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Jib halyard sheave: height on the mast-> Any wisdom

dinghy jib technical

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#1 pidry

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Posted 28 August 2012 - 02:29 PM

Dear SA community and maybe biggest knowledge base in the sailing world!

The framework:
I have a dinghy, which I sometimes use for racing, and sometimes I get these ideas: Why is this or that done as it is done? And why not differently? So now I have this jib, which has halyard, which has a sheave on the mast. In my boat, this sheave is about 25 cm below the forestay/shrouds attachment point. Rig tension is applied via the jjib luff wire.

The question:
Why is the jib halyard sheave lower than the forestay? Why not at the same height? Why not lower?

Obviously the minimum height is such that I can still hoist & tension the jib ... as it is now, I have about 15 cm space between the head and the mast, so plenty of room for (at least theoretical) experimentation.

Now, please give me some thoughts & ideas!

Thanks. Have fun.

#2 JimC

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Posted 28 August 2012 - 02:33 PM

basically its all about mast bend behaviour...

#3 Reht

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Posted 28 August 2012 - 04:21 PM

And rake, and all sorts of stuff. Basically, changing the halyard height on a jib where the jib luff tensions the rig will significantly change how the boat handles and how controls affect performance. Changing the height of any halyard will make a lot of differences.

#4 Mr. Swordfish

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Posted 28 August 2012 - 04:58 PM

Since you have a working forestay (ie the jib luff wire becones the forestay when you tension the jib halyard) it doesn't matter where the forestay attachment is - it's slack when you're sailing.

As for the displacement between the jib halyard and the shrouds, as JimC says it's about mast bend. And likely constrained by class rules.

#5 Major Tom

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Posted 28 August 2012 - 06:12 PM

Your jib is also probably cut to the existing halyard exit height, change this and the leach length will be too long, also the haliard exit should be at east about 100 mm above the head of the jib or else the leach at the head of the jib will be too close to the mast and destroy he slot effect at the top of the sail.

#6 pidry

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Posted 29 August 2012 - 08:33 AM

Ok, so lots of things change when moving the halyard attachment point (hoist height). But rake will stay more or less the same as long as I don't change the shrouds. So it will be mostly about bend.

But the question is: What is desirable? Putting the halyard sheave low, to get more bend? Or put it high to get less bend (e.g. only bend from the spreaders)?
I assume that one of these is good, so it should be either as low as possible (-> limited by the jib luff length) or as high as possible (limited by the forestay height). As it is now, it is right in the middle ....

Class rules don't restrict it in my case!

#7 fastyacht

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Posted 29 August 2012 - 09:14 AM

Just to be clear here:

If I understand you correctly, you don't give a hoot what the class rules say, you just want to make the rigging work.

If that is the case, you have truly opened Pandora's Box and you might end up being an expert in the end haha!

The mast bend induced by the shrouds and spreader is affected not only by the offset of the halyard and shroud attachment, but also by spreader length, sweepback, and chainplate location, and rake. As you rake the mast, assuming you have provision to change shroud length (like a 505) then with non-swinging spreaders, the rake induces bend.

How much bend? It depends on what you are trying to do and why, and what other controls are at play.

#8 Reht

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Posted 29 August 2012 - 07:20 PM

And on top of it all, what's desirable also has to take into account how the sails are cut. Changing the bend shape, rake, etc. will all result in changes to sail shape, and the best sail shape will be (should be) what the sails were cut to.

#9 pidry

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Posted 07 September 2012 - 12:42 PM

Hi guys,
so far, thanks for your replies. So what I have learned is what I have to take into account when moving the jib halyard heigt. Class rules do not mention anything about the jib halyard, which is why I started this thread. Moreover, new sails will soon be necessary anyway, which makes all kind of things possible.
So, let's move a step forward and have a look at the advantages and disadvantages of having the halyard high or low or somewhere in between.

Any ideas?





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