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  #1  
Old 07-18-2012
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VMG with lots of tacks

I'm not a racer, so this question may be a little naive. In making your way to a windward mark, you can get there in two long tacks or any number of shorter tacks. When you tack, you momentarily are going directly toward the mark. Are there some boats that accelerate fast enough that this is actually faster than just staying on the same tack? (assume no windshifts)
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  #2  
Old 07-18-2012
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Re: VMG with lots of tacks

Some boats, like America's Cup boats with expert crew. Acceleration is, of course, the issue - you might be momentarily heading for the mark, but tacking comes at a cost of speed. It takes time to reset the sails on the new tack and it take time to get up to speed. Also, each tack comes with the risk of somthing getting fouled up which steals even more time. On our decidedly amateur boat, we hold our tacks longer than we should, probably. When our VMG goes negative, that is when we start thinking of tacking. Yeah, we sail a longer course to the mark, but we put in fewer tacks.
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Old 07-18-2012
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Re: VMG with lots of tacks

To my knowledge there are no boats that sail fast enough, or accelerate fast enough, including the new AC45's (that accelerate from 4kn to 18kn in 80 foot) that justify just tacking. The first guys to the weather mark will be the ones that tack in the shifts. So the tack on throws. Ideally this means they are always in phase with the wind shifts, and may sail a 10-20% shorter course than someone heading off to a corner.

The only reasons to head to a corner is 1) you know there is a wind shift there 2) more breeze (particularly good in light air), 3) you are way behind and hoping to get lucky 4) tactical reasons involving a specific boat or boats).

Think about it like this... Two boats start together, and are sailing at the same speed parallel to each other. Then a five degree knock comes in, boat A tacks, boat B stays. The boats are now sailing 10% apart from each other, with boat A sailing 10 degrees closer to the mark.

A few minutes later there is another shift five degrees the other way. Boat A tacks, so the two boats are now again on parallel courses, but A has gained significantly to windward on the other tack. Boat A is also moving back to the center of the course, while boat B is headed further to the right side.

Another shift and A tacks again. B stays put. Again A is sailing 10 degrees closer to the mark than B is.

This goes on until B hits the layline and tacks. A however has constantly sailed in a lifted breeze and so is in the middle of the course. Any further lift for B is waisted since they are on layline, while A can still take advantage of it, and a knock for B hurts them since they may no longer be able to make the mark, and are sailing out of phase with the wind.



Of course knowing how to tack the boat properly, and knowing how to get it back up to speed is critical for this to work. As well as having a feeling for the speed of the shifts (how fast the wind is clocking), and how long it takes the boat to get up to speed. So a shift that I might tack on in a 12' ultralight skiff might not be worth it on a 50' heavy cruiser because of the amount of time it would take to return to full speed.
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  #4  
Old 07-18-2012
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Re: VMG with lots of tacks

Quote:
When our VMG goes negative, that is when we start thinking of tacking.
Will that only happen when you get past the lay line? I like to think in terms of VMG (just trying to get upwind), rather than VMC (velocity made good toward a specific point). Then the VMG should never be negative (if I understand correctly).

Quote:
So a shift that I might tack on in a 12' ultralight skiff might not be worth it on a 50' heavy cruiser because of the amount of time it would take to return to full speed.
That is kind of what I thought. Other than screwing up the tack, it is really a matter of how long it takes to get back up to speed that you could have been running on the original tack.
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Old 07-18-2012
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Re: VMG with lots of tacks

Yes. When VMG is negative, you are past the layline, assuming the mark is straight upwind(although you can be past the layline and still have postive VMG). Sometimes it is worth it to me to sail away from the mark for a little while if doing so will allow me to make the mark with one tack.
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Old 07-18-2012
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Re: VMG with lots of tacks

There is always a price to pay for tacking. Sure there are tricks to minimize that price, but if there is no good reason to tack, then don't!

Of course the assumption of no wind shifts is a pretty big one! That would be the number one reason a boat would throw in a lot of tacks on a windward leg. Playing the lifts and headers is where there are big gains to be made. There is no benefit to tacking for the sake of tacking. (Unless perhaps they are using illegal roll tacks in light wind!)
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Old 07-19-2012
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Re: VMG with lots of tacks

A reason to do short tacks is to stay out of the tide and/or current.
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Re: VMG with lots of tacks

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Originally Posted by ScottUK View Post
A reason to do short tacks is to stay out of the tide and/or current.
yes, that would count as a "good reason"!
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Old 07-20-2012
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Re: VMG with lots of tacks

Quote:
Originally Posted by johnnyquest37 View Post
Yes. When VMG is negative, you are past the layline
Remember that the layline shifts every time there is a knock or a lift. If you are doing a typical near shore buoy race the wind shifts off the shore (not to mention if you are sailing in the evening the wind shifts from sunset) may be changing your laylines on a regular basis. As pointed out although technically the layline doesn't shift current can change your strategy too.

A general rule - in changing winds stay near the middle, in steady winds tack on the laylines.

Like all general rules sometimes this will get you a flag and sometimes a Bronx cheer.
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Old 07-20-2012
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Re: VMG with lots of tacks

Quote:
Originally Posted by Stumble View Post
To my knowledge there are no boats that sail fast enough, or accelerate fast enough, including the new AC45's (that accelerate from 4kn to 18kn in 80 foot) that justify just tacking. The first guys to the weather mark will be the ones that tack in the shifts. So the tack on throws. Ideally this means they are always in phase with the wind shifts, and may sail a 10-20% shorter course than someone heading off to a corner.

The only reasons to head to a corner is 1) you know there is a wind shift there 2) more breeze (particularly good in light air), 3) you are way behind and hoping to get lucky 4) tactical reasons involving a specific boat or boats).

Think about it like this... Two boats start together, and are sailing at the same speed parallel to each other. Then a five degree knock comes in, boat A tacks, boat B stays. The boats are now sailing 10% apart from each other, with boat A sailing 10 degrees closer to the mark.

A few minutes later there is another shift five degrees the other way. Boat A tacks, so the two boats are now again on parallel courses, but A has gained significantly to windward on the other tack. Boat A is also moving back to the center of the course, while boat B is headed further to the right side.

Another shift and A tacks again. B stays put. Again A is sailing 10 degrees closer to the mark than B is.

This goes on until B hits the layline and tacks. A however has constantly sailed in a lifted breeze and so is in the middle of the course. Any further lift for B is waisted since they are on layline, while A can still take advantage of it, and a knock for B hurts them since they may no longer be able to make the mark, and are sailing out of phase with the wind.



Of course knowing how to tack the boat properly, and knowing how to get it back up to speed is critical for this to work. As well as having a feeling for the speed of the shifts (how fast the wind is clocking), and how long it takes the boat to get up to speed. So a shift that I might tack on in a 12' ultralight skiff might not be worth it on a 50' heavy cruiser because of the amount of time it would take to return to full speed.
Tacking does result in momentary increases in VMG, but as detailed above its just about impossible that the increase would not be offset by the speed lost during the tack.

"Shooting the line" is a technique that skippers who know their boat really well have used to shave a few seconds, or fractions there of, off an upwind finish by starting a tack just at the finish line so you gain the benefit of the increased VMG as you come through the wind, but are across the line before the speed bleeds off.
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