Have you measured your leeway/windage in anyway?

[jimbouy]

Having just covered Leeway and Windage on the dayskipper theory course I was wondering what you guys have done, when you have taken on a new boat, to gain some measure of the two.

Or have you just carried on as normal and "picked it up" as you go along, learning from experience rather than carrying out specific exercises to gain a measure.

(Hope that makes sense to you)


Jimbouy

<hr width=100% size=1>"If it weren't for the last minute, nothing would get done."<P ID="edit"><FONT SIZE=-1>Edited by jimbouy on 26/11/2003 14:24 (server time).</FONT></P>

 

[qsiv]

Re: Have you messured your leeway/windage in anyway?

A quick look at the difference between your wake and heading should tell you.

Based on my own experience, leeway falls from about 8 degrees to about 4 as the wind increases to about 5 knots. Thereafter about half a degree extra for each 5 knots of windspeed, perhaps more above 30 knots.

<hr width=100% size=1>

 

[Robin]

Re: Have you messured your leeway/windage in anyway?

Sometimes the leeway is counteracted by the boat heading up in gusts, especially if on autopilot. In rough weather on the wind I have factored in leeway only to find sometimes I end up upwind of the target. This is where GPS off-track information is useful when compared against that predicted beforehand for tide/leeway. In the 'good' old days we aimed to miss on the high side (upwind or uptide whichever was greatest), that way when you sighted land you knew if you should turn left or turn right.

<hr width=100% size=1>

 

[dulcibella]

Re: Have you messured your leeway/windage in anyway?

The grip of your keel on the water will affect "rule-of-thumb" guesstimates - a full long keel will make considerably less leeway than a bilge keel. Follow the previous suggestion of checking the offset of your wake under various conditions until you have a good "feel" for what your boat will be losing.

<hr width=100% size=1>

 

[quaelgeist2]

Observed our leeway (in fact not just that but the resuklt of leeway and heading up in breezy conditions, and...) or what we thought it was on a couple of longer trips and now take that into account.

Our Moody keeps nicely with the RYA material I have seen ;-) i.e. 5deg in low winds and flat water, 10deg in rough conditions.

That said, I am always cross-referencing to GPS track and thus correct it regularly anyway. I still feel you should have an idea, since pure GPS control might make you go funny curves in tidal waters.

chris

<hr width=100% size=1>

 

[pampas]

One method of measuring leeway is tow about 50ft of fishing line astern (with a small weight) from a point inboard along the c/l of the vessel and with a cheap protractor measure the angle, really needs a smooth edge to work properly, bit old fashioned but works well when set up, the inboard fixing point need to be about a foot and no more than 3 feet. I have used this method for years and have found it to be very useful. have

<hr width=100% size=1>

 

[pampas]

One method of measuring leeway is tow about 50ft of fishing line astern (with a small weight) from a point inboard along the c/l of the vessel and with a cheap protractor measure the angle, really needs a smooth edge to work properly, bit old fashioned but works well when set up, the inboard fixing point need to be about a foot and no more than 3 feet. I have used this method for years and have found it to be very useful. have

<hr width=100% size=1>

 

[jimbouy]

Heath Robinson but effective !!

<hr width=100% size=1>"If it weren't for the last minute, nothing would get done."

 

[peterb]

I use the same technique, with one addition. The line needs to be long (mine is about 25m) so it's difficult to see the end. I tie a yellow bath duck on the end, with a small weight on the line about a metre ahead of the duck. The weight makes sure that the line is immersed, and the duck makes sure that the end is visible. And we get some very funny looks from other boats!

 

[claymore]

Hmm - not sure about this - won't the duck have a bit of windage and therefore create its own leeway?

<hr width=100% size=1>regards
Claymore
/forums/images/icons/smile.gif

 

[jimbouy]

Perhaps it's a high tech aero duck. with spoiler.

<hr width=100% size=1>"If it weren't for the last minute, nothing would get done."

 

[Peppermint]

Re: Leeway changes by the minute

It's not really worth measuring you just need to be aware of it.

With GPS you can see if your being pushed below your course.



<hr width=100% size=1>

 

[peterb]

When we first tried it we didn't have any weight on the line, and yes, the duck drifted sideways. The effect was to reduce the amount of leeway we measured. Putting a weight on the line, so that the line stayed immersed, seemed to stop the drift.

I usually use orange polyprop floating line, about 4mm diameter, rather than fishing line, to improve visibility.

Ross Garrett, in his book "The Symmetry of Sailing", suggests that leeway angle depends on the side force on the boat, but is inversely proportional to the square of the speed. Since it's the side force that produces heeling, it should be possible to estimate leeway angle as a constant multiplied by angle of heel (more accurately, the sine of the angle of heel) and divided by the square of the speed. The difficulty, though, is working out the constant, which is different for every boat. But once you've measured the leeway under one set of conditions, you can use the relationship to work out the leeway angle under any other set of conditions. Hence the attempts at measuring leeway using a rubber duck!

<hr width=100% size=1>

 

[TheBoatman]

Sorry folks I just press a button on the GPS (a couple of times) and it "chats" to the autohelm....result...never of the track line, always end up where I expect to be?
Sad isn't it that having been taught all this "old" nav stuff I have succumbed to modern technology?
Who else here can remember sitting for hours at the nav table feeling decidedly iffy, whilst on a passsage, doing all those calcs, checking the trailing log wire angle for leeway, but now just presses a couple of buttons and adjourns to the cockpit with a beer knowing that the boat is dead on track and with a couple of extra button presses can instantly find out XTE, TTG, VMG, etc. and with a repeater in the cockpit I don't even have to go below.
Now I actually have time to keep a "good lookout".

The wonders of modern science?

I must admit that now I only check myself against the GPS (electronics) to keep my hand in and to have some fun trying to beat the Bas***d

<hr width=100% size=1>

 

[charles_reed]

get on a constant bearing with a shoremark and use a sextant to measure the angle of the wake.

With a wing keel this is virtually nil at anything but low windspeeds, but the old rule of thumb of 5 degrees worked with most deep keelers to windward, but needs doubling for bilge keelers.

In these days of GPS and electronic plotters such very approximate corrections are really of only esoteric importance.

Probably the most significant variation is caused by the main trying to hook a boat up to windward on a broad reach - this can be as much as 12deg on a wing keeler.

<hr width=100% size=1>

 

[mldpt]

As long as the COG remains the same as the bearing to the destination you are going to get there, but thats not necessarily the shortest route, due to tides.
Mike

<hr width=100% size=1>www.sailingforfun.co.uk