Tacking Downwind (faster than the wind)

Discussion in 'Sailboats' started by ancient kayaker, Oct 29, 2008.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    I have read several sailing articles that advocate tacking downwind to gain speed, but is it possible to exceed the wind speed in the course made good (i.e., directly downwind) direction? It sounds crazy, but a quick bout of vector analysis suggests there is no theoretical reason why this is impossible.

    As a simplistic example, a wind of speed W due North and a heading of 30 deg at speed W.sqrt(3)/2 ... would create an apparent wind speed on deck of W/2 due East, which could be harnessed by a sufficiently efficient sailboat to further increase speed. Sounds like something a foiler or cat would have done by now. Or is there a reason why not?
  2. DGreenwood
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    DGreenwood Senior Member

    ??? I don't meant to sound like a smartass but maybe I am missing your your point. This is a concept that has been understood by gaffers of 100 yrs ago. I dare say that a carbon cat is going to see improved SMG sailing at hotter angles. Maybe you need to clarify your question more?
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    Seems clear enough to me. I've always assumed sailing downwind was limited to less than windspeed. I am quite aware that down wind SMG can be improved using a zigzag course, but can a boat can tack down wind with a speed made good faster than the wind speed? I didn't know that. Please confirm; perhaps I've led a sheltered life. A simple yes or no would suffice.
  4. Stumble
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    Stumble Senior Member

    The simple answer: YES.

    The semi-complete answer:

    Lets say a boat is trying to go directly downwind and has a hull design that will allow it to plane and take maximum advantage of the wind it experiences. Plus the sails / crew are all optimized...

    Starting at a beam reach the boat starts to accellerate and generate more aparent wind. Which will be a vector of the true wind speed and the forward direction of the boat. This new apparent wind will be coming from further forwards that the true wind speed and at a faster speed.

    From here the boat turns downwind to keep the apparent wind speed on the beam of the boat. As it does so the boat continues to speed up, and the now new apparent wind continues to accellerate while moving further forward.

    The boat responds by turning downwind another five degrees and the cycle repeats over and over, until the boat could be sailing completely on its' own aparent wind actually beating into a headwind that exists completely in its' own mind.

    The boats that do this the best are ice boats that are always sailing upwind to their apparent wind regardless of their point of sail on the compass, and can reach rediculous SOG due to the effectively zero drag.

    The problem with maintaining this type of thing in the real world is that if a boat slowes down and looses the aparent wind, or is effected by an event that robs is of velocity (running into the back of a wave for instance) the boat now has to start the cycle over again. This leads to a practical limit on this, but not a theoretical one.
  5. lazeyjack

    lazeyjack Guest

    nice , stumble as usual
    my last was on Darwin Indo rally, this is a rally for cruising yachts(137 boats) bit off topic, but they reported back to me, they had beat the fleet on every leg, using SAIL TO VANE on the BAND G pilot, must put that question to em
  6. gggGuest
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    gggGuest ...

    To agree, yes, its not an issue for a sufficiently efficient craft, and ice boats are the obvious real life example. Some folk get amazingly wound up about this, claiming its perpetual motion or other fatuous arguments.

    However Stumble's claim that the boat can end up going dead downwind on its on apparent win isn't true. There does need to be some angle to maintain speed and apparent wind. I'm sure gusts and windshifts can give the impression that's happening, but that's another story.
  7. Omeron
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    Omeron Senior Member

    Search Sailing Anarchy under DDFTTW.(Dead Downwind Faster Than The Wind). It was one of the most discussed subjects ever.
  8. bistros

    bistros Previous Member

    18' skiffs (as in Sydney Harbor, Australia) are capable of far exceeding windspeed for about a 30 degree segment of their polar diagram. If you take a good read through "High Performance Sailing" by Frank Bethwaite, you'll quickly get an understanding of polar diagrams and performance expectations. New designs not covered in this book include the development surrounding Moth hydrofoilers and hydrofoilers in general.

    The 18' skiffs dominate and can maintain an average VMG ABOVE windspeed AROUND a complete triangular racecourse. F18 cats and C-Class cats are capable of truly amazing speeds around the racecourse as well. Foilers in general are capable of amazing speeds if the wind and sea conditions are perfect, but do not exhibit the all round performance to be spectacular in light winds or serious breeze.

    I've sailed a DN iceboat at over 70 miles per hour, it is a truly amazing experience going deep down wind on a broad reach (not Dead DW) with the sail sheeted block to block. gggGuest is absolutely right.

  9. rfnk
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    rfnk Junior Member

    Sailboards can reach speeds of close to 50 knots (I'm not quite sure if the 50 knot mark's actually been reached by a sailboard yet) in winds considerably lighter than 50 knots (around 20 knots). A fast sailboard with a competent sailor will regularly exceed speeds of 25 knots in 15 knot winds, especially on a broad reach. 18 foot skiffs, quite a few other fast, planing monohulls, and lots of cats achieve boat speed well beyond wind speed. The maxi racing yachts competing in races like the Sydney to Hobart also well exceed wind speed on broad reaches.

