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Downwind sailing--faster than the wind? Finding the Weta's groove

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  • Tim Corcoran
    I d like to follow up with some of the comments in the recent thread which started in response to Bruce s CYC writeup, and veered off on the topic of
    Message 1 of 64 , Oct 1, 2011
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      I'd like to follow up with some of the comments in the recent thread which started in response to Bruce's CYC writeup, and veered off on the topic of electronics. As one of those noobs in the back of the pack, I've been trying to understand the Weta better. One key area (of many) is downwind sailing. Having done a little poking into true wind and apparent wind calculations (US Sailing site, Wikipedia, there's lots out there), I'm struck with the fact that I clearly haven't found the Weta's groove.
      I started with the assumption that Marc echoed, that tack and gybe angles of about 90 degrees were a good beginning. If zero degrees is the heading with the bow pointing straight into the wind, then my estimated downwind heading is 135 degrees. The performance data I can directly measure in this context during a race is my wind indicator, pointing the direction of the apparent wind. The boat speed shifts the apparent wind forward, a quick calculation shows that if the boat speed equals the wind spreed at this heading, then the wind indicator should point at about 55 degrees from straight toward the bow. My wind indicator is usually showing closer to 90 degrees, hence the boat speed is much less than the wind speed. (The current wind indicator is a cheap one, so I don't want to press the numbers too hard here).

      In reading Frank Bethwaite's "Higher Performance Sailing" he shows the polar diagram of the 49er--there is a notable bulge to downwind at roughly 135 degrees at which the boat is clearly going faster than the true wind. It is sailing the apparent wind, and he calls this "apparent wind sailing". Other modern skiff do this too. Chris Kitchen indicated the Weta was intended to be an apparent wind sailor (but sorry, no polars available). Clearly I haven't found that spot where it performs like one. Does the Weta indeed sail faster that the true wind downwind? I realize this is a tricky question, since we usually aren't measuring the true wind, though we can get data from nearby weather stations. I can see things that would make a difference here would be stuff like sail trim, boat balance, and picking the right heading to start with.
      So, Weta wise men, any suggestions for us noobs?
    • Robert S
      ....and this is one of the reasons why I am sooooo glad that I have all year round sailing - I don t have the off-season to indulge in the laminar flow deep
      Message 64 of 64 , Dec 30, 2011
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        ....and this is one of the reasons why I am sooooo glad that I have all year round sailing - I don't have the off-season to indulge in the laminar flow deep thought process - if I have time for that, then I have time to go sailing !

        Oh, and as for Bruce - he breaks the rules of physics when he sails. He is not a big guy - and should have blown away in the big wind in SFO, so I guess sailing skill beats physics.... Sorry - discussion too long - I'm off for a sail....

        Oh, and speaking of Bruce - someday he will share the plans for his top secret sailing boot drier / aerator with us lesser mortals.

        Happy 2012 to the whole Weta community.

        Robert

        Sent from my iPad

        On 31 Dec 2011, at 04:23, "George" <wetabix0947@...> wrote:

        >
        > Hmmm.....all very complicated. But we should be able to work out a fix for the trolley!
        >
        > Rgds
        >
        > George
        > --- In Weta-Trimarans@yahoogroups.com, "Tim Corcoran" <tim.corcoran@...> wrote:
        > >
        > > Hi George
        > >
        > > Since the data were taken during a race, you can be sure that Bruce was not holding DDW for any extended period, he doesn't race that way. Hence, the only time he had heading of 180 degrees was while gybing and hence the speed is quite unrepresentative, so you're right on that. Likely DDW speed is worse that the plot indicates.
        > >
        > > Bethwaite's polar curves still look far too smooth to me, and he explains nothing about how they were processed. They look hand drawn or very highly smoothed since there is no visible noise in them whatsoever. I find that cause for caution. Doubtless they are based on averaging a large pile of the best data available. As I said, I'm beginning to see that a polar plot only captures part of the story anyway.
        > >
        > > One point Bethwaite makes very clear, and I think it's extremely important is that at no time is there a single value for wind speed, period. It not only varies with height above the water, but varies in space and in time quite significantly. For example, "average speed 5 knots, gusting 10" is just a brief attempt to capture the fact that ata fixed point in space the wind speed in changing on timescales from milliseconds to hours. What period one chooses to average over and how one defines gusts are fairly arbitrary choices. Hence Bethwaite's work on automatic rigs.
        > >
        > > Tim
        > >
        > > --- In Weta-Trimarans@yahoogroups.com, "George" <wetabix0947@> wrote:
        > > >
        > > > Hi Tim,
        > > >
        > > > The least believable part of the plot is the DDW speed of six and a bit knots in a five to ten knot wind. The Weta has a slippery hull but the rig is awful for 'blow downwind' sailing. Only about a third of the mainsail is presented at anything like 90 degrees to the wind and the other sails are pretty feeble. A boat with a boom and a decent spinnaker would go at close to its hull speed in ten knots of wind and rather less in five - say 4kts max in the latter case. And there's a Weta barreling along at over six! The big drop in speed at 90degrees to the true wind doesn't look right either - I reckon that at 60 degrees apparent you would be fair humming - not sure what the TWA would be.
        > > >
        > > > It took Frank Bethwaite 10 years to get his wind data using a chase boat, GPS and Sydney university. He is a professional boat designer and former test pilot and aviation engineer. It's probably quite difficult. And then there's the business of the difficulty of establishing the true windspeed in a laminar boundary layer......zzz
        > > >
        > > > Happy New Year - do you own an ice boat?
        > > >
        > > > Rgds
        > > >
        > > > George
        > > >
        > > > --- In Weta-Trimarans@yahoogroups.com, "Tim Corcoran" <tim.corcoran@> wrote:
        > > > >
        > > > > Hi George
        > > > >
        > > > > I agree that the data are not great, I've relied on the fact that there were a LOT of data points (plus I calculated boat speed and heading over 30 second intervals, the data points are 2 sec apart) to average the wind speed fluctuations, but that's just a hunch I haven't proven. Better data costs money. I disagree with how you've interpreted the plot--the projection of the speed at 140 degrees onto the 0-180 degree axis shows a clear advantage even in the smoothed plot (as well as the unsmoothed) for this heading compared to DDW. Going to angles larger than the "magic angle" shows clearly that VMG drops off (if these data are to be believed). The difference between doing 6 knots and 7 is over 15%, a significant improvement in a race, so I'll take it.
        > > > >
        > > > > However, I was reflecting on a critical weakness of the polar plot. It gives one the impression that if one sails at a certain angle, and assuming proper trim, then the boat should make a certain speed. What is ignored in such data is how one arrives at the particular heading you're on. The issue is that boat speed affects apparent wind, and vice versa. It's not just what course you're on and what the current true windspeed is, it's also what was your boatspeed when you reached that current course: two different boatspeeds yield two different apparent winds, the faster boat will benefit more from a particular heading if properly handled. That's why it was Bruce Fleming's tracks I analyzed, not my own (he consistently places at or near the top in California regattas). Bruce will have to speak for himself as to just when he deploys the genneker, he's usually too far ahead of me for my report to be definitive (I've seen him pull it out rounding the windward mark many times but have never noticed him try to beat with it).
