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

[I.Grind]

Thanks hoofsmit, I just had to clear that up. Over 100 posts and nothing getting sorted was starting to make me crazy. I try to stay out of these things, but I got sucked in.

[Hoofsmit]

Originally Posted by Hoofsmit

In the above scenario , you can now use some maths or trig and your boats polars to work out if pinching is more productive with a particular tide to wind ratio that turning on a loosing tack

Seaworthylass : did you read this?

[I.Grind]

I missed that one, you did beat me to it!

[Hoofsmit]

Originally Posted by Seaworthy Lass

Explains a lot LOL (Sorry, GBN, I am no angel. I have been so good lately, but I just couldn't resist that )

Oooooer that a dig at me or the RYA ....... I know I'm not perfect

[Hoofsmit]

Originally Posted by Hoofsmit

Oooooer that a dig at me or the RYA ....... I know I'm not perfect

Maybe it's the beans..... . Thus more apparent wind..... Sounds a good theory to me

[Seaworthy Lass]

Originally Posted by Hoofsmit

Oooooer that a dig at me or the RYA ....... I know I'm not perfect

The RYA.
We have a history .

Hoof, I may point out your errors, but I would never have a dig at you
(I know, I know, I should be better behaved with the RYA too.)

[Dockhead]

Originally Posted by I.Grind

When sailing upwind lee bow to the current, the current will sweep you towards the mark increasing your VMG. While theoretically the apparent wind will indeed shift aft, giving a lift, in my experience the effect is not nearly as noticeable as that of being swept by the current towards the mark.

.

I don't understand "sweeping you towards your mark". If the current is adding a velocity vector towards your mark, then it's going to "sweep" you there in any case, isn't it, regardless of whether your lee bow, windward bow, or transom is presented to the current?

Maybe you can explain what you mean, and what it has to do with lee bowing.

[Seaworthy Lass]

Sidetrack here:

Has anyone considered that the creation of a favourable tack doesn't just work if the current is on your lee bow? It works all the way around to roughly 135 degrees off your bow.

Shall we rename it the 'lee bow and amidships effect' now LOL?

I.Grind, I need to thank you for this thought. I was in the middle of replying to your post about being swept to the mark, when I got distracted by this thought. Throwing thoughts around is really helpful, even if they are not correct. It often leads to new things .

[europaflyer]

Dare I enter the fray? Can't resist...

I think people complicate the lee-bow effect somewhat. The apparent wind is the vector sum of the true wind, the headwind (equal and opposite to the boats speed) and the current induced wind (equal and opposite to the current). By putting the 'current induced wind' on the correct side of the boat, you will be able to point closer to the true wind (but, of course, no closer to the apparent wind... which is where a lot of the confusion comes from I think).

The effect is most pronounced in light airs, as the 'current induced wind' represents a far greater component of the 'apparent wind' than it would if the true wind were stronger. Take for example a scenario where true wind is 10kt, at 90 degrees to a 5 knot current. Boat speed is 5 knots. (diagram below... hopefully)

It can be seen that the 'lee bow' boat points much closer to the true wind than the 'weather bow' boat, perhaps by around 60 degrees. This results in a course through the water much more direct to windward, and therefore substantially greater VMG to windward for the lee-bow boat. This is what the lee bow effect is all about. It is worth noting that this difference would reduce as true windspeed increases, but would still remain significant.

So... even though the motion of the tide does not itself contribute to the VMG to windward of either boat, the change in apparent wind it produces makes a substantial difference in how close the boats sail to the 'true wind', and therefore their VMG to windward.

The contribution of the tide to the COG (COG = course through water + tide), further helps the 'lee bow' vessel makes a COG which is more direct to windward. This, although it may be of use in the real world, DOES NOT affect course through the water and therefore VMG to windward, and should not be confused with the lee bow effect.

I lost track WAY back of who I am agreeing/disagreeing with here, so I look forward to trying to defend what I've just written!

[Seaworthy Lass]

Originally Posted by I.Grind

When sailing upwind lee bow to the current, the current will sweep you towards the mark increasing your VMG. While theoretically the apparent wind will indeed shift aft, giving a lift, in my experience the effect is not nearly as noticeable as that of being swept by the current towards the mark.

Huh? You are being lee bowed when the current is on your lee bow when you are close hauled. Unless the current is more than 90 degrees off your bow (ie not off your bow any longer) it will be pushing you away from the mark. Not towards it.

