Hydrofoil Sailing Basics

Hydrofoils can really make a boat fly

A sailboat can only go as fast as the wind, right? In fact, given the resistance of drag through the water, and the boat's hullspeed (created by the displacement waves fore and aft), most sailboats can't even approach the speed of the wind that moves them, right?

Right?

Well, of course that was a loaded question. And as you've probably already guessed, the answer is, "Wrong!"

But how can it be possible? How can a sailboat travel faster than the wind that is pushing it?

In a word: Hydrofoils.

Hydrofoils are a set of rigid underwater wings attached to the hull of the boat. (At least, they start out underwater). As the boat begins to move, the hydrofoils begin to "fly" through the water just as airplane wings fly through the air. And in just the same way, hydrofoils create lift.

The result? The boat is lifted out of the water. As its speed increases, so does the lift created by the hydrofoils, and the boat is lifted still higher. As less and less of the hull is in contact with the water, both drag and hullspeed become less and less of a factor. And the faster the boat can go.

Eventually, the hydrofoils themselves begin to be lifted out of the water. They reach a point where enough of their wing shape is out of the water that the foils are unable to generate more lift. The boat then cruises along at a steady rate, which can be 1.5 to 2 times the speed of the wind.

Still, we have not explained how a sailboat can travel faster than the windspeed. So what if the hydrofoil lifts it out of the water? How does that enable the boat to go faster than the wind which provides the motive force?

Well, without going into a lot of eye-crossing engineering, the key to understanding this is to realize that this is a theoretical possibility of any sailboat capable of tacking into the wind.

Surprised? It's because the sails themselves are foils. The sails create lift, too -- just like an airplane's wing. But unlike an airplane wing, which are mounted to the plane horizontally, the sailboat's sails are mounted vertically, on the mast. This means that the lift generated by the sail does not pull the boat up (like a plane), but rather forward.

So in other words, your sailboat is not being pushed by the wind blowing on it from behind; it's being pulled by the shape of the wind flowing over your sail. This is why you are capable of sailing to windward. And, adjustments to the airfoil shape of your sail can actually create a lift force which could result in a potential boat speed greater than the wind itself.

Non-hydrofoil sailboats cannot usually approach this theoretical speed, however. The drag of the water on the hull, and the displacement waves fore and aft, limit the possible speed of the sailboat.

Ah-ha! you say. Yes, here is where the hydrofoils come in. The hydrofoils reduce these limiting factors of drag and hullspeed, thus freeing the inherent lift capacity of the sails themselves. The sails are now free to pull the boat forward at close to their theoretical speed -- which could be faster than the wind speed.

But this has got to be some brand spanking new high-tech gadget, right?

Again -- Wrong!

In fact, the idea of the hydrofoil has been around a long time. About 100 years ago, Alexander Graham Bell teamed up with his business manager Casey Baldwin and Enrico Forlanini to develop the first hydrofoil watercraft. They were able to reach a then-astonishing 90 mph.

But what about heeling? How do the hydrofoils work then? In fact, believe it or not, the presence of hydrofoils actually prevents heeling -- no matter what the weather. This is because as the boat begins to heel, the leeward foil would be submerged to a greater depth in the water beneath it. But then with more of its foil shape below the water, the leeward foil would also exert a stronger lift, and raise its side of the boat back up. Result? Little or no heel!

So why doesn't everyone use a hydrofoils? The drawbacks are the challenge in maintaining stability in choppy waters, and the difficulty in steering. With the stability of the rig depending on maintaining the hydrofoils at a steady depth in the water, it's clear that waves of any size can really play havoc with the boat's handling. Also, for multihull boats, the hydrofoils themselves protrude into the water below to a much greater depth than the hulls. So the multihull boat can no longer go into shallow waters with impunity.

Still, the sailing hydrofoil is an astonishing piece of technology. If you ever have a chance to check out one of these rigs in action, you won't be sorry.