Getting a ship to move through the water is more difficult than it looks
A European project is developing new Air Support Vessel (ASV) hull designs that allow watercraft to ride on a cushion of air to greatly reduce friction between the hull and the water, resulting in more hull speed for less power than conventional designs. The project is part of a EUR10,000,000 (approx. US$13,225,000) project funded in part by the European Union, the Norwegian Research Council and Innovation Norway, and Norwegian company Effect Ships International AS has recently completed tank-testing in Sweden of two ASV hull models.
Getting a ship to move through the water is more difficult than it looks. Though it’s very easy to move a floating object with the push of a hand that would be impossible to shift on land without a bulldozer, water itself has friction that even the smallest craft must work against. We think of water as being slippery, which anyone who has gone into a skid on a wet road can attest to, but as far as ships are concerned, it is actually quite viscous. It tends to stick to things like ship hulls and form a boundary layer that is dragged along with the ship like invisible seaweed. You can’t see it, but it drags at the surrounding water and slows the ship down. This effect isn’t noticeable on small craft going a low speeds, such as row boats, but in high-performance craft where every factor counts or in large ships were the effect is very large, it is very noticeable and very expensive in terms of speed and fuel consumption.
Also, when a conventional hull moves, it pushes the water ahead of it out of the way. We like to talk about a ship “slicing” its way through the water like a knife, but what is really happening is that the ship is pushing the water ahead of it. When the ship is going slowly, the water can easily fall off to either side, but as the ship goes faster, the water starts to pile up on itself until forms a wave. This wave swells and falls away constantly to form transverse waves along the hull that slow the craft down. Eventually, this effect becomes so large that the ship simply can’t go any faster. Putting more power behind it just makes the waves bigger without any increase in speed. The limit is calculated by the formula Hull Speed = 1.34 * (LWL)1/2 where LWL is the length of the hull at the waterline in feet.
Put simply this means that the only way to make a conventional hull go faster is to make it longer, so that, all things being equal, a 50-foot boat will outrun a 30-footer. Of course, the friction also increases with size, so the two tend to balance out. These two problems, among others, puts a real limit on the design of maritime vessels and the only way around it is to abandon conventional hulls in favor of ones that either reduce the friction or the bow wave.
Boundary layers and hovercrafts
Reducing friction on a hull means getting rid of or reducing the boundary layer. Golf balls do this in the air with their dimpled surface that sets up currents that break up the layer, allowing them to fly straighter, while dolphins do the same in the water by altering their skin to form similar depressions when they swim at speed. This isn’t practical for a steel-hulled ship (though some companies have tried using ridged rubber mats for the same effect), so Mitsubishi has developed a system for breaking up the boundary layer by blowing air bubbles under a ship’s hull. This is a sort of air lubricant that makes the hull more slippery in water, but that’s different from what Effects Ships International (ESI) is doing. Mitsubishi is trying to mitigate the problems of traveling through water. ESI is trying to avoid them altogether.
Read more . . .
Bookmark this page for “Air Supported Vessel” and check back regularly as these articles update on a very frequent basis. The view is set to “news”. Try clicking on “video” and “2” for more articles.