bollard pull/ propulsion and torque in a tug/towboat

Discussion in 'Powerboats' started by tugboat, Jan 26, 2012.

  1. tugboat

    tugboat Previous Member

    I have been trying to experiment(using a prop calc which gives torque ratings at both the engine and the shaft) for my tug using different engine combo's and gearing for the tug-
    here is what im wondering:

    bollard pull of a tug- could it be based entirely on torque at the shaft or is it torque at the engine--??

    for example if i took 30 hp at 3000 rpms and geared this 4.5:1 ratio- it would equal the torque at the shaft of a 200 hp engine at 3000 rpms using a 1:1 ratio.of course the prop would be 57 x 90 and not theoretically able to be put under a 26 ft boat-
    does this make sense? in other words buy a smaller engine which uses less fuel and gear it to give high torque at the shaft/prop.

    in theory it seems that any engine could be as strong at the shaft as any other depending on what gearing is used??
    to grossly exaggerate this: case in point--if i used 26 ft tug 20 hp@3600rpm with a gearbox of 1:25- thats 686ft lbs at the prop/shaft!
    and if it tried to tow a vessel with 200 hp with the same boat same displ. but running at 2:1 gear ratio-puitting out 549 ft-lbs of torque at the prop/shaft..then would it be fair to say the smaller engine could outpull the larger?
  2. SailDesign
    Joined: Jan 2003
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    Location: Jamestown, RI, USA

    SailDesign Old Phart! Stay upwind..

    Here's a spreadsheet to play with - it's old, but then again so is the calculation.

    You'll notice the only real outputs are diameter and horsepower.
    If the torque and diameter are balanced, then the bollard pull can be achieved.


    Attached Files:

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  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    My general rule of thumb for bollard pull of a standard B or K series prop is 35 lbs bollard per 1 shp in. Nozzels will increase this 10-20%, props less than ~48" diameter will reduce it 10-20%. Note this shaft horsepower (shaft torque*shaft rpm) delivered, not rated hp. There are ways to measure shaft torque, thrust, and rpm; but those are usually only done as contract deliverables. Generally, modern "high speed" engines cannot provide sufficient torque when loaded up and must be geared down, similiar to steam turbines. Old steam and heavy fuel engines, on the other hand, developed maximum torque no matter what the rpm so you could get a 5 rated hp engine pushing a 200-300 ton ship along at a good clip. The difference is the weight of the plant, 300-400 lbs for a lightweight high speed vs a couple of tons for an old slow speed.

    Edit: Xpost with SD, but his numbers are similiar though it looks like that wheel was not optimized for pulling and may have a different P/r distribution.
    Last edited: Jan 26, 2012
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  4. tugboat

    tugboat Previous Member

    Thanks for that--where did you get that calculator?

    it is good--so if im reading correctly--the amount of pull in force is the shaft hp? which i can get using a hp calc since i have the torque calcs?
  5. tugboat

    tugboat Previous Member

    thanks--i understood most of what was said--some of it is beyond me such as a k series etc..however steam engines are great--i have thought of using two steam engines at 8 hp each running of one boiler. but the reversing of it would be an issue. I remember reading somewhere that twins- reduce efficiency of bollard pull by 20% so it agrees with what your saying that the kort nozzles would increase bollard pull to balance out the losses? like usual the more questions get answered the more others crop up--does a kort nozzle need to be foil shaped? btw-the props on mine are 22 x 19..and are suited for hydrualics turning 585 rpms with 12.5 hp ea. im guessing this is pretty good hp at the props...i prefer to think like a steam engineer and use torque rfatings but pls steer me in the right direction if that wouldnt apply here- the main engine drives two cont. duty hydraulic motors 12.5 hp ea at 585 rpms with about 114-140 ft-lbs of torque. ill do the calcs for hp at the shaft but i think since its 1:1 its still 12.5 hp..but the low rpms means they develop that power at low rpms...creating pretty strong pulling I on the right track here?

    i loved the old fairbanks morse engines--one i saw was an IHP of 400 and ran at 350 rpms--it was one monster of an engine and must have weighed 6-7 tons...
  6. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    We used to use this when I was a "real" naval architect, not the Yacht Designer I've become. ;-)
    It boils down to having the correct disk area to absorb the torque.
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  7. tugboat

    tugboat Previous Member

    Ok i think i see what your saying(i hope) the diameter of the prop- if the diameter is balanced with the torque then it turns the prop at just the right speed to produce the required power or bollard pull??

    Where did you study??.i was looking at westlawn and Macnaughton doing Macnaughtons drafting course--
  8. tugboat

    tugboat Previous Member

    here is the calcs for my set-up

    Waterline length in feet: 26.4 feet

    Beam at the waterline in feet: 10 feet

    Hull draft in feet (excluding keel): 3.5 feet

    Vessel weight in pounds: 10000 lbs

    Engine Horsepower: 12.6 HP
    Number of engines: 2
    Total Engine Horsepower: 25.2 HP

    Engine R.P.M. (max): 585 RPM
    Gear Ratio: 1:1
    Shaft R.P.M. (max): 585 RPM

    Number of shaft bearings (per shaft): 2
    Desired speed in Knots: 7 knots

    Horsepower Calculations
    This will calculate the maximum horsepower and torque available at the prop(s).

    Total available horsepower at the engine(s): 25.2 HP
    Total available torque ft/lbs at the engine(s): 226 ft/lbs
    Horsepower loss of 3% per gearbox: - 0.8 HP
    Horsepower loss of 1.5% per shaft bearing: - 0.8 HP

    Total horsepower available at the propeller(s): 23.7 HP
    Total torque ft/lbs available at the propeller(s): 213 ft/lbs

    Speed & Power Calculations
    Basic displacement speed and horsepower required
    Displacement hull speed (1.34 X sqrt of waterline length): 6.89 Knots
    Minimum horsepower required at propeller(s) for Hull speed: 19.8 HP

    Calculations based on desired speed and available HP
    HP required at propeller(s) for desired 7 knots speed: 21 HP
    Estimated speed with existing 25.2 horsepower:
    This is the speed we will use for the propeller size. 7.28 Knots

    Propeller Size
    Number of blades Diameter (inches) Pitch (inches)
    2 Blade 23.8 X 23.6
    3 Blade 22.7 X 23.4
    4 Blade 21.3 X 22.9

    The propeller sizes shown above do not contain calculations for cavitation or blade loading.
    If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

    Sd- i am using 22 x 19 inch 4 blade props but wondered how much i could tow---??
    they run off a 30 hp essence i suppose the hydraulics act as a gear reduction...but the advantage for me is twin screws and instant reversing etc...could a kort nozzle just be a shroud over the props or are they specially designed?

  9. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    Kinda. ;-) for an efficient prop, there is a "perfect" diameter/hp that works. I studied at Southampton College, before it morphed into Solent university, and then went to work for a naval architectural firm in Alton called Keel Marine. After moving back to the States, I went back into yachts, and forgot most of my shippie past.
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