[PSUBS-MAILIST] Re: [Int P&M-Subs] Propulsion

Diesel-electric has the obvious advantage that the primary engine / generator set need not be located near the stern, or even in line with the prop shaft, allowing you greater flexibility with both vessel stability and interior layout planning. The system I am most familiar with is AC - the primary generator produces three phase power which is run through a rectifier bank to produce DC power at a higher voltage (typically sqrt(2)*generator AC voltage) on a bus which is common between all motors in the system, and indeed between any additional inverters, such as might be used to supply your hotel loads (110V outlets). Motor drives (variable frequency drives) are attached to the DC bus as needed (propulsion motors, thruster motors, pump motors, windlass motor, etc.). These drives are essentially inverters that change the output AC frequency in order to drive AC motors at their most efficient speeds and torques as the application demands. DC motors would use the DC voltage from the bus through similar DC drives, but instead of varying the AC frequency these drives use pulse-width modulation (rapid switching of full voltage) to control speed at maximum torque. The battery bank(s) on board would connect directly to the DC bus through a charging circuit, so that running the genset on the surface would both charge the batteries and make propulsion power available on the bus (and the available amount of reserve propulsion power is at the discretion of the designer - if you can afford it, size your genset for your vessel's surface hull speed plus the desired time to charge your batteries, given their capacity which will be a function of your desired range and propulsion motor size). Running submerged, the DC bus is powered by the batteries directly, but this has no effect on the availability of your AC and DC devices - only the maximum available voltage as the batteries deplete. The advantage of a system such as this is the ability to use both AC and DC devices (ie propulsion motors) on the same system, by simply changing the motor drive, and also that your genset can be sized smaller than your actual propulsion power requirement, since the shortfall can be made up by the batteries, and the genset in that case is sized to charge your batteries within an acceptable amount of time.

-Sean

On Jun 2, 2011, John Scott <john@maine-scuba.net> wrote:

Thinking about the propulsion for my sub, and I had a question for you all. I am considering the pros/cons of a Diesel-Electric Drive -vs- a Direct Drive power plant. (for anyone un-clear on the differences, you can find clear diagrams and a good explanation here: http://www.fleetsubmarine.com/propulsion.html).

I had originally thought of employing a standard diesel engine and prop for surface propulsion, and two electric (separate) motors attached to the hydroplanes for when the sub is submerged. I now am beginning to feel that a diesel-electric arrangement would be easier and cheaper overall. It would eliminate the need for the two separate electric motors, and simplify the whole system by using the same single prop for all propulsion - surface or submerged.

Does anyone have an inside scoop on either of these approaches - pitfalls, gotcha's and the like?

Thanks.

________________________________
John "JB" Scott
http://maine-scuba.net/sub.htm
Email: john@maine-scuba.net
" To strive, to seek, to find, and not to yield.” -- Tennyson

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