Rig tension theory

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Bruce
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Re: Rig tension theory

Postby Bruce » Thu Mar 08, 2018 3:42 pm

Raked forward of my normal settings was slow downwind. I haven't tried raking back further of ‘normal’ as getting under the boom is challenging enough, without putting all my weight on the stern.
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Re: Rig tension theory

Postby DanV193 » Thu Mar 08, 2018 5:37 pm

Bruce,

Interesting - although I wonder if that is the slot effect, I have never measured it, but I would have thought the gennaker leech is a long way from the main (does it get within 1m? when sailing downwind) - Note if you are reaching across the top of a trapezoid course trying to go as high as possible with the spinnaker sheeted tightly and the main ragging, then the 'slot' if it could be called that is a lot closer, but that is not what you are talking about.

The slot between jib and main on a dinghy can be fairly narrow on some boats (I guess a close sheeting boat like a Fireball probably gets jib leech within 30 cm of the nearest point on the mainsail.

I had always assumed that the rig settings had pretty much evolved around what was best for upwind sailing, and that downwind, it all made so little difference that we all decided to compromise. Perhaps I am wrong (wouldn't be the first time :? )
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Re: Rig tension theory

Postby Rick » Thu Mar 08, 2018 11:09 pm

Maybe it is the change in the leech profile rather than any slot effect. The sheeting angle and tack are fixed so moving the head around would change that.
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Re: Rig tension theory

Postby paul manning » Sat Mar 10, 2018 10:01 am

Or it could be the change in balance on the rig / boat means the hull needs to be trimmed differently to get a balanced helm and the hull shape isn't as quick at this particular pitch /angle..
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Re: Rig tension theory

Postby SELSBowbitch » Tue Mar 13, 2018 11:53 am

Some interesting stuff here so here's my two penneth (to quote my mother)

In the past I put the go pro on the deck and pointed it skywards to have a look at the effects of the spreaders etc on the bend (mainly side bend) and found that you needed longer spreaders further forward to stop the middle of the mast popping to windward too much as the leech loads up (generally destroys the shape in the middle of the main).
With this, theory says that you should be able to decrease the amount of pressure on the shrouds (I did to 28) as you are providing the support needed by moving the spreaders forward and lengthening. Interestingly this is what is happening in the 14's, they carry super length spreaders and quite low rig tensions in comparison with 10 years ago (Andy and my first boat carried 36 on shroud, new one 30 and far longer spreaders)...more on this below

I'm a big fan of telltales up the luff of the main as it allows you to see what is going on with regards to bend, when I used to bother the front of the fleet a bit (A long time ago....) I used to find that this gave a great indication of when to change gears with regards to lower tensions. If you have a spreader height telltale about 1 foot off the luff tape it shows the depth in the sail well at that point. From my experience in really light stuff a bit of bend is a good thing as it opens up the leech and prevents having to use too much downhaul which can over flatten the sail if you're not careful (Cue Sten to disagree with this!). As you get to power up conditions that telltale starts to stall as the luff entry is too flat and more lower is required to make it fly again. As the breeze builds, the point of max depth starts to move aft on the main (making the boat feel a bit unbalanced) so you can do one of two things, if it's a variable day then use a bit of downhaul to counteract this or if it's looking like building breeze then back the lowers off a bit to allow bend, both have the effect of pulling the point of max depth forward.

Now, after that blabbering, here's where it gets interesting....

As the breeze builds you use more kicker which bends the mast more therefore putting more pressure on the lowers which can't cope with the added load, you get a bit more lower bend than you had, so in power up conditions you might need a bit more than you think to keep that in check.

Then there's also the impact that bend has on the shrouds, the more bend in the mast, the less tension you have on the shroud, think of the tip of the mast to the kicker point, as you put bend in that distance decreases, it's a similar (although less effected) at the hounds. The question is do you want the mast to twist off from the spreaders upwards to allow the topmast to be more gust responsive? Personally I prefer it to be pretty solid to the hounds(I avoided stiff...!) as if you have too little tension the shrouds start to go a bit loose and it always worries me what the shock loads are on the various components when that starts happening, particularly in waves upwind

Then, add the impact of weight on the wire. In flat water and flat wiring it's good because the rig is pretty stable and the settings you have remain in place, when the waves build you move aft (this bends the mast slightly more by increasing forestay tension and decreasing shroud tension, making the rig more bouncy generally) things start to change and this can impact the main shape etc again. What I've done is increase the shroud tension slightly to keep everything stable, probably to 30/31. So if you take a waif like Palf vs a hunk of beef like Ashby you would find Palf will need less additional tension whereas Ashby would need to reinforce his shroud points!

For me I think there is more to be had in the rake debate than anywhere else and I'm interested in looking at mast foot position too. I'm guessing that the more rake you have the further forward you need the mast foot (is it legal to put the foot forward on a pin so that it's ahead of where most people have it now?) And having used the board position a lot in the 14 and the 18 but never bothered in the Musto apart from light stuff downwind, there has to be more to be had there too....I believe our Aussie mates have been up to that

Another thing....the slot debate is effected a lot by the rake, more rake = tighter luff of kite = less leech tension = more twist = less choking of slot. Some of the early pioneers in the class (Bov, Kev etc) used to have a bit of kite halyard showing at the top of the rig to counteract this. I'd be interested to hear from Hyde to see if there is a specific distance between the tip of the bowsprit and the kite haly block they think is optimum?
In breeze I'd guess that it would be better to drop a bit of power out of the kite, but the slot (I'm guessing here) might get more open anyway as you tend to carry more mainsheet when you're in hanging on mode to stop the bow digging. Again, not sure if the extra kite twist will be good or bad, personally I like the extra power even when it blows 25+.
Again the 14's have been playing in this area too, with the new Lennon sails you basically don't ever ease the kicker, this keeps the slot open a treat, however the 14 generates a far larger amount of apparent angle and has a square top (did somebody mention square tops?) and a jib which may be the logic behind this. Vague recollection that Mr Hiscocks tried some kicker and less mainsheet on downwind at Garda, not sure if it worked but I'm guessing gybing was interesting!

Still convinced there's more to be had, just wishing I realised it 10 years ago!

See you on the water soon :-)

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Re: Rig tension theory

Postby olisouth » Sat Mar 17, 2018 11:00 am

thought it was worth posting, i used 360 cameras whilst learning - saves on buying loads to do the same job - but this video gives an view up the mast as well as to what i was doing in the boat, fast foward to 8:23 for the hoist.

https://youtu.be/H2gt09uesGI
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Re: Rig tension theory

Postby DanV193 » Sun Mar 18, 2018 3:42 pm

Interesting video Oli, I am not so sure how useful it would be for forensic analysis of the rig dynamics. The 360 degree lens does distort objects particularly near the edges.

How useful did you find it as a training tool?
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Re: Rig tension theory

Postby olisouth » Wed Mar 21, 2018 2:14 pm

Dan,

yes the distortion would be problem for detailed analysis at this resolution, im sure the newer cameras have a much higher resolution that with markers on the spreaders / sails etc you could get some usable data from.

as a training tool i found it very useful as you can watch almost every aspect of the session (depending on camera position) over time from 1 upload. i was by no means any good but watch the Dan Henderson video https://www.youtube.com/watch?v=_gVod89w1cw&list=PLO6CApCL12WNFlIF6unBURxOgaNDSbbT3 and follow his movements through the gybes - best viewed on mobile and if you have them a VR headset.
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