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-   -   Jib flapping in higher winds (http://www.sailnet.com/forums/seamanship-navigation/75323-jib-flapping-higher-winds.html)

MarkSF 06-15-2011 03:19 PM

Jib flapping in higher winds
 
Last weekend I was on a starboard tack, close hauled, and the wind was about 18-20 knots. The jib was noticeably oscillating, similar to leech flapping but seeming to be the whole sail. It doesn't do this on a port tack. I would like to tune it out as I don't think it's healthy for the rig.

The boat has roller furling. I did notice that the port jib track was set further aft than the starboard ones.

OK, I admit it, I'm not sure what to do with them. I've just set them in the middle.

So my questions are, how to tune out the jib flapping

Secondly, how to set up the jib leads in general.

Faster 06-15-2011 03:28 PM

As a starting point, draw an imaginary line from mid luff through the clew to the deck. Where that 'line' hits the genoa tracks is a good place to start.

Then, under sail, watch your tell tales... when you 'pinch up' into the wind if the upper telltales stall first, move the lead forward. If the lowers stall first move it back. Ideally all three telltales should 'break' at the same time.

In heavier breezes like those you were in it sometimes is a good idea to move the lead aft to intentionally spill some power from the upper part of the sail.

In the case described was your jib partially rolled up? Since the cloth comes off one side of the roll it's not unusual to have the sail behave differently on one tack than the other since the bulk of the furl affects flow differently on each tack.

turbulicity 06-15-2011 03:32 PM

Jib car location was discussed in several threads recently. It has to do with sail twist. If you seem to be getting luffing first near the head, move the car forward (this will pull the clew down, and tighten the leech). In lighter winds when you want more bagginess, move the card aft.

A general guide is that the sheet should be more or less aligned with an imaginary line from the middle of the luff to the clew.

Everything being equal if you are getting oscillations on only one tack, a wild guess would be that your shrouds may need to be tuned up.

MarkSF 06-15-2011 04:44 PM

Quote:

Originally Posted by Faster (Post 740815)
As a starting point, draw an imaginary line from mid luff through the clew to the deck. Where that 'line' hits the genoa tracks is a good place to start.

Then, under sail, watch your tell tales... when you 'pinch up' into the wind if the upper telltales stall first, move the lead forward. If the lowers stall first move it back. Ideally all three telltales should 'break' at the same time.

In heavier breezes like those you were in it sometimes is a good idea to move the lead aft to intentionally spill some power from the upper part of the sail.

In the case described was your jib partially rolled up? Since the cloth comes off one side of the roll it's not unusual to have the sail behave differently on one tack than the other since the bulk of the furl affects flow differently on each tack.

Yes, the jib is partially furled. It's a big 150 or 160% and it's usually furled down to say 120% or less, 100%. If this is normal I get the impression I would be better off getting a 125% or so to start with.

Faster 06-15-2011 05:04 PM

Quote:

Originally Posted by MarkSF (Post 740839)
Yes, the jib is partially furled. It's a big 150 or 160% and it's usually furled down to say 120% or less, 100%. If this is normal I get the impression I would be better off getting a 125% or so to start with.

Amen to that!! You're always going to be better off with the 'right sized' sail over a furled one, esp going to weather.

padean 06-15-2011 05:10 PM

It isi possible that the problem is not Jib car placement at all. It sounds like what you are describing is the entire sail oscillating back and forth, I assume over a short distance.

This is often not a sail trim issue, but improper rig tuning. I have seen this happen when trying to go to weather with the backstay too loose (hence the headstay too loose). This happens occasionally when we are racing and someone forgets to tighten the backstay rounding the leeward mark on a large boat. One way to confirm this is to site the headstay along the mast when going to weather. There should be little or no bend or sag in the headstay - in other words, it should be straight, like the mast. If the furler/headstay sags3-4 inches on this size boat, tighten up the backstay and see if this eliminated the oscillation of the headsail.

chris_gee 06-15-2011 05:49 PM

Taking the imaginary line from 1/2 way up the luff works better with a hanked on sail. It is not so accurate with roller furling or a yankee cut where the clew may not be inches above the deck rather a metre or more.
In that case the best I can figure is that the angle of the lead should be 1/2 the angle at the clew minus the angle of the foot to the deck or the leech to the vertical which is probably the same if clew is a right angle.
More simply the line can be taken as bisecting the sail angle at the clew. The first approach will put it about 5 deg lower. Either puts you in the ballpark for then testing and adjusting the shape.
The important thing is that the lead will be rather further back than with a standard sail.
The unrolled part of the jib that will have an effect with the air flow attachment on that side. In effect if it is on the port side you will get an effect (it seems to me lol) of the attachment if any moving back lessening power. That might cause the difference.
I am not a fan of large genoas on furlers unless you sail in predominantly light winds.
There is a further factor of wind sheer differing on each tack. In the northern hemisphere it is greater on starboard tack and can require the port genoa car set further back. This was the setting initially on your boat. However wind sheer effects are greater in light winds and also close to shore. In 15-20 knots the effect would be less, and moving the car forward would put some more tension on the leech. Ocean Sail Articles: Sail Twist and Wind Shear
It is also possible that you were pointing higher on that tack because of the rolled up luff. Pinching could give you some sail shaking.

MarkSF 06-15-2011 06:17 PM

Thanks everyone for the replies, looking forward to doing some experimenting next time I take her out.

sailingfool 06-15-2011 09:06 PM

From yesterday's thread:
To set your jib lead position correctly, sailing closehauled, head up:
- if the upper windward telltale lifts first, move the block forward,
- if the lower windward telltale lifts first, move the block aft.

In light air you may want to adjust the proper position a little forward for fullness, in heavy air, a little back to flatten the bottom of the sail.

This technique works equally well with a partially furled genoa, other than the telltales may disappear into the turns...

__________________

nolatom 06-16-2011 10:35 AM

If you have the cars set right (as in the telltale example above) and it still "oscillates", then you may want to make sure you don't have too much headstay sag as Padean suggested above. That kind of oscillation will shake your whole rig, not a good thing.


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