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

[europaflyer]

Originally Posted by Seaworthy Lass

Valid.



Now that made me take a second look. And a third and a fourth. I have even tried holding it upside down and back to front. Standing on my head did not help either. Did I really say that sector had the current on the windward side?

What was I thinking?
It actually stemmed from a diagram days ago and I didn't look again this time around, just merrily reached for the yellow texta (it did feel so good colouring it in LOL).

The only thing I can say is that thankfully no bets were accepted. Whew! Bedtime here I think. Maybe I can just scrub a few of my posts before I go to bed . All that voluntary hard work behind the scenes must have some rewards.

Not to worry.

I'm really not trying to dispute that a lifting current can come from the windward side of the boat, we are in total agreement on that point. Note that I am talking about lifting compared to the other tack - it is worth noting that once you factor in headwind there is a big zone in the middle of your 'yellow zone' where both tacks are being knocked compared to at slack water (in the same way that when the current is against the true wind there is a zone where both tacks are lifted compared to slack). It took me a while to realise that there is a difference between talking about the effect relative to the other tack, and relative to slack water. It happened yesterday as your point about the 'bow effect' finally struck home, and I feel it is key to understanding this problem. I hope you understand this point too, in fact you probably got there ahead of me.

There is no 'lee bow effect' in the literal sense, but the most pronounced cases of 'the effect' as we are now talking about it are when the current happens to be on the lee bow... yes? This doesn't preclude a lifting current to windward, but the effect isn't nearly as strong in these cases.

[Dockhead]

Originally Posted by europaflyer

Not to worry.

I'm really not trying to dispute that a lifting current can come from the windward side of the boat, we are in total agreement on that point. Note that I am talking about lifting compared to the other tack - it is worth noting that once you factor in headwind there is a big zone in the middle of your 'yellow zone' where both tacks are being knocked compared to at slack water (in the same way that when the current is against the true wind there is a zone where both tacks are lifted compared to slack). It took me a while to realise that there is a difference between talking about the effect relative to the other tack, and relative to slack water. It happened yesterday as your point about the 'bow effect' finally struck home, and I feel it is key to understanding this problem. I hope you understand this point too, in fact you probably got there ahead of me.

There is no 'lee bow effect' in the literal sense, but the most pronounced cases of 'the effect' as we are now talking about it are when the current happens to be on the lee bow... yes? This doesn't preclude a lifting current to windward, but the effect isn't nearly as strong in these cases.

And we slouch towards total consensus

What a great thread. I wish I had time to make a YM article out of it.

[europaflyer]

Originally Posted by Dockhead

And we slouch towards total consensus

Phew, its been a heck of a ride.

[Seaworthy Lass]

Originally Posted by Dockhead

And we slouch towards total consensus

What a great thread. I wish I had time to make a YM article out of it.

Yes, it was a great thread. I really enjoyed it . Many thanks for raising the issue.

I think that we do finally have consensus, although a few stragglers still maintain that pinching to put the current on the lee bow is beneficial. The fact that at the swap over point having the current on the windward bow provides a lift as well, in my opinion truly finally blows that myth out of the water.

I think that is me done for with the LBE issue now. Many thanks to all those who posted. It was a real pleasure exchanging ideas.

Dockhead are you planning to open a new thread on the issues of when to tack with current or just continuing here?

[Hoofsmit]

Originally Posted by s/v Moondancer

So after 581 posts you finally worked out that you put the tide on the bow that allows you stay closest to the rhumb line and closes the tacking angle...great work! Now Seaworthy Lass can continue with her massive thread proving that she knows more about crossing the Channel than the RYA. Moderators, like children, should be seen and not heard!

That's harsh
At least she is trying

[Hoofsmit]

Swl....... Please not yet

Just pop in and dispute mine...... I'm getting there
At this time , one comment ,
Why have you changed from windward to weather bow Dockhead, waves by any chance
Not hull resistance surely?

Or are you all chicken ?
( tongue in cheek...... Again)

[DumnMad]

Count me with the stragglers. I can't get past the fact that hull resistance in the water slows a boat down, that changing the heading will change the wetted hull form, and that a hull is a foil or fin that has lift when at an angle to the apparent water flow.

[Dockhead]

Originally Posted by DumnMad

Count me with the stragglers. I can't get past the fact that hull resistance in the water slows a boat down, that changing the heading will change the wetted hull form, and that a hull is a foil or fin that has lift when at an angle to the apparent water flow.

