Leeway

[Dougie_the_Mate]

As a newcomer to cruising - at least in our own boat - I felt it advisable to get some knowledge of navigation etc.. and completed the Yachtmaster Theory course over the winter just past. I found the various aspects of Tidal Theory, plotting a course, Colregs, passage planning etc.. fascinating and managed the exercises pretty well.

When trying to keep my skills up to scratch while waiting delivery of the boat, - as a leisure dinghy sailor I usually just point where I hope to go and work it out on the way, - I have been planning prospective passages using all the relevant information, but am slightly lost when it comes to leeway.

In the exercises during the course, you are told what the Leeway is likely to be and then factor it in to the calculations - next step - right or nearly right answer.

It occurs to me however that in real life you won't actually know what the effects of wind are until you get out beyond the harbour or river mouth or breakwater or whatever - so how do you calculate Leeway to ensure that the course to steer is approximately true. The explanation the RYA navigation handbook makes no sense to me despite having read it several times.

Is there a simple rule of thumb which experienced sailors apply which tells them roughly how much Leeway to consider when setting a course? I apologise if this seems like a dumb question, and assure you that I do try to stay out of folk's way so don't worry unduly about this novice crashing into you while on your weekend cruise. I would be happier in my own mind however if I understood this better. In the Dinghy we simply react to the wind and we are not going so far that it really makes a difference, but it seems to have greater significance when on longer passages.

Any advice welcome.



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[boatless]

Well... It really is a complex fudge varying with boat design/keel type, point of sailing, sail trim/quality, wind strength, sea state and cleanliness of hull. Amongst other things.

So, upwind - which is where leeway has the greatest effect, it can vary from say one degree for a high tec racing boat in flat water and 10kts true wind, to as much as 15 degrees for a tired old wide beam, shallow draft, iron keel, roller sails and weedy - bottomed Benjen struggling uphill in 35kts.

In a decent modern boat you could allow say 10 degrees for a beat into 30 knots, and then give yourself a pat on the back for good helmsmanship if you do better. Best thing is to overestimate, and then build experience.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[Peppermint]

Re: Very few of

our calculations actually survive contact with reality.

Thats why Nav is an art not a science. Leeway can be calculated, by taking back bearings of your wake or a fixed point on land with your hand bearing compass, but like course, boatspeed and tidal stream its effects are constantly changing.

On a simple cross channel trip it's more something to be aware of as a factor than worthy of constant re-calculation and or adjustment.

Before GPS it was good tactics to arrive at the destination with a known error. So you knew to turn left or right with certainty, to find your chosen port. Best if your up tide too. It was often possible to just leave leeway to create your error for you.

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[qsiv]

With our boat heavy cruising boat I simply use 1/3rd windspeed as a leeway allowance when anything closer than a fetch ...

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[snowleopard]

you're supposed to find out what leeway your own boat makes under different conditions. fortunately with gps giving you course over ground it's easy to work out the leeway, i.e. the difference between the course steered plust tidal allowance and actual COG.

the one factor they never mention in rya exercises is helmsman's error, there will always be a tendency to crab to windward on a reach/run and to sag to leeward on a beat so watch out for that!

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[Stemar]

One thing that seems to be forgotten on theory course is that steering a yacht especially on a bumpy sea is hardly an exact science either. You're taught to work out your leeway, variation and deviation, figure in your inside leg measurement and come up with a course to steer. "Heading 279 degrees please" Yeh, right."

I find I'll get something between 270 and 290 on my old tub, so for local sailing I don't worry about it, just point her somewhere in the right direction and adjust as I go. Obviously this would be risky in poor visibilty without regular fixes from the GPS and a regular eye on the chart, but most of the time it's fine

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[Robin]

Lots of sensible answers already but another factor is the windward screw (that'll get 'em going), especially if the course is set under autopilot. Humans and APs will screw upwind in gusts and in some cases enough to counter the leeway. We used to have an ST50 Compass which was kept standalone from the other instruments and pilot, when asked nicely it could show the average deviation from a preset course (ie + 3 degs or - 2 degs etc) which could then be adjusted for and re-checked after a suitable time. It doesn't work if connected to the autopilot since it just displays the locked course on the pilot,, shame because it is a nice feature.

Going back to leeway, if on a passage like cross channel we will always aim to err on the side of being uptide and upwind, if they are opposed then it depends on which is more hassle to correct when close in. How much allowance depends on the course and really we only add a bit if closehauled and it's choppy, it can be monitored and adjusted for after a few hours by comparing predicted off track from the passage plan versus the actual from the GPS.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[claymore]

Navigation

Is neither art nor science but an estimate added to a guess to produce a supposition

<hr width=100% size=1><font color=purple>regards
Claymore<font color=purple>
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[claymore]

"so how do you calculate Leeway to ensure that the course to steer is approximately true. "
If you take back bearings on a fixed point whilst sailing hard on the wind when your boat will create the greatest amount of leeway then you will have a rough idea of what to factor in - SO - if you sail down the Clyde with Cloch Point behind you aiming say at Toward point which is a beat and say after 15 minutes the angle of your back bearing on Cloch Point has opened up by 20 degrees then allowing for helming errors - you could reasonably say that when hard on the wind the boat makes around 15% of leeway in 1/4 hr - or you are 15 degrees left or right of a fixed point whilst doing say 4 knots. So in an hour if you continue that line it is possible to plot how far to the left or right of your track you are going to be.
So next time you clear Cumbrae and have a long upwind sail to Sanda you have a vague notion that if your desired heading is say 300 degrees and the best your boat can manage is 45 degrees then your track is going to be around 300 less 45 which gives you around 255 less the 15 degrees of leeway your boat makes so that brings your most optimistic track to around 240. By plotting that on your chart you can begin to form an idea of the shape your course is likely to take and from that you can begin to shape in where you might need to put in your tacks. This is all helped by the fact that (broad generalisation) the tides are not too strong and run straight up and then straight down the Clyde. All these calculations are further complicated by cross tides - so for example if you were to sail across to Ballycastle then a much stronger tide would be predominantly on your beam and would need to be factored in.
In reality when cruising on the Clyde or anywhere else in sight of land we just have a squint and say something like "lets stick in a tack and see where it gets us"
That is if things are so desperate that we are actually being forced to sail up wind


<hr width=100% size=1><font color=purple>regards
Claymore<font color=purple>
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[Cotillion]

I found leeway depressing when doing the course. Having determined your allowance for magnetic (three degrees W(ish)). Taken an allowance of 1 degree for compass deviation. Worked from the tidal diamond to lay off the set, say 313 degrees true for 7/10ths of a NM. Having admired the precision of the vectored course to steer, say 102C, which you probably wont be able to keep to anyway. To then be told, "Allow about 5 - 10 degrees for leeway." Seemed to render the exercise virtually null and void.

It kind of reminded me of when I went to a builders merchants to by a yard of 2 x 2 to be told it was sold by the metre. When I asked how much it was they replied "£1 per foot".

kim

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