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Submarine Propulsion Placement
Name: Rachie
Status: Student
Grade: 9-12
Location: CA
Country: United States
Date: August 2006
Question:
In an underwater submarine is it easier to control
the sub by pulling it or pushing it? All of the subs I see are
pushing the sub with the propeller, but it seems like it would be
easier to control it by pulling it with the propeller in the front of
it.
Replies:
Hi Rachie,
Your question sounds a little like the theory 80 years ago
that proposed that rockets would work better if the rocket
motor were placed in front of the rocket's nose, as Robert
Goddard's first liquid fuel rocket was.
In fact, a propeller at the front of a submarine would be
very inefficient indeed. The high velocity backwash from the
propeller would impinge directly in the front of the
submarine. This would result in several things. The high
speed "jet" of water that the propeller creates behind it
would encounter drag from flowing over the hull, resulting in
a net force on the hull backward, slowing the submarine down.
This propeller backwash will be deflected sideways to some
degree as it tries to flow over the hull, creating energy-
sapping turbulence. Another problem is that modern submarines
go to extreme lengths to be silent. The high speed flow of
water from the propeller against the hull and the resulting
turbulence would result noise that would be easily detectable
by the enemy.
Maneuverability of the submarine would also suffer because
the action of the rudder to try to swing the rear out to
change direction, would simply result in the rear being
pulled back into line again, making turns extremely
difficult.
Then there is the obvious question: how can it fire torpedos
if a propeller is in front of the torpedo tubes? Could get a
bit messy!
So you can see from this that there is good reason why for
centuries, all power boats and submarines have always been
driven from the rear.
Regards,
Bob Wilson.
Rachie,
Great point -- only one problem. Most naval submarines do not have
windows to see where they are going. So they rely on SONAR devices
to actively and passively get sound waves back to "see" what is in
front of them. SONAR does not work well through the noise of a
propeller -- so naval submarines cannot "hear" what is directly
behind them. And most folks figure it would be better to locate the
SONAR devices up front (and the propeller aft) so that you have the
best chance of detecting something before you run into it.
If you could design a SONAR device that can work through the noise
of the propeller, it may make a lot of sense to put the propeller up front.
Regards,
Todd Clark
Hi, Rachie.
It indeed may be easier to control that way. There are model
submarine (and human powered submarine competitions) at some
colleges, and I'd encourage you to find some and ask there,
too. Maybe you could build some models to experiment with.
I believe it is usually in a pusher configuration for other
reasons. Efficiency is the main one. Water has much more drag than
air, and the blunt, rounded shape of a sub's nose is the most
efficient at moving through the water. Then the aft end is tapered
gradually, to come back together at the aft end of the sub. The
idea is make only small, gradual changes to the direction of flow of
the water to minimize drag. If the direction or speed changes
abruptly the water can cavitate (form vapor bubbles). These bubbles
make noise when they collapse, which means other subs can hear
you. The water in the pusher arrangement is moving over the surface
of the sub at the same speed as the sub. When the propeller speeds
it up to push the sub along, it does not have anything behind it to
force it to move.
If you put a propeller in front, now the water it speeds up is
moving across the surface of the sub at a higher speed, making it
more sensitive to cavitation, which can happen if there are
protuberances on the outside of the sub, or in some maneuvers, for
example. You could get around this by making it a bigger, slower
propeller, but you can only fire torpedoes if the sub is bigger
around than the propeller. A smaller propeller also means it has to
turn faster, again increasing the risk of cavitation. You also take
an efficiency hit, because the water, which is now moving very fast,
immediately has to change direction and go around the sub.
Another reason is that the propeller would be more prone to damage in front.
David Brandt
Rachie,
If the motor and propeller are firmly mounted to the hull of the
submarine, then whether you are pushing or pulling does not really
matter. However, because of the chopping action of the propeller
blades against the water, all of the water behind the propeller is
very turbulent. If the propeller were at the front of the sub, then
the rest of the sub would have to pass through this turbulent area,
which would be very uncomfortable for those inside as the sub would
be rocked about and pounded on by the turbulent water.
Ryan Belscamper
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