how many lbs of thrust is equal to 1 hp?

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

  1. deermaster
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    deermaster New Member

    how many pounds of thrust in a trolling motor is equal to one horsepower of gas motor? thanks for any help.
  2. marshmat
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    marshmat Senior Member

    The power absorbed by a hull is its resistance (wave drag, skin friction, etc.) multiplied by the speed at which it is travelling.

    The actual power produced by the engine is substantially larger, as much of the engine power is lost to inefficiencies at various points in the system. For example, a gearbox might take 3% off the engine's brake horsepower, and a typical propeller would convert 50-60% of the horsepower that reaches it into useable thrust. (Really good, well-matched props can sometimes be 70%+ efficient.)

    When you look at a trolling motor rated for something like "55 lb thrust", the figure given is often the maximum thrust that motor will give. Let's say for the sake of argument that you'll be going at 2 knots, or 1 m/s with the trolling motor at full power. If you're getting the full 55 lbf of thrust at this speed, then your hull has 55 lbf total resistance at this speed. 55 lbf = 0.245 kN, so (0.245 kN)*(1 m/s) = 0.245 kNm/s = 0.245 kW. That's about 1/3 hp.

    Now, once again for the sake of argument (these figures might not be representative of your actual setup) let's say your propeller efficiency on that trolling motor is 50%. Then it's actually producing 0.49 kW or 2/3 hp. Electric motors aren't perfect, let's say our example motor is 80% efficient at full power, so we're up to 0.62 kW of electric power that's actually being sent into the thing. At 24 volts that's about 26 amps; at 12 volts, more like 52 amps.

    To compare against a gasoline engine is a little harder. An outboard's SHP rating is usually at full power (very near WOT) and usually measured at the propshaft. To get the same 55 lbf of thrust as above, again with a 50% propeller efficiency, we get the exact same 0.49 kW or 2/3 hp. Shaft power has the exact same meaning regardless of what is turning the shaft.

    The question becomes, then, what gas engine will produce 2/3 hp at the prop shaft. You'd probably find that a 2 hp outboard at about one-half to two-thirds throttle would be roughly equivalent.

    Now, all of what I've just said rests on a big assumption- that we actually know what thrust the trolling motor is making at a given speed. Short of putting strain gauges on its mounting bracket, we generally don't- and can't- know this value accurately.

    But the key message in all that math I just posted is this: You cannot make a direct comparison between thrust (force) and power, unless you also know the speed of the vessel when it is absorbing that thrust or that power.
  3. PAR
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    PAR Yacht Designer/Builder

    Depending on which math formulas and efficiency assumsions you want to accept, a single HP is some where between 75 and 110 pounds of thrust.

    For an average trolling motor, figure towards the high end of this scale. If using a better prop, then comes with the typical trolling motor, then the some place in the middle.

    The 105 pounds of thrust, 24 volt motors are about a single HP.
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    FAST FRED Senior Member

    I know formulas vary , but I always use 20 lbs for most boats , 25 for really good prop setups and 30 as the dream of "perfect".

    Perhaps Rick will bridge the huge difference between 25 and 100 lbs per hp.

  5. Guest625101138

    Guest625101138 Previous Member

    HP and thrust are indirectly related. As Matt points out:
    Power = Force X Velocity

    If you were solely interested in bollard pull without actually moving then you would go for the biggest possible prop and gear it down. So the water velocity needed to generate the thrust is small and the area that it acts over is large. This is what you see with tugs. The power required to achieve a certain bollard pull for a zero loss system is:
    Power = (4/pi/9/rho)^0.5 x Thrust^(3/2)/D in Watts

    Thrust in Newtons
    rho = 1025 for salt water
    D prop diameter in metres

    Realistically the power will be roughly twice this level to make up for drag of the blades through the water.

    If you actually want to move then the prop size will reduce and gearing ratio will be lower so it spins faster. The force will be lower but you now have the blade advancing through the water plus the added slip to generate the thrust.

    The smallest commercial combustion engine outboard I know of is 2HP. This is more powerful than any of the Minn Kota range of trolling motors as far I know. However the trolling outboards tend to have larger diameter props than the little combustion outboards so one of the larger trolling motors will produce similar thrust to a 2HP outboard. It is possible to get larger diameter props for many of the small combustion outboards for low speed, high thrust applications.

    The original question posed does not make much sense unless there is an application in mind. The thrust rating of trolling motors is nonsense anyhow unless you want to have tug-of-wars. If you actually want to move then it is POWER that matters.

