Does anyone have 'polar' data/diagrams for Mirrors?

Thanks for all those replies. I think two of you have misunderstood my question (an easy thing to do) which was restricted to the business of maximising progress downwind. Clearly to go upwind you have to tack, but my question was only about going downwind.

The following link shows polars for 6 different kinds of boat:-

http://www.thebeachcats.com/OnTheWire/westnet/_lpm/hobie/archives/v1-i3/...

Note that all those diagrams are slightly wrong, resulting in them being distorted - this is because the scale has 1 at its centre instead of 0, but the shapes of the curves are reasonably correct. The lowest part of the line on the diagram indicates the angle at which you should sail downwind in order to maximise "progress made good", so it's easy to see that in a Tornado you would make the fastest progress on a downwind leg by zigzagging at 130 degrees to the wind, thereby making huge gains by the bottom mark over another Tornado which is running directly downwind. In the days when I raced a Mirror in a handicap fleet, there were Nacra 5.2s (similar to the old rig Tornado, but a bit smaller) which also ran directly downwind on downwind legs, and they would have benefited substantially from zigzagging if only they had known it. They were crewed by people who previously sailed Fireballs and who went on to sail Tornado; people who competed in national championships.

The Tornado is an extreme case, of course. If you look at the two monohull diagrams at the bottom of that page you will see that there is nothing like as big a gain for them in zigzagging downwind, but they could still make small gains by zigzagging at anything up to 140 degrees to the wind, and they could increase further on those small gains by gybing on windshifts, much as you can tack on windshifts to take shortcuts upwind.

Here's another set of diagrams for four boats:-

http://www.yachtblick.de/www.yachtblick.de/wp-content/uploads/2012/02/Bo...

Again the gains for the monohulls there are tiny, but they are non-zero, and races can be won and lost by tiny margins.

Here's an interesting set of polars which shows that they vary for different wind conditions:-

http://www.sailonline.org/static/var/sphene/sphwiki/attachment/2011/06/2...

In some wind conditions it is faster for some of these types of ship to sail directly downwind, while in other conditions they should zigzag. With the Class C it alternates back and forth as the windspeed goes up. The Class D may gain just about nothing at all by zigzagging (apart from getting out of disturbed wind from boats behind), and this may apply to the Mirror too for all I know, but all the polars I've seen for other monohulls with the same general sail plan as the Mirror show small gains by zigzagging.

"Have you asked the racing fraternity at UK Mirrors or one of the other class associations who race regularly?"

So far I've only found two forums relating to Mirrors, this one, and one that doesn't let you register, so I assume you have to be a member of the Mirror Class Association to use it, and there is also almost no activity on their forum at all. They have subforums about racing, but I can't post there without paying to join, and I'm reluctant to do that until I've actually got a Mirror again that I can sail.

"Leaning forward to get the transom out of the water seems to help"

Yes, and you can even put your weight so far forward that the front transom digs in because the extra drag from there is more than cancelled out by the reduction in drag from the rear section of the hull. I learned that during a Mirror fleet race at Findhorn when the one boat that had no spinnaker was able to keep up with the rest of us by doing that to an extreme while the rest of us just stared at them in disbelief.

"A hundred hours under sail in mixed conditions will refamiliarise you with the Mirror."

The memories I have are as clear as if all that sailing happened yesterday, so I don't think I need a lot of refamiliarising. I've still got all the parts of a Mirror except for a hull, and I have every intention of building a new hull as soon as circumstances allow. The old hull was already badly rotten when we got it and it was also officially no longer a Mirror because a new hull had already been made by its previous owners with the same hull number (15345). I'm surprised that that was allowed, but it happened. Anyway, we got four years of sailing out of it before it finally gave way in a heavy sea (we regularly sailed in huge waves in Stonehaven bay, NE Scotland) and the buoyancy tanks filled up with water - we sailed it back into harbour with the hull almost completely underwater. The hull really needed to be taken to pieces and rebuilt to fix it, but it simply wasn't worth doing as it was so rotten, and all the less so given that it could never officially be a Mirror dinghy again.

"Ransome's books were incredibly detailed. The boat handling techniques he described made his work seem authentic. If he had made significant errors in these areas I would have found his books unreadable."

I agree. He knew what he was doing and got the details right. I want to get all the details right too, and that's why I need to find out what the current state of the art is in relation to sailing them downwind, but it may only be the people racing at the top level that know the answer. I don't want to suggest that zigzagging downwind can be beneficial if it isn't, and I don't want to suggest that it isn't if it is, so it's really important to know what the truth is. It's a difficult thing to test too when the differences are likely to be very small, so it's only the people sailing at the elite level who are likely to be able to do the experiments to find out.

Thanks Curlew - I've only just seen your post as it came in after I started writing mine. Even though it doesn't address the issue I'm asking about, the document you linked to is very interesting and I'm very glad you pointed me to it.

Nothing to apologise for - I can well imagine how spammers drain your patience and time, leading to you having to rush to get things done. When I reread my original post it was all to easy to see how it could be misinterpreted. I see I made an error in my next post by talking about "progress made good" instead of "velocity made good" - I'm only just trying to catch up with recent developments myself having been out of sailing for a long time due to pressure of work and lack of money.

