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Beating Mainsail Sheeting angle

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dave_c_v View Drop Down
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Post Options Post Options   Quote dave_c_v Quote  Post ReplyReply Direct Link To This Post Topic: Beating Mainsail Sheeting angle
    Posted: 05 Aug 11 at 12:55pm
Hi - I am trying to get my head around the best sheeting angle for the main (on a beat) on both single sailed dinghys and jib/main dinghys.
I own
1) a Supernova which is fully battened with the centre main attached to the boat on a centre bridle that can pull the main in past the quarter deck.
2) an Enterprise which has very fixed and wide jib sheeting position.

Lets do a bit of theory - the main sail effort is at right angles to the sail so the more you pull it in the more the air is turned over the sail and the more reaction and power is produced by the sail. However the direction and power moves more and more onto the side of the dinghy rather than pulling the boat forward.  Therfore there must be the ideal position pulling in to increase the power before the direction of the thrust all goes to heeling the boat rather than powering it forward.  (mmm difficult to descibe without diagrams)

First on the simple single sail dinghy - comparing Supernova to the Laser the main on the Laser is on a rope traveller so there is NO possibility of bringing in the main more than the rear quarter.  On the super Nova you can do more but I "ASSUME" that the Laser has it right and I should set up the bridle so that it does the same.

>>>> For a single sailed boat having the main/boom over the rear quarter is about right?   
>>>> How do you judge boat-to-boat where that single sailed magic angle is?

OK now to having two sails. I know the jib does a lot more work than its smaller size would indicate. Also the two sails should be actually considdered as one "complex" sail with the two interacting with one another. With boats such as Merlins and Tasers I have seen the mains pulled in almost onto the centreline.  I would of thought this would produce low power and massive heeling. It appears a lot more complex to decide where those magic angles are going to be,  

Sticking with the Enterprise though we have a very fixed sytem.  A wide jib sheeting angle and a wide slot with the main. "I am told that the magic angle for the main is over the rear deck slightly more that you would get with the Laser"  This is because of the wide slot?   The problem is looking on Youtube I can see people sailing with far tighter mains.

>>>> Again how do you determine what this magic angle is going to be for a jib/main dinghy?

Phew - hope you can help?
Dave
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Scottish Scrutineer View Drop Down
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Post Options Post Options   Quote Scottish Scrutineer Quote  Post ReplyReply Direct Link To This Post Posted: 12 Aug 11 at 7:43am
Good question. I'm looking forward to the explanations as I have darted racing a Laser but also crew on a Medina.
Renny Thomson
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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Aug 11 at 8:06am
It depends on a lot of factors...

Consider the mainsail as a whole, not just the boom. The sail will always be twisting off as you go higher amd what you want is best compromise over the majority of the sail. The best angle will be different for different sail cuts/sails from different sailmakers too.
Then the main sail effort is *not* precisely at right angles to the sail.
Its also probably a mistake to consider that much about the setup on an Ennterprise or a Laser translates well to other boats - the Enterpise is heavily restricted by one design rules and the Laser is notoriously idiosyncratic in tuning.
How do you determine the best angle. Well its not easy. The very best way is to do what is called two boat tuning. Two identical boats with crews of the same ability sail against each other for hours at a time, varying one setting and then another, so they can see if they go faster against the other boat. Right: if you are not a full time Olympic sailor this may not be practical.

Failing that it still comes down to practice. Your most valuable tool is your ears: listen: sheet the sail in a little closer, a little farther out and listen to the wake. More noise, more speed. Try tighter and looser kicker as well as sheeting angles. Some boats are better off with a tighter kicker and looser main, some vice versa.

It all sounds very difficult. It is. That's why sailing is a sport for life. I always say that by the time you are old enough, experienced enough and cunning enough to have all the nuances of racing sailboats the body is starting to give out. So sometimes old and cunning beats young and fit, sometimes young and fit beats old and cunning. That we we all get to sail together no matter what age:-)
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 12 Aug 11 at 12:58pm
Most 2 sailed boats can sheet to the centreline because the jib slot changes the angle the wind is coming onto the main sail. However, this is a subject about which some very long and complex books have been written.

Also, take a look at some of the tuning guides for the Europe to give you an idea of sheeting angles for a singlehander and how they are affected by wind strength, as the traveller in the Europe allows for great adjustment.
Firefly 2324, Lightning 130, Puffin 229, Minisail 3446 Mirror 70686
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17mika View Drop Down
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Post Options Post Options   Quote 17mika Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 11 at 6:05pm
I think too that most 2 sailed boats with reasonably flat main have almost always to be sheeted to the centerline upwind. The exception IMHO could be for fat mains with like the one of a 420, in low wind, but I've never raced 420 so it's just a personal feeling not backed up by sufficient experience. 
 
I'd say the main parameter to watch on 2 sail boats are leech ribbons. On the 4k and the 29er 99% of the time (1% would be wind under 5 knots in glassy water)  main doesn't stall at all with boom on the centerline. Yes, lift is directed almost sideways, but the turbin blade effect on the jib makes the difference.
 
And I remember that in the 4k, even when I tried in low wind to ease a bit the main to keep the upper main working (i.e. not stalling), I lost too much height because the lower main wasn't "cooperating" enough with the jib.
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