Upping the efficiency
How wind conditions and boat design affect performance
The poor windward capability of some early generation multihulls tarnished the reputation of these craft. This bad impression is perpetuated somewhat by the lack-lustre performance of some of today’s ‘floating apartment’ cruising catamarans that emphasise amenities much more than sailing efficiency. Catamarans and trimarans, designed for good performance can sail about as close to windward as monohulls with similar design objectives.
Efficient boats
If wind and water were moving at the same speed in the same direction it would be impossible to sail at all. Exploiting well the difference in flow between wind and water makes a boat sail well. The part sticking into the air (sails, rigging and topsides) must be made to work efficiently, as must the part sticking down into the water (hull, keels, rudder).
Aerodynamic forces – good and bad
When sailing close, air blowing over a sail produces ‘lift.’ This mans that the sum of all the forces on the sail is not in the direction of the wind, but at a substantial to it. If it were not, there would be no windward sailing. It help understand what is going on by splitting the force on the sail into driving force, which is along the axis of the boat and heeling force, which is at right angles to it and tending to roll the boat over. As the diagram shows, healing force is always a lot bigger than the driving force, but, as long as the force on the sail is pulling at least a little forward we can expect to move to windward. The amount of forward force compared with heeling force will depend on the design of the sail and how well it is set. There will be an optimum setting which will give the biggest driving force. If steered to sail closer and closer to the wind we will get to the point where no matter how we set the sails we cannot get any driving force at all. This is the absolute limit of windward capability. An efficient boat can still sail effectively at 35-40 degrees off the wind.
However, the air is not only blowing over the sails. It is also blowing through the rigging and over the exposed hull and topsides. The force on these parts will be just about directly downwind. If we split these forces into heeling and ‘driving’ force we see that the ‘driving force’ is negative. This is, it subtracts from all the good work of the sails and adds to the heeling force. The vector representing the total aerodynamic forces on the boat is angled back a bit compared with the one just representing the sails alone. This means that the boat will not be able to sail as close as previously predicted. How much worse will depend on the amount of ‘windage.’ Every boat has windage, but multihulls typically have more that monohulls. Two or more hulls will usually have more windage than one with equivalent freeboard.
Moreover, multihulls designed for maximum comfort will have higher topsides than monohulls, because of the strong temptation to have standing headroom in a bridgedeck cabin. If the sole of the bridge deck is 2.5 ft above the water, the roof of the bridgedeck will be more than 8 ft above the water line. Such a cabin would mean a lot of windage in a 32 footer.
Windage becomes more of an influence at higher winds when sail is shortened. Under shortened sail or storm canvas the proportion of forces from windage will be greater, and will increase as the sail area is decreased. Consequently the windward ability of the vessel will be worse in heavy weather and could be a safety consideration in these conditions.
Lower windage multihulls
The smaller the boat, the harder it is to minimize windage since, whatever the boat size, the crew remain the same size. Providing standing headroom in the hulls of a catamaran will determine the amount of freeboard. This will be more than in an equivalent displacement monohull, which lies lower in the water. Rounded topsides reduce air drag by reducing disturbance to the airflow.
The charms of a bridgedeck saloon with standing headroom are hard resist; space to spread out with shelter and plush comforts; panoramic visibility for looking out while the self steering does the helming. The central saloon integrated the two hull cabins into one walkthrough, covered living space.
Highly rounded ‘igloo’ designs are one approach to get standing headroom in the bridgedeck saloon with minimum penalty from windage.
Trimarans, with a main central hull, do not have this particular design challenge, and superstructures are generally lower.
Efficiency below the water
Cheaper, protecting, less vulnerable and don’t obstruct internal space, but 5-10% less good to windward and add something like 30-50 cm to draft.
Besides tending to tip the boat over, the strong side forces on the sails tend to push the boat sideways. When beating to windward, the side force from the sails, trying to make the boat skitter downwind, are much higher than the driving force they produce, pushing the boat forward. Efficient keels will reduce this to a modest few degrees of leeway, visible in the angle the boat’s wake makes with the direction it is pointing.
Just like what is going on in the air, under the water the most efficient shape will generate a lot of side force and minimize drag. So like the sails, the hull will be producing a lot more side force (lift) than drag. The most efficient shape to generate ‘lift’ underwater, is much the same as above the water, that is a deep foil shape. (Though, because water density is 1000 times greater that air, the size can be a lot smaller than the sails.)
Therefore deep, fin keels are good, though much shallower keels also work, however substantially less efficiently.
As a multihull needs no ballast, lightweight centerboards and daggerboards are used for the best performance. These can be raised to reduce area at higher speeds (reducing drag) and raised completely to sail in very shallow water.
For catamarans, fin keels are a very reasonable alternative to daggerboards when it is acceptable to sacrifice some performance for improved convenience and cost. Daggerboards take up cabin space (though clever design can prevail); are more expensive and ultimately prone to collision damage.
Daggerboards can give about 5 degrees better performance to windward compared with fixed fin keels (depending a lot on design, of course). This would amount to a 9% difference in speed made good to windward.
