November 8, 2011

Allowing for leeway

I SOMETIMES WONDER how many of the sailboat skippers I see blithely cruising among the San Juan Islands are making any allowance for leeway. If you don't make any allowance, there can be a surprising difference between your chosen arrival point and the actual place where you end up. And that's even without taking the tidal current into account.
  Leeway is sideways drift, of course, caused by the wind hitting the side of the boat and sails. All boats make leeway to some degree on all courses except two: when the wind is either dead ahead or dead astern.
  You can assume that a close-hauled sailboat will make between 3 and 5 degrees of leeway in a 7-knot breeze, and 8 degrees or even more in 20 knots. So, if you want to maintain a set course over the ground, to avoid reefs and rocks, you must head your boat toward the wind to offset leeway.
   It's not always easy to estimate how much leeway you're making, but you'll get a fair idea if you look aft along the centerline of the boat and compare that imaginary line with the line of the wake. If you have a hand bearing compass it will give you the difference in degrees, so you'll know how much correction to apply next time.
   Powerboats often make more leeway than sailboats because they present a greater area of topsides to the wind, compared with the amount of hull under water. But it doesn't affect their course over the ground so much because the sideways component is mostly a small percentage of the powerboat's forward speed.
   In any case, it's wise to be aware of leeway because it means your boat is not actually going in the same direction she's pointing at. It's not at all obvious, but she's actually crabbing along to leeward, and if you don't compensate by pointing higher, you might be in for a nasty surprise.

Today's Thought
To be conscious that you are ignorant is a great step to knowledge.
— Benjamin Disraeli, Sybil.

Little shots of whisky
Little drops of gin
Make a lady wonder
Where the hell she’s bin.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Aaron Headly said...

There's a serious probability that those boats around the San Juan Islands know their accurate C.O.G. (Course Over the Ground) to the degree. Every GPS calculates it. My cellphone has at least two programs capable of decent COG reporting.

Further, a lot of people turn on their GPS just to figure out their leeway, then steer by the binnacle compass (saving their GPS batteries for other things. Like phone-calls).

I was taught always aim down-wind of obstructions, if possible, anyway — by a captain that loved his GPS.

s/v Windward said...

I was thinking about leeway during a recent couple of weeks sailing Pamlico Sound and its environs. I keep the chartplotter set to display current magnetic heading (COG), and finally made a point of comparing it more systematically to the heading claimed by the compass. For simplicity, I'd been making my paper log entries of current speed, average speed, lat/lon and heading all from the chartplotter's display. I've now started also noting the magnetic heading from the compass to help me get a better sense of "normal" leeway for my boat.