Originally Posted by I.Grind

The only example I can conceive of where pinching would be beneficial, would be if one were not quite fetching the windward mark by a very small margin in light air and strong current. By pinching forward momentum would be decreased allowing the current to sweep the boat past the mark whereupon falling off would allow for a rounding. This only applies when wind and current are such that if the boat were tacked the lost ground could not be regained.

Huh? How can the current on your lee bow sweep you to the mark if is is on your bow. Have you managed to somehow overshoot the mark?

[Seaworthy Lass]

Originally Posted by europaflyer

Dare I enter the fray? Can't resist...

I agree with all you have said. It does not contradict anything I have posted, but you say it more eloquently .

Thanks for the lee bow definition. It is just purely the creation of a favourable tack then, not an advantageous one.

I am still thinking about the advantageous tack issue .

[Hoofsmit]

Originally Posted by europaflyer

Dare I enter the fray? Can't resist... I think people complicate the lee-bow effect somewhat. The apparent wind is the vector sum of the true wind, the headwind (equal and opposite to the boats speed) and the current induced wind (equal and opposite to the current). By putting the 'current induced wind' on the correct side of the boat, you will be able to point closer to the true wind (but, of course, no closer to the apparent wind... which is where a lot of the confusion comes from I think). The effect is most pronounced in light airs, as the 'current induced wind' represents a far greater component of the 'apparent wind' than it would if the true wind were stronger. Take for example a scenario where true wind is 10kt, at 90 degrees to a 5 knot current. Boat speed is 5 knots. (diagram below... hopefully) It can be seen that the 'lee bow' boat points much closer to the true wind than the 'weather bow' boat, perhaps by around 60 degrees. This results in a course through the water much more direct to windward, and therefore substantially greater VMG to windward for the lee-bow boat. This is what the lee bow effect is all about. It is worth noting that this difference would reduce as true windspeed increases, but would still remain significant. So... even though the motion of the tide does not itself contribute to the VMG to windward of either boat, the change in apparent wind it produces makes a substantial difference in how close the boats sail to the 'true wind', and therefore their VMG to windward. The contribution of the tide to the COG (COG = course through water + tide), further helps the 'lee bow' vessel makes a COG which is more direct to windward. This, although it may be of use in the real world, DOES NOT affect course through the water and therefore VMG to windward, and should not be confused with the lee bow effect. I lost track WAY back of who I am agreeing/disagreeing with here, so I look forward to trying to defend what I've just written!

Yep ...... I see my understanding of the term is simplistic, re. Your second to last paragraph,

Your explanation of a better pointing angle makes sense to me ( mainly because the terminology relating to true and apparent wind is more along the lines of which I understand it)

Thanks ....... I might now leave the thread to the other views on here

[europaflyer]

Originally Posted by Seaworthy Lass

Sidetrack here:

Has anyone considered that the creation of a favourable tack doesn't just work if the current is on your lee bow? It works all the way around to roughly 135 degrees off your bow.

In my mind it works for any angle of current, other than two points (roughly, to windward and to leeward) where it makes no difference to the direction of the apparent wind vector. At these points, it switches from helping to on one tack (coming from your leeward side) to hindering you (coming from windward).

It is a very, very subtle point, that I can picture but not quite explain properly (yet), that these 'switchover points' are not exactly to windward and leeward. Picturing it using my 'sum of vectors' approach for the wind as above, it can be seen that these points where the 'current induced wind vector' does not affect the 'apparent wind vector' are when the tide is just slightly to port of windward and leeward when on starboard tack, and just slightly to starboard when on port tack. As I say, this is a VERY subtle (ie. totally unimportant!) point, that I may try to support with a diagram if I have the time later. No, I'm not sure if I'm right either.

Oh and by the way Seaworthy, what is this favourable vs advantageous tack issue? Probably being thick but I don't quite understand the difference...

[Dockhead]

Originally Posted by europaflyer

Dare I enter the fray? Can't resist...

I think people complicate the lee-bow effect somewhat. The apparent wind is the vector sum of the true wind, the headwind (equal and opposite to the boats speed) and the current induced wind (equal and opposite to the current). By putting the 'current induced wind' on the correct side of the boat, you will be able to point closer to the true wind (but, of course, no closer to the apparent wind... which is where a lot of the confusion comes from I think).

