| Bruce Roberts classic 45 Valencia, Spain |
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| Free mast for Merlin Rocket - has a bend! Guildford |
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| Rossiter Pintail Mortagne sur Gironde, near Bordeaux |
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List classes of boat for sale |
Rise Of The Ok etc |
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Rupert
Really should get out more
Joined: 11 Aug 04 Location: Whitefriars sc Online Status: Offline Posts: 8956 |
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Topic: Rise Of The Ok etcPosted: 03 Oct 14 at 3:08pm |
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I think you have to teleport to get to the other side on your boat, Jim.
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Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686
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Bootscooter
Really should get out more
Joined: 15 May 07 Location: United Kingdom Online Status: Offline Posts: 1094 |
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Posted: 03 Oct 14 at 7:31pm |
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The OK is an awesome boat, and one that teaches you so much about how rigs work. BTW, there's more space boom-to-floor in an OK than a Laser.
I love mine and can't wait to get started on refurbing my REALLY old one - it's going to be a rocket when done! |
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iiitick
Far too distracted from work
Joined: 04 Jun 14 Location: gb Online Status: Offline Posts: 478 |
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Posted: 03 Oct 14 at 7:42pm |
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Medway Maniac
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Joined: 13 May 05 Location: United Kingdom Online Status: Offline Posts: 2788 |
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Posted: 03 Oct 14 at 10:54pm |
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I like Dan's point about lowering the centre of effort more than my observation about induced drag, but I can't accept your reasoning, Jim. ![]() No way that's an 18" inch gap between the bottom of the boom and the deck at the mast (where it matters most, as you know); more like 5" or 6". And there's a deck flush with the gunwales beneath the boom. Whereas on the Aero: ![]() Put it another way, if you were running a wind tunnel test with the foils with those sorts of ratios of gap to floor:span, would you be happy there was no end-plate effect in either case? I'd say there would be an effect with both rigs, but considerably more with the OK. Marchaj recorded a 10% improvement in L/D by moving a Finn sail from 10" to 6" above the floor.
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Blue One
Far too distracted from work
Joined: 09 Nov 13 Online Status: Offline Posts: 317 |
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Posted: 04 Oct 14 at 12:48am |
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But how much of that 10 % is theoretical, and how much is real when sailing in all the ever changing conditions that happen in a race? Also what percentage is the increased drag of a higher boom in the total drag of the boat and sailor? And if an average sailor is more comfortable and quicker though the tack with a boat with a higher boom does this also out way the theoretical advantage?It's like bikes, a manufacturer will tell you it's new bike is 10 % more aerodynamic, but a bike is less than 1 % of the total drag of the bike and rider. So what do good riders do, go in a wind tunnel and get tested and this is where it gets messy. His best aerodynamic position, might not be fastest on the road, because it's too uncomfortable. Most of these things are a compromise.
Edited by Blue One - 04 Oct 14 at 12:50am |
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mongrel
Far too distracted from work
Joined: 27 Aug 08 Online Status: Offline Posts: 304 |
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Posted: 04 Oct 14 at 5:35am |
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I think the end plate effect between the boom and deck is limited. It may look worth while in a wind tunnel, but in the real world, its acknowledged that there's not a great deal of wind between the water level and 1 metre above it. So unless you've got a metre of freeboard its probably better to give yourself a bit more room beneath the boom for ease of handling.
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Daniel Holman
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Joined: 17 Nov 08 Online Status: Offline Posts: 997 |
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Posted: 04 Oct 14 at 7:54am |
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How come my hat always gets blown off then? Accept wind gradient in the light stuff but it scrubs sown to sea pretty well in any breeze else you wouldn't be able to see it on the water. |
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Medway Maniac
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Joined: 13 May 05 Location: United Kingdom Online Status: Offline Posts: 2788 |
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Posted: 04 Oct 14 at 8:42am |
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Good point about the hat, Dan, mine does too! And of course it's in windy weather that L/D becomes most important, when you want max. thrust/heeling moment.
Blue One, I share your scepticism about manufacturers bike tests. Results depend on how they weight yaw, the wheels they fit and whether the dummy's legs are pedalling. Nonetheless, measurable improvements have been made, meaning you cover a set distance faster and/or use less energy, so an aero frame is handy if you have a flat stage. Likewise I'm sceptical about claims made for small-area, high aspect boards in, say, the 505 class. There are so many other, major contributors to drag upwind - hull hydrodynamic and aero - and high-aspect foils stall more easily. Nonetheless, I'd be loath to risk turning up at a 5-oh worlds with a large area, low aspect board. That 10% gain was experimentally measured, not merely theoretical, and the difference between Aero and OK is greater than that. We've had testimony from two people that the OK has plenty of room under the boom, too, so I find it hard to see any advance in the Aero rig over that of the OK, quite the contrary in fact. |
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Daniel Holman
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Joined: 17 Nov 08 Online Status: Offline Posts: 997 |
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Posted: 04 Oct 14 at 8:50am |
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The size of foils on most boats is such that you can get out of tacks relatively easily - so when you are applying max righting moment but are only doing 2 or 3 kts, the whole lot doesn't stall.
The flip side of this is that especially for the quicker boats like 505s that can do approaching 9kts upwind in breeze, that they are working at very low lift coefficients, and are therefore carrying around a lot more WSA than they need to. A high aspect foil is working at its best at about 4 deg AOA - best compromise between reducing induced without having too much frictional drag. About 6 years ago the 14 class gravitated to skinnier foils, so daggers went from 280 chord to 220, rudders from 220 to 170 chord, so a fairly large change. I reckon that it made a minute an hour or more difference depending on breeze. And makes you have to get good at tacking / downspeed manoeuvres!! |
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sawman
Far too distracted from work
Joined: 04 May 11 Online Status: Offline Posts: 205 |
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Posted: 04 Oct 14 at 9:11am |
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I have always had a soft spot for the OK, I actively sought some when I jumped out of open circuit 2 man boat racing - but for some reason I ended up with a Vareo (with which I never really gelled)
If my daughter gets fed up sailing the miracle with me and I cant persuade her little brother that sailing is better than rugby I will probably scratch that itch
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