Let's Get to the Bottom of the Lee-Bow Effect Once and For All

[I.Grind]

Originally Posted by Hoofsmit

this brings us right back to post *4 bt Dockhead who posted a link to this Lee-Bow Effect: This brings up the famous lee-bow effect. If the current is coming at you at an angle that is very close to the course you are sailing and if, by pinching just a little bit, you can get your lee bow into the flow of the current, the movement of the water is going to push against the hull, the keel, and the rudder, and it is goign to drive you up to windward even though you are going slower over the bottom. If you are on the other tack the current is going to be hitting you broadside and pushing you down. If you can get the lee-bow effect to push you to windward, I feel you also increase the wind pressure on the sails. If I am on the tack that goes across the current I feel I am losing speed and distance to the mark. That is why, unless there is an obvious way to get out of the current entirely, or at least to a slower flow, I think you should always make your longest tack to the next mark sailing in the lee-bow position. And I would do this even if it meant pinching a bit to do so.

Aaaaaahh now I get it.

Wait, people believe that?

[Seaworthy Lass]

Originally Posted by I.Grind

Aaaaaahh now I get it.

Wait, people believe that?

They certainly do, as they say it works in practice. I can only think that if they find that it gets them to the mark more quickly than the rest of the fleet, it is because the rest of the fleet misjudged their final tack (possibly because of wind swinging and or current altering). It has absolutely nothing to do with which side of the bow the current is on.

See this discussion and read how many people report it works in certain conditions:
https://groups.google.com/forum/m/#!...ng/TFWS6xe55KQ

[Hoofsmit]

seaworthy, thanks for the diagram

as I see it, there are only two vectors on each tack that could be described as lee bowing the current
are we agreed... that any other vector is not what the term describes ?

[estarzinger]

Just about the "boat floating in the current, so the current dies not matter" thought . . . . Airplanes float in the air but the wind does make a difference when they are landing. You always land into the wind so that your ground speed can be slower. And if there is a cross wind you crab in.

So if the destination is a ground reference destination point, the wind/current does certaintly matter.

Similarity, sailing on the lifted side of shifts (wind or current) and tacking on headers will get you to a (ground referenced) mark faster.

Probably the reason 'lee bow' works 'in practice' is that current is rarely steady across a course, so it makes sense to be on the lifted tack when it is strong enough to be noticed and then the other tack where the current is less.

[Hoofsmit]

Originally Posted by estarzinger

Just about the "boat floating in the current, so the current dies not matter" thought . . . . Airplanes float in the air but the wind does make a difference when they are landing. You always land into the wind so that your ground speed can be slower. And if there is a cross wind you crab in.

So if the destination is a ground reference destination point, the wind/current does certaintly matter.

Similarity, sailing on the lifted side of shifts (wind or current) and tacking on headers will get you to a (ground referenced) mark faster.

Probably the reason 'lee bow' works 'in practice' is that current is rarely steady across a course, so it makes sense to be on the lifted tack when it is strong enough to be noticed and then the other tack where the current is less.

totally agree it is a tactic not a cts calculation

[Hoofsmit]

really do view the term as 'edgeing your bets'

or minimalising unforeseen negative affects of a course to steer to a static point

[foolishsailor]

Angela directed me to this thread as she knows I love me some racin' - especially in the Solent which has crazy tides and tidal gradients.

I thought I would throw my two cents in, but want to clarify a bit about difference between tactics and strategy.

Tactics is the use of boat on boat actions to create advantage. An example of this would be when you are in the lee position very close to another boat you could pinch up to try to force them to tack away or to try to put them into bad air and slow them down. This can also be when you are not close to boats. If you feel that even though you are on an advantageous tack and are in the lead and you see the rest of the fleet, or boats that are in contention, go the opposite direction you might consider that abandon your current strategy and "cover the fleet" by tacking over - even if you feel this is not the optimal direction. You have kept yourself between the fleet and the mark and will experience the same wind and current as the fleet so no advantages can be gained be a mistake in your strategy.

Strategy is the use of a sum of vectors to sail the shortest distance and the highest VMG to a given mark. The vectors are Wind Direction and Speed, Current Direction and Speed, and most importantly the ΔT (change over time) of these vectors - which brings me to the reason of my post.

If a course is static (which doesnt exist in real life) and there is perfectly constant wind direction and speed and current direction and speed across the whole of the course for the whole of the race then there can be no gains found for any boat with any strategy that give it an advantage over another boat.

i.e. The lee bow effect does not exist.

