I think not. Torque aside, 30-40 RPM's will barely move a cargo ship from the dock, much less cross an ocean.

Compensating with prop size? That would be a really, really big prop.

Torque aside, 30-40 RPM's will barely move a cargo ship from the dock, much less cross an ocean.

Not quite so my friend........this rpm on a large vessel could equate to 6-10 knots, particularly on some of the large 9-14 inline cylinder engines where max rpm maybe from 95-120rpm and at this rpm a lot of these vessels can travel at 18-25 knots, Generally these engines are direct drive and to obtain astern movement the engine is stopped and restarted in the opposite direction of rotation.......medium sized vessels have engines running at constant speed using controllable pitched propellers for manoeuvring ahead and astern.

This is quite different for passenger liners which can cruise at 28+ knots. These are diesel eclectic and are powered by 2 or 4 Azipods, these give propellers 360⁰ rotation and with bow thrusters give the vessels a high degree of manoeuvrability.

What? "The engines that I see now run at 5,000,000 foot pounds of torque"

http://www.pwctoday.com/f29/new-large-watercraft-engine-264157.html

Wow! I need to get out more.

pretty neat huh?

For the same hp, 3,000,000 lb-ft at 30(40) rpm would equate to 1,000,000(750,000) lb-ft at 120 rpm.

The largest screw I could find is 9.1m (~30') diameter. If it is "square" the pitch would also be 9.1m (~30'). Further, if it is 70% efficient, it would then advance through the water ~6.4m (~21') per turn. At 30(40) rpm, this would come to ~6.2(~8.3) knots.

This all sounds "in the ball park"; but beware it is only a thumbnail estimate.

I don't think that was implied. Permission to contemplate it further, maybe; but not a green light to build it. This behemoth would be in the class of the Knock Nevi, and there would be many tight/shallow places she couldn't sail.

Like this from HOWSTUFFWORKS:

The World's Biggest Ship

­­No cruise ship that has ever been built can compare to the enormity of Freedom Ship. Imagine a mile-long stretch of 25-story-tall buildings in New York City; now imagine that floating on the water. If you can picture that, then you get the­ general idea of Freedom Ship's size. At 4,320 feet (1,317 meters) long, 725 feet (221 m) wide and 340 feet (103 m) tall, the ship is taller than the length of a football field and wider than two football fields put together. And not only can a ship that size float on water, but it may be navigating the world's oceans as early as 2005.

Freedom Ship will dwarf any ocean-going vessel operating today -- it will be more than four times longer than any current cruise ship. Here's a comparison of Freedom Ship to Royal Caribbean Cruise Line's Explorer of the Seas, the largest cruise ship as of December 2000:

Freedom Ship will be built on top of 520 airtight steel cells that will be bolted together to form a sturdy base. Each cell will be 80 feet (24 meters) tall, between 50 and 100 feet (15 and 30 m) wide and between 50 and 120 feet (15 and 37 m) long. These cells will be assembled to form larger units that are about 300 x 400 feet (91 x 122 m). These larger units will then be taken out to sea, where they will be put together to form the ship's nearly mile-long base. The rest of the ship will be constructed on top of this base. Norman Nixon, who developed the idea of a floating city, has said that it will take about three years to finish the ship once construction begins.

It will take a tremendous amount of engine power to push the gigantic ship through the water. The vessel will be equipped with 100 diesel engines that can generate 3,700 horsepower each. Developers project the cost of each engine to be about $1 million. That may give you an idea of how expensive the project is, although the total cost of Freedom Ship has not been released. The ship's high construction cost will be passed on to residents, who will pay up to $11 million to purchase living space on the floating city. In the next section, you'll find out what these residents will get for such a price.

And they wonder why the sea levels are rising......

shut up lynch!

Not cool guest.

Thank you for your reply,

Could a larger custom screw be developed that could leverage that power?

