Rig tension theory

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DanV193
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Rig tension theory

Postby DanV193 » Mon Feb 26, 2018 6:25 pm

After a brief hiatus in the Moth class I have returned to the Musto Skiff fold (hurrah), and I have decided that rather than revert to what I used to do with rig settings, or copy Bruce (yawn) that I would start with a fresh sheet and try to develop some rig settings from basic theory, maybe even having a light, medium and heavy setting. I think I understand the action that most of the variables have on the rig as follows:

Lowers: more tension = less bend at gooseneck = straighter mast = more power
Spreader length: longer = more poke force keeping middle of mast in column = straighter mast = more power
Spreader deflection: more = more distance fore and aft between mast and shroud = bendier mast = less power
Rake: more = rakier = less power = more pointing = better for lightweights and/or strong winds (bit vague on rake)
...
But what is the theory on shroud tension?
Is it more = mast less likely to flex until higher wind forces = straighter rig = more power?
Is there a relationship between weight of helm (on the trapeze wire) and ideal rig tension? What does Dave Poston/ Richie Roberts use vs Mike Palfreman/ Iver Ahlman?
If you see the windward shroud slacken (when you are on the wire) - is that a sign that you need more rig tension?
Why don't we use more than 30 - 31? Is it because the boats can't take it, or because above that you start to compress the mast causing more bend lower down?

Any answers, theories or insights all gratefully received because the generic sailing literature doesn't seem to cover it well (at least nothing I have found), because I guess the Musto Skiff rig is fairly unusual (most trapeze + shrouds + lowers + spreader rigs are also supporting jibs).

Note: I know the benefits of a single general purpose rig setting that works reasonably well in everything, and I anticipate that for 80% of the time that is probably where I will be, but just interested in experimenting. I am rig-curious.
Dan Vincent
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Re: Rig tension theory

Postby paul manning » Mon Feb 26, 2018 7:22 pm

Hi Dan

I think that setting up from first principles would be an interesting exercise.

I've added a few additional thoughts and promoting questions...


Lowers: more tension = less bend at gooseneck = straighter mast = more power
(Maybe more power, but why and what is the drawback of under /overdoing it?)

Spreader length: longer = more poke force keeping middle of mast in column = straighter mast = more power
(This is tied to the shroud tension)

Spreader deflection: more = more distance fore and aft between mast and shroud = bendier mast = less power
(This is tied to the shroud tension, rake, heel position etc and is all relevant in matching the flying shape of the sail with the corresponding mast bend)

Rake: more = rakier = less power = more pointing = better for lightweights and/or strong winds bit vague on rake
(Why do you think more rake means less power?)

...
But what is the theory on shroud tension?
Is it more = mast less likely to flex until higher wind forces = straighter rig = more power?
Is there a relationship between weight of helm (on the trapeze wire) and ideal rig tension? What does Dave Poston/ Richie Roberts use vs Mike Palfreman/ Iver Ahlman?
If you see the windward shroud slacken (when you are on the wire) - is that a sign that you need more rig tension?
Why don't we use more than 30 - 31? Is it because the boats can't take it, or because above that you start to compress the mast causing more bend lower down?
(Typically you only need high rig tension to maintain an acceptable forestay sag for the jib. A una rig therefore typically doesn't need such high rig tension)
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Re: Rig tension theory

Postby Rick » Tue Feb 27, 2018 11:01 am

If you compress a flexible spar it will deflect ... the more you compress it the more it will deflect.

If you use attachment points on the spar to control the deflection (spreaders and lowers) you will deflect the spar into a certain profile.

The more tension you apply to the stays the more compression you get and so the rig will become "harder" i.e. the profile created by compression and control will be more rigid in its set-up.

I think there is a point of diminishing returns on rig tension and when you get to a certain load you don't really change much apart for stressing the boat..

So, you will get a fixed sail shape with more rig tension; if you use less rig tension the profile of the spar can change and flex.

I wonder what the boat would be like if you sailed with really lose rigging? I'd be worried about the foot jumping out ...
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Re: Rig tension theory

Postby Paul Gilbert » Tue Feb 27, 2018 12:33 pm

Hi Everyone,

I'm new to the class, swimming loads, learning fast and meeting some interesting and hugely helpful people. Improving every time I go out, but a long long way to go!

