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What about rigs leaning to leeward?


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#1 MartinRF

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Posted 10 June 2018 - 12:57 AM

Some racing multis, mostly French ones, cant their rig to windward but the rest of us don't bother with that as complication and cost goes through the roof.

 

As things (shrouds, crossbeams...) have limited stiffness our rigs lean over to leeward instead and the lee shroud goes limp. In my case it is very limp:

 

Attached File  rigglutning_meas_2005b.png   608.42KB   3 downloads

 

I have not thought much about this untill last weekend. This morning I tried to find photos I could use for measurement but only found a couple and all with me singlehandling (= low stability). The one above is the best since I am just about to fly a hull (in a guestimated 10 knots of wind).

 

Questions:

 

* How much rig leaning do you see?

* Efficiency takes a hit but only a small one I guess. What about handling, like gust respons? (This comes from old Tornado hands who have told me how much easier to controll super stiff Martström Tornados were compared to the cheaper/softer boats they usually started out in.)

 

Edit:

 

* A 'back-of-the-envelope' calculation I just did indicate that the 'measured' rig leaning angle above is in rough harmony with what I should get when taking wire stiffness and beam stiffness into account. Disclaimer: It is getting close to midnight here.

 

* The photo was shot by a friend sailing a Marström A-cat.

 

/Martin


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#2 TimB

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Posted 11 June 2018 - 02:22 PM

Seems no one else is concerned or noticed this Martin.

 

However I do remember the likes of Split Enz and Afterburner have rigging leading to forward ends of their hulls to avoid the windward (rear) shroud pulling the windward stern way up twisting the whole structure, which leads to the rig leaning even more to leeward. Split Enz also had massive rubber things to take the slack out of the very loose leeward shroud.

 

That's another problem of the GBE style open deck cats and flexible trimarans. I remember Malcolm Tennant taking about "tension structures", ie all the loads are spread around using wires or tension only members, with a few compression members here and there....eg beams and hulls.

 

Tb


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#3 Hurts

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Posted 12 June 2018 - 09:34 AM

A few trimarans here cant their rigs to windward - Vodaphone/Frank racing (the ORMA) seem the most dedicated practitioners... sailed a few times on F82 Sledge (need for speed/Predictwind) the effect between canting and non canting in gusts is quite remarkable - with canting you seem to lean less and accelerate much faster... but it is a lot more work on maneuvers...


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#4 Ed

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Posted 12 June 2018 - 10:11 AM

Rig canting often used on tri's (big and small) due to the dihedral leaning the rig over to leeward all the time, and the lift from the rig pushing the boat into the water rather than driving it forward, its relatively easy to set it ip so its about right most of the time.

 

Cats are a bit more difficult in that regard as they sit level, so the systems aren't as widespread.

 

Looking at the photo, the rig is bending on the top half, this can be a god thing for gust response, but if you are looking for more power then a set of top mast side stays may help support the rig above the hounds, this will stop that section falling away.

 

We usually rig these with a 12-16:1 cascade purchase so they are adjustable and can be wound on as required


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#5 MartinRF

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Posted 16 June 2018 - 10:08 AM

Judging mast bend from that photo may be tricky. There is a 100 mm pre-bend set-up by help of swept spreaders and diamond tension and the mast is rotated beach-cat style.

 

Here is another photo in similar wind but as we were sailing two-up I could sit on the windward bow while shooting this. Still not a great photo but I think the mast-top bending windward-and-aft can be seen.

Attached File  mastböj_2014.png   847.15KB   2 downloads

 

This what it does when the mast is fully rotated and the main sheet is fully in. If I reduce mast rotation the top will start to fall off to leeward and the pre-bend lines up with the sail 'cloth'. Both these and reduction of the mast's contribution to sail depth de-powers the rig quite a bit. Flattening and opening up the jib slot completes the de-powering.

 

/Martin


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#6 Freedom GBE

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Posted 17 June 2018 - 12:18 PM

Simply try not to look up when flying a hull.

 

Our raft goes 10% faster and 10 degrees higher when we fly a hull. All the small tweaks give us around 1% or 1 degree, not enough to worry about unless we can fly a hull.


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