Vessel Hull Types

There are different vessel hull types for different uses. The efficiency of one may or may not cross over into the other. Some hull design benefits are obvious, but their drawbacks may not be entirely understood by the average wet boater. Keeping these facts in mind, I will just try to keep the pros and cons of the various hull design types simple. I’ll start with the most basic one first.

Hull or Monohull

The displacement hull or monohull is simple in its form and function. Its name says it all. The hull sits in the water and pushes the water away. The hull thus displaces the water. The water doesn’t take this displacement attempt lightly. It pushes back on the hull causing buoyancy and resistance. The good thing about this type of hull is that it’s simple to design and build. That means that it is relatively inexpensive to build as well. It also affords the most space to utilize within the hull for compartments, equipment, cargo, and people. With enough compartmentalization, this hull is the most buoyant and hardest to sink. The drawbacks are simple to understand too. If you’re displacing so much water, it’s very inefficient to propel it through the water. The hull also has a maximum speed no matter how much power you put in it. That’s called the hull speed.

The tumble home hull is a type of displacement hull that you might commonly find on a simple canoe. It was employed in large wooden warships of old to deflect cannonballs, because the hull is angled inward toward the main deck just as soon as it leaves the water. This ancient deflection technology has been resurrected once again for use in large modern warships. This time it's to deflect radar waves instead of cannonballs.

Planing Hull

 

The planing hull is what you might choose to call the opposite of the displacement hull. The planning hull is like a flat plane or flat hydroplaning hull. It sits on top of the water and hydroplanes or skips along like a flat stone skipping over a calm lake. It solves the inefficiency problem of trying to move all that water aside, but there are some drawbacks. The buoyancy is much less than a displacement hull, so you can’t put too much weight on it. It doesn’t have as much physical inner hull space either. It may be fast, but it can’t go nearly as far as a displacement hull.

Semi-Planing Hull

Now, if we combine a displacement hull with a planning hull, we get the best of both worlds. That hull is called a semi planing hull. These hulls are perfect for patrol boats. They can carry enough fuel and supplies for long range missions, yet can get up and go on a fast plane when more power is added. The bow or front of the hull is most commonly shaped like a displacement hull, yet the after part or rear of the hull is shaped more flat like a hydroplane. It’s a kind of magic hull that displaces water at slow speed and skips on the surface at high speeds. There’s another type of hull that does that magic trick.

Hydrofoil

The hydrofoil hull is a displacement hull at slow speeds just like the semi planing hull is. It’s also fast at high speeds just like the semi planing hull is. The difference is that the semi planing hull’s rear flat portion skips along the surface of the water. That results in a certain amount drag. The hydrofoil hull is completely lifted out of the water. This eliminates all hull to water drag. How can you lift a hull completely out of the water without using a crane? You use a type of underwater wing called a hydroplane. The hull is equipped with these fixed or deployable hydroplane fins. They lift the displacement hull up just like an airplane wing lifts a fuselage up in the air. What are the down sides to this great idea? They are mechanically more complex and therefore more expensive and subject to higher maintenance. Due to these reasons, they are most commonly found on smaller vessels. The advantage of small vessels is that they can get into shallow waters. Hydrofoils tend to have at least some of their hydroplanes fixed down. This limits their shallow water advantage. Many hydroplanes become easily damaged and bent unexpectedly running aground. There’s also an unexpected vibration issue that causes health problems to long term crew members. In all fairness to their skippers, if you hit even the least benign object either on the surface or just below the surface at high speeds, you’re probably going to bend a hydroplane.

Catamaran

A much less complex and less expensive way of lifting a hull mostly out of the water is to use two narrow displacement hulls instead of trouble prone hydroplanes. Yes, the two hulls have the inefficiency of displacing water, but if the hulls are long and narrow you eliminate most of that problem. Now you connect the two hulls together by a bridge. Then you build most of the rest of the vessel on top of that bridge. The result is a fast and stabile hull design know as a catamaran. If you have a lot of superstructure to build with a lot of weight, you might want to add another long narrow monohull. Now you have a vessel known as a trimaran. Both a catamaran and a trimaran have some drawbacks though. Their pontoon type of long and narrow hulls tend to be very cramped in space. They flood quickly when breached. If this happens in the case of a catamaran, you can lose half of your buoyancy instantly. Fuel storage capacity and range is limited for power catamarans. Even though their long twin monohulls are constructed as efficiently as possible, they still have a maximum hull speed. If you could only lift a catamaran a little higher out of the water without having to use those damage prone hydroplanes, then you could get an even faster and more fuel efficient catamaran.

M Hull

One way to lift a catamaran a little higher out of the water is to capture some air under the bridge spanning the two narrow monohulls. Now you’ve got a semi air cushion lift hull. This hull is called an M hull. They call it that because the underside of the bridge as well as the inside portions of the monohulls form a sort of M shape. This unique shape captures the air at high speeds and directs it in such a way as to help lift the hull out of the water. It’s good to get less drag, but an M shape hull has to be more precise in its design and construction. It also has all of the same limitation as the catamaran hull with a little less drag. The vessel has to travel fast for all of that air cushion assisting to work. This hull is what you might call a limited surface effect ship or SES. A true SES hull sits completely on a cushion of air without any part of its hull on or in the water. Those are known as hovercrafts.

Hovercraft

Hovercrafts sit on a cushion air that they create through powered ducted air. That means that they don’t have to move forward quickly to capture air like an M hull does. Hovercrafts can hover at a standstill. They can also hover over many flat obstacles like ice and smooth land. This makes them very versatile. They also happen to be highly complex and expensive to build and maintain. They are usually always loud and kick up a lot of spray. They also can’t handle rough seas. That task is best left to the SWATHs.

SWATH

SWATHs are Small Water Area Twin Hull vessels. Yes, they are catamarans, but with a twist. The twist is that instead of the twin catamaran type hulls floating on top of the water, they float under the water. How can something float under the water? Simple, just add more weigh to shove the pontoons under the water. Now extend the pontoons down deep so that the bridge connecting the pontoons stays high enough out of the water. The result of this design is that the whole vessel is actually floating on small pontoons under the water. That means that to a large degree it doesn’t matter what the surface waves are doing. The ship remains very stable despite the rough surface waves. I was around when these SWATHs first came out. I was also around when many of them went right back to unscheduled dry docks to reinforce their long deep pontoons and repair all of their stress fractures. They have all of the catamaran drawbacks with the addition of these high stresses. They’re also more expensive to construct.

WIG

Is it a boat or is it an airplane? It's really a little of both that's called a Wing in Ground effect or WIG. It's more related to an M hull in it's concept of the physics going on here, but it's 100% out of the water like a hovercraft. This unique surface effect craft deserves an honorable mention, because eventually it has to come down and get wet! Maybe you can design a unique wet boat too.

By Captain Marc Deglinnocenti

OldArmada@Gmail.com