Yes it could, but it comes down to what is practical. The larger the wheel, the deeper the draft required and the larger the ships' transom design to accomodate it for tip clearance and water flow, larger shaft to carry the weight and torque...... it goes on and on. On cargo vessels and tankers, a major consideration is what the draft will be when the vessel is in ballast and the size of the wheel plays into that.

This site may have some useful information. http://www.globalsecurity.org/military/systems/ship/container-types.htm

Would it not be simpler to use a gear reduction to bring the speed down to the normal range (which would increase the torque by the same ratio as the speed reduction minus the efficiency loss of the gearbox.)

a larger prop requires MORE torque, not less to turn. and if you turn a prop too fast, you just cavitate and damage your prop. the speed of the prop is going to be a function of the pitch of the prop and it's diameter. small pitch and small diameter props can turn faster than a larger higher pitch prop. But they don't move enough water which is where the rubber meets the road.

There is a model of a ship in the Museum of Science and Technology here in Ottawa. The ship is called the "Turbina", and the drive shaft has several propellers mounted on it...spaced out. (Actually I believe it had three shafts, each with three propellers mounted on them.) Here is the link to the plans.

I always presumed they did this because they didn't want to run big slow propellers, but rather small fast moving props. But then, I never researched it.

Do you suppose a similar design solution could be used to deal with msandrock's problem?

Hi Yusef1.

Turbinia was as the name suggests a steam turbine powered launch. She was the brainchild of Charles Parsons and was his way of demonstrating the merits in terms of speed and performance of his reaction turbineover the then traditional triple expansion engine, at the turn of the 20th century.

Most famously he sailed her from Newcastle,leaving at breakfast time to the Spithead Review arriving early afternoon. He then proceeded to 'blast ' his way up and down the lines of moored naval vessels in order to demomstrate to 'their Lordships' what the future of naval propulsion should be! Needless to say her speed left any patrol boats well astern and ruffled the composure of some top brass. It was not long after that the stuffy naval architects had to accept thst there were benefits in using turbines althouh initially they were used as the 4th stage of expansion after the main engines (see RMS Titanic). Some time later the high speed geared versions made an appearance.

On Turbinia the prop shafts rotated at 600RPM and hence the need for multiple small diameter propellers to absorb the power and remain underneath the shallow draft hull.

She was a fascinating piece of technology and clearly showed the way forward.

Massey.

six hundred RPM! Oh my! Well, it was an answer to a problem for sure. Maybe not the OP's problem, but perhaps an adaptation.

Enclosed gear boxes can be built to give you the RPM desired.

Of course, you could just put your smaller engine in a smaller cargo ship.....

no. or yes. or maybe. do you mean could the faster engine be slowed down to 30-40 rpm and provide 3M foot pounds? or do you mean could an engine that at maximum output puts out 3M foot pounds at 30-40 rpm do the same job as one that puts out 5M foot pounds at 120 rpm?

when you formulate your question more precisely, you may find that you have answered it yourself.

Yes, I suppose I worded that wrong. I guess I meant could a 3,000,000 ft lb torque 30-40 RPM engine propell any kind of boat or smaller cargo ship.

"...any kind of boat..." Only ones large enough to carry it, for a start! And I think that calls for a ship, as opposed to a boat. Some smaller (half the length?) ships should meet the criteria.

Propellers are not the only possibility. A sidewheeler or sternwheeler with an effective wheel diameter* of about 16 feet would move forward about 50 feet per revolution (P1 x D). At 30 rpm, that's 1500 feet/minute, or 90,000 feet/hour, which translates to just under 15 knots, or roughly 17 mph.

* By effective wheel diameter I'm thinking the equivalent of the pitch diameter of a pinion in mesh with a rack; this means that the actual size will be larger. Slip between paddles and water will further reduce distance actually traveled per revolution, which would likely be countered by enlarging it further. It is likely to take some VERY large paddles to absorb that kind of power. (If there is a proper technical term for what I'm calling effective wheel dia., I don't know it - anyone who does, please chime in!)

BTW: torque is measured in pound-feet, NOT foot-pounds!!!