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Debunking Dodgy Sailing Theories |
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Oatsandbeans
Far too distracted from work Joined: 19 Sep 05 Location: United Kingdom Online Status: Offline Posts: 382 |
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Topic: Debunking Dodgy Sailing Theories Posted: 20 Oct 11 at 10:47am |
Following the great success of the Lee Bow Effect thread ( we can all now see what utter cr*p that one was) I got to thinking about other unsound theories that get peddled around sometimes by those that should know better.
Well I will start this one off, by a comment from a post that I saw yesterday saying that the benefit of having twist in sails is so that in instable wind conditions at least one section of the sail is set correctly. Well it is surprising how many sailors hold this view.
Well here goes- Rubbish- If you think a the sail as a number of horizontal strips, say 4, that due to the twist are at different angles of attack and only one section is set correctly. This means that 75% of the sail area is set incorrectly either not enough angle or too much angle of attack this is obviously very inefficient (and slow). The reason that twist works is because the sails have to operate in different conditions at different heights. The effects of wind speed variation, wind shear, and the large effect of the jib on the lower sections of the sail all mean that a sail with no twist will lead to a stalled head and or a undersheeted base. The optimum twist will depend on the boat, the conditions (wind and water) and what the helm is trying to do at the time ( foot, squeeze,or depower). A good indicator of when the twist is right is streaming of the leach tell tales with the top one one the point of stalling (30%). Hopefully this will bring some comments and maybe even some more unscientific sailing "theories". |
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G.R.F.
Really should get out more Joined: 10 Aug 08 Location: United Kingdom Online Status: Offline Posts: 4028 |
Post Options Quote Reply Posted: 20 Oct 11 at 10:48am |
Never studied the action of power boat propellors then OaB?
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Oatsandbeans
Far too distracted from work Joined: 19 Sep 05 Location: United Kingdom Online Status: Offline Posts: 382 |
Post Options Quote Reply Posted: 20 Oct 11 at 11:27am |
No I haven't - fill me in please!
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JimC
Really should get out more Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6649 |
Post Options Quote Reply Posted: 20 Oct 11 at 11:47am |
Why do you think one precludes the other? Twist can do a lot of things for you. Yes, twist to deal with wind sheer is the most important thing most of the time, but that's not all... An overtwisted rig is more tolerant in highly unstable winds when its impractical to have the sail sheeted correctly and correct twist set all the time. Twist is also a tool in the armoury for depowering the rig, especially if you have a fully battened rig. For example I sail on a raised reservoir, so the wind is very unstable on the final approach to the windward mark. I find it faster to release some kicker and overtwist the sail for the last few yards of the beat. |
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2547
Really should get out more Joined: 11 Aug 11 Online Status: Offline Posts: 1151 |
Post Options Quote Reply Posted: 20 Oct 11 at 12:43pm |
He's one ... the V Turd will be faster round the cans than a Laser
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RS400atC
Really should get out more Joined: 04 Dec 08 Online Status: Offline Posts: 3011 |
Post Options Quote Reply Posted: 20 Oct 11 at 9:07pm |
Power boat propellors have twist in the blades because for a constant pitch distance (the distance the prop would move forward with no slip in one turn) the pitch angle varies with the radius along the blade. The tip moves forward say 1ft in 1 metre of circumference, halfway alon the blae, it moves forward the same distance but runs through half the circumference, hence it must be at a steeper angle. But it's made more complicated by other effects I'm sure. |
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Menace
Far too distracted from work Joined: 16 Oct 10 Online Status: Offline Posts: 296 |
Post Options Quote Reply Posted: 20 Oct 11 at 9:09pm |
Another load of crap, even pedalled by the RYA guys from time to time is that the jib causes a venturi effect accelerating flow through the slot gap. It's the other way round, creates an area of high pressure, hence the reason you get backwinding in the main when the slot is wrong and the wind is up. What effectively happens is the slot increases the stall angle of the main, allowing the main to operate at higher lift coefficient.
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Xpletive
Far too distracted from work Joined: 28 Jan 06 Online Status: Offline Posts: 320 |
Post Options Quote Reply Posted: 20 Oct 11 at 9:37pm |
Reminds me of a little known theory regarding the flight of birds actually being related to the discharge of wind from the anus. The wing flapping bit is to get as far away from it as possible.
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Post Options Quote Reply Posted: 20 Oct 11 at 9:44pm |
Aspect ratio, what it is and what effect it really has strikes me as generally poorly understood.
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Xpletive
Far too distracted from work Joined: 28 Jan 06 Online Status: Offline Posts: 320 |
Post Options Quote Reply Posted: 20 Oct 11 at 9:50pm |
Coincidentally, I am one of those who have an appalling understanding of what aspect ration is.... and, for that matter, what effect it really has.......I am humbly aware of how evident this may be in my posts.
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