# Propulsion

A propeller is basically a convertor. The motor supplies it with mechanical energy. It should convert this energy into thrust. This is done by accelerating the water that enters the propeller. Force (which is the same as thrust) is defined as change of momentum per unit time. When you accelerate the water, you are changing its momentum. What do you get? Thrust! But how can the thrust be increased? Let's go back to elementary physics. Newton's second law says that force = mass x acceleration. Increase either the acceleration of the water flowing through the propeller or the rate of mass flow, and you get more thrust. The latter is easier to do, since the condition can be easily satisfied by designing a propeller with a larger diameter.

One of the most efficient propellers in current use is the ducted propeller. Therefore, we shall use this type in our submersibles. We decided to have two thrusters for up and down motion, four for horizontal motion and one for rotation of the submersible around its axis. Most research submersibles spend lots of time in vertical motion as they descend into the deep sea and ascend back to their support ships. However, our submersible is more likely to spend most of its time in horizontal motion since it will be supported by the underwater base. This means we have to focus more on forward/reverse thrusters, and that is the reason why their number is the greatest.

To provide the easiest means for maneuvering the submersible, the propellers will be controlled by the pilot using joysticks.

References:
1. Submersible Vehicle Systems Design, written by a group of authorities and edited by E. Eugene Allmendinger.
2. The online ALVIN User Manual.

Posted by Waseem Bakr on 09/25/2001