Types of Boat Hulls

Or Planing vs Displacement vs Semi-Displacement hulls. What’s the difference?

A long time ago, in a city far away, King Hieron II of Syracuse ordered a crown of pure gold made. The king thought the crown maker had tricked him and used silver to build this crown. So, he asked Archimedes, a mathematician, to determine whether the crown was pure gold. Archimedes got some gold and silver, both equal in weight to the crown. He filled a container of water to the brim and put the gold in the water. Archimedes measured how much water was displaced. He then did the same to the silver. The silver displaced more water than the gold. So, he refilled the container with water and put the crown in. The crown displaced more water than pure gold but less than silver. So showed that the crown was not pure gold but was mixed with silver. Archimedes discovered this theory while watching the water in his bath rise. He ran outside naked, shouting, "Eureka! I've found it!" I don't think it was THAT he found.

This story led to the Archimedes Principle, the theory behind how boat hulls float. This principle states that a body at rest in a fluid is acted upon by a force pushing upward called the buoyant force, which is equal to the weight of the fluid that the body displaces. If the body is completely submerged, the volume of fluid displaced is equal to the volume of the body. If the body is only partially submerged, the volume of the fluid displaced is equal to the volume of the part of the body that is submerged.

A boat hull floats due to the hull being equal in weight to the water it displaces. The ball is too dense and so sinks.

And breathe!

In easy-to-understand English, please. A floating object must displace the amount of water equal to its weight. The force of the water is pushing up on the thing, with a force equal to its weight pushing down.

Let's say you have a block of wood about a meter square, and let's say it weighs 50kg. If you drop this lump of wood into water, it will move down until it has displaced 50 kilograms of water. That means 50kg of water pushes back up on the block and makes it float. But a small piece of lead weighing 50kg won't displace as much water. It is too dense, and so it sinks.

 There is a bit more involved such as stability and manoeuvrability, but this is the basis of how boats work.

What is a boat hull?

A boat hull is the watertight part of the boat that sits on and in the water. It can be open on the top, such as in a dinghy or rowboat, or closed. It could have many superstructures, funnels, cabins or more sitting on top. There can also be many levels above the hull. Have you seen some of these cruise liners?

 I was sailing in the Solent one night, and one came past. It was like a skyscraper! I counted 13 levels!

 Describing a hull is the easy part. But what type of hull is best for you? That depends on what you plan on doing with the boat. A hull for a barge on the river will be very different to one used for water skiing, deep-sea fishing, or a liveaboard with sails.

 To make this more straightforward, there are three basic categories of hulls, with these having sub-categories. We will be delving into the three main categories in this post—displacement, Semi-displacement and planing hulls.

 You will find sub-categories in these hulls, such as a multihull, monohull, flat, V, deep V, pontoon, and more. But that's a topic for another post.

Displacement Hull

Let's start with the displacement hull, as we still have Archimedes' principle in mind.

The name does give a bit away. The displacement hull sits in the water and uses its design in buoyancy to displace its weight and move through the water by pushing it aside.

 A displacement hull is stable in rough weather as it moves with the waves. A displacement hull is great for cruisers, such as most sailboats. Another bonus of a displacement hull is its cargo-carrying capacity. Compared with a planing hull, you can take anything, including the kitchen sink, with you. And more! Just pile it all in. There will come a time when the weight is too much, but you can pack a lot.

 A displacement hull is built for comfort rather than speed. You won't get anywhere fast in a displacement-hulled boat. As a general rule, the maximum speed of any displacement hulled vessel, commonly called the hull speed, can be worked out by this formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet. So, for example, if you have a 35-foot boat with a water line length of 28 feet, its hull speed will be around 7 knots. That's it. You can stick big engines in there and rev it to buggery; all you'll get is a lot of noise and a hefty fuel bill. A little more speed, though.

 Now, I know there will be people screaming rubbish. My boat is 15 ft and can do 10 knots. Yes, this may be true. The formula is a theory, and clever hull design can help increase the theoretical speed. But this is the theory and is a good rule of thumb.

 Imagine you're lying in the ocean. It's a glorious sunny day, and you're just floating about. Suddenly your friends come past in their boat, shouting "shark" and throw you a line. You grip this line very tightly. As the speed of the boat increases, you feel more force on your body until you eventually can no longer hold on, and after your panic ends, you see your friends laughing. You reached your hull speed, where the drag increased so much that you couldn't increase speed, even with larger engines.

So, what are the advantages of displacement hulls?

• They are incredibly stable.

• They handle well in rough waters.

• They can be heavy, which could be suitable for your use.

• They can be large.

• They can carry large amounts of cargo.

Disadvantages of displacement hulls:

• It has a maximum hull speed which may be slow.

