Re: Unmanned sub crosses ocean

From: Alan Browne (alan.browne_at_FreeLunchVideotron.ca)
Date: 11/22/04 

Date: Mon, 22 Nov 2004 12:38:48 -0500

Peter wrote:
> Alan Browne wrote:
>
>>> Hans-Georg Michna wrote...
>>
>>
>>>> From that point of view I find a sailboat much more fascinating,
>>>> because it uses freely available energy rather than carrying a
>>>> battery to propel itself.
>>
>>
>> A sailboat requires control of sails (via winches), rudder, etc. An
>> energy outlay similar to that required by the device described for the
>> ride it gets for free from gravity v. buoyancy.
>
>
> The difference is that you can actually extract energy from the wind
> while the gravitational field is conservative, meaning that no energy
> can be extracted by traveling in any path where the endpoints are at
> the same gravitational potential. So in going on a complete cycle of

Note that I said: "gravity v. buoyancy".

> surface to maximum depth and back to surface there can be no net energy
> extraction from the gravitational field. In a stationary ocean (i.e.
> no currents), all the energy to drive the sub must come from the
> batteries - the gravity/bouyancy and wing-like fins are just means to
> transform the battery power into forward motion. In contrast, it's not
> hard to build a small sailboat where the power to operate the sails,
> rudder, and controller is also extracted from the wind and no external
> power source needs to be provided.

I couldn't disagree more.

Moving the oil in the vehicle does no work at all related to moving up and down.
  It simply changes the density of the whole object with respect to the ocean
water causing it to sink or rise. As it sinks or rises (velocity) the wing acts
to change the direction of the vehicle. That's where the work is done: the
velocity made by buoyancy differential is partly vectored into horizontal
velocity. None of this work comes from the battery, it is not 'working' at the
time that work is being done.

If you're on the surface and you increase the density, then shut off the tranfer
motor, there's no limit to the depths you can sink to [there's no limits to the
heights you can rise]*. The two sets of work are unrelated.

[Note: the amount of work on or near the surface to move the oil will be so
little as to be unaccountable; but the deeper you go, the more work it will take
to "inflate" the bladder with oil against the ocean pressure... that being said,
it is still unrelated to the velocity extracted from the vertical motion. Yet
the same velocity can be extracted going up as going down despite wildly
different amounts of work to transfer the oil.]

The work done to move the density-change working fluid is tiny compared to the
work done to move the vehicle.

Anyway, I'm passing this on to a friend who teaches physics (University).

Cheers,
Alan.

*Some may recognize the lyrics. 

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