Why?

Boats A,B and C all sail the exact same distance and spend the exact smae amount of time on each tack.

1. In an environment where there is no change in vectors over times all boats sail the same distance and time on each tack and therefore would all benefit equally. Regardless of where they go or how many tacks they make they are all equal (taking out any loses made during the tack itself in large boats or gains made by roll tacking in smaller boats)

2. There is no "current" - you are sailing through still water. All boats are moved the same distance by the current regardless of sailing angle or tack. This was covered very well by the "Tablecloth" analogy in the other thread

3. All boats experience the same amount of header and lift effect with the wind due to the current since they spend the exact amount of time on each tack.

4. Pinching up to get the "current" to push your lee bow isnt possible. The "current" doesnt push your boat. You are sailing through still water and the whole body of water (including all the other boats) is moving as a single mass over the ground. You are decreasing your VMG by slowing the boat down by pinching.

5. There is no benefit gained by staying on a given tack longer to take advantage of the lift caused by the current as all boats have to sail the same distance on the same tack - on a race course with marks where there are no changes in wind and current over time or geography every boat will spend the same amount of time over a given leg on starboard and port tacks whether you sail to the layline on a long tack or short tack up the middle it is irrelevant - smae distance and time for every boat.

Once you introduce ΔT to the equation then you are no longer talking about the mythical "lee bow effect" but instead discussing Wind and Current Strategy. Once there are differences in wind and current on the location of the course (one side has a stronger tidal stream gradient) or based on time (there is a fixed period 30degree wind oscillation) then various strategies work - but not the lee bow effect.

I dont know anyone personally who believes in this effect, they would be more proponents of Positioning and Strategy which is planning for and estimating the change in location and time of these vectors and planning how to be in position to take advantage of them.

In fact I would venture to say those that do believe in it and claimed to have won races with it are actually merely benefiting by placing themselves in the right place at the right time benefiting from the right combination of vectors of force.

edit: for clarity on the image attached i am assuming the following:

1. These are race boats with a tacking angle of 70°
2. The boats have a STW of 6kts
3. The current of 2kts is coming parallel to the starboard layline and add ~33% to the Port Tack layline - meaning to lay the mark your COG on Port Tack would be 35° + 33% = 46.55°

[Seaworthy Lass]

Originally Posted by estarzinger

Just about the "boat floating in the current, so the current dies not matter" thought . . . . Airplanes float in the air but the wind does make a difference when they are landing. You always land into the wind so that your ground speed can be slower. And if there is a cross wind you crab in.

So if the destination is a ground reference destination point, the wind/current does certaintly matter.

Hi Evans
Yes, ground referenced current and wind certainly do matter . But if the current and wind are steady then the only way they matter is in determining your CTS. If you can't achieve this you need to tack. You pick the favourable tack (ie the one closest to the CTS) first only because it makes judging your final tack easier and you may pick up a lift along the way allowing you to sail even closer to the CTS. The favourable tack is not necessarily the tack that is lifted by the current if the mark is not directly into the ground wind. All this has nothing to do with the current being on the lee bow. The lifted tack by the way is the one with the current closer to being off the bow compared to the other tack - again nothing to do with the current being specifically on your lee side either.

Originally Posted by estarzinger

Similarity, sailing on the lifted side of shifts (wind or current) and tacking on headers will get you to a (ground referenced) mark faster.

Yes, I fully agree. But none of this has anything to do with the keeping the current on your lee side either.

Originally Posted by estarzinger

Probably the reason 'lee bow' works 'in practice' is that current is rarely steady across a course, so it makes sense to be on the lifted tack when it is strong enough to be noticed and then the other tack where the current is less.

Does it work in practice? It all seems to be anecdotal from casual racers. If the current is not steady then the tack you must start on is the one which takes you to the side if the CTS where you are going to be knocked later (ie where the true wind will alter adversely due to a change in ground wind direction or a change in current). Again picking the correct tack has nothing specifically to do with the current being on the lee side.

So how does this famed Lee Bow Effect actually help you?