    The question would be better framed if the intended purpose was given.

    Rick W

    thrust will be roughly the same.A mid range troll
  6. PAR
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    PAR Yacht Designer/Builder

    740 watts is a single HP. It's not a simple nor direct conversion (HP to pounds of thrust).

    Lets take Fred's example and say 25 pounds = a single HP. At 1 pound of thrust per amp (not my assumsion, but Minnkota's, which I feel is less then half right) a 24 pounds (24 amps) of thrust motor times 12 volts, equals 288 watts. Clearly this isn't right, so a more effective measure might be .35 pounds per amp. Of course this doesn't count slippage, speed (your not performing work unless you're moving) nor any other variables.

    Thrust is a static measurement in pounds. HP is a measurement equal to 550 foot pounds of work per second.

    Most trolling motors use a prop designed to provide quick acceleration at the sacrifice top speed. Most are "geared" to reach about 4.25 knots (around 5 MPH). You can trade off acceleration for top speed, with a different prop.

    Since the conversion isn't direct, it would be much more helpful if we had more understanding of the application.
  7. CDK
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    CDK retired engineer

    If you manufacture outboard engines with outputs below 1 hp and advertise them as such, sales figures will be much lower than if you advertise them as having, say, 80 lbs thrust.
    Of course the thrust figure is meaningless, but for the public 80 lbs is a considerable weight to lift, so an engine with that much thrust is regarded as powerful. Horse-powers we relate to the figures we know from combustion engines, there anything under 4 hp is almost nothing.

    Only jet engine output is measured in lbs thrust, but for totally different reasons.
  8. jehardiman
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    jehardiman Senior Member

    Last edited: Nov 17, 2008
  9. bruceb
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    bruceb Senior Member


    Back in the 80s, both evinrude/johnson and chrysler (maybe others) rated their longshaft "sailboat" engines at about 150lbs thrust for a 6hp and 250lbs for a 10hp. From experience, it did seem close to real world performance. Bruce
  10. Joakim
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    Joakim Senior Member

    Power = V * F. Typical system has roughly constant maximum thurst (F) with all speeds. Thus it is enough to calculate the thrust at max speed.

    F = Pmax /Vmax * efficiency. For normal, not so good, applications a good first guess for efficiency might be 50%.

    Now to stupid non-SI units...

    Thus 1 HP (735 W) would acchieve a thrust of 370 N/V[m/s], which would be 160 lbs/V[kn]. Thus for a propeller designed for 3 kn speed (typical electric system) the thrust is ~50 lbs/HP, but for a 6 hp outboard designed for 6 kn it would be less than 30 lbs/HP. If we take a 20 outboard propped for 30 kn speed, we have only 5 lbs/HP.

    For a 12 V system 1 HP equals to 60 A, which is close to normal 1 lbs/A of thurst.

  11. daiquiri
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    daiquiri Engineering and Design

    This thread illustrates well the nonsense of rating an engine in lbs of thrust instead of giving the power data.
    It is similar to a question one guy, a sheet metal worker, has asked me once: "how many square meters are there in 1 cubic meter?"
    Last edited: Nov 24, 2008
  12. bruceb
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    bruceb Senior Member

    I think the engine builders were avoiding the issue, but at least you can use lbs of thrust to compare similar engines. As for the different measurement systems, the USA has been trying to convert for 40 years- we just haven't quite got it yet.
  13. marshmat
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    marshmat Senior Member

    Hey Bruce, we Canucks are only slightly better.... you still see odd hybrid units like "milligrams per pound" here, and the grocery stores often make the $/lb price bigger than the $/kg price when they have sales on so that food looks cheaper than it really is.
    And then there's the infamous 4x8 foot sheet of 6-millimetre plywood. (Some of our mills have converted to metric in one dimension and stuck with feet in the other two.)

    But back to the original topic.... to be honest, I don't think the folks who sell trolling motors really care what their actual power rating is. They just want to know which motor will push what size of bass boat, and how soon the battery will die.
  14. bruceb
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    bruceb Senior Member


    I think it depends if the fish you are trying to catch are measured in inches or kilowatts. Here in the south, we are mostly concerned if the beer will run out before the battery dies.

  15. PAR
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    PAR Yacht Designer/Builder

    We've been using metric tires on cars for a number decades now, except the rim diameters are still inches.
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