Gusts

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I've been wondering what to do when I see a gust coming across the water.  Avoid it at all costs?  Head straight for it?  Tack in order to reach it?

I've constructed a "model" of "cats-paw" gust in an attempt to see what's happening.  The model of the cats-paw assumes that the gust hits the water from "up there" and then spreads out.  It presumes that the wind in a gust is always veered relative to the prevailing wind; and that within the cats-paw the wind on the left is more veered than the wind on the right.  (This seems about right for where I sail, but if you want some other presumptions, make your own diagram!  The books I read tell me that, in the Southern hemisphere, the wind will back in a gust.)

The track of a yacht is made up of a succession of line segments.  Each line segment on the diagram represents the yacht sailing for a boat-length or two or three, and the little tag on the line represents the local wind direction.  So the line is angled at 45 degrees to the tag, on the assumption that you are roughly beating at an angle of 45 degrees to the incident wind.

The first diagram shows two boats, one on port and one on starboard, coming off the start line and heading for the windward mark.  On the way, they pass through a gust.  I've arranged the boats so they pass equally through the gust, starting at its centre.

One boat is lifted nicely, the other is headed moderately.  When they emerge, and tack for the mark, the lifted boat finds itself well ahead.

So the first conclusion is to head for the gust, and when you get there (or it gets to you!), make sure you are on the lifted tack.

The second conclusion is to favour the side of the gust where the wind has bent the most.  According to the books, you should therefore take the gust on starboard in the Northern hemisphere, and on port in the Southern.

The second diagram shows a gaggle of boats, labelled A to F, beating through a gust.

I've shown two boats A and B who think they are going to miss the gust, so they tack over in the hope of reaching it.  B just gets a piece of it, while A misses it completely.  Nevertheless, the modest time that B spends in the gust puts her clearly above A and in a better position for the mark.  (The "++" signs on their tracks indicate they have tacked twice.  It reminds us that they've lost a little more in the process than is represented on the diagram.)

So the conclusion here is to head for the gust even if you don't get all of it.

Boats C to F are in an evenly-spaced line before the gust hits and, in theory, are equally distant from the mark, though it is clear that F has the tactical upper hand.

After the gust has passed, two things are obvious.  The first is interesting, that the boats have become more bunched up.  The cats-paw has squeezed them together.

The second is that the boats are now "line astern", having started "line abeam", and the boats which stayed in the gust the longest have benefited the most.  Though C experienced a stronger lift than D, D experienced her lift for longer, and has emerged the clear leader.  F has gone from hero to zero through no fault of her own.

2005-12-18


2011 Lester Gilbert