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What does the kicker do?
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NickA View Drop Down
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    Posted: 30 May 14 at 1:43pm
The V3000 has the boom joined to the mast using a hole and pin arrangement (as do the Vago and RS500 tom name but two) 

http://www.seldenmast.com/en/products/dinghies_only/booms/gnav_system/__lang_en.html

This is SO much better than the standard "square pin into the boom" of a traditional boat in that the boom can never come off the gooseneck.  

I wish we had the same arrangement on the Javelin on which the boom is only held on to the mast by the outhaul and kicker (which are not always tight).

I guess a GNAV will work better on a boat with a low boom EG a musto skiff - though I think the boom is at deck level to maximise low down sail area rather than to maximise GNAV efficiency.

Partly fashion, partly ergonomics, but evidently little difference in performance given by the variety of boats using the two systems:

Vang:  B14, RS800, RS700 
Gnav:  49er,  i14, Laser 4000, MPS, RS500
 
Interesting to note that the mast does bend in a different place under GNAV and VANG, pulling fullness out of the sail in a different place; so sails cut for one may not work their best with the other.  (The V3000 sails were cut for a GNAV presumably).
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Medway Maniac View Drop Down
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Posts: 2788 		Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 28 May 14 at 12:36pm
Originally posted by iGRF

Why however do you feel a conventional kicker wouldn't tighten the leech?

I didn't anticipate that interpretation of my wording!

What I meant was "using a conventional kicker instead of a gnav on the EPS would have little effect on leech tension".

When I said "might be a little softer even", it was on account of the longer unsupported length of mast above the gooseneck implied by a conventional kicker compared to the length above the gnav attachment point; the longer the unsupported length, the greater the tip deflection for a given load.  But as I said, it's hardly likely to make a huge difference, given that the change in length would be small.


Edited by Medway Maniac - 28 May 14 at 12:37pm
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 28 May 14 at 10:43am
Originally posted by Medway Maniac

Indeed, a completely different situation on an unstayed rig. So far as I can see, on the RS300, if the boom is to rise then the mast has to bend below the gnav attachment point, but the leech tension should be maintained as the mast tip also moves forward. Likewise the EPS.

Using a conventional kicker on the EPS would have little effect on leech tension (might be a little softer even), but it would result in less rig movement, since the boom end wouldn't lift as much. Would be interesting to try it. Would also be interesting to know designer Yves Loday’s rationale in adopting a gnav; maybe just to avoid fouling the raised daggerboard?


I imagine it was fashion rather than function in the case of the EPS, particularly since the 'lowers' act well below the upper impact point of the GNAV strut. It seemed to be the de rigeur device of choice about the time I came on the scene, almost every boat I've owned has one and it was only when they rigged the V2 with a proper kicker I got to see the difference, which was not the best option on the spaghetti RS700 mast we used.

Why however do you feel a conventional kicker wouldn't tighten the leech? I did flirt with rear sheeting for a while which occasionally I'd like to revisit, I'm sure my windward work was better, but without an elastic band the downwind clew first stuff doesn't happen so easily in light air having all that warp off the boom end, you could almost do with some sort of variable system that could be changed on the fly.
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Medway Maniac View Drop Down
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Posts: 2788 		Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 11:35pm
Indeed, a completely different situation on an unstayed rig. So far as I can see, on the RS300, if the boom is to rise then the mast has to bend below the gnav attachment point, but the leech tension should be maintained as the mast tip also moves forward. Likewise the EPS.

Using a conventional kicker on the EPS would have little effect on leech tension (might be a little softer even), but it would result in less rig movement, since the boom end wouldn't lift as much. Would be interesting to try it. Would also be interesting to know designer Yves Loday’s rationale in adopting a gnav; maybe just to avoid fouling the raised daggerboard?
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Posts: 1459 		Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 10:32pm
Lot's of oversimplification in the diagrams.  Because much of the force is applied to the mast through the sail, pulling 'downwards' (down the leech) on the upper mast.
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 9:46pm
I have to use the Gnav on that EPS to make the entry of the sail shallower so the damn thing points worth a damn. It's even worse on the soft sail I've had made, I have to have almost max kicker just to get the sail to even look half normal.
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Posts: 106 		Post Options Post Options   Quote pondlife1736 Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 9:13pm
Interesting. 
What about an unstayed mast?
I was looking at an RS300 (which of course has a gnav) at the club on Sunday and noticed that the goosneck is virtually at deck level giving that characteristic angled boom. I'd never really thought about it before but I suppose that was done to limit any boom induced mast bend?
My EPS has the mast collar just under the boom, so the mini-forestay would give the same effect.

Nick, does your V3000 have something like a transverse pin connecting the boom to the mast? 
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Posts: 2788 		Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 7:24pm
The problem is not actual reverse bend at the gooseneck, but rather uncontrolled bend/movement.  

If you push up on the end of the boom (to simulate wind loading), there is no positive restraint on the gooseneck moving back a little (not necessarily to reverse the direction of bend, but to reduce the forward bend you previously had).  With a conventional kicker, the gooseneck is held by kicker-induced boom thrust against the positive restraint of the lowers adjacent the gooseneck; the only way the sail clew can then move upwards is through stretch in the kicker or bend in the boom.

As a result, for a given kicker setting, the gnav will allow more upward movement of the boom (permitting increased twist) during gusts than a conventional kicker would.
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NickA View Drop Down
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Posts: 784 		Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 14 at 6:14pm
If anyone is still interested ................

Here are a couple of pics of my V3000 with nearly full mast rake and full GNAV on (more than I'd use when sailing I think) first without the lowers disconnected and then with the lowers on and coming tight at half GNAV.

The boom certainly does NOT move back (other forces won't let it).  Putting on the lowers moves the point of max bend (and minimum sail fullness) up the mast and also causes the GNAV to pull the leach in more.

Think my spreaders are swept back too far tho!
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Posts: 784 		Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 19 May 14 at 1:15pm
You can work out the resolved forces yerselves.  I only showed the initiating force!

Have to remember that the fixed points are the mast foot and the tops and bottoms of the shrouds; everything pivots around those.

Shall meddle with my GNAVed boat and send photos; I'll be surprised if that bi-directional twist is significant compared to the bend induced by the boom pulling the leach down - but in a spirit of inquiry will be pleased to be found wrong.

Indeed, low floored boats with big centre-board cases and kickers are a recipe for making a big boat seem stupid small inside.  Especially with a 15 stone crew wearing bouyancy and a harness!  Nearly lost my nose a couple of times in early crewing days, until I learned to tack facing backwards.  
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