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Have you ever wondered why IMOCA 60s like Hugo Boss have those strange looking outriggers?
Well those outriggers are one of the reasons that Alex Thomson was able to establish a new 24 hour record for a 60 foot sailboat of 540NM at an average speed of 22.49 knots.
Compare this to the overall monohull record held by 100 foot long Comanche of 618 NM at an average speed of 25.75 knots.
But how is it that Comanche can be more than 50% bigger but less than 10% faster?
This is where the outriggers start to come into play: IMOCA 60s have what are called rotating wing masts - the mast is actually part of the sail.
On the other hand Comanche uses a more traditional fixed mast like those found on most monohull sailboats, which uses spreaders attached to the mast to increase the leverage of the shrouds to keep the mast upright.
Because they move, rotating masts can’t use spreaders attached to the mast, so they have to find another way to get the leverage the shrouds need to keep the mast upright.
Without spreaders on the mast you have to connect the shrouds to a much wider base, and the boat on its own simply isn’t wide enough to provide enough leverage to keep the mast up. So the outriggers primary job is to effectively widen the boat to provide leverage to keep the mast up. So in one way, you can think of them as simply being spreaders attached to the boat, instead of to the mast.
But why go through all this hassle for a rotating wing mast?
It’s because rotating wing masts are far more efficient than the fixed masts for a number of reasons.
First of all, because the mast is actually part of the sail, it increases the effective sail area of the boat, and bigger sails generate more lift than smaller sails.
Second, for maximum sail efficiency you want the wind to flow evenly over both sides of the sail, and in order to do that you want to align the leading edge of the sail with the airflow. But a fixed mast gets in the way of the wind and creates turbulence over the outside of the sail, which reduces the lift. A rotating wing mast aligns with the flow of the wind so sails of the same size are more efficient than with a fixed mast.
Third, the maximum lift generated by the sail is at the point of maximum curvature of the sail. With a rotating wing mast being part of the sail, the maximum curvature of the mast-sail combo is much further forward than with a fixed mast, and much more aligned with the direction of travel of the boat, so the energy of the wind is more effective in pushing the boat forward, rather than heeling it over.
In addition to sail efficiency, by putting the spreaders on the boat instead of on the mast, you are reducing the weight of the rig aloft, and lowering the centre of gravity of the boat, which allows the boat to carry more sail in any given wind.
A rotating wing mast is like a turbocharger for a sailboat, in that it makes an already powerful engine - the sails - even more powerful, especially in reaching conditions.
Performance wise, all sailboats could benefit from rotating wing masts, however the outriggers required to create a wide enough mast base on monohulls are highly impractical for most purposes, which is why rotating wing masts are so rare on monohulls.
However most racing multihulls, like the 100 foot long trimaran IDEC Sport, also use rotating wing masts for the exact same reasons, but they are already wide enough that they don’t need outriggers, so it’s far more practical, and much less noticeable.
So the next time you see a racing multihull check to see if it has a rotating wing mast like Hugo Boss - it probably does.
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