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Board » Technical Support » Gain or loss by using TWA

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Posted by
Rod


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2011-05-29 14:41:40 - Gain or loss by using TWA
I have observed that, when I set up an exact course to a specific point using the TWA, and then compare it with a CC course to the same point, that the CC course is slightly faster when measured by the predictor line.
My tentative conclusion is that, while the CC course is faster in one part of the course, it is slower in the other part. As the distance traveled on a curved course (TWA) is greater than on the straight (CC) course, it is therefore faster to use the CC course to any point.
Would anyone care to comment? Am I correct?

--- Last Edited by Rod at 2011-05-29 14:43:59 ---
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Posted by
Chappo


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2011-05-30 02:50:42 - Re: Gain or loss by using TWA
I have taken advantage of the speed difference of CC or TWA in different races and yachts. I am not an expert but I think the differences come from various factors such as the shape and position you are sailing on the polar, whether you are sailing into a heading or lifting wind change and whether you are going upwind or downwind. So even though the CC is a shorter course, that advantage can often be offset by the fact that the last part of the CC course is so much slower than the TWA course (when sailing upwind into a header, or at the start of a CC course into a freeing wind change, the CC course is much slower than the TWA course.
Posted by
ShamrockPirate


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2011-05-31 21:29:29 - Re: Gain or loss by using TWA
Rod
As we have worked through these same ideas over the past months, I think the answer lies somewhere in the world of VMC (an as yet magical and confusing place for the Pirate!)

Check out the excellent blog from 76T. I have read it 10+ times and am SLOWLY starting to get the concept.
VMC Example

See you kn Vancouver!

SP
----------
edited to change url to link to fit inside column

--- Last Edited by RainbowChaser at 2011-06-16 22:36:22 ---
Posted by
AGage


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2011-06-01 16:08:03 - Re: Gain or loss by using TWA
There is just no way to make such a generalisation. Sometimes CC will be best, sometimes TWA. Of course if you try a CC and TWA course and predictor shows same location in 6 hours, or whenever you are intersting in, then sure: sail the CC course - it is also often safer over a wx update, although you can often get to a much better location using a combination of both.
It also depends on what the TWA course is doing exacly AFTER the point of interst (ie end of predictor) - if a CC course will sail high, then this is not good and you should either use the twa, or set a dc to change you at that time. same thing if the TWA is going to lit you toward a mark - you need a very good reason to sail further off the rhumbline than is necessary - either land (the ultimate reason!) or more favourable winds - pressure or a shift.
As earlier stated, TWA is useful when sailing either upwind or downwind VMG in a persistent shift. The few times we get oscilaating shift in SOL it is not so simple - it depends on the magnitude of the shift.
Those times you see top SOLers follow a gentle curving path - they are usually NOT sailing on a twa - but sitting at their computer for 14 hours changing course by .25 of a degree every 20 minutes, or setup a heap of DCs to do it for them - and yes they do spend that much time for the shorter races where every 0.05 of a nm counts! Just ask WINSTON ;-)
I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by
Chappo


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2011-06-02 01:46:19 - Re: Gain or loss by using TWA
Good points Aaron, it is easy to work with confidence when plotting within 6 hour weather cycles, much more challenging after 6 hours so CC is a safe option. We have all seen some big shift between weather updates. I note you mentioned you have written a small tool that calculates both max VMG (for upwind & downwind) for a given TWS – , and also maximum VMC given input of TWS, TWD and a CC to the waypoint. Does it link to an uploaded polar or does it just require manual input based on different angles and BS from the polar at the time you choose to do a calc. Is that tool available for download somewhere? I use my simple worksheet attached for my max VMC calcs. I find it pretty good downwind, can be misleading upwind.
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Posted by
76Trombones


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2009-08-03 18:04:56
2011-06-02 02:10:27 - Re: Gain or loss by using TWA
I'll generalise :-) (standard caveats apply)

If we ignore difference in windspeed, we have two regimes: target inside tacking or gybe angles, target on a pure reach.

Polar hop is same idea as up/downwind, but sideways.

CC on reach. Straight line is shortest. The fast boats often sail a gentle curve if there is wind speed variation, but the gains are small.



TWA (VMG) up/downwind unless you have a good reason to do otherwise. The red path above is TWA/VMG, blue VMC and Black CC. Both the TWA and CC tracks have a single steering input, while VMC adjusts continuously. TWA is usually faster and CC has to be very careful to not sail above VMG. If you're going to be paying that close attention might as well sail TWA?

The huge bonus for sailing TWA is that the predictor tells you 90% of what you need to plan your strategy.

If you're going to sleep, TWA might have you going backwards, while CC might have you stopped. I suppose it depends on how stable the weather is. CC is pretty conservative, but won't ever be fast vs the fleet (might beat TWA). TWA at least you have a chance of ending up somewhere good!
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Posted by
Chappo


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2011-06-02 02:54:50 - Re: Gain or loss by using TWA
Excellent summary 76. I presume it is not to scale but broadly highlights the distance differences. It certainly makes a good case for twa but as Aaron notes, just because the fast yachts have curved tracks, often actually reflects continuously adjusting VMC sailing rather than TWA sailing.
Posted by
A2


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2011-06-02 02:56:21 - Re: Gain or loss by using TWA
--- Last Edited by 8mR Who at 2011-06-02 09:02:20 ---
None so blind
Posted by
Chappo


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2011-06-02 02:58:29 - Re: Gain or loss by using TWA
8MR, I see the diagram embedded in the post and as an attachment too?
Posted by
A2


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2011-06-02 03:04:16 - Re: Gain or loss by using TWA
--- Last Edited by 8mR Who at 2011-06-02 09:00:44 ---
None so blind

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