Multihulls, Catamarans and Trimerans

There is a lot of conjecture and mis information "out there" about multihulls. I was curious and this article is the result of my notes.

Benefits of multihulls

• They are comfortable. Keelboats like to sail at an angle and can heel quite a lot, while multihulls tend to stay flat. They rarely heel more than 5 or so degrees. This means you don't have to walk on a crooked floor, tie all your stuff down, or be careful how you cook. Of course some people will sail smaller cats with one hull out of the water but that is not necessary, it's a choice.
• There is less wave motion because of a wider base. Waves tend to cancel out and the boat moves less. This is more comfortable while underway and at anchor. If you are sailing with kids, pets, or folks with less balance, then it's maybe safer. The reduced motion means less sea sickness. Less motion also translates in safer conditions for crew to move around.
• In a multihull cruiser most of the living space is on the same level wich increases the comfort. Fewer stairs, better ventilation, nicer views because living quarters are not below the surface. Having 2 or more hulls also means that sleeping quarters are more private. Because of the wider deck, it's easier to carry stuff like Scuba tanks, or a tender. Surprising to me was the fact that trimerans are often sailed as performance boats and are often not as roomy and comfortable as catamarans.
• Because Multihulls derive their stability from a wide base rather than ballast, they don't need keels. Making away with the deep keels means that multihulls can sail in shallower water and many models can be beached. Being able to go in shallower water means a cruising boat can anchor in shallower water (as well as deeper water of course). More choices in anchorages.
• Doing away with the keel means a much lighter boat. This results in a more easily moved boat. Wheather it's sail driven or powered, less energy is required. A lighter boat also means it will float better even if it takes in water. Many multihulls have a great deal of buoyancy and will not sink if damaged or overturned.
• Because the stability of the boat does not depend on having wide hulls, the individual hulls can be quite narrow. Narrow hulls are faster and create less wake.
• They are FAST. Cruising Catamarans will be 25-30% faster than a similar length monohull keelboat. Some Trimerans have been clocked at double the speed of monohulls. Wind force is not used to make the boat heel, rather it all goes to the sail. Explanation of why Catamarans are faster
• Catamarans have 2 rudders and typically 2 engines. This results in improved manoeuvrability. Having 2 engines gives a level of safety and redundancy in case one fails. Easily driven hulls result in fuel efficiency.
• Multihulls can be designed to be displacement OR planing hulls.
• Because multihulls avoid the complications of having a heavy keel or even a centreboard, in some ways they are simpler to build that monohulls. There are many plans and kits geared to the home builder.

There are some disadvantages to multihulls

• Because multihulls don't heel much, wind is not spilled. If a large gust comes, a keelboat heels over, safely spills the wind, and goes on. In a catamaran the force of the wind acts directly on the sails and riggin, increasing the load tremendously. The crew must be aware of the weather and be ready to spill the wind in gusts or reef if the wind looks like it will increase. Pressure on the sails quadruples as the wind speed doubles.
• Because of the extra width of the boat it is more expensive to moor since it takes a lot more space. It is more difficult to move on land, and more expensive to store. Ramps are not always able to accommodate the extra width of multihull day sailors. All kinds of mechanisms have been devised to fold or tuck in the hulls but this increases the complexity of the boat. Multi hull Dinghies often need to be disassembled in order to move them.
• Even weight distribution is harder to achieve in a catamaran. Because the centre of gravity of a Multihull is not necessarily where the weight is placed, then it's easy to unbalance the boat by putting too much weight in one hull. Unbalanced loading affects trim and performance.
• It's harder to tack and easier to get into irons. On top of that it's trickier to get out of irons in a multihull than a monohull. There is less feedback from the controls and it takes a while to get used to the handling. They are said to be less responsive. This means that in close quarters it is sometimes difficult to maneuver a cat particularly in light wind.
• When the water is rough there can be pounding on the underside of the deck.
• Although multihulls are faster, they usually cannot sail as close to the wind as monohulls.
• When anchored a multihull will typically swing around more than a monohull.
• Although Catamarans have proved themselves very stable and cruising boats have very rarely capsized, if they do, they will not self right as will a keelboat (as long as it's not full of water). If they capsize they will float because they lack the heavy keel.
• Smaller catamarans are often raced and pushed beyond safety limits. They do capsize. If they capsize and turtle they are very hard to right.
• I don't think catamarans and trimarans are as pretty as monohulls. Proas are lovely though. This is purely personal opinion!!

How fast can they go?

On 24 November 2012 Vestas Sailrocket 2 recorded a top speed of 68.33 knots in a 25-29 knots wind, averaging a speed of 59.23knots over the 500 metre course.