Wave powered propulsion for ships

Wave Powered Propulsion for Ships
Fish, marine mammals, and humans wearing flippers all propel themselves through the water the same way. That way is to use a semi flexible fin or tail that is driven sideways relative to the direction of motion of the user. The motion of the fin causes it to bend and the force exerted on the water by the leading side of the fin pushes the water backwards as well as sideways. When the fin is driven in the opposite direction, it bends the other way and once again there is a backward as well as a sideways force pushing the water. As the fin is driven from side to side, the sideways forces balance each other out, but the backwards forces add and the result of driving the water backwards is that the fish or whatever is driving the fin from side to side is propelled forwards.
Now, it’s a funny thing; it doesn’t matter if the fish is pushing the fin or if the water is pushing the fin. Think about it: if a dead fish somehow had the water behind its fin moving left and right (I will leave it to your imagination exactly how this might come about), the result would be the same. The fin would still bend, and the elasticity of the fin will cause it to try and straighten, which means that the elastic force is pushing back against the water. Again, the side to side forces balance out, but the backward forces add. You see?
So let’s consider the case of a fin that is mounted on the back of a boat but is well under the surface of the water. We will picture a horizontal fin that is strongly connected to the structure of the boat. If the boat is in water that has waves, the waves will exert a force on the fin that causes it to bend. Sometimes the fin will be bent down and sometimes up. As the waves bend the fin, the force bending it is also partly directed towards the front of the boat. If the fin is semi flexible (or springy), it will try to bend back again and so will produce a force on the water that is partly pushing the water away from the boat towards the back. Again, this produces a force towards the front of the boat. Of course, it also pushes up or down, but, again, those backwards forces are the only ones that aren’t balanced out. So the result is that the boat is pushed forward as the water is pushed back.
Now imagine that similar fins are placed on both sides of the boat. They are all horizontal and all connected to the boat, but the ones on the sides are actually connected to vertical poles that are sticking straight down into the water but a few feet away from the sides of the boat. The poles are connected to the boat so that they move along with the boat while remaining straight up and down. The fins are connected to the poles and are facing the back of the boat. These fins are also well beneath the surface of the water and the front of each fin is connected to the pole so that it is horizontal and the front part of the fin is connected so that it stays horizontal even when a wave bends the back part of the fin up or down. Each of these fins now act just like the one connected to the back of the boat that we described in the last paragraph. Each of them produces a force directed towards the front of the boat. A boat can have several of these poles on each side, and each pole can have several fins mounted on it.
The following is a description of a possible commercial use of this basic principal.
Ships travelling through waters with significant wave activity experience a strong Up/Down component of water motion relative to the sides of the ship due directly to the waves and also to the motion of the ship as it rocks and pitches. This motion may be used to provide thrust to propel the ship either alone or by assisting the ship’s engine. There is considerable potential energy in waves and some of this energy can be used to propel ships.
The propelling force is generated by connecting flat plates horizontally below the water level so that the connected edge of the plates face the front of the ship and are connected to a vertical shaft that is parallel to the side of the ship but removed from it so that the plate can move up and down. The plates are connected to the shafts by strong springs that hold the plates horizontal but permit them to move up or down when strong wave action pushes them. When the rear parts of the plates are pushed up or down, the springs will exert a force on the plates attempting to restore them to a horizontal position. A part of that force will be directed towards the front of the ship. The fraction of the total force on the plate that is directed toward the front of the ship is given by the sine of the angle that the plate makes with the horizontal. There will always be a forward force even for a small deflection of the plate, for example if the engine is also being used, the plate could still increase the speed of the ship.
One way of configuring this system would be to have strong horizontal poles extending from the side of the ship. At the outboard end of each pole there would be another pole that extends down vertically several feet into the water and is rigidly fixed to the first pole. On this pole are mounted several plates with springs. The poles that extend horizontally from the side of the ship are held by bearings that make them free to rotate, but limit their motion, so that the vertical poles at the end can tip towards the back of the ship but not forward. This permits the system to be pulled up out of the water when it is not needed and also protects it from damage if it is hit by large floating debris. In this case the vertical poles would simply move back and rotate up out of the way. Obviously, there would be many such poles on both sides of the ship.
This system would not drive a ship as fast as would a powerful engine, but it does not use any fuel. As Fossil fuels become more scarce and costly, the wave propulsion system becomes more economical, particularly for cargos that do not have a large time value such as bulk commodities. If all bulk commodity shipments were transported in this way, the total number of ships used would have to increase because of the greater transit time. However, there is currently a large surplus of unused ships which could be retrofitted with this system and put back into service.
By Bill Isecke,
July 14 2011