How Torque, HP, and max RPM relate to speed/cruise? How do Gas/Diesel compare?

Discussion in 'Powerboats' started by leaky, Oct 20, 2015.

  1. Barry
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    Barry Senior Member

    Torque does not absorb horsepower/power as torque is a static load. As Adhoc and Philsweet stated, propellors absorb power.




    So after NavalSArtichoke states that you chose the prop based on how much torque that you want it to absorb, his second quote above says you look at the torque and the speed that it is delivered, which is power.

    Which is of course correct
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    In the context of a propulsion system, torque is not a static load. It varies, depending on several factors, including engine RPM. The propeller is a device which converts the torque of an engine into thrust, which then drives the vessel.

    If the propeller selected cannot absorb the torque the engine wants to deliver at the RPM at which it is running, there must be some adjustment in operating conditions. Something changes to bring things back into balance.

    No one is saying that power is not related to torque or RPM. As I said in an earlier post, power and torque are two sides of the same coin.

    One method of propeller selection uses torque as one of the elements in determining propeller output, another uses power. At the end of the day, if each method is used properly, the propeller selected should be the same.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Again slightly misleading and not strictly correct.

    Since one only needs the Kt curves if the vessel is predominately used for pulling, i.e tugs etc. Since the speed of advance is next zero or zero in some cases. In which case standard Bp-delta charts cannot be used. In all other cases Bp-delta charts are used determine the design of the prop by the 3 parameters, that of: rpm, power and speed of advance.

    Thus it is not a choice of one or the other in the deisgn of a prop. The Kt is is only used when you cannot use the Bp-delta charts owing to the different modus operandi of the prop. That being bollard pulls as its MO not top end speed.
     
  4. Barry
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    Barry Senior Member

    Torque is a static load as the units are simply pounds force times feet. The propeller is a device which converts the torque of an engine into thrust, is just not correct. You can say that the propeller is a device which converts the torque of an engine into thrust at certain a RPM, which is power.
    Or just say that the propeller converts the power of the engine into thrust. Mind you, you can say whatever you want but to be technically correct, the prop absorbs horsepower.
    Certainly, the prop shaft sees torque loading but the minute you enter shaft RPM into your selection, you are dealing with power.

    If Torque is not a static load, what are its units?
     
  5. fcfc
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    fcfc Senior Member

    Yes BUT :

    1) Engine installation have the SAME weight. Otherwise, lighter boat will go faster.

    2) Suitable gearing can be found to have the same prop speed at engine max power speed. (Technically not an issue, but commercial availability may differ).
     
  6. powerabout
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    powerabout Senior Member

    the gas engine will have torque with a curve over a wider range than the diesel which is another reason they go faster as more rpm will net more power
    BUT if boat is not really planing then the tug boat engine might keep up
     
  7. Joakim
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    Joakim Senior Member

    What do you mean by "static load"? The crank shaft is rotating it's not static. Yes the units is Nm (of lbs-f * ft or whatver). But the unit of thrust is N (or lbs-f), so in your reasoning that must be "static" as well?

    It's equal to say that propeller converts torgue to thrust or propeller converts shaft power (torque * omega, kW, HP or whatever) to propulsive power (thrust * speed, kW, HP or whatever).

    In any case the propeller need to be chosen based on shaft RPM, shaft power or torque, speed and resistance. You can calculate all this using torque and thrust or using only power.

    Propeller can be seen as a gear. The conversion from shaft torque to thrust will be almost inversionally proportional to pitch, but also varies depending on operating conditions.

    So if you have two engines with equal power, but different RPM and thus different torque, you obviously need different gearing and/or different propeller (mainly different pitch). With properly chosen propeller these two engines will deliver about the same thrust and thus top speed. The only things making then possible different are the efficiencies of the gear boxies and propellers.
     
