Comfort in multihulls
The multihull ride
It is a formidable challenge to build a boat that can bear us along gracefully as well as sustain us with a full compliment of essential facilities and creature comforts, considering the operating environment of sun, waves, wind and remoteness. There are limits to how good the ride can be. However, though originally exploited for their speed, the last 50 years of development have proved that multihulls have the edge in tending to our sensitive side.
Level sailing
The wide beam and low displacement of multihulls makes their movement in a seaway quite different from the narrower beam and heavier displacement of most monohulls. For instance, multihulls do not roll as much. Sea conditions aside, multihulled boats heel only a little with the wind, prompting some multi aficionados to call monohulls 'leaners'.
On a catamaran, the stabilising force, counteracting the heeling force due to the wind blowing over the sails, reaches a maximum at around 7 degrees of tilt, when the windward hull is on the point of lifting out of the water. For a trimaran, this happens at about 15 degrees, when the lee float is on the point of being submerged, or when the main hull is about to lift out. Compare this with a monohull which has its greatest righting force at around 60 degrees. Monohulls, therefore, need to heel at much greater angles than multis to balance the forces on the sails. Under the same conditions, a monohull might be sailing with 25 degrees of heel, a trimaran with 10 and a catamaran with 4.
Being able to sail more on a level is a key advantage of multihulls, though some monohull sailors find flat sailing lacks a certain 'something'. Still, the ungimballed stoves and lack of fiddles on tables and work surfaces in multihulls are evidence of more comfortable living conditions. There is a well-worn story of the skipper of a catamaran on a windy ocean crossing who went on watch leaving a glass of wine on the saloon table. He returned from his watch to find not a drop spilled. A nice illustration of a multis virtues, though perhaps it is more important to note that crew tire quicker, and find it harder and harder to do tasks as heeling angle increases. Above 15 degrees of heel their efficiency is already severely impaired - level sailing is safer as well as more comfortable.
Movement of a different kind
When there is wave action, multihulls respond differently from monohulls. Because of the high buoyancy concentrated at the beam ends, multis are very 'stiff' in the rolling direction, riding the waves like raft. Their reaction is short and quick and they do not have the exaggerated wallowing roll that can be very unpleasant in a monohull, particularly when running downwind, or at anchor in a swell. The livelier motion of multihulls can nevertheless still cause seasickness, and some mono sailors find the multihull motion initially more unpleasant than the one to which they have become accustomed.
The other movements likely to cause much discomfort are heave and pitch from following or head seas. Heave is the vertical acceleration of the whole boat, and pitch is the rotation of the bow up (or down) and the stern in the opposite direction. Both pitch and heave usually happen together. When beating into waves, pitching and heaving may be accompanied by slamming or pounding, which describes what happens when the boat crests a short, steep wave and makes a violent contact with the next one.
Some early catamaran designs pitched badly, their hull forms encouraging the nose up and down rocking called 'hobbyhorsing'. Too much weight in the ends of the boat, too little reserve buoyancy in the bow and stern, Vee sectioned hulls aft, and too much hull rocker all combined to magnify this vice. Designers have since learned to draw catamarans that have an easier motion to windward, as comfortable as any sailing vessel.
Slamming, particularly from waves hitting the underside of the bridgedeck, remains an enduring problem of catamarans. Although this problem can be designed out by having sufficiently high bridgedeck clearance above the waterline, for example, doing so has its costs. It is difficult to get standing headroom in the bridgedeck saloon, especially on a small boat, if the deck is high and the cabin roof is to be kept low to avoid a disproportionate amount of windage. Cats not designed for ocean sailing might have insufficient clearance to avoid slamming in unfavourable conditions.
