What’s in a Gust

A few days ago I was checking the weather forecast for that day on San Francisco Bay.  It was a pretty mild wind forecast for late August, winds from the SW at 15 with gusts to 25, however it got me thinking about what that really meant.

Winds from the SW at 15 … wow … the perfect wind for sailing under an un-reefed main and full jib.  But, what about those gust to 25?  Here are a couple of things to keep in mind about wind in general and gusts in particular.

First, the difference in the power of the wind is not just 10 knots (a 66% increase).  The power of wind doesn’t increase in a straight line, it increases exponentially.  25 knots of wind contains nearly 300% of the force of a 15 knot wind.  That well balanced 15 knot sail plan is way over powered in 25 knots.

The second thing to keep in mind is wind direction.  Although this can be altered by the geography of the bay, in most cases, gusts come from the right of the wind’s true direction.  In other words, that SW 15 wind may be gusting at 25 from the W.  If you are on a port tack in the gusts your sails are probably “under trimmed” based on the wind direction, which may be a good thing while you ride out the gust.  On the other hand, if you are on a starboard tack, when the gust hits, you are over trimmed in 3 times more wind!  No wonder things get a little hairy out there during gusts.  One last thought regarding the starboard tack idea … what is going to happen if you are on a run before the gust overtakes you?  At best, you will be sailing by the lee … at worst; the gust gets on the wrong side of the main sail, and WHAM!  An accidental jibe in 25 knots can ruin an otherwise perfect day of sailing!

So, here is the question.  In those conditions, do you reef, or not?  Unfortunately, the answer is not a simple yes or no.  How many gusts are there?  How long are they lasting?  What is my tack and point of sail?  I do know that I am more likely to reef if I’m on a starboard tack with the wind forward of the beam and less likely on a port tack or with the wind abaft the beam.  I probably won’t worry too much about being on a run on a port tack, but will rarely go on a run on a starboard tack during gusty conditions.

It basically boils down to this … if I find myself (or someone else on the boat) asking “should I reef?”  The answer is YES!!  It’s very easy to shake it out later if I didn’t really need it.

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1 Response to What’s in a Gust

  1. Ernest Galvan says:

    Thanks. Very timely information.

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