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Old 24-06-2014, 07:09   #1
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VMG - rhumb line vs. banging the corners

There's an ongoing debate on our racing team about banging the corners vs. tacking along the rhumb line to a windward mark.

Assuming calm seas with no current and no wind shifts:

One side says tacking along the rhumb line is a shorter distance than banging the corners. The other side says the distance is the same.

One side says the VMG is better tacking along the rhumb line than banging the corners and you risk overstanding the mark on a longer layline. The other side says the VMG's average out to be the same but the extra tacks along the rhumb line slow the boat down too much.

What say you and does anyone have a link to an article that definitively addresses the issue?

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Old 24-06-2014, 07:16   #2
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Re: VMG - rhumb line vs. banging the corners

Well, from a geometrical perspective they are the same. But in the conditions you stated (which aren't very realistic) it is better to make shorter tacks because on the long tack to the corner you may overstand the mark. With short tacks you are much closer to the mark when you call your last tack and the consequences of overstanding because you were able to sail a few degrees closer than expected are much less.

David
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Old 24-06-2014, 07:23   #3
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Re: VMG - rhumb line vs. banging the corners

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Originally Posted by djmarchand View Post
Well, from a geometrical perspective they are the same. But in the conditions you stated (which aren't very realistic) it is better to make shorter tacks because on the long tack to the corner you may overstand the mark. With short tacks you are much closer to the mark when you call your last tack and the consequences of overstanding because you were able to sail a few degrees closer than expected are much less.

David
Good point and one which is part of the debate. I edited the argument to include your comment.
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Old 24-06-2014, 07:31   #4
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Re: VMG - rhumb line vs. banging the corners

"Assuming calm seas with no current and no wind shifts"

Therein lies the rub.

IMNSHO:

In that extremely unusual situation, "the other side" is correct. There is no difference in distance or overall VMG. The geometry is quite simple.

Banging the corners will mean less tacks and every tack loses a bit of ground, so banging the corners is a good tactic in that situation


In the more common situation where you do get currents and wind shifts, banging the corners removes the possibility of taking advantage of the changing circumstances and the extra tacks to take advantage of current and wind conditions generally more than pay for themselves.
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Old 24-06-2014, 07:34   #5
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Re: VMG - rhumb line vs. banging the corners

Seems like it's generally accepted that tacking slows the boat down, more tacks less speed, therefor banging the corner equals faster.

Given there are so many variable involved in the real world I have to ask what the context of the debate is? Favored side of the starting line, favored side of the course, current, variable wind direction, etc.
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Old 24-06-2014, 07:47   #6
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Re: VMG - rhumb line vs. banging the corners

Also depends on boat and course length. Hobie cats and 49ers are fast and are slow to tack, so every tack you do more than your opponent makes them farther ahead. The polar curve on Hobies are pretty flat near optimum upwind vmg. You hardly give anything away overstanding a little, especially compared to doing another two tacks.

Mathematically with no time lost tacking banging corners vs rhumbline is no different. Get a piece of graph paper and prove it to yourself. I don't think you'll find an article on something that basic. All articles are going to be on shifts, currents, tactics, strategy, overstanding because you can't tell from a distance, etc.

Stuart Walker's Advanced Racing Tactics is a nice short 400 page dense introduction to the topic:
Advanced Racing Tactics - Stuart H. Walker - Google Books

Dave Perry's Winning in One Designs is good also:
Winning in One-Designs - Dave Perry - Google Books
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Old 24-06-2014, 07:48   #7
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Re: VMG - rhumb line vs. banging the corners

If you are in a race to a windward mark and are concerned about this, it seems like you are either in the lead or way in the back. For everyone else, wouldn't they have other tactical concerns dictating how to sail the mark?

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Old 24-06-2014, 07:51   #8
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Re: VMG - rhumb line vs. banging the corners

Quote:
Originally Posted by Delancey View Post
Seems like it's generally accepted that tacking slows the boat down, more tacks less speed, therefor banging the corner equals faster.

Given there are so many variable involved in the real world I have to ask what the context of the debate is? Favored side of the starting line, favored side of the course, current, variable wind direction, etc.
For the sake of the argument, we are assuming all the factors you mentioned are neutral.

