High-Speed Sailing

The speed-sailing contenders have various approaches to going fast and I thought a look at the merits of these might be of interest. We can sort our contenders into two groups one being "planing" and the other "foiling". What are the advantages/disadvantages of each?

Let me back up a bit and point out that a major barrier to high-speed sailing is the water. This is due to the density of the water and its resistance to an object moving through it at high speed. Don't overlook the fact that aircraft have the same problem in moving rapidly through the air; it's just that air is less dense than water so there is less resistance until very high speeds are reached.

I have been saying that the #1 problem for high-speed sailing is stability, but the #2 problem is drag (the resistance to the motion of an object through a fluid, be it water or air). I will discuss both of these problems a bit. Stability has traditionally been handled by using weight in some form or another to counter-balance the sail. Let's assume we have adequate stability for our craft but we want to go faster. Our problem now becomes how do we minimize drag. Since we know that the water itself is the biggest barrier, it is easy to see that the less we contact and disturb it, the less drag we will have.

We need to minimize water contact to decrease drag, and we have a few ways to do this. One would be to decrease the craft's weight. Another would be to raise the craft partly out of the water (we can't totally disconnect from the water and still go fast as I explained in earlier posts). There are four ways to raise our sailing craft that I can think of. One is by using "negative weight", such as Bernard Smith's use of helium in his "Sailloons, one is by hydro-planing, one is by hydro-foiling and one is by aero-foiling.

I'll not discuss "negative weight", since none of the contenders appear to be using it. All of the contenders are hydro-foiling with respect to leeway resistance, but only Hydroptere and Wotrocket are hydro-foiling to raise the hull out of the water. Earlier Trifoiler was hydro-foiling the hull up, but is not currently a contender; there have been others and there will be more. The hydro-planers include Innovation, Sailrocket, windsurfers and kite-boarders. Additionally, Sailrocket is using aero-foiling to raise the craft (the cross-arm foil) and they managed to fly the craft but demonstrated how that's not the fastest way to sail.

The advantage of planing is that the water surface is what you are planing on, and with a good set-up the craft will be stable since it remains in contact with the surface. Generally speaking, planing is the simplest way to raise the hull and keep it stable. The advantage of hydro-foiling is that it provides a better lift/drag ratio than planing (a foil will have less drag than a plane, when lifting the same amount of weight). The correct aero-foil used for raising the hull should produce less drag than a hydro-foil at high speeds (due to less density).

The disadvantages of planing are a poorer L/D ratio and any chop in the water will "pound" against the plane, increasing drag (hence the search for ideal speed-sailing conditions with smooth water). Hydrofoils are not so dependent on smooth water, since they work below the surface of the water. For this reason Hydroptere and Wotrocket are able to sail in rougher water. A disadvantage of hydrofoils is that some means must be used to keep the desired ride height constant to maintain stability. Trifoiler has a good solution with its "feelers" that reference and automatically maintain ride height. Surface-piercing hydrofoils can be used to maintain proper ride height, since as the craft rises; less foil is in the water producing less lift.

Using aero-foils to lift the craft again provides us with the problem of maintaining ride height. We were given a dramatic demonstration of what happens when the ride height is not maintained with Sailrocket's flip.

At this point I'd like to once again state that in my view, stability of a high-speed sailboat is a designer responsibility and the crew's job should be to control direction, acceleration/deceleration and trim.

Bob