Diesel hp rating not always what's advertised!

Discussion in 'Diesel Engines' started by Chuck Bates, Feb 4, 2005.

  1. fredrosse
    Joined: Jan 2005
    Posts: 399
    Likes: 55, Points: 38, Legacy Rep: 56
    Location: Philadelphia PA

    fredrosse USACE Steam

    Torque & Horsepower

    The fundamental relationship between Torque and Horsepower is as follows:

    HP (USA STANDARD) = Torque (Ft-LBf) x RPM x ( 2 x 3.14159 / 33,000 )

    Where :
    HP = USA STANDARD = James Watt Standard = 33,000 Ft-LBf / min
    Torque = (Ft-LBf)
    RPM = Revolutions per minute

    Rearranging for conveinence:

    HP (USA STANDARD) = Torque (Ft-LBf) x RPM / 5252

    Torque (Ft-LBf) = 5252 x HP (USA STANDARD) / RPM

    Reduction gears change the RPM output, and the output torque, horsepower remains essentially constant, except for some small loss (nominally 1 to 3%) across each gear reduction.

    If you can give further details as to the boat displacement, hull type, waterline length, and desired speed, the proper propeller diameter and pitch can be selected. Depending on the availability to mount the ideal prop, some compromise is ususlly met, adjusting prop size and pitch to fit the constraints of the hull.
  2. gonzo
    Joined: Aug 2002
    Posts: 15,242
    Likes: 948, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    40 HP is 40 HP regardless of brand. The RPMs at which the power is rated will indicate what the torque is. However, with a higher revving engine and a higher ratio reduction, it is possible to turn the same wheel. There is no difference.
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "We will still only inject 10 mm3 thus the btu content for the 10 mm3 @ 35C° will be lower. Lower btu per injection event = less power."

    Even the lowly mechanical govenor on a Detriot 2 stroke will compensate for thin hot fuel.

    The rack simply calls for MORE FUEL to keep the rpm the same , weather the load changes or the fuel does.

    They were no dummies in 1936!

  4. Karl2
    Joined: Feb 2005
    Posts: 67
    Likes: 1, Points: 0, Legacy Rep: 34
    Location: Right Coast

    Karl2 Junior Member


    Yes - And all mechanical engines with a governor will do the same. BUT only at partial load. At full load (and this is where your power loss due to fuel temp is noticable) the mechanical governor can do nothing to compensate.


  5. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    The standard for measuring engine horsepower output is "Brake" horsepower (which, as Gonzo describes is measured by torque at RPM acting on a water brake). I have, however, seen horespower published in terms of "shaft" horsepower, and in another way that I can't quite recall (Similar to Drawbar horsepower).

    I have had diesel engines Dyno tested, and actually have never seen one fail to meet the publilshed values. I have heard of them though.

    One interesting point is that some manufacturers will sell the same engine as a commercial engine and as a recreational engine. The commercial engine has a lower reccommended continuous operating RPM and therefore a lower Max Horsepower rating. (But it's the same engine)
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.