[Hoofsmit]

pauls quote:
Now place your boat, sailing the same speed but in a current. Your entire boat will be swept by the current, but your movement through the water will be identical. The flow diagram will be the same. Your movement over the ground will be affected by the current, but not your movement through the water.

ok trying to explain the logic

you are on a swinging mooring in a tidal stream on your nose
you put your helm to port - you move to starboard your mooring line goes under tension and your laterial position is held.

now put the current on your port bow by say 10 degrees and say the wind is 35 degrees on your starboard side you put up you sails and the wind speed affect is just right to keep you in the same position, you let go the mooring and stay in the same position .

where is the current now hitting my foils?

yes there is forward current diversion but on my hull and foils but the lift to windward is still happening, its just being counteracted by leeway induce laterial movement to port

am I wrong?

[Seaworthy Lass]

Originally Posted by foolishsailor

Angela directed me to this thread as she knows I love me some racin' - especially in the Solent which has crazy tides and tidal gradients.

I thought I would throw my two cents in, but want to clarify a bit about difference between tactics and strategy.

Tactics is the use of boat on boat actions to create advantage. An example of this would be when you are in the lee position very close to another boat you could pinch up to try to force them to tack away or to try to put them into bad air and slow them down. This can also be when you are not close to boats. If you feel that even though you are on an advantageous tack and are in the lead and you see the rest of the fleet, or boats that are in contention, go the opposite direction you might consider that abandon your current strategy and "cover the fleet" by tacking over - even if you feel this is not the optimal direction. You have kept yourself between the fleet and the mark and will experience the same wind and current as the fleet so no advantages can be gained be a mistake in your strategy.

Strategy is the use of a sum of vectors to sail the shortest distance and the highest VMG to a given mark. The vectors are Wind Direction and Speed, Current Direction and Speed, and most importantly the ΔT (change over time) of these vectors - which brings me to the reason of my post.

If a course is static (which doesnt exist in real life) and there is perfectly constant wind direction and speed and current direction and speed across the whole of the course for the whole of the race then there can be no gains found for any boat with any strategy that give it an advantage over another boat.

i.e. The lee bow effect does not exist.

Why?

Boats A,B and C all sail the exact same distance and spend the exact smae amount of time on each tack.

1. In an environment where there is no change in vectors over times all boats sail the same distance and time on each tack and therefore would all benefit equally. Regardless of where they go or how many tacks they make they are all equal (taking out any loses made during the tack itself in large boats or gains made by roll tacking in smaller boats)

2. There is no "current" - you are sailing through still water. All boats are moved the same distance by the current regardless of sailing angle or tack. This was covered very well by the "Tablecloth" analogy in the other thread

3. All boats experience the same amount of header and lift effect with the wind due to the current since they spend the exact amount of time on each tack.

4. Pinching up to get the "current" to push your lee bow isnt possible. The "current" doesnt push your boat. You are sailing through still water and the whole body of water (including all the other boats) is moving as a single mass over the ground. You are decreasing your VMG by slowing the boat down by pinching.

5. There is no benefit gained by staying on a given tack longer to take advantage of the lift caused by the current as all boats have to sail the same distance on the same tack - on a race course with marks where there are no changes in wind and current over time or geography every boat will spend the same amount of time over a given leg on starboard and port tacks whether you sail to the layline on a long tack or short tack up the middle it is irrelevant - smae distance and time for every boat.

Once you introduce ΔT to the equation then you are no longer talking about the mythical "lee bow effect" but instead discussing Wind and Current Strategy. Once there are differences in wind and current on the location of the course (one side has a stronger tidal stream gradient) or based on time (there is a fixed period 30degree wind oscillation) then various strategies work - but not the lee bow effect.

I dont know anyone personally who believes in this effect, they would be more proponents of Positioning and Strategy which is planning for and estimating the change in location and time of these vectors and planning how to be in position to take advantage of them.

In fact I would venture to say those that do believe in it and claimed to have won races with it are actually merely benefiting by placing themselves in the right place at the right time benefiting from the right combination of vectors of force.

edit: for clarity on the image attached i am assuming the following:

1. These are race boats with a tacking angle of 70°
2. The boats have a STW of 6kts
3. The current of 2kts is coming parallel to the starboard layline and add ~33% to the Port Tack layline - meaning to lay the mark your COG on Port Tack would be 35° + 33% = 46.55°

Brilliant post! Many thanks for contributing. So good to hear from a serious racer.
My use of the term 'tactics' was wrong, so thanks for clearing that up.

I agree totally with everything you have said (gosh, I rarely say that even to my hubby ). It reinforces what I have been jumping up and down about in the last few posts LOL (ignore some of the guff I wrote earlier before I drew that cone diagram).

