The Secrets of a Catamaran’s Performance: The Different Types of Hulls

23 July 2018 | | Reading time 4 minutes

In the last article, you have learned the key elements to define a hull. Our new article will deal with the different hull classification.

The displacement hull:

Cargo Photo © CMA-CGM

Comme son nom l‘indique, ce type de coque déplace en permanence 100 % du volume et de la surface immergée, quelle que soit sa vitesse.

This hull displaces (as its name implies) permanently 100% of the volume and of the underwater surface, never mind the speed of the boat.

The length/width ratio stays the same when the boat is sailing. This kind of boats can support a heavy load so consequently, the water displacement is proportional and the underwater volume is important. We find this shape of hull on the cargo ship, the ship tank, etc. but also on the cruise liner. The shape of their midship section is rectangular, so the underwater area is large. Regarding the entry line, they are wide because these ships try to minimize the draft (the depth). They need to balance the load near the bow and the stern.

The semi-displacement hull:

©Romain Claris This 42 under medium winds, is in situation of semi-displacement, her hull is lifting off from the sea.

A part of the underwater area and volume can get out of the water thanks to the speed.

The speed allows lifting off from the water, which is comfortable for sailing, even in strong seas. The sustentation effect reduces the underwater surface and so the friction. In this situation, the boat can overtake the hull’s speed (cf article: length of Water line). With the same habitable volume, which is important for the cruising catamaran, this sustentation effect releases one part of the static underwater volume, procuring a less important submarine volume, which is ideal for the performance. The bow shape and the water entry lines are designed to participate at the general sustentation of the boat. This hull is perfect to combine performance and habitability.

Narrow hull:

Extreme 45 fleet photo Xaume Olleros

The main characteristic of the Narrow hull is that she leaves almost no wake behind her, because she has a tiny bow wave. The only resistance comes from the friction (designers try to reduce the underwater surface thanks to the hull shape with circular arc sections, and thanks to the chosen material, always seeking to get the boat lighter) and the aerodynamic resistance: the boat is going fast enough to consider this important resistance. This hull has a small underwater volume and a minimized depth, thanks to a light weight and to a good balance of the volumes along the hull.

The planing hull:

A planing racing 60 feet IMOCA boat with downwind, Photo ©François Van Mallenghem

A variable, but important, part of the underwater surface and volume is regularly outside the water when the boat is overtaking a certain speed. The planning hull is surfing on the water thanks to her hull and speed. In reality, we can consider that the boat is surfing her own bow wave. This planing’s feeling can be experienced on a motor yacht, when the boat rears because of the bow wave, and then succeed in takeover it and surf it. This has for consequences to reduce the friction area and allows the planning boat to be fast. The water entry lines are generally narrow but it is not really important because they are out of the water. The negative point of these hulls is their instability.

Nowadays, the performance race leads to get completely rid of the hull thanks to foils, which make the boat “flies”. Once in flying position, the underwater surface gets spectacularly smaller, because only one part of the foils gets into the water. If this kind of sailing is the best to win speed contests, it is not the solution for the cruising boats!

Photo ©Holland Composite