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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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