Minimum Width rudder can be built yet remain
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=12298
Printed Date: 08 Jul 25 at 1:49am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Minimum Width rudder can be built yet remain
Posted By: iGRF
Subject: Minimum Width rudder can be built yet remain
Date Posted: 02 Feb 16 at 5:13pm
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..effective.
So following the gybing plate thread and foil drag etc, do we have any case history of very narrow high aspect rudders and the reduction in drag they bring versus their efficiency at turning the boat (although I constantly read you're not supposed to use them for that if you're doing it properly). Having reshaped the main foil of this minisprint I am left with this chunk of an outside privy door nailed to a bit of wood that I'm minded to get medieval with since that is clearly its origin. I'm wondering just how savage I can be in reducing it's plan form. ------------- https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals https://www.corekite.co.uk" rel="nofollow - New Core Kite website |
Replies:
Posted By: realnutter
Date Posted: 02 Feb 16 at 5:30pm
| Class rules are open on the subject.... go down to a 3mm alloy sheet if you want to... |
Posted By: Jack Sparrow
Date Posted: 02 Feb 16 at 5:31pm
about 10% of the chord http://www.pinterest.com/pin/create/extension/?url=http%3A%2F%2Fwww.yachtsandyachting.com%2Fforum%2FRTE_textarea.asp%3Fmode%3Dedit%26POID%3D1389876%26ID%3D902&media=http%3A%2F%2Fwww.yachtsandyachting.com%2Fforum%2Fuploads%2F934%2FScreenshot_2016-02-02_17.32.54.png&xm=h&xv=sa1.37.01&xuid=G7fW2MyGV_BM&description=" rel="nofollow - ------------- http://www.uk3-7class.org/index.html" rel="nofollow - Farr 3.7 Class Website https://www.facebook.com/groups/1092602470772759/" rel="nofollow - Farr 3.7 Building - Facebook Group |
Posted By: laser193713
Date Posted: 02 Feb 16 at 6:40pm
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Rudders have been known to go down to 6% sections but you start to move into the area where exotic materials are needed. It really depends on the chord length of the rudder, a huge long chord length means that you can go down to lower %ages but you might not want a long chord length for other reasons, mainly drag and weight on the helm. I suspect an oppy rudder has less than 6% section but then it is basically square, so it is inefficient for other reasons. A vague rule is also that the thinner the rudder relative to its chord length the earlier it will stall as the angle of attack increases. Given a high chord length this is not necessarily a massive problem as the rudder will have enough of a barn door effect to overcome this, but in that mode it will have massive amounts of drag. In short, it depends what you want to achieve and what materials and plan form area you have to achieve it with. |
Posted By: iGRF
Date Posted: 02 Feb 16 at 7:04pm
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Well logic says the less there is, the faster through the water we should go.
Reason also suggests any extra width should be at the bottom, so it still engages when the boat is up on its ear, so I'd thought of a narrow down shaft with maybe a wider section at the tip. ------------- https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals https://www.corekite.co.uk" rel="nofollow - New Core Kite website |
Posted By: RS400atC
Date Posted: 02 Feb 16 at 7:45pm
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Most of the time, you don't need much rudder, but I seem to recall that MiniSails sometimes need a lot of steering downwind, as the boom is long and the hull is not keen to be under the rig? The hull itself is fairly ambivalent about direction of travel, particularly if you succeed in keeping it flat. I had one many, many years ago, I bought a new sail for it from Great Western Sails, about 10 minutes before the CA started allowing battens. How about a dagger-box type rudder and pull it up a bit if you feel too much under control? |
Posted By: JimC
Date Posted: 02 Feb 16 at 7:55pm
| Fatter sections tend to be preferred on rudders because they stall less easily. The choice of section too is relevant, usually something more forgiving than the rudder. I wouldn't change the area too much away from stock because of the rudder's role in balance and lateral resistance, but having said that the minisail may not have had optimally set up foils out of the box. |
Posted By: Rupert
Date Posted: 02 Feb 16 at 8:20pm
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The Minisail rudder can be very, very heavy off wind as the boat tries to broach if the balance is wrong. Too narrow a blade will simply let the boat spin out, I suspect. However, I've really not figured out what works best. The standard rudder is pretty poor, just a piece of ply. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Guests
Date Posted: 02 Feb 16 at 8:31pm
| Form drag, due to the shape and thickness of the foil, will be no more than 5% of the total drag of the rudder. It's not worth going thin to reduce drag, and thin foils stall more aggressively. |
Posted By: RS400atC
Date Posted: 02 Feb 16 at 8:53pm
IIRC the original MS sails were quite horrible by the time they were a year or 6 old, full, stretchy, often funny colours... I'd hazard that optimisation of the rudder was somewhat limited. Also, if the rudder was very big/deep, it would probably have bent the stock all over the place, as this was ludicrously shallow to suit the transom? Things may have changed over the years, I must have sold mine in the early 80's (?) and it was a bargain boat then, bought to keep sailing while mending something else... It is one of the few boats I've had that I can't recall the sail number. It was GRP with a wooden, clamp-on sliding seat. |
Posted By: Rupert
Date Posted: 02 Feb 16 at 9:01pm
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The old sails are now much, much older and haven't improved. My nice new sail is just amazing compared. It does need a new rudder to compliment it, but as RS400 says, the stock has virtually no pintle spacing. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Daniel Holman
Date Posted: 02 Feb 16 at 9:18pm
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If you want to lose drag, lose planform area, but as someone else said, respect the ratios of areas between the dagger and rudder. Rudders are ultimately limited by needing to go to max cl out of tacks or dodgy manoeuvres or keeping control on windy days. AC foil designer I know said that if you're going below 12% thickness you're better off losing chord (and planform area /WSA and thus friction) whilst keeping a more stall tolerant shape. You can reach structural limits too. I just built a rudder that can take a tonne of force. 19mm thick, of which ~2mm HM unis in 50mm strips staggered out. Carbon or HD foam shear web. Weighs little more than a kilo. ou could, if building solid carbon, go to maybe 6 or 8mm thick, but unless it literally is the chord of a windsurf fin, it'll have sh*t stall performance. |
Posted By: I luv Wight
Date Posted: 02 Feb 16 at 9:19pm
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< up on its ear> You see the good guys sailing like that? ( unless it's a scow ) The further down the fleet, the more tippy uppy they are. I've made very long, very thin rudders... seems faster for the faster sailors. ------------- " rel="nofollow - http://www.bloodaxeboats.co.uk" rel="nofollow - http://www.bloodaxeboats.co.uk Andy P foiling Int Moth GBR3467 Freedom 21 Codling |
Posted By: iGRF
Date Posted: 02 Feb 16 at 10:39pm
I used to have a nightmare with that RS500 with the rudder not functioning out of skewed up tacks or gybes.. Long and deep could work and I have an old wooden storm foil from a windsurfer that might fit in the stock. I've seen Merlin rudders with an extra trailing tip which I assumed was to counter steerage issues if up on their ears, but then, that wouldn't work would it, I wonder what they're for. Makes you think the ideal shape might be a scimitar reversed.. ------------- https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals https://www.corekite.co.uk" rel="nofollow - New Core Kite website |