What are Catamarans?
Derived from the Tamil word ‘Kattumaram’ meaning ‘logs bound together’ that were actually the log-rafts, the modern catamarans features two exact demi hulls spaced across. It is often referred to as an improvement over the conventional mono hulls of same displacement in terms of stability and resistance. It features shallower draught and also has reduced hydrodynamic resistance compared to equivalent monohulls given the slenderness of the hulls. This results in reduced propulsive power. The widely separated hulls reduce both heeling and wave induced roll motions as compared to a mono-hull and also gives reduced wake.
The question arises why catamarans fare better to mono-hulls. So let us now don the hats of a naval architect and dwelve into this.
In case if you do not know about the terminologies and the stability of mono-hulls, we recommend you to read these articles. They provide an in depth understanding of the intact stability of mono-hulls. And those terms will be used right away assuming you know them.
Catamarans offer better stability than equivalent mono-hulls. How?
In comparison to the mono-hull, placing demi-hulls away from the centerline, provides an increased second moment of area and hence an increase in BM. This improves GM i.e metacentric height and hence provides an increased restoring moment arm GZ. Thereby improving the stability curve against heeling and capsizing
The resistance of a catamaran is lower than the equivalent mono-hull. How?
When it comes to resistance, the slender nature of hulls reduces the wave making component at high speeds (Froude Number, Fn > 0.4). Also the tendency to form wakes in minimized. Hence it can be said that the total resistance drops. In case of low speeds (Fn < 0.2), the wave making component although small is relatively lower than an equivalent monohull, hence reducing the resistance. Proper spacing also reduces the interference effects (cancels in some cases) between the wave systems of both the hulls.
Then the question arises why they are not used every-time?
Well, the primary reason is the challenges in maneuvering and the placing of the propulsion system. Although research has been carried out with regard to the use of catamarans for heavy tonnage ships, their usage is much preferred to smaller ones. This is due to the complex engineering involved and the size of propulsion units that could impact the size of the demi hulls. Nevertheless, they are a really good option for small tonnage vessels where speed is a major factor.