Facebook

Login

Support Sailonline

If you haven't already - join the SAILONLINE YACHT CLUB!

Please also consider making a donation - all amounts are greatly appreciated!

Board » Technical Support » Gain or loss by using TWA

Page: 1 2 Next

Posted by
Rod


Users avatar

Posts
392
Registered at
2010-06-24 22:30:48
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


Users avatar

Posts
53
Registered at
2009-04-17 12:00:35
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


Users avatar

Posts
189
Registered at
2010-12-03 17:49:57
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


Users avatar

Posts
411
Registered at
2008-12-27 10:31:57
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


Users avatar

Posts
53
Registered at
2009-04-17 12:00:35
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.
Attachments
Posted by
76Trombones


Users avatar

Posts
362
Registered at
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!
Attachments
Posted by
Chappo


Users avatar

Posts
53
Registered at
2009-04-17 12:00:35
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


Users avatar

Posts
347
Registered at
2009-10-26 00:39:42
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


Users avatar

Posts
53
Registered at
2009-04-17 12:00:35
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


Users avatar

Posts
347
Registered at
2009-10-26 00:39:42
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

Page: 1 2 Next

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Koshiki PYOC Sprint 2025


For the first time since 2017 we return to the islands and shima’s offshore Nagasaki, Japan. This time our crafts of choice are X-35’s. Just 23nm in a straight line but choose your course wisely.
Race 1954
INFO by brainaid.de
X-35 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
SPQ3 - SPRCH - SUPSOL - SYC
Race starts: Sep 20th 15:00 Registration Open!
▶ Flash
GO TO RACE

Silverrudder 2025


Ahoy, SOLers! Get ready for the epic Silver Rudder – Challenge of the Sea! This 125nm single-handed race around Denmark’s island of Funen is back—shallow waters, narrow straits, and shifting winds await. This year, we trade up to our single-handed X-99s—can you master their power where precision counts most? It’s the world’s largest solo regatta, and your virtual helm is calling! Chart your course, trust your instincts, and race for glory in this ultimate test of seamanship!

It is imperative that you read AND understand these special Starting Conditions for SILVERRUDDER 2025:
Starting Conditions!!
Race #1948
INFO by brainaid.de
X-99 Single-handed PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC
Race starts: Sep 19th 09:00 Registration Open!

▶ Flash
GO TO RACE

Rome to Tripoli 2025


The next race in the ARCH series will take us almost straight from Rome to Tripoli. But don't expect this 550nm race to be an easy one. The Mediterranean always offers some surprises. So let's prepare your Lagoon 55 with due care and... Fair Winds.
Race #1943
INFO from brainaid.de
Lagoon 55 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
ARQ3 - ARCH - SUPSOL – SYC
RACE CLOSE: Wednesday,
September 24 at 2300 UTC.
Race starts: Sep 15th 17:00 Registration Open!
▶ Flash
GO TO RACE

Carib Rum Run 2025

Welcome to the Caribbean and Sailonline’s legendary Carib Rum Run! This year we trade tall ships for pure speed as we race our sleek Santa Cruz 70s on a 1759nm slalom through the rum islands. Yer mission: plunder barrels from the region’s finest distilleries and deliver ‘em to the rum-thirsty sands of Cancun, Mexico. Finish on or before International Talk Like a Pirate Day, 19 September, to etch yer name in legend! Join SYC before race end and ye scallywags can disguise yer boat with a pirate name for this race only! Hoist the sails, sharpen yer cutlasses—the rum run awaits!
Race #1944
INFO by brainaid.de
Santa Cruz 70 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC
Race starts: Sep 15th 00:00 Registration Open!
▶ Flash
GO TO RACE

Roslagen TIMED Sea Race

Welcome to quite possibly the most challenging TIMED race of the year. This month finds us in Sweden with lots of islands and narrow passages when we sail our Seacart 30 trimaran over the 86nm from Lidingö outside Stockholm to the beautiful coastal village of Öregrund. Consider yourself a winner if you complete the course nonstop! This is a TIMEDrace so you may RE-REGISTER HEREto try again after finishing a run. You will have 13 days and 11 hours to test your skill and decision making after the race opens.
Race #1945
INFO by brainaid.de
Seacart 30 Particulars
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ3 - TRCH - SUPSOL - SYC
RACE CLOSE: Saturday,
20 September at 23:00 UTC
Race starts: Sep 07th 12:00 Registration Open!
▶ Flash
GO TO RACE

Great Nicobar to Marion Island 2025

The Adventure Continues – Leg #6 of the RWW Series.
We’re setting sail on the next stage of the RWW Series: a 4,400-nautical-mile passage from Great Nicobar to Marion Island. This demanding leg will test endurance, strategy, and seamanship at the highest level. As part of the 2025 Ocean Championship, this marks the ninth legendary challenge in the series — and one of the most exhilarating yet. The race will be contested aboard the Archer 78’ ketch, a vessel built to push sailors to their limits across the vast Indian Ocean.
PRIZE: SMPF
Race #1942
INFO by brainaid.de
Archer 78 PARTICULARS    
WX Updates:
0430 / 1030 / 1630 / 2230    
Ranking: OCQ3 - RWW - OCCH - SUPSOL - SYC
RACE CLOSE: Thursday,
September 25 at 2300 UTC.
Race starts: Sep 01st 11:00 Registration Closed
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member vida
  2. Sailonline Yacht Club Member WRmirekd
  3. Sailonline Yacht Club Member CriticalHippo
  4. Sailonline Yacht Club Member rafa
  5. Sailonline Yacht Club Member KaSToR
  6. Sailonline Yacht Club Member Kipper1258
  7. Sailonline Yacht Club Member BRENTGRAY
  8. Sailonline Yacht Club Member Sax747
  9. Sailonline Yacht Club Member Panpyc
  10. Sailonline Yacht Club Member CollegeFund

View full list

Series

Mobile Client

SYC members have the benefit of access to our mobile/lightweight web client!

The mobile client