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HSH Yacht Articles

Reading the wind


Jeremy R. Hood

I'm thinking of fitting new instruments aboard my Rival 32 Melos but I've decided against having wind instruments. Its not that I don't like them but they make me lazy and somewhat dependant upon them when in reality I managed for thousands of miles without them. So why do any sailors fit wind instruments. Well they do have there uses. When racing, the helmsman or skipper can see easily which is the most advantageous tack and how close to the wind the boat is able to point, and racing and cruising sailors alike may use the wind speed as an indication of when to reef or change headsails. But it has to be remembered that most wind instruments are giving a value for the apparent wind angle and apparent wind speed. Only with some of the more modern (and expensive) electronics where the wind instrument is interfaced with a knot log and fluxgate compass can they show true wind speed and direction. Before I set out from England I wanted to know what strong winds would feel like so I arranged for a friend to drive 50 miles per hour down the road while I stuck my head out the sun roof. I figured that it would give me an idea of what a 50 mile per hour wind would feel like at sea. Well it did, but the wind I experienced in the car was the apparent wind caused by the motion of the car.

When sailing it is important to understand apparent wind. It is the combined effect of the true wind and the wind caused by the speed of the boat. Like in a car, wind caused by the speed of the boat always comes from ahead but the true wind will blow from any direction. The combined effect of these is what you feel on your face. For example, you are sailing north at five knots on a beam reach with the wind blowing from the east at 10 knots. The apparent wind you experience aboard does not come from the north or the east but somewhere between the two. And the speed of the wind is a combination of the two also. Fig 1 shows a vector diagram of these winds. The apparent wind that affects the sails and you feel on your face is coming from 064? and blowing at 11 knots. Because the boats speed is always forward, the effect of this is always to make the apparent wind come from more ahead than the true wind. The apparent wind moves forward as the boats speed increases and when sailing close to the wind it is important to recognize that this is happening. However if the true wind increases (Fig 2), as it may during a gust, then when sailing close to the wind, the apparent wind moves aft and the wind frees (ie. you may temporarily head higher during the gust).

Knowing the apparent wind speed is important in making decisions about reefing. If you are sailing closehauled (tacking) then the apparent wind will feel stronger than the true wind (Fig 3). Conversely when sailing downwind the apparent wind speed will be less than the true wind speed (Fig 4). Thus a boat may feel overpowered by having up too much sail area when beating yet comfortable when sailing off the wind. Because the apparent wind is lessened by the boats speed when it is blowing from behind the beam the boat will be able to carry more sail then, but beware; when you need to reduce sail you will often need to come up in to the wind to do so and then at this critical time you may find yourself severely over canvased. In my experience one of the most difficult decisions required of the skipper is how much sail to carry when sailing downwind or when to reduce sail in these conditions. As a starting point it is good practice to only carry an area of sail downwind that would also be appropriate for upwind sailing. This may be a little conservative but erring on the side of caution is not that bad though you won't win races this way! One saying regarding this situation is that when beating the boat can take more than the crew, when running the crew can take more than the boat. The point is that when beating, the motion of the boat will become uncomfortable for the crew before the boat is in danger but running downwind the crew may be lulled in to a false sense of security by the reduced apparent wind so that sails may be left up too long with the result that something may break or the boat broach!

If using wind instruments remember to allow for the speed of the boat in determining true wind speed and then make decisions appropriate for the conditions. A forecast wind of 15 knots will appear as a 20 knot wind if heading in to it at 5 knots but as a 10 knot wind if doing the same speed downwind. And remember that the forecast wind is an average wind speed. An average of 15 knots may well include gusts of 20 knots which could be experienced as 25 knots apparent wind and it will probably include lulls of 10 knots which could be experienced as 5 knots apparent. And it is because of these wide variations in apparent wind speed that I choose to look at the sea state and follow the Beaufort scale in determining what sails to carry. Developed by Admiral Sir Francis Beaufort, Hydrographer to the British Navy, this scale is still a valid way of ascertaining the wind speed by its effect on the sea. Originally developed in the days of square rigged vessels I have included his comments regarding these vessels in the Table 1. The scale goes right up to winds and seas of hurricane force but I am only including the first eight as these are the only ones really relevant to the small boat sailor.

Beaufort force Description Appearance of the sea Wind (knots) Beauforts criteria

0 Calm Sea like a mirror   <1 Calm

1 Light air Ripples with the appearance of scales are formed without foam crests 1-3 Just sufficient to give steerage way

2 Light breeze Small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break 4-6 That in which a well-conditioned man-of-war with all sail set and 'clean full' would go in smooth water from 1-2 knots

3 Gentle breeze Large wavelets. crests begin to break. Foam of glassy appearance. Perhaps scattered white caps 7-10 as above but from 3-4 knots

4 Moderate breeze Small waves, becoming longer; fairly frequent white caps 11-16 as above but from 5-6 knots

5 Fresh breeze Moderate waves, taking a more pronounced long form; many white caps are formed. Chance of some spray 17-21 That to which she could just carry in chase 'full and by' Royals, etc

6 Strong breeze Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray 22-27 That to which she could just carry in chase 'full and by' single-reefed topsails and topgallant sails

7 Near gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind 28-33 That to which she could just carry in chase 'full and by' double-reefed topsails, jib etc

8 Gale Moderately high waves of greater length; edges of crests begin to break into the spindrift. The foam is blown in well-marked streaks along the direction of the wind 34-40 That to which she could just carry in chase 'full and by' triple-reefed topsails etc.

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