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Old 27th April 2009, 05:18 PM   #12
C two four nine
Posts: n/a

I think the reason that C Class cats use wing masts but other boats don't has been explained by top aerodynamicist Mark Drela, whose foil designs hold things like the human-powered flight and boat records.

Mark notes that with a board or boat, the issue is not the l/D of the rig in isolation BUT THE LIFT DRAG OF THE RIG AND PLATFORM (ie the rig and the hull or hulls). Looked at it that way, everything comes together.

A glider consists of a huge wingspan with a tiny bit of parasitic drag caused by the fuselage, so going for the lowest-drag wing is vital. In contrast, a boat or board consists of a smaller (proportionately) "wingspan" and a massive amount of parasitic drag caused by the hulls. No matter how efficient the L/D of the rig, the L/D of the whole boat is dominated by the huge drag of the hull. Therefore a lot of the time, it's more important to maximise the lift of the rig, even at the expense of maximising the drag of the rig, because the rig drag is a small fraction of the total drag of the entire boat or board.

For example, a C Class cat is an inherently low-drag beast for its size and sail power. Therefore the low drag of the wing sail is very important (and it helps that the boat is so big that it rarely crashes and blows the wing!). So wings work in Cs.

At the other extreme is a cruising yacht; it's got an inherently high-drag hull and therefore maximising lift is vital, and since rig drag is a small amount of overall drag, an inferior L/D in the rig is no problem if L increases.

In boards, we go from longboards in light winds (high drag compared to windspeed) to speed boards in strong winds (low drag compared to windspeed) and the fastest sails follow the same rules; increase lift when your hull drag is high even at the expense of higher aero drag, and vice versa in other conditions.

I'm no expert in this, just passing on theory that matches practice 100%.
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