The key word here is apparent water flow. You've almost got it Everything you write is true; you just have the wrong frame of reference. It's the water -- apparent water flow, as you wrote. Not the ground. You are floating in the current, not tied to the ground, so you don't feel it.

If you have sails down and no way on, and zero boat speed, and you're being swept by a 10 knot current, in what way does your boat behave differently, than when you have sails down, no way on, zero boat speed, but slack tide? Answer: in no way is it different. It's exactly the same. You need apparent water flow -- just as you wrote! -- to have any interaction with the hull, keel, or rudder.

[DumnMad]

Who on earth was talking about a drifting boat? And, who on earth is talking about feeling the ground? I thought we were talking about a boat on the move.

[Paul Elliott]

DumnMad and Hoofsmit, can you please show us a diagram, or describe with numbers, a case where the current generates hydrodynamic forces on a sailboat (or powerboat) that are any different than those generated when the boat is moving through slack water? You can have the boat sail in any direction. Let it tack if you like, or just have it sail in a straight line. In each case let the wind speed and direction be identical as measured relative to the water.

This is one of the many reasons why we we use water-referenced wind when analyzing sailboat performance. If our boats were on land and had wheels, we would use ground-referenced wind.

Nobody is disputing that keels, rudders, and hulls generate lift and drag. Of course they do, otherwise we could do nothing but be blown dead downwind.

[DumnMad]

The current causes the sailor to change the angle at which the boat is aligned to the wind power in order to get to the destination. This results in a changed wetted hull shape with respect to the apparent water flow and therefore changes the forces on the hull.

[Paul Elliott]

Originally Posted by DumnMad

The current causes the sailor to change the angle at which the boat is aligned to the wind power in order to get to the destination. This results in a changed wetted hull shape with respect to the apparent water flow and therefore changes the forces on the hull.

If that's your point then I don't think anybody here would disagree.

When there is a current our optimum course can be different from the no-current course. Current and Ground Wind affect the True Wind (water-ref), and (as if I need to say it) they also affect the Apparent Wind as seen on the deck of the boat. Course changes also affect the Apparent Wind. Any change in Apparent Wind will change the angle of attack (or leeway) of the foils, and change the shape of the immersed hull, thus affecting the hydrodynamics.

Can you show me where anybody has claimed otherwise?

[Seaworthy Lass]

Originally Posted by s/v Moondancer

So after 581 posts you finally worked out that you put the tide on the bow that allows you stay closest to the rhumb line and closes the tacking angle...great work!

Thank you . That is very kind of you.

You don't have it quite right yet though, so obviously 581 posts were not enough for you to understand.

- The correct conclusion is: "The tack most lifted by the current is the one where the current is closest to being on the nose".

- The lifted tack need not necessarily be the one you will select if conditions are not stable or the current does not cancel out on a leg.

- I also believe: "The rhumb line has nothing to do with it necessarily", it is the tack that is closest to the CTS that you need to consider. You may well be laying the mark for one close hauled position, so there is actually no need to tack at all.

That last comment is really very important and may stop you zig zagging unnecessarily around the course. Showing this is the next thing we are turning our minds to.

Originally Posted by s/v Moondancer

Moderators, like children, should be seen and not heard!

Did you get put on the wrong side of bed yesterday? Hope you are feeling cheerier today .

If a thread or its contributors annoy you, then please put the thread on ignore.

[Dockhead]

Originally Posted by DumnMad

Who on earth was talking about a drifting boat? And, who on earth is talking about feeling the ground? I thought we were talking about a boat on the move.

Look, think about it like this:

No current, zero boat speed -- zero forces.

10 knots of current, zero boat speed -- zero forces - identical.

No current, 6 knots of boat speed -- certain forces.

10 knots of current, 6 knots of boat speed -- those exact same forces.


All the forces come from the relative motion between boat and water. I brought up the boat dead in the water only because it would be most obvious in that case.

[Dockhead]

Originally Posted by Seaworthy Lass

- I also believe: "The rhumb line has nothing to do with it necessarily", it is the tack that is closest to the CTS that you need to consider. You may well be laying the mark for one close hauled position, so there is actually no need to tack at all.
.

Interesting question --

Some Channel sailors believe that the LBE works because it shortens your distance sailed over ground. This is obvious nonsense, and it threw me off -- that's why when we started this thread, my hunch was that there is no such thing.

Now we know that there definitely is such a thing, and lee bowing, at least in the standard Channel situation does keep you closer to the rhumb line.

Interesting coincidence.