Interesting topic this, I’ve come from Merlin Rockets (with a few years off in between). They have fully raking, very bendy rigs with a huge amount of control over the rig set up. The crew can control pretty much everything live and rakes fore and aft during the race as the wind changes strength. As a result we used rake all the time to de-power and power up as the wind increased and decreased - it is highly effective. I think for a number of reasons, some relevant to the MPS, others not:

1 -As you rake aft with a fixed jib sheeting angle there is less downward force on the jib leech, allowing the jib to twist. This opens the slot allowing more air to flow round the back of the main, decreasing power and improving helm balance - not relevant to us
2 - As you rake aft the centre of effort of the rig moves aft. On the Merlin we would raise the centre board a little as we raked to maintain balance. Bringing the CoE aft encourages the boat to pinch up into the wind, reducing lee helm.
3 - More rake with fixed spreader angle and length = more mast bend up top = flatter main up top = less power, much less tipping moment and much less drag. This last point is important, we found drag not only slowed you down but it made the boat harder to fight to stay in control. Raking the rig got rid of the power and drag where you don't want it (up top) but we could continue to control the power lower down using the lowers (less tipping moment).
4 – Raking aft lowers the boom (ouch!) meaning the main sheet only controls boom angle, not leech tension
5 - Letting off the lowers let the mast bend low down but straightened it higher up (equal and opposite effect above the other fixed point - the spreaders), so this was a double edged sword and I struggled tbh, this was a fine balance between pointing, power and drag. Tuning the lowers (and kicker) was more about looking at the leech tell tails to get them flying 80% of the time and working out what felt right.

In a blow we would "blade" the sails off, lots of kicker to bend the mast, lots of twist and very fine entry maintaining pointing and balance. The lighter we were the earlier we raked. Also, sailing the boat free in these conditions made a huge difference to our overall pointing, not sure why, perhaps something to do with lift off the foils increasing as speed increased...

As to rig tension, we were able to control this whilst sailing and only ever tensioned up so the leeward shroud was taught, any more and we felt that only bent the hull.

I imagine they have moved on a lot since I sailed then but I’m sure there are some things that apply to the skiff.

See you at Rutland.

Paul

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Re: Rig tension theory

Postby Gavryn » Tue Feb 27, 2018 3:13 pm

I to am trying new things as when you look on the class web site everyone is all in the same ball park and although I’m not going far from those settings I am trying some new stuff especially to me my settings so far as stands from Sunday but yet to sail are.

Mast rake-7290
Lowers-12
Shroud-32
Spredder deflection-115
Spredder length-395 (5 holes showing)

Being 94kg at the mo I wanted a more powered up rig and will tweek from there.

Gavin
Stokes bay 414

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Re: Rig tension theory

Postby PaulS » Tue Feb 27, 2018 9:35 pm

I've got a feeling the more you rake a musto rig the less kicker you'll be able to pull on due to the angles.... might be wrong. No idea about speed. I seem to max out on kicker pretty easily at the moment.

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Re: Rig tension theory

Postby PaulM » Tue Feb 27, 2018 9:51 pm

PaulS wrote:I've got a feeling the more you rake a musto rig the less kicker you'll be able to pull on due to the angles.... might be wrong. No idea about speed. I seem to max out on kicker pretty easily at the moment.


Rake should not affect the kicker max as it will pivot with the mast.

If you are maxing your kicker maybe check the pins on the bar are in the correct position, and also that they have not worn and elongated the holes.
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Re: Rig tension theory

Postby DanV193 » Tue Feb 27, 2018 9:58 pm

Interesting stuff. I think that has already been identified it is not easy to take the lessons from one class to another.

Spreaders require rig tension to work. If there is no tension in the shroud the spreader connected to it won't have any effect on the mast position.

By sketching it out on the back of an envelope I think I agree with Paul G above, and that the spreader doing most work in our rig, should be the windward one, pushing and preventing the middle of the mast from bending too far to windward. My guess is also that the leeward lower shroud is more loaded than the windward one?? Is that correct?

I guess that a heavy sailor putting weight on the trapeze will take some of the weight off the windward shroud. Presumably a highly loaded rig will be less effected by this then a low rig tension setting. A bit like Rick said a high tension rig will be more locked in.

But I could be completely wrong.
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Re: Rig tension theory

Postby Rick » Wed Feb 28, 2018 10:05 am

I have observed the windward shroud being slack with heavy sailors and the opposite with light sailors.
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Re: Rig tension theory

Postby DanV193 » Wed Feb 28, 2018 6:20 pm

It definitely used to happen to me (weight 83-85 kg) on fairly standard settings (28-30 rig tension), normally when windy and the rig was heavily loaded.
Occasionally the shroud would go visibly slack, but I guess even when it is not obvious the tension in the windward shroud is significantly reduced.