The effect is most pronounced in light airs, as the 'current induced wind' represents a far greater component of the 'apparent wind' than it would if the true wind were stronger. Take for example a scenario where true wind is 10kt, at 90 degrees to a 5 knot current. Boat speed is 5 knots. (diagram below... hopefully)

It can be seen that the 'lee bow' boat points much closer to the true wind than the 'weather bow' boat, perhaps by around 60 degrees. This results in a course through the water much more direct to windward, and therefore substantially greater VMG to windward for the lee-bow boat. This is what the lee bow effect is all about. It is worth noting that this difference would reduce as true windspeed increases, but would still remain significant.

So... even though the motion of the tide does not itself contribute to the VMG to windward of either boat, the change in apparent wind it produces makes a substantial difference in how close the boats sail to the 'true wind', and therefore their VMG to windward.

The contribution of the tide to the COG (COG = course through water + tide), further helps the 'lee bow' vessel makes a COG which is more direct to windward. This, although it may be of use in the real world, DOES NOT affect course through the water and therefore VMG to windward, and should not be confused with the lee bow effect.

I lost track WAY back of who I am agreeing/disagreeing with here, so I look forward to trying to defend what I've just written!

Very interesting.

This would be easier to follow if you would use the modern types of wind, as our instruments call them:

Ground Wind: The speed and direction of the wind from the point of view of someone on land, or on a boat which is docked or anchored. Wind in relation to the ground.

True Wind: Wind in relation to the water. So what our instruments will report as True Wind.


So True Wind already includes the "current-induced" wind component, so we can talk about how the current changes True Wind in relation to Ground Wind, and we can even talk about VMG to Ground-Windward, which could be a useful concept here.

If you lump current influences with boat speed influences all into Apparent Wind, you get a mess.



Otherwise, this sounds pretty much like what Seaworthy and I have stumbled towards in the last part of our discussion.

I liked your phrase "get the current-induced wind on the right side of the boat" -- an excellent, pithy, summary of the problem.

[Paul Elliott]

Originally Posted by europaflyer

Dare I enter the fray? Can't resist...

I think people complicate the lee-bow effect somewhat. The apparent wind is the vector sum of the true wind, the headwind (equal and opposite to the boats speed) and the current induced wind (equal and opposite to the current). By putting the 'current induced wind' on the correct side of the boat, you will be able to point closer to the true wind (but, of course, no closer to the apparent wind... which is where a lot of the confusion comes from I think).

The effect is most pronounced in light airs, as the 'current induced wind' represents a far greater component of the 'apparent wind' than it would if the true wind were stronger. Take for example a scenario where true wind is 10kt, at 90 degrees to a 5 knot current. Boat speed is 5 knots. (diagram below... hopefully)

It can be seen that the 'lee bow' boat points much closer to the true wind than the 'weather bow' boat, perhaps by around 60 degrees. This results in a course through the water much more direct to windward, and therefore substantially greater VMG to windward for the lee-bow boat. This is what the lee bow effect is all about. It is worth noting that this difference would reduce as true windspeed increases, but would still remain significant.

So... even though the motion of the tide does not itself contribute to the VMG to windward of either boat, the change in apparent wind it produces makes a substantial difference in how close the boats sail to the 'true wind', and therefore their VMG to windward.

The contribution of the tide to the COG (COG = course through water + tide), further helps the 'lee bow' vessel makes a COG which is more direct to windward. This, although it may be of use in the real world, DOES NOT affect course through the water and therefore VMG to windward, and should not be confused with the lee bow effect.

I lost track WAY back of who I am agreeing/disagreeing with here, so I look forward to trying to defend what I've just written!

You are using "True Wind" for what we have been calling "Ground Wind". We have more or less agreed to call True Wind the wind that is referenced to the water. Some people call this the "Sailing Wind".

In any case, it is critical that we differentiate between the ground-referenced and the water-referenced winds. Otherwise we will be forever arguing in circles.

The True Wind (water) is the vector sum of Ground Wind and the current-induced wind.

The Apparent Wind is the vector sum of True Wind (water) and the speed/course of your boat through the water.

If we are going to discuss the Lee-Bow Effect, we should not be discussing changing currents, or changing Ground Wind, or waves. Ockham Instruments describes the mythical lee-bow effect on their website:

Most people think that when sailing in current there is a “lee bow effect”; current “striking” the leeward side helps while from the windward side hinders performance.

(Lee Bow Effect « Ockam Sailing Instruments Ockam Sailing Instruments) They then proceed to show why this a myth, using the same logic (and math) that some of us here have been using.

We can discuss changing currents, winds, waves, Course To Steer, etc, but I hope we can all agree that there is no lee-bow effect as that term has been traditionally defined.