Re: 'Hot Body' Could Help Ships Reduce Drag

In article <0bisu6tv0vrhg3p2s03g65shtojgan6sji@xxxxxxx>,
fjmccall@xxxxxxxxx says...

Mark Borgerson <mborgerson@xxxxxxxxxxx> wrote:

In article <kd8ru6h66canbqnmjuk5a41ltoda9oodv5@xxxxxxx>,
fjmccall@xxxxxxxxx says...

Mark Borgerson <mborgerson@xxxxxxxxxxx> wrote:

In article <o96pu6dlfj8muqs2ev1iq9ordph0vi0eiu@xxxxxxx>,
fjmccall@xxxxxxxxx says...

Mark Borgerson <mborgerson@xxxxxxxxxxx> wrote:

In article <3fjou6530ksipsqs73csobus6dsjm8c1jn@xxxxxxx>,
fjmccall@xxxxxxxxx says...

Mark Borgerson <mborgerson@xxxxxxxxxxx> wrote:

In article <rb3ou6l40949jr4pi7vul2cd5e2mq1fdp5@xxxxxxx>,
fjmccall@xxxxxxxxx says...

I really question doing this with a ship, since wouldn't it lose
buoyancy as well as drag in the flashing of water to steam?

I think the film of steam will have about the same pressure on the
hull as the water outside the steam film. The hull still displaces
as much--or perhaps more water. In a reductio-ad-absurdum argument,
would a ship sink if you painted the hull with a few tenths of a
millimeter of paint that is much less dense than water?


The steam is going to tend to escape at the waterline, which means you
sink, a few millimeters at a time.

Neither of those conclusions seems obvious to me. It may well be that
the steam will recondense by contact with the surrounding water---as
often happens in a pan of water brought to the boil. The steam doesn't
get to the surface until all the water is near the boiling point.

Why would the ship sink any more than it had been raised by the extra
water displaced by the steam? The steam transmits the pressure of
the water as well as the water itself---and it is that pressure that
floats the ship.


The pressure holding the steam in place against the heat of the very
hot hull is the water pressure. As you come up the side of the ship
the water pressure decreases and the steam will tend to 'flow' upward.
At the surface there is practically no pressure. Why do you think
WATER will flow up around the hull and steam will not? That seems
very counter-intuitive, so I assume you have some principle in mind
when you believe that it will not do so.

Whether the steam flows up along the side of the hull or is condensed
as it undergoes turbulent mixing with the water does not change the
pressure on the bottom of the hull or otherwise a function of
depth below the surface.. Thus, there is no loss of
buouyancy.


Of course it does.

No there isn't.


Of course it does.




At the water surface it is not a situation of "practically no
pressure" it is a situation of 1 atmosphere of pressure.


And big steel boats won't float in air in the general case.

This is not a general case. Whether the fluid surrounding the
hull is water or water separated from the hull by a thin film
of steam, the pressure differential from the water surface
to the bottom of the hull is the same. That film of steam is
a fluid that conducts the pressure of the surrounding water
without significant change.


You're ignoring the fact that the steam is less dense and is going to
tend to escape upward along the sides of the hull. If the steam was
ONLY under a (flat bottom) hull, your analysis would be correct.
However, that's not what we're talking about here.


You would only see a loss of buoyancy if the average density
of the fluid surrounding the hull was significantly lower,
leading to a significant change in the pressure as a function
of depth. That isn't going to happen with a thin film of
steam. Adding a thin film of steam around the hull does
not significantly change the volume of water displaced, and
Archimede's law has not been repealed.


What keeps the steam where it is?
If it is a very thin film, it condenses on contact with
the water---and more has to be generated to replace it. There
won't be large steam bubbles until you have heated the water
around the ship nearly to the boiling point.
However, thick the film is, it still conducts the surrounding
water pressure to the hull---and doesn't change the buoyant
forces.

If the ship is moving (after all, the goal is to reduce drag---
which isn't a problem at the pier), you will never have
the problem with rising bubbles---any steam bubbles will
be moving aft much faster than they are rising.

You can probably think of the steam film as a thin film of
gas that has to be continuously regenerated by heat
from the hull.



Where did I say that WATER will flow upward around the hull?
That seems to be your statement---not mine.


Nobody said YOU said it. It is, however, what the stuff does.

If water is flowing upward around the hull, then the hull should
float a bit higher---as a result of the drag against the hull
presenting an upward force.


the difference being that water tends to stay where it is, what with
being heavier than air and all, while steam does not.

True. However, steam bubbles won't last long enough to move very far in
seawater at 20C.


I tried a simple experiment: I heated a 100W soldering gun to about
230C(soldering temperture), then stuck the tip about 1cm in to room
temperature water. No rising steam bubbles--just a few around the
tip which quickly cooled below 100C. If that tip had 1cm sq of area
and can't maintain a steam film with 100W input, think of the
energy requirements for a ship with ~10^7 sq cm of hull area in
contact with the water. If I haven't slipped a decimal point,
that means distributing a gigaWatt of heat energy around the
hull.


If you want to reduce hull friction with a gas film, it would
probably be more efficient to use a version of the masker air
bubble system You have the advantage that the bubbles persist
longer due to the low solubility of nitrogen. (Although the
surface tension inside small bubbles does push gas into solution
and the water can become supersaturated at the bubble surface.

I wonder if the ships running the masker system noted any
significant loss in buoyancy. Did the stern settle significantly
because the aft half of the hull was moving over a film of
bubbles?


Mark Borgerson



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