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

[Seaworthy Lass]

Originally Posted by Dockhead

I was not joking at all. It is of course a technique that works. I have a degree in philosophy

It is equally valid in math, and is sometimes called "one of the mathemetician's finest weapons" Reductio ad Absurdum -- from Wolfram MathWorld

See also: Proof by contradiction - Wikipedia, the free encyclopedia

Have you thought then about the "zero wind, cancelling current over the duration of a journey" situation? Would you agree you can make progress?

[tamicatana]

Maybe this is the same thing as you describe:

In a race recently, current is heading east to west. All boats are heading from north to south towards a mark, all beating to a southerly wind

We go onto the port tack for awhile (westward, as it were) and notice that the boats on starboard seem to be lifted. We then get onto the starboard tack (more eastward) and experience this lift for ourselves. The competition all ignored us for whatever reason, going onto the port tack covering each other. Sometimes it's nice to be ignored.

Staying on starboard, we got picked up all the way to the mark. It was a huge, huge, lift. The competition, having stayed on port, were pushed westward, we assume by the current, and had an amazing amount of distance to cover to get to the mark.

Won the race. The owner/skipper and tactician had seen it, though. Never doubt experience over youth and enthusiasm

[Seaworthy Lass]

Originally Posted by tamicatana

Maybe this is the same thing as you describe:

In a race recently, current is heading east to west. All boats are heading from north to south towards a mark, all beating to a southerly wind

We go onto the port tack for awhile (westward, as it were) and notice that the boats on starboard seem to be lifted. We then get onto the starboard tack (more eastward) and experience this lift for ourselves. The competition all ignored us for whatever reason, going onto the port tack covering each other. Sometimes it's nice to be ignored.

Staying on starboard, we got picked up all the way to the mark. It was a huge, huge, lift. The competition, having stayed on port, were pushed westward, we assume by the current, and had an amazing amount of distance to cover to get to the mark.

Won the race. The owner/skipper and tactician had seen it, though. Never doubt experience over youth and enthusiasm

Tami, there is lots of anecdotal stuff. The boats concerned could have been in a patch of more favourable breeze or current (both either in terms of direction or strength), or slicker, or better balanced with crew weight, or the helmsman and sail trimmers could have been more skilled etc. Tales like this are not proof that the effect is real.

[carstenb]

taking transmitterdans logic to its absurbian limits, lets say you have zero wind and a 50 knot current. magically you can hold your boat just pinched with a bow to the current. you have all your sails up and are moving with the current at a 50 knot speed.

will you get forward motion of the boat due to apparent wind?

actually, no. the apparent wind generated by your current caused movement will be coming at you from the rear, so there will be no forward movement. you may have some apparent wind resistance, causing to move backwards at slower than 50 knots, but it is not forward motion generated by sailpower.

I would venture to propose that if you have apparent (or true) wind from forward, the backwards motion caused by the current will cause a reduction in the forward force your sails make as the backfilling will cancel out some of the forward attack.


applause please

[Seaworthy Lass]

Originally Posted by carstenb

taking transmitterdans logic to its absurbian limits, lets say you have zero wind and a 50 knot current. magically you can hold your boat just pinched with a bow to the current. you have all your sails up and are moving with the current at a 50 knot speed.

will you get forward motion of the boat due to apparent wind?

actually, no. the apparent wind generated by your current caused movement will be coming at you from the rear, so there will be no forward movement. you may have some apparent wind resistance, causing to move backwards at slower than 50 knots, but it is not forward motion generated by sailpower.

I would venture to propose that if you have apparent (or true) wind from forward, the backwards motion caused by the current will cause a reduction in the forward force your sails make as the backfilling will cancel out some of the forward attack.


applause please

Start with the situation where there is zero wind and the current is partly with you. Make it a say a five knot current (not impossible), generating a five knot headwind that you need to tack into. Yes, as you get moving, your boat motion is will alter the angle of this "wind", but there will still be a some angle you can sail at. You will progress forward (as well as the progress with the current that will of course cancel out once the current turns).