Rick, when you say you have observed the opposite happening to lighter sailors, do you mean the leeward shroud slackening?

My guess is that it happens when the mast is compressed by downhaul and bent by wind force and kicker so that the hounds are getting closer to the deck, combined with a helm weighing 80+kg dangling off the wire causes the windward shroud to unload.

I guess that with a higher rig tension that is less likely to happen.

What I don't know is whether it is a good (fast) thing or not. Presumably as the load reduces in the shroud the spreader is less effective and the mast is able to bend more, this might allow the top to flatten off and twist (good gust response) or it might be having a negative effect??

Sooo... if its a bad thing and we could reduce its effect with higher rig tension perhaps that is worth trying.... but if it is a good thing and/or the hulls couldn't cope with the higher tensions perhaps not.
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Re: Rig tension theory

Postby Rick » Wed Feb 28, 2018 6:41 pm

DanV193 wrote:Rick, when you say you have observed the opposite happening to lighter sailors, do you mean the leeward shroud slackening?


Yes ... when breezey....

I wonder if there is a maximum design rig tension?

I know when we sailed 470s some people used massive tensions and the boats used to last 5 minutes; fortunately the MS if far better designed than a 470 for longevity ...
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Re: Rig tension theory

Postby DanV193 » Fri Mar 02, 2018 12:41 pm

In the interests of science and progress, I will do a bit of empirical 1-boat testing in uncontrolled and highly variable conditions and report back.

I'll try with low (24) medium (28-30) and high rig tension (33) and see if I can spot any differences (highly subjective).

The challenge will be fixing other variables, so rake (7250), lowers tension (12), spreader length and deflection, but this will obvs require a bit of fiddling around with pins and turnbuckles, and a whole load of luck regarding wind and waves. Ideally there will be some other Mustos out to race against.
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Re: Rig tension theory

Postby DanV193 » Wed Mar 07, 2018 11:04 am

Part one of the test - Sunday racing at Stokes Bay - with baseline rig set up

- 2 races - wind started at about 6 kts got up to 12-13 kts by the end of the second race. Sea state was pretty flat for Stokes, due to wind direction (SE), also aligned with tide.

I didn't have time to fiddle with settings so just went with current set up as a baseline-

Rake - 7270
Rig tension - 31
Lowers - 14
Spreaders - 4 holes showing - haven't measured deflection yet.

Observations - Start of race one I was barely wiring- more hanging in the cockpit than out of it. Shroud tension was visibly taut. Speed seemed OK compared to the competition, but we were sailing a short course, which was less than ideal for 2 boat testing.

As breeze increased and full trapezing became the norm, the rig remained visibly taut.

Breeze increased further and more downhaul and kicker were applied and the rig started to look a bit floppier. Both windward and leeward shrouds were alternating between taut and just off- taut, they were sort of vibrating, they weren't flapping in the breeze. When I looked up there was no obvious movement at the spreaders.

Overall speed was very hard to judge, but I never felt slow and had fairly good height against Pelly, Kilsby Gouldy, Wrighty, Southwelly and Moysy plus an assortment of 700s and Tarby in an 800.

I'll try with something a bit tighter in the next couple of weeks, BTW did i read on FB that Neil Ashby runs rig tension 37 as standard?
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Re: Rig tension theory

Postby Bruce » Thu Mar 08, 2018 12:25 am

Hi Dan,

I definitely don't understand it all, but here are some observations:
- the rig is incredibly dynamic at least at Stokes Bay in 15knots. The top of the rig moves considerably over each wave and the leech opens at the same time. You don't notice it so much when helming, but with a camera at the top of the mast it is very obvious. I believe all the rig settings will impact this, but I can't explain how.
- rake affects jib slot on other boats, but similarly on Musto it affects the kite slot. Changing rake made a huge difference to my downwind speed, far more than it did to my upwind speed.
- when trapezing the leeward lower has all the load. For me the leeward shroud is the one that goes slack in breeaze and I am not light.
-plenty of lower tension is good in less than 8knots but anything more that that and the boat seems to stall in gusts rather than accelerate.
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Re: Rig tension theory

Postby Rick » Thu Mar 08, 2018 11:08 am

Bruce wrote:- rake affects jib slot on other boats, but similarly on Musto it affects the kite slot. Changing rake made a huge difference to my downwind speed, far more than it did to my upwind speed.


In what way ??

More rake = faster or slower downwind?
Rick Perkins. GBR


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