        > > > >
        > > > > Hence, the simple comparison of two boats on different courses doesn't capture all that's important about this problem, it's way more complex than that. But isn't that what makes sailboat racing interesting?
        > > > >
        > > > > Tim
        > > > >
        > > > > --- In Weta-Trimarans@yahoogroups.com, "George" <wetabix0947@> wrote:
        > > > > >
        > > > > > I've had another look at that plot. Assuming that the wind recorded three miles away at who knows what height above ground level means anything and ignoring the fact that it was quoted as '5kts gusting 10' which makes the plot almost meaningless and accepting that you've done a huge amount of work to try and get some usable data, look at the DDW case - the boat is doing about 6kts. Between DDW and about 140 it is doing pretty much 6kts VMG and and the magic angle it is doing just a teeny bit better. Sure there's a spike in the unsmoothed plot (of about half a knot of VMG) but the adjacent unsmoothed plot is two knots less. The plot looks very like what I would expect a plot from a two sail dinghy such as an Albacore to look like. It sort of shows that anywhere between DDW and 45degrees off (which would put the apparent wind on the beam) will do about the same and that any attempt to be greedy and heat it up any further will be hugely counterproductive.
        > > > > >
        > > > > > It would be interesting to see to the plot at what angle you first deployed the genneker. I did a bit of two boat testing in October to see if going fast to windward under genneker was faster than a Wayfarer beating. It wasn't.
        > > > > >
        > > > > > Rgds
        > > > > >
        > > > > > George
        > > > > >
        > > > > > --- In Weta-Trimarans@yahoogroups.com, "Tim Corcoran" <tim.corcoran@> wrote:
        > > > > > >
        > > > > > > Very cute! Fascinating concept. But yes, I was referring to sailboats.
        > > > > > >
        > > > > > > Tim
        > > > > > > --- In Weta-Trimarans@yahoogroups.com, "Lee" <oceans3101@> wrote:
        > > > > > > >
        > > > > > > >
        > > > > > > >
        > > > > > > > "You cannot get an enhancement in apparent wind going DDW."
        > > > > > > >
        > > > > > > > I know you meant your statement for the Weta, but have you considered a propellar, probably a lot of propellars.
        > > > > > > >
        > > > > > > > http://www.youtube.com/watch?v=5CcgmpBGSCI
        > > > > > > >
        > > > > > > >
        > > > > > > > --- In
        > > > > > > > Weta-Trimarans@yahoogroups.com, "Tim Corcoran" <tim.corcoran@> wrote:
        > > > > > > > >
        > > > > > > > > Hi All
        > > > > > > > >
        > > > > > > > > There are some actual recorded data to inform this discussion.
        > > > > > > > >
        > > > > > > > > On Oct 5, 2011 I posted a message and a file ("Weta Polar Plot.pdf") Those data indicate that even at roughly 5 knots wind, the Weta does indeed go faster than the wind and makes better VMG going at about 140 degrees from the wind rather than going 180 degrees, i.e., DDW. This angle may change with wind speed. The improvement was about one knot under these conditions. This polar plot resulted from analyzing and averaging several hours of Bruce Fleming's GPS data during racing along with wind speed and direction data from a station a couple miles away. [Changes in wind direction were accounted for. Though wind speed fluctuated, enough data were averaged--many hundreds of points--to give a fairly smooth curve] Hence, a winning sailor's tracks show that this is likely to be the best strategy. You cannot get an enhancement in apparent wind going DDW. The polar data also indicate that there is plenty of performance to be had on the downwind leg, it's not all about the upwind game.
        > > > > > > > >
        > > > > > > > > Hence I'd say that Bethwaite's general point about apparent wind sailing is still applicable to the Weta, although we won't get skiff-like speeds and we won't pull the apparent wind so far forward.
        > > > > > > > >
        > > > > > > > > Getting real-time, right on top of the boat wind data required more instrumentation than I care to pay for. Since I can't use it in racing situations I'd rather not build it into my training approach. Since they don't adjust for the fact that wind velocity and especially direction are not constant, I think spot VMG reading from a GPS give a less informed perspective.
        > > > > > > > >
        > > > > > > > > When in need, I found I could goosewing going near DDW by pulling the jib over, instead of the gennaker--this is faster to do, and helps keep the gennaker filled even with the main on the same side. Since I take the view that going DDW is something to do briefly for traffic/positioning reasons, I prefer to adjust the sails quickly.