If one accepts that VMG and distance are the same for both alternatives (primary argument) then you are left with the tactical question of whether the additional flexibility to adjust to changing conditions but the time lost on the extra rhumb line tacks is better than the risk of missing the mark on a longer layline (secondary argument).
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Old 24-06-2014, 08:06   #9
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Re: VMG - rhumb line vs. banging the corners

Seems like a weird argument to have (while excluding all variables) since the answer is obvious. We're talking line-of-sight here and if you can't pick a layline from far out I guess you go up the middle.

The only times I can ever recall grossly overstanding from a corner has been the result of windshifts that occurred after tacking to the mark.
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Old 24-06-2014, 08:09   #10
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Re: VMG - rhumb line vs. banging the corners

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Originally Posted by OldFrog75 View Post
For the sake of the argument, we are assuming all the factors you mentioned are neutral.

If one accepts that VMG and distance are the same for both alternatives (primary argument) then you are left with the tactical question of whether the additional flexibility to adjust to changing conditions but the time lost on the extra rhumb line tacks is better than the risk of overstanding the mark on a longer layline (secondary argument).

For your secondary argument it's a how many angels can dance on the head of a pin question. What boat are you sailing? How many seconds or minutes does it take to get back up to speed? How long is your course? What is the probability of overstanding? Overstanding how much? Compute how much time you lose tacking vs time lost overstanding, pick the shorter one. Good luck computing that.

That said even Hobies don't bang the corners on a really big course for them, but keeping the tacks down to minimum is important.

Dinghies like Lasers can come out of a tack faster than they went into it. I don't think twice about tacking a Laser for any excuse on the upwind leg.
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Old 24-06-2014, 08:13   #11
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Re: VMG - rhumb line vs. banging the corners

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Originally Posted by cal40john View Post
Also depends on boat and course length. Hobie cats and 49ers are fast and are slow to tack, so every tack you do more than your opponent makes them farther ahead. The polar curve on Hobies are pretty flat near optimum upwind vmg. You hardly give anything away overstanding a little, especially compared to doing another two tacks.

Mathematically with no time lost tacking banging corners vs rhumbline is no different. Get a piece of graph paper and prove it to yourself.
We (my crew and I,) pretty much proved it during 4 races in a lake in Northern Mississippi during the Broken Mast Regatta of 1997 (steady wind/no current) against 17 other Hobie 16's. We were the only boat banging the corners.............(but also in clear air).

My crew (my son) was 14 at the time with very good vision and 4 years calling tacks so we were on the layline pretty much every time we tacked for A Mark.

Races 5 and 6 on day two there were wind shifts which we tacked on successfully at least once to win one of those races. With the throw out, we had 5 firsts and 1 second to win the regatta.

Then we moved to Pensacola and had to start from scratch against better competition, lots of wind, (in early spring at least), lots of current, lots of shifts, and lots of land effect on the wind due to new highrise condos on the beaches..
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Old 24-06-2014, 09:29   #12
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Re: VMG - rhumb line vs. banging the corners

In steady conditions, we always bang the corners, too much time and headway lost tacking, where there are windshifs then we tack on the wind shifts.

dave
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Old 24-06-2014, 10:08   #13
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Re: VMG - rhumb line vs. banging the corners

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Originally Posted by Delancey View Post
Seems like a weird argument to have (while excluding all variables) since the answer is obvious. We're talking line-of-sight here and if you can't pick a layline from far out I guess you go up the middle.

The only times I can ever recall grossly overstanding from a corner has been the result of windshifts that occurred after tacking to the mark.
It might seem like a weird argument but some on the team are adamant that Distances and VMG's are different while others argue that they are the same. Without graph paper on board it's hard to convince one or the other who's right and who's wrong.

And without agreement there then the multiple tacks vs. longer layline argument never gets off the ground.
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Old 24-06-2014, 10:34   #14
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Re: VMG - rhumb line vs. banging the corners

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Old 24-06-2014, 10:52   #15
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Re: VMG - rhumb line vs. banging the corners

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Originally Posted by cal40john View Post
tacking pic
Thanks for taking the time to post that. Now do you have a formula that also proves the average VMG's are the same? Just kidding...sort of.

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