[Hoofsmit]

In fact I would venture to say those that do believe in it and claimed to have won races with it are actually merely benefiting by placing themselves in the right place at the right time benefiting from the right combination of vectors of force.

yep .......that would be me

probable my local harbour had those " right combination of vectors" on so many occasions , it made me believe the term applied

[foolishsailor]

Originally Posted by Hoofsmit

pauls quote:
Now place your boat, sailing the same speed but in a current. Your entire boat will be swept by the current, but your movement through the water will be identical. The flow diagram will be the same. Your movement over the ground will be affected by the current, but not your movement through the water.

ok trying to explain the logic

you are on a swinging mooring in a tidal stream on your nose
you put your helm to port - you move to starboard your mooring line goes under tension and your laterial position is held.

now put the current on your port bow by say 10 degrees and say the wind is 35 degrees on your starboard side you put up you sails and the wind speed affect is just right to keep you in the same position, you let go the mooring and stay in the same position .

where is the current now hitting my foils?

yes there is forward current diversion but on my hull and foils but the lift to windward is still happening, its just being counteracted by leeway induce laterial movement to port

am I wrong?

The problem is you are not including the Vector of force created by your mooring attachment. Instead of thinking Wind, Current, etc. - think magnitude and Direction.

Think of the Mooring attachment in this way:

The tidal stream is 5 knts and your are held in place by the mooring attachment. The mooring attachment is exerting a force equal to that required to move your boat through the water at 5kts.

It is the exact same as if you are being pulled through a perfectly still lake by a motorboat at 5kts. The exact same effects of moving the rudder and lift gained on the foils by being pulled through the water as being held in place by a fixed object and the water moves past you at 5 kts.

The confusion here is tied to understanding frame of reference. Your boat is "attached" to the water. The current affects how you boat interacts with the wind as it generates an apparent wind shift, however lets take a motor boat so that we can ignore tha issues with apparent wind, which are affected by current.

If your motor boat is has a STW of 10kts regardless of what direction you move through the water in relation to the waters direction of travel in relation to land your STW will be the same. Your SOG will change but your STW wont.

Since your STW is the same regardless of your orientation to the current it means that there is no additional "flow" over the foils of rudder and keel regardless of orientation to current.

[foolishsailor]

Originally Posted by Hoofsmit

In fact I would venture to say those that do believe in it and claimed to have won races with it are actually merely benefiting by placing themselves in the right place at the right time benefiting from the right combination of vectors of force.

yep .......that would be me

probable my local harbour had those " right combination of vectors" on so many occasions , it made me believe the term applied

Your harbor didnt have anything magical - you were the one who put yourself in the optimal position at the optimal time to take advantage of changes in wind, current direction and speed. But it has nothing to do with a lee bow effect.

[Hoofsmit]

thinking about it , in my bi weekly racing in my local harbour this was the case on two out of the 3 legs we would sail, both had renowned wind veers from the harbours geography that would force you to pinch to make the mark. keeping the tide on your lee bow ensured you would make it. tack on the veer and you where dead meat.

[foolishsailor]

If you are pinching to make the mark then that means you tacked short on the lay line, knowing in advance that there is a consistent geographically induced veer that left you pinching to reach the mark previously should tell you that the next time you are tacking on the lay line for the mark that you should extend further to allow you to lay the mark under optimal VMG.

There is never a time that pinching is a viable strategy for speed.

Pinching can be and is used frequently tactically however. Usually to recover from a bad tactical decision made earlier though. For example if you end up inside boat approaching the windward mark but are below the lay line and have overlap with the boats to windward and can't tack if you are close enough you can call for room and pinch up to clear the mark.

The situation you describe still has nothing to do with Lee Bow Effect it is just about current and wind - you are not gaining an advantage on other boats because of the orientation of your bow to the current - you are gaining advantage because you are entering a wind veer on optimal tack.

edit: Also Veer is when the wind moves clockwise and Back is counterclockwise so if you are approaching a mark on starboard tack for a port rounding a veering wind would lift you above the mark meaning you would have to foot to make it versus pinch and the opposite for a port tack starboard rounding. Starboard roundings are less common for anything other than gybing marks, gates or non-windward or leeward marks that dont involve major course changes to the fleet as they present dangerous crossing situations due to Port-Starboard rules