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Author
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Topic: Current vs Leeway
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Dan Cline
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posted July 03, 2006 08:50 AM
Lake Michigan has very subtle circulatory currents -- in the neighborhood of 0.3 knots. Can you recommend methods for detecting currents of such small magnitude and for distinguishing the effects of current from the effects of leeway?
From: Midland, MI
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David Burch
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posted July 04, 2006 09:06 PM
The main problem with low currents like that is they are comparable to leeway, so the main task is distinguishing leeway from current.
Both leeway and current can cause you to move in a direction the boat is not pointed, but these two effects have some important distinctions.
First, leeway causes actual motion through the water--as opposed to current,which is motion of the water. In other words, moving by leeway is just sailing without sails.
Everything in the water has some leeway... a log or a battleship. we will come back to the nature of leeway later on -- for the time being, let me reference our Emergency Navigation book which covers the subject thoroughly.
for now we just note that leeway is a form of sailing, meaning an actual speed that can be measured with a knotmeter. usually it is thought of as slipping, rather than sailing, since you are generally not moving in the direction the boat is pointed, but rather slipping downwind.
So two things that characterize leeway is actual motion read on a knotmeter or GPS as SOG and the COG from the GPS is always downwind.... so if you are pointed toward 090 in a northerly wind, you would find a cog of say 100. the amount of slipping depends on wind speed and vessel draft and vessel windage for the most part.
to measure a very weak current you will have to have no leeway, which means essentially calm air. An easy method would be to just stop, and then watch the SOG and COG, which would then be the drift and set of the current.
But it is important that the knotmeter reads precisely 0.0. if there is any speed on the knotmeter then what you see on the GPS is some combination of leeway and current.
currents in the great lakes can be studied here http://www.glerl.noaa.gov/res/glcfs/
sometimes in seich conditions they can reach into the knots range, especially in Lake Eire. see http://www.glerl.noaa.gov/seagrant/glwlphotos/Seiche/SeicheHome.html
From: Starpath, Seattle, WA
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