Over to you .

[Hoofsmit]

Really not that up on this subject but, just wondered if by having current on you lee bow could this push the bow to windward thus less rudder thus more speed ?

[Seaworthy Lass]

Originally Posted by Hoofsmit

Really not that up on this subject but, just wondered if by having current on you lee bow could this push the bow to windward thus less rudder thus more speed ?

No. Think of the current as a tablecloth you can slide around. The boat in current is just like an object on the tablecloth. The object on the table cloth does not know if the tablecloth is moving, except for the apparent wind generated.

[Paul Elliott]

If we're going to discuss the zero-wind case (or any wind, for that matter), we need to define our planes of reference. If we are referring to zero wind relative to the water, then we can't fill our sails and we drift with the current. If, as seems to be the case, we are talking about ground wind, then of course the current can make a huge difference.

Much of the disagreement is due to differing planes of reference, and things can be clarified significantly if we use water-referenced wind. Once we understand this, we can then start thinking about land-referenced waypoints.

[olepedersen]

Lee bowing works simply because it increases the apparent wind
Ole

[Seaworthy Lass]

Originally Posted by Paul Elliott

If we're going to discuss the zero-wind case (or any wind, for that matter), we need to define our planes of reference. If we are referring to zero wind relative to the water, then we can't fill our sails and we drift with the current. If, as seems to be the case, we are talking about ground wind, then of course the current can make a huge difference.

Much of the disagreement is due to differing planes of reference, and things can be clarified significantly if we use water-referenced wind. Once we understand this, we can then start thinking about land-referenced waypoints.

I am discussing the situation of zero ground wind. The presence of current would mean that there would be true wind.

[Paul Elliott]

Originally Posted by olepedersen

Lee bowing works simply because it increases the apparent wind
Ole

Not by any definition of Apparent Wind that I am familiar with.

Apparent Wind is the wind seen by your boat as a result of its motion through the water. In a constant current, the Apparent (water-referenced) Wind will be the same on either tack. Your progress towards a fixed (land-referenced) mark will be affected by the current.

I think we could all agree on this if we could first all agree on the terminology and reference planes.

[transmitterdan]

Originally Posted by Paul Elliott

Not by any definition of Apparent Wind that I am familiar with.

If apparent wind means anything other than the speed and direction of the wind as viewed by a person (or instrument) stationary on the boat in question then how can it be a useful concept? The boat is the only frame of reference the sails have. It matters not whether that wind is due to a difference in air pressure or the boat is drifting in a current or the boat is being propelled by the sails. The net effect of everything sums up as apparent wind.

[hellosailor]

"keep the lee bow into the tide. "
TIDE refers to the vertical change in water level. As opposed to the horizontal movement, which is CURRENT. So if someone tells you to keep the bow into the tide, they're confused and a grain of salt may be needed.

But assuming your boat has a keel and rudder, it has airfoils [sic] in the water, and any time you place an airfoil at angles to the air(water) flow, the foul will be pushed across that flow. Contributing to the "forward" speed of the boat, and yes, that means there is a favored side and an unfavored side, relative to the direction you might want to be going in, depending on how that matches the direction the current is or isn't coming from.

Kayakers also do this all the time, in order to get "upstream" to a favored side faster.

Like gybing, instead of running downwind, it may not seem obvious but it usually is the faster way to go. Just depends on how the vectors add up, depending on the relative directions of the current and your course to the goal.

[Dockhead]

Originally Posted by Seaworthy Lass

Have you thought then about the "zero wind, cancelling current over the duration of a journey" situation? Would you agree you can make progress?

Sure, it's pretty obvious that you drift with the current, right? I wasn't sure what the point of that Gedankenexperiment was.

You stay in one place relative to the water. Right? Or what was the point?

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

Current moves the boat through the air and creates an apparent wind.

Just looking back over my terminology and found that I need to amend the above sentence to:
Current moves the boat through the air and alters the apparent wind.