How much HP to push a barge??

I'd go for torque... I reckon you need Clydesdale's not Show Ponies JMO.

 

I can supply an alternative forumula that never fails.

Current bank balance - [(Width x length x height of the barge) x cost of barge per cubic feet] - X HP = 0

 

There seems to be some confusion with respect to torque versus horsepower. The barge in question will no doubt use an engine to cause propulsion. So let us speak of engine output. Horsepower is a calculated quantity, it is not directly measureable on the testing device such as a dynomometer.

Imagine that you have a brake mechanism such as a caliper, and a disk firml y mounted on the output shaft of the engine. A lever is attached to the brake caliper. When the brake is applied the caliper will tend to rotate and the lever can restrain the rotation. Attach a scale to the end of the lever so that we can read the thrust of the lever in pounds. ( sorry about the imperial measure for all you sensible types who use the metric system) The engine is turning some RPM. Let us say 1000 RPM. The lever is five (5) feet long. The scale reads 100 pounds and the engine cannot stand more brake pressure without losing some rotational speed. Now we have 5 feet multiplyed by 100 pounds and we have 500 foot pounds of torque. Horsepower is calculated by multiplying torque times RPM and dividing by the arbitrary number 5250 SO...(500 x 1000)/5250 = about 95 HP You can see that HP is linked to RPM. The HP thing gets a bit spooky as revs are varied.

A 350 cubic inch V8 (Chevrolet, Ford, Dodge, etc) that has been tinkered with just a bit will make about 400 foot pounds of torque at around 5250 RPM. Do the arithmetic and you will see that the V8 is making 400 HP. When a HP curve is plotted on the same sheet that plots the torque curve, the lines will intersect at 5250 RPM. The arithmetic demands that it do so. Horsepower is related to torque but the two quantities are far from the same.

Way back in the day, before engines, there was talk of horsepower. "My horse is stronger than yours" ran the arguments. Many wagers were thus made. Somewhere along the way the old fellows needed to strike an agreement on what exactly constituted a horsepower. It came to pass by some agreement or other that any machinery, gearing, or tackle rigs could be used to permit the following: One horsepower is when 33,000 pounds can be lifted one foot in one minute. I leave it to your imagination to conjure up whatever rigging that might have been used for such a test. Sure enough a Clydesdale could lift that weight more readily than a race horse. Clydesdales probably could muster more than one horsepower. The old guys did not speak of torque.

It is enough to say that an engine that produces a certain horsepower at a low RPM is more torquey than one who produces the same HP at a higher RPM. Indeed a John Deere may have more torque than a Porsche, but the Porsche may have more ultimate HP than the JD. You would not figure to plow with the Porsche but you would not figure to road race with the JD. It's all a matter of arithmetic. To push the barge I would opt for an engine of large displacement that turned a very modest RPM and was thus capable of twisting a really big prop. You might get the same thrust from a smaller engine that turned a lot of revs. The low speed engine is the choice because it will probably last longer.

 

Thanks for that very well written and thought out post. I agree that a diesel engine would be the way to go.

 

Yes, HP is HP, doesn't matter if the engine is large and slow or small and fast - both can turn the same prop and get work done equally well (with the right gears). But it makes a difference for other issues such as weight, longevity, power band, fuel economy, etc.

Horses are perhaps a confusing example - no torque involved and hard to use mechanical advantage for most of the things they are used for.