What is the purpose of the mast being able to twist?

Just wondering.....

Its basically just another sail control.

For instance. With my boat in really light air, in order to achieve proper sail shape I have to use a ton of downhaul to get the top of the mast to bend off and to keep the leach at the top from hooking in too far. Without that flexability I would not be able to make that adjustment. Kinda works the same way with high winds too, but now there is enough to actually push the top of the sail off so its not making much power, and also in the worst spot.

Sail shape... The foil shape of the mast leads into the shape of the sail when rotated. Less windage and better sail shape.

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Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

when the mast turns it points into the wind and creates a basic aerofoil shape (due to the teardrop shape). an area of high pressure is created on the inside of the sail (the part you look at) and an area of low pressure is outside the sail or between the main and the jib (opposite side of sail). this creates lift as things generally move from an area of high pressure to an area of low pressure this is trapped partly by the jib creating the sleigh ride/pant crapper of a ride they call catamaran sailing.

you should makes sure, however, that the mast doesn't turn too much

long story short the mast needs to turn unlike a monoslug

sorry to get all dry and sciency

You weren't sciency.

This is sciency:

It's not that the mast "needs" to rotate on a catamaran and that it doesn't "need" to on a monohull - every rig would benefit from having a mast that is able to rotate.

The problem for monos is that they tend to run (relatively) thinnner mast sections than cats, and these mast sections need to be supported in some cases by multiple spreaders which are tied into the shrouds. The spreader systems prevent the mast from rotating. In addition, monos don't use mainsheet load to tension the forestay (in order to counteract headsail sag). Instead, they use backstays and/or very high shroud tensions to keep the forestay tight and these don't permit the mast to rotate. The shrouds also need to be kept very tight in order to keep the mast "in the boat". On cats, we can get away with running a sloppy rig which is conducive to having a freely rotating mast.

It's really a matter of the difference in overall design philosophy between conventional monohull rigs vs. conventional catamaran rigs (and to an extent the difference in boat designs as a whole) that has lead to rotating versus non-rotating masts.

sm

The Hobie mast rotates to provide an aerodynamic profile to the wind. Most monohulls don't due to the construction mentioned above. In addition to my H-16, I also sail a monohull, a Cape Cod catboat. The sail is connected to hoops on the mast which are free to rotate. The gaff saddle also rotates so that even though the sail is in a slot in the gaff, it turns to provide the desired profile. There are no shrouds or backstay on the catboat mast, just a forestay. When winds are light I sail the catboat and when it gets up, I sail the Hobie.

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Howard

I agree, that is the primary purpose for mast rotation - to provide a cleaner entry for the airflow across the mainsail.

Essentially, a round mast or a non-rotated teardrop mast creates a large disturbance to the wind and causes the airflow to separate from the rig just behind the mast. This is because the air can't stay attached to the mast/sail around the corner that is created by their intersection. This separation occurs exactly where you DON'T want it to and the result is a large loss of potential power from the rig.

The rotated teardrop profile mast (a round profile would have no benefit) attempts to align with the incident angle of the aparant wind and provide a smooth transition from mast to sail. This reduces the disturbance caused by the mast. The smaller the disturbance, the sooner the air reattaches to the sail's surface (there is usually still some separation) and so more power is produced.

The fact that the mast bends differently on it's major vs. minor axis is a beneficial side effect of the teardrop profile that can provide another level of sail shape control.

sm