  8. Barry
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    Barry Senior Member

    There is much misunderstanding about horsepower and torque curves because the two are usually placed on the same graph and hence create opinions that both have an rpm component

    A dyno measures static torque or the amount of static foot pounds (lbf*foot) that the engine COULD put into a shaft if the engine could produce it without rotating at a particular RPM

    Then the dyno, or tech could run the static foot pounds number through the equation to come up with the horsepower that the engine is producing at the various RPM'S tested.

    Say you take a 2 foot torque wrench and attach it to a nut on your cars wheel that is concentric with the axle. The brakes are on, and you pull on the wrench at 50 pounds. You have then put 100 foot pounds of torque into the nut. There is no movement, it is static.

    Then you have someone release the brakes slightly and you rotate the wheel one revolution. You then have done 50 pounds times the distance that the 50 pounds moves through 2 feet times 2 times 2 feet of radius time pi which equals 628 foot pounds of work.
    It can take one minute or 60 minutes, but you have done
    628 foot pounds of work. (ignoring acceleration)

    Horsepower is the rate of doing work. If you are able to make one revolution in one minute you will be producing 60 times more horsepower than if it takes 60 minutes to do th work.


    It takes a specific horsepower value to move a boat at 30 mph. Force through distance with respect to time. This is the output side of the equation. For later reference lets set the effective horsepower at 200 hp.

    At 30 miles per hour, this is the thrust (force) generated by the propeller through the distance with respect to time. The same amount of horsepower as the output.

    My point was to dispute the comment that a contributor made " you have to choose the prop based on how much torque you want it to absorb"

    A person can take a 10 horse power engine and with enough gearing can create 200 foot pounds of torque. This will not move the above boat at 30 mph no matter what prop you select. ie based on torque

    Or take the an engine properly geared and matched to a propeller that will produce the thrust through distance with respect to time ( horsepower) which will equal 200 horsepower (effective output to the water) to move the boat at 30 mph.

    The above wording is tricky as you might need a 250 horsepower engine to turn the prop to produce the thrust that will create the thrust, distance, time that produces a 200 horsepower input into the boat.

    Back to the wrench on a car. The brakes are on and you pull the wrench at 50 pounds at 2 feet. This is torque, it is static, and while it might not feel like it, the nut is not absorbing any torque.

    A propeller absorbs power. Torque times RPM

    If the equation is Horsepower = Torque*RPM/5252
    Then HP*5252 = T *RPM

    There is also often confusion when reading a HP curve in that what is often forgotten is that this is the maximum amount of power that the engine CAN produce at each measured RPM.

    The only time that an engine reaches A maximum HP output is when the throttle is advanced to full throttle and the RPM will not increase.
     
  9. DCockey
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    DCockey Senior Member

  10. powerabout
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    powerabout Senior Member

    are we splitting hairs to say on a dyno ( its moving) torque is measured statically as the sensor is on a torque arm versus shaft twist measured
    same result
    Electric motor the only motor that can produce torque without rpm
    if the boat moves its absorbing 'power'
     
  11. Barry
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    Barry Senior Member

    Torque can be measured on rotating shafts by many different methods.

    The most common is the absorbing dyno where a braking force is applied to the shaft by friction (old style), hydraulic resistance, electrical resistance, and many others

    The other way is to measure the strain (twist) in the shaft
    from a no load situation to a loaded situation.

    The strain guage system that DCockey linked is one and another is a two light transducer system. This system incorporates two light pickups that provide two light impulses at the exactly the same time, but when a torque is produced which causes the shaft to strain, the impulses come at a different interval and calculating the time, structural parameters of the shaft, a calculation can be made to determine the torque that the shaft is carrying.
     
  12. Barry
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    Barry Senior Member

    Or a hydraulic/air motor possibly a steam engine?
     
  13. rxcomposite
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    rxcomposite Senior Member

    Rubber power also. And the coiled spring/flywheel energy storage on toy cars.
     
  14. powerabout
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    powerabout Senior Member

    In all that stuff you dont need a dyno.
     

  15. powerabout
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    powerabout Senior Member

    Good point
     
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