• It can be susceptible to rolling.

• It can capsize if it has no keel.

• A keel can add considerable draught to the hull, reducing the harbours or anchorages you can visit.

So now, let's move to the opposite side of the spectrum.

Planing hulls

Whereas displacement hulls minimise the disturbance in the water. By design to make it more efficient, designers want them to be as streamlined as possible. A planning hull will maximise disruption. This hull design will push water down. When this happens, the water pushes back up against the hull. This causes the hull to rise out of the water and create a lift to ride its own bow wave.

A planning hull starts in displacement mode. Obviously. It's sitting in the water. An aircraft starts off sitting on the tarmac before it has generated lift by the air passing over its wings fast enough. As power increases, the boat speeds up. Once it has reached planing speed, it has created lift, and the bow wave it was pushing is now sitting on top of and riding.

A planing hull needs a different shape than a displacement hull. Whereas you could think of a displacement hull as similar to a barrel, a planing hull needs to have a V or Deep V at the front to cut through the water, tapering to being as flat as possible at the stern to help keep as much of the hull out of the water as possible. They may have strakes and other devices to help increase the lift generated by the hull. What does this flattening of the hull mean? That's right. Less carrying ability. Well, that's one reason. Another is weight. Planing hulls need to be kept light. And relatively small in comparison to displacement hulls.

Why is this? Behold the curse of the square law. Mass scales up by the cube size of the vessel. Double the length, and the weight increases eight times. We need to balance the weight increase with the planing surface of the hull. Weight increased eight times, but the planning force only increased four times. We don't have enough planning force to support our weight. We can't get onto the plane. This is why most planning hulls top out at around 20-24m.

So, with this weight issue, we can now see that planing hulls can't tolerate much weight. After the weight of the hull, superstructure, those big engines, internal fittings, etc., you only have a little weight left for the payload. Planing hulls will never be cargo hauliers. Another factor is that weight distribution is constant on a displacement hull with a big heavy keel. On a planing hull, the centre of effort moves aft as the hull rises onto the plane; this forces the designer to balance most of the weight near the transom. Planing hulls require constant vigilance to balance. 

So, balance is an issue even in smooth conditions. Let's add a force 7 or 8 to that and ocean waves. For a displacement hull, this would be a gentle rise and fall with the waves. Running at 30 knots, a planing hull would become airborne. So, in anything over a force 4 or 5, planing hulls will be forced to reduce speed in anything over moderate waves to stop this from happening. But now, the wide bow introduces very rough ship motions. Planing in moderate seas also introduces operator fatigue. It is tough working keeping a planning boat under control in these seas. Not good on a long voyage. So, a prudent captain of a planning hull vessel will choose to avoid going to sea in rough conditions.

So, what are the Advantages of a planning hull?

Advantages

• Fast

• Lots of speed in a small package

• Extremely manoeuvrable

Disadvantages of a planing hull.

• Low deadweight

• Weight matters

• Seakeeping/sea limits

• Slamming

• Operator fatigue

And finally!

Semi-Displacement hulls.

To summarise. A displacement hull will displace the water around it. It is good in rough weather but is slow and limited to its hull speed. A planning hull is fast and stable but can't handle anything over moderate seas at speed and can't carry much in the way of stores.

Now, imagine there was a boat that combined the best qualities of the two types of vessels. There is! And that is the semi-displacement hull shape.

Generally, planing hulls are fine and flat aft and displacement hulls are bulky and round. A semi-displacement combines these two hull shapes. The bow is wedge-shaped and gets flatter as you move aft. They look like a sailboat from the front, and from the back, they look like a powerboat. The bow has to be able to displace water and provide lift at the same time.

A semi-displacement hull displaces water at low speed but can generate some lift at cruising speed. It is more stable than planing hulls but faster than displacement hulls, and it planes at a lower rate than a planing hull. When I say the plane, it semi-planes. It can ride its own bow wave but is too heavy to plane. This is a good thing. As it's heavier than a planning hull, it is more stable and can carry more stores. And it is better in rough waters, which makes semi-displacement hulls an excellent option for trawlers or cruising motorboats.

So, there are three hull types.

 A displacement hull displaces the water around it and is slow, stable and good in rough conditions.

A planing hull rides its own bow wave, is fast but can't carry much in the way of cargo and can't handle anything over moderate seas.

A semi-planing hull, while it isn't the best at any of these characteristics, is a good hybrid of both, giving faster passages, decent load carrying and good stability.

Which hull type is right for you depends on your needs. For smooth high-speed exhilaration, a planing hull is your go to any day.

A displacement may be best for long voyages in all sea types.

Or check out a semi-displacement and see if that fits your needs.