PS Advisor November 1, 1999 Issue

PS Advisor

Tacking Degrees
What fudge factor can I use to deal with the following upwind claims:

1. The brochure of an older, narrow, deep, fin-keel boat with a racing heritage says, “…points up to 25° from the apparent wind.” The next sentence says, “…while reaching under spinnaker with 30-35 knots of wind she fetched 16 knots with perfect control.” Are they saying that this moderate displacement hull was surfing?

2. A reviewer looking at a slightly beamier boat described it as “sailing within 40° of true wind.”

3. Another describes a different boat as tacking through 95°.

4. A fourth boat is described as tacking through 110°.

All the boats have a similar waterline lengths. How would one deal with these claims?

Paul McQuillan
via e-mail

A non-sailor has no trouble figuring out what makes a sailboat go downwind, but is mystified when the boat goes against the wind. Those who race sailboats wish they knew everything about the latter. In fact, everybody’s still learning.

Basic sailing theories change from time-to-time…much like whether you should brush your teeth with vertical or horizontal strokes.

You can spend a lifetime studying the works of C.J. Marchaj (an early pioneer and author); academic experts like German-born Heiner Melder and a Canadian named Tom Schnackenberg; Arvel Gentry (the great Boeing aerodynamicist), and even try to understand Ted Hood, of whom it is said…no one in the world has ever had a better eye for sails. Before Hood, the reigning sail genius was Ernest A. Ratsey, whose grandfather made the foretopsail used at Trafalgar in 1805 by Lord Nelson.

Getting deep into the complications, you could get yourself tangled up in the Bernoulli effect (named for a Swiss mathematician 1700-1782), the Kutta condition, form drag and as many vortexes as you like.

What you won’t learn is whether to sail low and fast with full sails or high and slow with flat sails. In his book, The Art and Science of Sails, Tom Whidden says that is among the most complicated decisions in sailing. Before Whidden’s book, the “bible” on sailing theory (if you didn’t want to take on C.J. Marchaj’s 1964 Sailing Theory and Practice) was Wally Ross’s Sail Power, published in 1975.

How close-winded are various boats? How fast do they really go?

To begin, the curvature in a sail is what makes it work while going to windward. Without the curvature, there’d be no pressure differences. The leading edge of the curve must be in line (or nearly so) with the apparent wind.

The apparent wind is not the true wind. As a boat goes forward, the telltale shows that the wind moves forward, or seems to. The true wind doesn’t change. (The telltale on an iceboat at speed points almost aft, despite the fact that it is beam reaching.)

Any boat that can intercept the wind at 45° and make hull speed in optimum wind and water conditions is considered reasonable. The boat is said to tack through 90°

Some boats can tack through less than 90°, maybe as little as 80°, and still hit their optimum speeds. The 12-Meter boats used for the America’s Cup (and racing boats used primarily for around-the-buoys races) can do even better. Beamy boats used for downwind races like the Trans-Pac are not made to go well to windward.

Most boats are a compromise, trying for something acceptable between outstanding windward ability, fast reaching (perhaps planing) and scooting fast downwind (maybe surfing on the front of waves). And, of course, while doing so, they must meet all sorts of other requirements—be seaworthy, steer well, be comfortable, etc.

Commenting on the numbered items above:

1. It’s utter rubbish. No boat points 25° from the apparent wind. If it did, it would not, no matter what size it is, do 16 knots reaching.

2. Could easily be true, a good boat to windward; but it might eat dust on a reach or run.

3. Believable. Not great, though.

4. Poor. Most good boats make 3° or 4° leeway, so add that (or more) to the angle of attack and you’ll see why you could spend a lot of time sailing back and forth to reach a goal to windward.

There are formulas that predict speed quite accurately.

For a more complete picture, a polar diagram for more than 600 individual boats is part of a Performance Package handbook available for $195 from The United States Sailing Association, 15 Maritime Dr., Portsmouth, RI 02871, 401/683-0800. To compare the speeds of different boats, the USSA also sells for $45 the PHRF Handicap Book, which contains data (derived from years of round-the-buoys racing) for more than 3,300 different boats. If comparative boat data interests you, the book is a bargain.

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