Dalí foils explained

That would be ideal, and in many designs that will surely be implemented.

But these boats are restrained by a design rule for their particular class, which is that they are only allowed to have 5 foils.

The foils on a boat are those appendages they have underwater which work like wings. The foils on this boat will be the 2 rudders, the keel and the 2 'Dali' foils.

The shape of the Dali foils make them impossible to retract, and in essence their shape are dictated by the 5-foils rule. As with most things with sailing, the way the rules dictate something as esoteric as this is complicated and technical.

Monohull sailboats always heel away from the wind. The more they sail into the wind, the more they heel. This is bad for the performance (speed) of the boat, because it means that the shape it presents to the water under the boat changes away from the hydrodynamic optimum, and the sail area presented to the wind becomes smaller than when the boat is upright.

If you can make the boat sail more upright, then you will go faster. So, about 15 years ago they started building boats with canting keels. The keel in a modern sailing boat is a long winglike structure on the centreline of the boat, with a torpedo like bulb at the bottom, filled with lead. When a boat heels to the one side, they will swing the canting keel to the other, higher side of the boat, to pull the boat upright again.

But the keel has 2 functions - the one is ballast (therefore the heavy bulb at the bottom) to keep the boat upright, but the other function is to act as a wing so that the boat goes only forward, rather than drifting sideways (drifting sideways is called 'making leeway').

So, if they swing the keel sideways to pull the boat upright, it loses it's ability to prevent leeway, and the boat will start making a lot of leeway. To prevent that, they introduced retractable daggerboards. A daggerboard is also like a wing, and a boat will have 2 of them, one on the left and one on the right. When the boat is heeled to the left, they will swing the keel to the right, which will lift the boat more upright. They will then lower the daggerboard on the left, to take over the function of preventing leeway which the keel can now no longer perform.

Then, about 5 years ago, they came up with this idea of adding lifting foils to a monohull sailing boat. The foils are one on each side, but horisontal to the boat, like an aeroplane's wings. The idea is that, when the boat heals to the left, you will push out the foil on the left, which, acting like a wing, will push the boat more upright and in the process even lift part of the hull out of the water, which reduces drag and make the boat go faster.

So, the optimum configuration is: 2 rudders, a canting keel, 2 (straight) retractable daggerboards and 2 (straightish) retractable lifting foils. (The only foils outside the boat will be the ones in use, the others will be retracted inside the boat).

But that is 7 foils, which for this boat is against the rules. That is why they came up with this 'Dali' foil idea, which is basically an attempt/compromise to combine the lifting foil and the daggerboard into a single foil, providing both lift and preventing leeway. Unfortunately, that shape makes them impossible to retract.




A) Diagram of a boat with a canting keel and retractable daggerboards. The boat heels to the right, the keel has been swung to left, and the daggerboard on the right has been deployed. The daggerboard on the left has been retracted into the hull and is not visible.




B) This is a boat with retractable lifting foils, without daggerboards and with a non-canting keel:



The 'Dali' foils are an attempt to combine A and B.

The problem I have with the last two America's Cup boats is that they are no longer seaworthy, you can't cross oceans with them. Not without incurring an excellent probability of dying, anyway.

So why should I have a problem with that? F1 Grand Prix cars can't race on normal roads either, so why should the sailing equivalent of F1 be any different? I don't know - I guess it is because for me it violates some of the things that make sailing fundamentally unique - such as that when you step onto a sailing boat, you can, in principle, go anywhere in the world - which is the only vehicle this can be said about.

So maybe they should introduce a leg in the America's Cup which involves the crossing of some ocean, New York to Portsmouth, for instance, using the same boats they use to race around a course. That should fix up the design nicely.


 

Because the sails have two different modes of operation:

1) Like a parachute. This is when the wind comes from behind and blows into the sails. In this mode, the boat will always go slower than the wind.

2) Like a wing, in exactly the same way as on an aeroplane. In this mode, the wind flows across the sail and generates a force perpendicular to the sail. This force is many times greater then the force generated in parachute mode, as in (1). In wing parlance, in parachute mode the wing (sail) is actually stalled.

The discrepancy in force is so great that racing sailors will almost never sail dead downwind, even if that is the shortest distance - the speed increase is so pronounced that they will rather sail a longer distance at an angle to the wind in order to operate in 'wing' mode, instead of parachute mode, and so get to their destination much quicker.

(2) is also the reason why modern boats can sail upwind, which square rigs (windjammers) such as the boats of Dias and Da Gama and Van Riebeeck and Columbus and Nelson could not do. Square rigged sails can only operate in parachute mode, never in wing mode.