        > > > > > > > >
        > > > > > > > >
        > > > > > > > > Tim
        > > > > > > > >
        > > > > > > > > --- In Weta-Trimarans@yahoogroups.com, Robert Spencer <mfcdubai@> wrote:
        > > > > > > > > >
        > > > > > > > > > Well discussed, and in my painful experience there is a crossover point, somewhere under 8 kts, where ddw gives the best VMG - with the main resting on the shroud on one side, and the gennaker held at arms length on the other side, and all the weight lifting the opposite ama. Feels slow (and is!), but gives a better vmg than tacking downwind (or so the gps says)
        > > > > > > > > > Over 10kts - blast away, as deep as I can without the gene collapsing, and chase the pesky monohulls...
        > > > > > > > > >
        > > > > > > > > > Sent from iPhone (probably without my reading glasses, so please excuse any bizarre word replacements that I did not see!)
        > > > > > > > > >
        > > > > > > > > > On 28 Dec 2011, at 13:17, "George" <wetabix0947@> wrote:
        > > > > > > > > >
        > > > > > > > > > > Richard,
        > > > > > > > > > >
        > > > > > > > > > > Time to stop talking about trollies and start talking about sailing?
        > > > > > > > > > >
        > > > > > > > > > > The first time I read your post I did not really understand your point. I have since read Higher Performance Sailing and now agree with you. I have long thought that the 'asymmetric revolution' was a con in most dinghies. The fastest way downwind in most boats is to point (nearly) at the mark and run square. Apparently even the Tornado used to do this and they are a lot faster than we are. The difference in speed between a Laser running and a 505 running is not great. Both boats go at their hull speeds which may be about three quarters of the wind speed. If they point up they may plane and go faster but not enough to cover the extra distance sailed. Only a boat at least as fast as a 29er will do that. The 29er is the same length as us and has a lot more sail but has to carry two people. Are we as fast as a 29er downwind? Well, I'm not! Of course our boomless mainsail is useless on a run so goosewinging is not an option so we HAVE to point up and tack downwind. But that doesn't mean we go faster than the wind or even faster than a Laser! It is our fastest route to the leeward mark but it is not particularly quick. Pointing up to try to generate skiff-like speeds where VMG is faster than the square running boats will not, IMHO, work. So basically we have to soak - ie sail as low as possible. I don't know what the correct angle is but I doubt if it's critical. The most useful thing you can do on a run is get the approach to the leeward mark and subsequent furl right. I have been practising this and it is just so easy to come into the mark too high and dog slow (watch the Americas Cup World Series to see this illustated).
        > > > > > > > > > >
        > > > > > > > > > > There may be exceptions at the extreme ends of the wind scale but basically, if you don't lose any places to a Laser downwind you have probably done as well as possible. The real money is made upwind (or on a broad reach if you are lucky enough to get one),
        > > > > > > > > > >
        > > > > > > > > > > Rgds
        > > > > > > > > > >
        > > > > > > > > > > George Morris
        > > > > > > > > > >
        > > > > > > > > > > Higher Performance Sailing by Frank Bethwaite and his several sons, designers of the 49er, 29er and Taser, available second hand in the UK through Amazon - though not cheap!
        > > > > > > > > > >
        > > > > > > > > > > --- In Weta-Trimarans@yahoogroups.com, "Richard Stephens" <richard@> wrote:
        > > > > > > > > > > >
        > > > > > > > > > > > > Does the Weta indeed sail faster that the true wind downwind?
        > > > > > > > > > > >
        > > > > > > > > > > > I think if the wind is about 10-13knt, you can sail the same speed as the
        > > > > > > > > > > > wind, on a reach. But that is not necessarily your best angle for VMG
        > > > > > > > > > > > downwind.
        > > > > > > > > > > >
        > > > > > > > > > > > When people say the Weta is an apparent wind boat, they mean the fastest way
        > > > > > > > > > > > downwind is to gybe, and not sail at dead downwind. I don't think it means
        > > > > > > > > > > > we sail faster than the wind.
        > > > > > > > > > > >
        > > > > > > > > > > > A fast skiff will "tack" downwind, meaning when you gybe, the apparent wind
        > > > > > > > > > > > is from in front of the boat. The Weta is nowhere near fast enough to do
        > > > > > > > > > > > that.
        > > > > > > > > > > >
        > > > > > > > > > > > Richard.
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