Centre of gravity (CG): This is the point at which all the boat’s weight can be considered to be concentrated. It depends on the weight distribution on board (equipment, crew, structure).

Centre of hull (CH): This is the point where Archimedes’ buoyancy is applied, corresponding to the centre of the submerged volume of the hull. It varies according to the trim and inclination of the boat.

Comparison between trimaran and catamaran

On a catamaran, the CG is generally located in the centre of the boat, slightly above deck or sea level depending on the design, while the CH depends on the distribution of submerged volumes between the two hulls.

The absence of deep ballast (as on a monohull) means that stability is based solely on the width and buoyancy of the hulls.

On a trimaran, the CG is generally more centred than on a catamaran, but can be slightly off-centre depending on the load and design. The CG is located in the central hull when the boat is flat, but can move sideways as the floats increase in volume when sailing.

Unlike the catamaran, the trimaran has a different dynamic stability:

In strong winds, the windward float can rise, shifting the CH towards the leeward float. The central hull plays an important role in longitudinal stability (less pitching than a catamaran).

Main differences in stability

1. Catamaran :

Purely transverse stability based on hull spacing.

Less variation in CG and CH, but risk of sudden capsize if one hull is suddenly lifted.

2. Trimaran :

Better stability transition thanks to the floats.

Less risk of instant capsize than a catamaran, as the leeward float gradually takes over.

Conclusion

The centre of gravity and the centre of the hull play a fundamental role in the stability of multihulls.

On a catamaran, stability is rougher and depends solely on the two hulls.

On a trimaran, stability is more gradual, thanks to the leeward float, which gradually increases in volume.

A trimaran has a more pronounced CH shift when sailing dynamically, offering a better distribution of forces.

In short, the two types of multihull use different principles to ensure their balance, but the trimaran offers an interesting compromise between the stability of the catamaran and the ability to better absorb variations in heel.