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Author Topic: Position of sheet attachment on jib sail.  (Read 7169 times)

g4yvm

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Position of sheet attachment on jib sail.
« on: August 30, 2010, 02:06:31 pm »

On my Odyssey the jib sheet runs to a point on the jib boom about 1/4 to 1/3 the distance from the tack, i.e. close to the front.  This has the effect of allowing the jib sail to balloon or kits as it fills with wind.  The kit does allow three holes for trimming, about 1cm apart, but to my eye the whole thing just flies way too much and on a full size yacht I have never seen the sheet fastened there, always at the clew.

By way of modification, as I am going through the boat after yesterdays "sea trials", I have moved the attachment point to about 1/3 of the way towards the clew, thus reducing the kiting dramatically.

Two questions arise:

One...am I totally wrong?

two...what effects should I expect?

David
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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #1 on: August 30, 2010, 05:16:33 pm »

Hi again David, when rigging up the sails to start with the main, the distance from the pivot point to the sheet attachment is such that the full in/out travel of the winch/arm will let the boom travel from close hauled to running free. Once that distance has been found the same distance is then measured from the jib swivel point aft. This will make the travel of both sails the same ie 'synchronous'


Mike
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g4yvm

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Re: Position of sheet attachment on jib sail.
« Reply #2 on: August 30, 2010, 08:47:10 pm »

Ah, alles klar!!

Thank you.

D
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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #3 on: August 31, 2010, 07:58:13 am »

Alles in ordnung !



pour nada


Mike   :-))
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Prophet

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Re: Position of sheet attachment on jib sail.
« Reply #4 on: August 31, 2010, 09:11:09 am »

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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #5 on: August 31, 2010, 09:21:24 am »

That is a great posting Mr P  O0 Is it some kind of hi - tec programme ? I have never seen such a good picture with written overlay and diagram, probably way beyond my restricted dabbling in things 'tinternet !

Mike
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Prophet

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Re: Position of sheet attachment on jib sail.
« Reply #6 on: August 31, 2010, 09:36:19 am »

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tigertiger

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Re: Position of sheet attachment on jib sail.
« Reply #7 on: August 31, 2010, 12:45:33 pm »

That is a great posting Mr P  O0 Is it some kind of hi - tec programme ? I have never seen such a good picture with written overlay and diagram, probably way beyond my restricted dabbling in things 'tinternet !

Mike

Hi Mike
It was done using Microsoft PowerPoint software. When the slide is finished save as a .JPG file.
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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #8 on: August 31, 2010, 12:53:31 pm »

Thanks for the info TT, very impressed ! I don't think I have Powerpoint, unless it is knocking about on my old computer that I inherited. Must have a look - see.

regards


Mike
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g4yvm

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Re: Position of sheet attachment on jib sail.
« Reply #9 on: August 31, 2010, 02:23:48 pm »

Brilliant, thanks.

Prophet, Why attach the jib sheet to the centre ring at all? If it runs through a ring on the boom end, then surely thats where the effort is and thus a sheet fastened to the boom end and ONLY to the boom end would be the same as the one in your diag?

D
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Netleyned

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Re: Position of sheet attachment on jib sail.
« Reply #10 on: August 31, 2010, 02:54:07 pm »



Surely the only reason the jib goes so far forward is that the sheet is too long.
It just needs to be set up so the jib does not go forward of the beam at 'full throw'
and comes in to about 5 degrees short of the fore and aft line.


Ned
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g4yvm

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Re: Position of sheet attachment on jib sail.
« Reply #11 on: August 31, 2010, 05:50:26 pm »

What happens if those angles are reversed?  Ie the gap between the jib and mainsail is narrower?  Aerodynamically that seems to make sense as it compresses the air producing more lift over the mainsail.  But I be there's more to it than that!


I only ask because Ive just rigged it wrong!
D
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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #12 on: August 31, 2010, 05:59:09 pm »

The Prophet he has spoken !!
joking apart the diagram is right, jib angle is slightly greater than main. You have it about right if the yacht will sail with the sails set on a broad reach and she slowly luffs up into the wind, hands off


Mike
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JayDee

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Re: Position of sheet attachment on jib sail.
« Reply #13 on: August 31, 2010, 06:27:51 pm »


Hello,

The key Statement of these post is " she slowly luffs up into the wind, hands off "

The ability to sail in a straight line, with no Rudder input, is exactly what is required in a yacht.

Set the sails as in the post, then sail across the wind - - on a reach, and see what happens.
IF the boat slowly turns down wind, move the Jib out very slightly.
Do not move the mainsail.

Sail again as before, move the Jib in, or out, in 1/8ths to get the boat to sail in a straight line on a reach.
When the boat IS sailing straight, mark the sheet settings for future use - - - in case the Rig has to be taken off !!!.

Real Good Thread !!!!.

John.  :-))  :-))  :-))
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tony23

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Re: Position of sheet attachment on jib sail.
« Reply #14 on: August 31, 2010, 09:42:07 pm »

Mmm, that's not excatly correct altering the slot between the jib and mainsail will alter the helm slightly but weather helm is more to do with the rake of the mast. As a guide rule for a boat this size the mainsail boom wants to be about 15-20mm off centre with the sheet fully in the gap between the jib boom and mast wants to be about 3 fingers wide this should give you near enough settings to try the boat with. Watch the boat sail and look at the mainsail when on a reach then adjust the jib slot closer to the mast in increments until you see a crease forming the length of the mainsail if you can see this crease let out the slot slightly until it's gone.
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g4yvm

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Re: Position of sheet attachment on jib sail.
« Reply #15 on: August 31, 2010, 11:56:54 pm »

Thanks all, excellent stuff!!

D
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MikeK

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Re: Position of sheet attachment on jib sail.
« Reply #16 on: September 01, 2010, 07:28:48 am »

Mmm, that's not excatly correct altering the slot between the jib and mainsail will alter the helm slightly but weather helm is more to do with the rake of the mast. As a guide rule for a boat this size the mainsail boom wants to be about 15-20mm off centre with the sheet fully in the gap between the jib boom and mast wants to be about 3 fingers wide this should give you near enough settings to try the boat with. Watch the boat sail and look at the mainsail when on a reach then adjust the jib slot closer to the mast in increments until you see a crease forming the length of the mainsail if you can see this crease let out the slot slightly until it's gone.

I thought I might have got away with that faux pas !      :embarrassed:  After I wrote it and realized it was two different things I was on about ie slot and moving the centre of effort, I had hit the submit button and it was out there for my superiors to dissect !  {:-{  4/10, more attention required !


Mike
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tigertiger

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Re: Position of sheet attachment on jib sail.
« Reply #17 on: September 01, 2010, 09:41:47 am »

Surely the only reason the jib goes so far forward is that the sheet is too long.


Not quiet.
If the jib sheet were shorter, then when the sheet is fully wound in it would be pulling the jib down through the deck. Or with the jib full in the main will still be 3/4 out.

This is because both the main sheet and jib sheet are on the same winch. And so both sheets will be retracted by the same amount.

The alternative is two winches controlled to turn at different speeds and with different end stop settings. Or tinkering with various diameters of drum. It can be done electronically as well, but you need to buy more stuff (apart from the extra winch).
Alternatively, if using a sail arm servo you can have holes at different positions on the arm.

Use one winch and keep it simple if you can.


I think (but stand to be corrected) the reason the sheet runs through the end of the boom is for increased leverage, and balancing the angle of boom swing (of both booms) with the length of sheet being hauled in.

After passing through the end of the jib boom, the sheet can then run to a point towards the front to allow for adjustment using a bowsie.
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