    Our Folkboat also achieves this but only on very dark nights when no one can see us.
  10. CT 249
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    CT 249 Senior Member

    I would very highly dubious that any boards can get close to 50 in 20 knots breeze; they haven't made it yet in a lot more wind than that.

    For example, 2 years ago the #7 ranked sailor on the GPS speedsailing rankings (1500+ sailors at the time, using sophisticated programmes to counter GPS errors) reached his best speed on medium-wind gear (31.9 knots IIRC) in about 25-32 knots wind.

    His higher speed were done on speed gear, in much stronger winds. Generally for boards, the bigger wind, the better the speed.

    However, longboards can and do achieve consistent speeds DDW that are higher than the apparent or true winds.....there is a trick to it.
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Oh, oh, I see I've started something here. There's already a good thread for 50K+ sailing.

    Thank you all for your inputs, I could see the theory but I had trouble believing it for the downwind case; it's much easier to understand for a reach.

    As I suspected originally, it seems to apply to foilers of both the hydro- and aero- persuasions and large catamarans, neither of which are too common in my neck of the woods. I did get one sail in a typhoon many years ago but the nightmares rarely occur these days ...

    It was always clear that a high efficiency boat would be necessary if twer' to be done. At least the F18 has a sail not a wing. All of the books and articles about sailing that I have read have always describe sailing gull-winged down wind, and tacking seemed to be done mostly for a square rigger to expose more sail area.

    It is clear that I have led a sheltered life, but then my introduction to performance sailing in particular and boating in general was rather late in the process. Now I am thinking about what it would take to make my own foiler ...
  12. bistros

    bistros Previous Member

    All extreme high performance planing dinghies are faster tacking downwind at angles than going dead down wind. 49ers, I-14s, 12, 16 and 18 foot skiffs, 29ers etc. are all single hull, soft sailed boats capable of far exceeding windspeed going downwind at angles.

    This is not the preserve of multihulls or hydrofoils only. Mutlihulls generally do not achieve a plane, and although extremely fast, they do not achieve the speed a skiff will on a broad reach planing. Hydrofoils have a very narrow window of conditions in which they are fast, usually 10-18 knots of wind, and it is very debatable if they could keep up to an 18' skiff around the track consistently.

    Building a foiler is certainly possible, but perhaps not the best initial introduction to high performance sailing. I'd recommend sailing on a trapeze boat first, then perhaps a two trapeze skiff, and then if you are still unmoved consider a foiler. Foiling is a relatively new art form, and it isn't without some potential for a very un-fun extended learning process. Foilers by their nature require very expensive exotic materials and expert build skills if you intend to succeed. The lowest end production foiler out there from Bladerider still uses vacuum bagging, high skill composite manufacturing and is not really a homebuild option. Read through Wind_apparent's thread about his build to get some perspective on what the task looks like. I have no doubt his project will foil as a homebuild - but he's assembled a lot of talent and engineering skill together.

  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    I have a project just getting underway to build a sailing kit for a kayak, my interests at present being at the lower end of the size range.

    I have been getting a lot of help on this and there are three threads, looking at foil profiles, lifting sail systems with Bruce foils, and an idea I had to have the sail and Bruce foil assembly separate from the hull and attached by means of a pivot (the infamous articulated sailboat).

    I want to combine these ideas to explore the possibility that tacking can be performed without waiting for the hull to rotate, since most kayaks are slower to turn than small purpose-built sailboats.

    Although I am of an inquisitive turn of mind, being nearly 70 I have to be rather more careful than a few decades ago, so I am not so sure I want to hike out on a trapeze these days. I probably should not try a foiler but the kid in me is trying to get out again ... I can certainly build a skiff though.

    Oh. I'm not a naval architect either; actually a retired robotics engineer, which is close enough, isn't it?
  14. Doug Lord

    Doug Lord Guest


    Bill you're a bit wrong on two accounts:
    1) There are many reports of F18's racing against aussie 18 skiffs and beating them. There was a race in San Fran a while back where both raced and the cat won. The story of that race by "Tornado Alive" from Sa is contained in post 709 on the "Moth on Foils" thread.
    2) Where do you come up with this ridiculous anti-hydrofoil bias? Your facts aren't even close. Moths can take off in 6-7 knots of wind and their recent Worlds in the UK was held in winds over 20. About 2 years ago Rohan Veal raced a fleet of F18's and won as he did against a fleet of top A Class sailors
    around 4 years ago.
    F18-see www.rohanveal.com june-july 2006 archives
    A Class www.rohanveal.com 2004 archive
    In the recent singlehander(monohull) yardstick regatta in Australia the Moth beat every other singlehander including the International Canoe taking the top 5 places. The Moth is-without a doubt-the fastest singlehander-mono or cat- in winds of 8-10k and up.

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    I hear the voice of temptation ... and then there's wingsails to be explored ...

    Bill, I found wind apparent's foiler thread, it's huge but I will work through it. I would like to see if a foiler can be done in wood, it's a great composite material.
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