6 Responses for "When a diesel-electric submarine is charging its batteries does it get worse gas mileage?"

1. Floyd G November 30th, 2008 at 12:07 am 1

   Logic would dictate the solution would be a yes to your question.

   Any time you add a load to an electrical source, the demand for energy increases. And running both the motors and charging batteries at once would represent an increased load.
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2. Lord Sesshomaru November 30th, 2008 at 12:57 am 2

   I did not do any research to answer this but i would guess you are correct. If the sub is sitting on top of the water running on diesel power but not moving, it will get worse gas mileage when charging the batteries. When the batteries are charging this puts a load on the generator charging them. To keep up with the load created on the generator the engine will have to run faster. However this doesn't exactly apply to a hybrid car the batteries are often charged by different means.
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3. ed c November 30th, 2008 at 1:20 am 3

   Its slightly complex to answer. While the diesel is running and charging the battery banks, then yes it will indeed use more fuel.

   Running on diesel alone, it will use less fuel, as you will not be charging the battery banks.

   But underwater the craft runs solely from the batteries, which means it is not using any fuel. So you'd need to offset the fuel used to charge the batteries against the distance traveled on battery power.

   Also remember that hydrodynamics dictate that a completely submerged craft will have less drag than a craft on the surface, due to water surface tension.

   Also there are tides and currents to think about, after all a diesel charging its batteries, running with the tide/current could easily use less fuel to cover a given distance when compared to a diesel not charging its batteries, but running against the tide/current. So mpg's are less relevant in this situation.
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4. Dave G November 30th, 2008 at 1:50 am 4

   The answer is a fundamental principal of science. In any form of energy conversion, you never get something for nothing. I don't know if you could qulify the energy loss as an issue of "Diesel Mileage", but for sure you use more fuel since an engine that turns the generator that charges the battery must be fueld.
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   I'm qualified in Guppy 1A submarines.

5. Yak Rider November 30th, 2008 at 2:39 am 5

   I don't know about fuel consumption, but I can tell you this, when they are using the diesel engine, whether to run on the surface or charge batteries, the sonar detection range is massive. The sub is radiating more noise than a freight train.
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6. jbarn_1999 November 30th, 2008 at 2:52 am 6

   Most diesel subs run totally off of electric power. The diesels are running generators that in turn run electric motors. The charging power comes from these generators also. Most engines on subs and ships do not run at max power because of overtorquing of the shafts.

   This combined with the fact that diesel engines run more efficiently at higher speeds, I would have to say that they get WORSE gas mileage when NOT charging their batteries.

   At any rate, it really does not make that much difference because they rarely run on all four of their engines (assuming it is a two shaft sub). Most of the time they are running on one shaft (trail shaft mode) on one engine while on the surface (or snorkeling). This is done specifically to save fuel - but it does limit maximum speed.
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