New Posts New Posts RSS Feed: Debunking Dodgy Sailing Theories
  FAQ FAQ  Forum Search   Register Register  Login Login

Debunking Dodgy Sailing Theories

 Post Reply Post Reply Page  123 9>
Author
Oatsandbeans View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 19 Sep 05
Location: United Kingdom
Online Status: Offline
Posts: 300
Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post 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".


Back to Top
G.R.F. View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 10 Aug 08
Location: United Kingdom/Hythe
Online Status: Offline
Posts: 4027
Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 10:48am
Never studied the action of power boat propellors then OaB?
Back to Top
Oatsandbeans View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 19 Sep 05
Location: United Kingdom
Online Status: Offline
Posts: 300
Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 11:27am
No I haven't - fill me in please!
Back to Top
JimC View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 17 May 04
Location: United Kingdom
Online Status: Offline
Posts: 6339
Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 11:47am
Originally posted by Oatsandbeans

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-


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.
Back to Top
2547 View Drop Down
Really should get out more
Really should get out more


Joined: 11 Aug 11
Online Status: Offline
Posts: 1130
Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 12:43pm
He's one ... the V Turd will be faster round the cans than a Laser  Wacko
Back to Top
RS400atC View Drop Down
Really should get out more
Really should get out more


Joined: 04 Dec 08
Online Status: Offline
Posts: 2957
Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:07pm
Originally posted by G.R.F.

Never studied the action of power boat propellors then OaB?


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.
Back to Top
Menace View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 16 Oct 10
Online Status: Offline
Posts: 296
Post Options Post Options   Quote Menace Quote  Post ReplyReply Direct Link To This Post 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.
Back to Top
Xpletive View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 28 Jan 06
Online Status: Offline
Posts: 320
Post Options Post Options   Quote Xpletive Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:37pm
Originally posted by Menace

..... a venturi effect accelerating flow through the slot gap
 
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.
Back to Top
Peaky View Drop Down
Really should get out more
Really should get out more


Joined: 15 Nov 08
Location: United Kingdom
Online Status: Offline
Posts: 2771
Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:44pm
Aspect ratio, what it is and what effect it really has strikes me as generally poorly understood.
Back to Top
Xpletive View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 28 Jan 06
Online Status: Offline
Posts: 320
Post Options Post Options   Quote Xpletive Quote  Post ReplyReply Direct Link To This Post 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.
Back to Top
 Post Reply Post Reply Page  123 9>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy