The secrets of a catamaran’s performance: the hull’s shape
|26 April 2018||Posted by Alex from Lagoon under Technical world|
After a first article about the Length on water line, we continue the series of technical articles allowing you to compare the catamarans performances. Our new article deals with the incidence of the hull’s shape on the performance of a boat.
Before anything, it is important to understand that 3 types of resistances are existing:
The resistance dues to the waves
The resistance dues to the frictions
The aerodynamic resistance (which does not concern the hull)
We will demonstrate that the hull’s performance is closely related to its capacity to minimize these different resistances.
How does a hull work?
The hull is the submersible part of the boat.
Several elements set the hull’s shape:
- Underwater area (or wetted area): this is the area constantly under the water. We are looking for the smaller underwater area to reduce the friction, which is one of the two hydrodynamic resistances. The second obstacle is the resistance due to the waves: starting slowly, a hull creates several waves along itself. When her speed is important and close to the hull’s speed ( last article), only one wave is created at the bow, the wave digs along the hull and goes up at the stern. Only one part of the energy spent to create this wave, will be regain for the stern’s wave. The bow pushes, the stern regains, but it is not possible to regain everything, and more the progressive speed’s increase is important, less it is possible to regain, until to reach the hull’s speed.
- Underwater volume: is the volume of the boat under the waterline. More the volume is important, more the displacement of water is important too and more the boat will be slow down.
- The Depth: represents the deepest point of the underwater part of the hull. An important depth, with equal displacement (weight), indicates that the volume is centered and that the prismatic coefficient is small. On the contrary, when the depth is spread out along the hull, it means that the volume is regularly spread on the entire hull. For a multihull, the best is to spread out the volume along the hull, because it is reducing the pitch.
- The midship section (midship beam): represents the maximal width of the hull (or of the complete boat if it is a multihull). The midship section at the waterline (or beam at the waterline: BWL) indicates the hull’s maximum width at the waterline: this is the Block coefficient of the hulls and so their ability to split the water by creating the less possible disturbances. Depending to the type of hull, it could be interesting to reduce the BWL, if we are not looking to glide. On the contrary, it could be interesting to increase the BWL to reduce the depth and make the hull glides.
In the case of a displacement hull only and on an equal displacement:
Hull A: The entry line is narrow and the water lines’ angle is small, so there is not much running resistance.
Hull B: The entry line is narrow but the water lines’ angle is becoming wider (larger hull) which is making us lose the hydrodynamism of the boat.
The shape of the hull combines all these elements and set up one part of her performances. You are now a hull’s expert! Do not hesitate to enjoy the boat shows and especially indoor events to compare the hulls. In the next article, we will introduce you to the different types of hulls.