ghostlymoron
Well-Known Member
Whilst reading another thread, I came across a reference to lee bowing. I've seen this before but never really understood it. Could someone explain please.
Lee bowing
- Thread starter ghostlymoron
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RichardS
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TLouth7
Well-Known Member
When making a passage upwind with cross tides (perpendicular to your desired heading) where the passage duration will be at least one tide cycle (~12 hours) you must make a choice about when to sail on each tack
The rule of thumb is to sail on the tack which puts the tidal stream towards your lee bow. So If you are heading North (into a Northerly Ground Wind) with tide from the West you would sail on Starboard tack. This gives an advantage because the True wind is shifted to Starboard by the motion of the water, effectively allowing you to sail in a lift for the duration of that tide. When the tide turns you tack onto Port (the new lee bow tack) and get a lift for the next 6 hours.
To visualise the lifting effect of the tidal flow imagine you sit stationary (relative to the water), head to wind. With a 4kt tide from the West and a 4kt Ground Wind from the North you would experience a 5.5kt True Wind from the North East. Thus on Starboard tack you could sail on a heading of due North, so clearly you have been lifted.
The rule of thumb is to sail on the tack which puts the tidal stream towards your lee bow. So If you are heading North (into a Northerly Ground Wind) with tide from the West you would sail on Starboard tack. This gives an advantage because the True wind is shifted to Starboard by the motion of the water, effectively allowing you to sail in a lift for the duration of that tide. When the tide turns you tack onto Port (the new lee bow tack) and get a lift for the next 6 hours.
To visualise the lifting effect of the tidal flow imagine you sit stationary (relative to the water), head to wind. With a 4kt tide from the West and a 4kt Ground Wind from the North you would experience a 5.5kt True Wind from the North East. Thus on Starboard tack you could sail on a heading of due North, so clearly you have been lifted.
TLouth7
Well-Known Member
I disagree slightly with the posted article. Just because you sail closer to the rhumb line on the chart it does not follow that you sail a shorter distance. This is clear if you consider the case where you are sailing a beam reach - we all know that you sail a shorter distance through the water by keeping a constant heading and thus deviating from the rhumb line.
The vector diagrams showing the effect of tide on True Wind are spot on though.
The vector diagrams showing the effect of tide on True Wind are spot on though.
johnalison
Well-Known Member
Lee-bowing can also help in river situations. Sometimes one is heading up to clear the inside of a bend, which is just about where the boat will point, against the stream. If you choose to sail full-and-bye you will be pushed further into the bank, when careful sailing and a little pinching will give you a lift to clear the corner. I won a race by miles in 12sq m Sharpies at Burnham by doing this.
lw395
Well-Known Member
The 'lee bow effect' means various different things to different people, particularly those who aren't terribly good at handling vectors.
It is a wonderful source of circuitous arguments at cross purposes, as each person defines a few things slightly differently and has different expectations about what stays constant.
Partaking in arguments on this subject is a bit like racing slow boats in Solent tides. Probably need a beer afterwards!
It is a wonderful source of circuitous arguments at cross purposes, as each person defines a few things slightly differently and has different expectations about what stays constant.
Partaking in arguments on this subject is a bit like racing slow boats in Solent tides. Probably need a beer afterwards!
Kukri
Well-Known Member
Just try, other things being equal, to sail on the tack on which the tide is pushing you to windward.
NormanS
Well-Known Member
That's always been my understanding.
RupertW
Well-Known Member
Yes, but lee bowing works most effectively when the wind is dead against you and the tide is on the side so in that case the tide is not pushing you to windward (on true wind )at all on either tack. But the tide is shifting the apparent wind as described above, making one tack (the one where your lee bow is facing the tide) giving you a better angle to the true wind, thus reducing the distance you need to cover.
lw395
Well-Known Member
I'm struggling to imagine circumstances where 'other things' could be 'equal' and that advice would be any good.
It's a good example of the vague statements which are trotted out on the subject.
Kukri
Well-Known Member
God give me strength!
This is really, really, simple stuff. It is intuitive to any child learning to sail in a dinghy.
I will try again:
If a boat is close hauled, she will make leeway. Put her on the tack on which the tidal stream or the current is on the lee bow and the tide will reduce the leeway that she makes.
Sorry, four words of two syllables, there.
lw395
Well-Known Member
This incorporates a fair range of vague and spurious concepts.
Is the wind that's 'dead against you' the true ground wind you'd measure when anchored or the 'true' wind you'd measure while stopped in the water?
If you think of it in a frame of reference moving with the water, the mark or destination is moving up tide because it's anchored, and it matters not which tack you start off on, provided the wind and tide stay constant.
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As johnalison suggests, pinch up in light winds to maximise the benefits of lee bowing. I was told by an Olympic coach these are the only circumstances in which sustained pinching can be justified.
lw395
Well-Known Member
Nope.
Leeway is a completely different vector, or angle if you prefer.
I'm sure you understand your personal concept of the 'lee bow effect', but if you drill down, you will find other people have different definitions of 'lee bow effect' ranging from subtly to grossly at variance with yours.
TLouth7
Well-Known Member
I am about to sail across the North Sea, a journey approximately East-West with tides that run North-South. This crossing will take me long enough that for some sections the tide will be heading North and others South. The forecast is for consistent Easterlies which means I need to beat the whole way. There is not very much to avoid on either side of my rhumb line. Thus I would say that, pretty much, other things are equal.
Thus I select my first tack based on the current tidal stream, and know that the best moment to tack back will be when the tide turns.
Absolutely, which is why the lee bow effect is only relevant when there is a change of tidal stream (either in time, or in space).lw395 said:
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lw395
Well-Known Member
Sail slowly into a current? good way to minimise VMG!
lw395
Well-Known Member
Now you're talking.
But this is an interesting point. If you take the ground wind to be constant, then the current is shifting the 'true' wind relative to the water, so what you are doing is simply sailing on the lifted tack, as observed relative to the water.
TLouth7
Well-Known Member
Absolutely, and once you can appreciate that you can also see that it is not necessary to pinch or otherwise sail weirdly in order to capitalise on the effect.
lw395
Well-Known Member
What's really significant is when there's also changes of wind.
When the tide is strong, a small windshift can make a big difference to VMG. A 5 degree shift takes you from lee bowing to hopelessly plugging tide.
RupertW
Well-Known Member
You are really not getting this are you - the full effects of lee bowing are when you aren’t sailing into a current - it’s at right angles to the true wind. Of course you can get a partial effect if it’s slightly different to a right angle.
There is nothing vague about an apparent wind shift when the boat is being bodily moved sideways by the wind, it’s a simple vector change. So if the wind is dead ahead and the tide is dead on the side then the apparent wind shifts a few degrees downtide (I.e. if the tide is coming from your port side, then the apparent wind is shifted from the true wind direction of dead ahead to being a few degrees off your starboard bow).
So if you go on a tack so that your lee bow is facing the tide then you can get a better angle to the true wind based entirely on the shift in the apparent wind. On the other tack the angle to the true wind is made worse by that same tide.
Of course the lee bow advantage increases with speed of tide and decreases with yacht speed but it’s still there and most evidenced in strong tides and slow boats.
