Yachting Monthly or any other test which measures bollard pull and top speed on any vessel, you would know this.

Your logic probably works quite well for bigger gas outboards, that are designed and propped for much higher speeds (15+ kts). Those will likely have about the same thrust at 0 and 4 kts. But this is not true for trolling motors, which loose their thrust rapidly with increasing speed.

Remember also that there are special high-thrust models/propellers for gas outboards. These will be much better at 4 kts than the standard models/propellers designed for higher speeds.

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### Mr EfficiencySenior Member

Remember also that there are special high-thrust models/propellers for gas outboards. These will be much better at 4 kts than the standard models/propellers designed for higher speeds.

There are props for the low speed thrust application, but the diameter is pretty limited and well short of ideal for the purpose. Gearcase ratios, too, are restrictive.

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