In the America's Cup they were foiling downwind at high speed relative to the wind, and the higher the speed you go at relative to the wind, the closer to going directly downwind you can go - there is no theoretical limit to this because if you eliminate all friction you can go on accelerating forever and still take power out of the difference in movement between the air and the water, getting closer and closer to going directly downwind with the sails ever closer to the centreline of the boat. In practice of course, the friction builds up fast and puts severe limits on it, but Oracle was foiling downwind at 150 degrees and would have been able to travel from the upwind mark to the downwind mark much faster than a hot air balloon. Even the non-foiling AC45s could just beat a hot air balloon in a race downwind if they didn't gybe too many times along the way.

It's clear from the monohull polars that I've seen that any possible gain from zigzagging downwind in a Mirror would be small (if it's there at all), but if there is any gain available it has to be worth knowing about. It's also safer to avoid going directly downwind, and the boat rolls less, so it would be worth knowing which angles still provide good VMG in different windspeeds. If it falls off continually like with the Class 4 diagram, it would be fastest to sail directly downwind, but if there's a broad range of angles which provide practically the same VMG, that gives you more options and could make a significant difference to any decisions you might make about heading over towards the side of the course where the wind may be stronger, or the current weaker/stronger - you might not think it worth sailing some way to the side to get over a channel with faster moving water in it, but if you can get there at an angle that loses you little or no VMG, it could well pay off. There are all sorts of ways this kind of knowledge could be applied in racing, but it depends squarely on what a polar for a Mirror actually looks like. There's also a new complication with Mirrors as there are two different rigs for it now, so there are two polars needing to be worked out. Those who race at the elite level will be using the new rig, so if they have a polar for the Mirror it'll be for that one. Even so, it's likely that it would still serve as a reasonable approximation for the old rig Mirror.

My Internet connection's slow so I had to give up on the longer video, but managed to get through the shorter one - that looks like a nice place to sail.

Ransome's books are a good read for adults as well as children - I'm reading through them again now myself and I still think that some of them are the best children's books ever written, in particular the two books set in the Norfolk Broads, Coot Club and The Big Six, so make sure you don't miss out on those. Swallows and Amazons is the best of the five books set in the Lake District - the other four may disappoint as there is less and less sailing in them. He never did write the book that I expected to follow Swallows and Amazons, perhaps because he'd put too many of the best ideas into that first book and it would have been hard to write more of the same without just repeating the same ideas. We Didn't Mean to Go to Sea has a very simple plot and it's hard to imagine that it could maintain your interest all the way through, but it does. I wouldn't want anyone to miss out on reading any of the four books that I've named here.

"wishing you success with the book project"

Thanks. It'll be tough to get it noticed, but it's worth a go.

[For any fans of Ransome reading this, I wasn't sure for a long time whether it would be the done thing for me to try to move into this area with a book when Ransome had made this genre so very much his own, but I recently came upon a book called Bevis by Richard Jefferies (available at gutenberg.org) which was written long before Ransome's books and which covers a lot of the same ground as Swallows and Amazons, as well as setting the scene for Winter Holiday. It tells the story of two boys who build a sailing boat (which they actually consider naming Swallow, though that was the name of a famous ship of the time) and who camp on an island in a lake. In the story there is a war against some other children where they have to try to take over each other's camp (though not on the island). Later in the story they take to the frozen lake on sledges with sails. Ransome must have read this: he was an expert on what had been written before him (as is clear from his earlier book "A History of Storytelling: Studies in the Development of Narative). This genre clearly does not belong to Ransome alone. (I should point out though that Bevis is a badly-written mess, and not a rewarding read.)]

Yes, that's a book that really ought to be on your bookshelf too.

(I heard that Racundra's Third Cruise was published recently too after the log was unearthed somewhere, but I haven't got a copy of it yet and can't tell you if it's a good read or not.)

I've found this: http://100races.blogspot.co.uk/p/polars.html

It appears to confirm that Lasers should sail more or less directly downwind, though in lighter conditions they may be able to explore other angles without losing ground - that also matches up well with how people actually race Lasers. The first diagram appears to be from a simulator, so it may not match up to reality - it suggests that sailing at 160 degrees to the wind may be best in 9-12 knots of wind.

This 470 polar may be a bit more relevant than the Laser ones as the rig has more similarities with a Mirror:-

http://www.tacticalsailing.com/gametips/features/boats/470er.html

There are diagrams low down on that page which show the angle for fastest VMG downwind to be 160 degrees in 5 knots of wind and 150 degrees in 10 knots of wind.

This suggests that the optimist could potentially benefit a tiny amount from going at 160 to 170 degrees from the wind, though it's just a simulation and can't be trusted:-

http://eprints.utas.edu.au/7388/1/BinnsBethwaiteSaunders2002.pdf

There are actually polars there for a variety of laser rigs there too, and a Byte (which I've not heard of before). In Fig 15 there's are two very faint lines showing the actual on-water performance for the laser at 9 and 12 knots which can be compared with the simulated performance. It shows higher downwind speed than the simulation, though the shape of the two curves isn't hugely different at the downwind end for 12 knots. The 9 knots on-water does not compare so well with the simulated 10 knots though, with the real results showing a loss of ground at angles other than 180 degrees.

Thanks for those good points from Gernot and Bruce about it being relevant to cruising. It's just occurred to me that if you're sailing into a current, any potential gain could be magnified greatly. I once took part in a race where the three Mirrors in the fleet spent two hours trying and failing to cross the start line due to a current which exactly matched our speed in the light wind. We weren't going downwind, but had the wind been behind us it might have been possible to make significant progress at some angles while making none at 180 degrees. The same could apply in cruising, allowing you to beat a current rather than by being beaten by it (though again it will only be possible if the boat really can make better VMG downwind by sailing at non-180 degree angles).