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#1
 
 
bollard pull/ propulsion and torque in a tug/towboat 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 pointif 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 ratiopuitting out 549 ftlbs of torque at the prop/shaft..then would it be fair to say the smaller engine could outpull the larger? 

#2
 
 
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. Enjoy! 
#3
 
 
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 1020%, props less than ~48" diameter will reduce it 1020%. 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 200300 ton ship along at a good clip. The difference is the weight of the plant, 300400 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.
__________________ A vessel is nothing but a bunch of opinions and compromises held together by the faith of the builders and engineers that they did it correctly. Therefor the only thing a Naval Architect has to sell is his experiences. Last edited by jehardiman : 01262012 at 03:19 PM. Reason: xpost 
#4
 
 
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it is goodso if im reading correctlythe amount of pull in force is the shaft hp? which i can get using a hp calc since i have the torque calcs? 
#5
 
 
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i loved the old fairbanks morse enginesone i saw was an IHP of 400 and ran at 350 rpmsit was one monster of an engine and must have weighed 67 tons... 
#6
 
 
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It boils down to having the correct disk area to absorb the torque. 
#7
 
 
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Where did you study??.i was looking at westlawn and Macnaughton school..im doing Macnaughtons drafting course 
#8
 
 
here is the calcs for my setup 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 diesel..in 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
 
 
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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