More input from the hive mind requested please. My understanding of what a kicker does lacks nuance. I've basically been using the kicker on my 600 as a 'go button'. I have two questions; 1) how does the kicker affect sail shape and 2) how does the kicker's effect on sail shape influence the boat's performance at different wind strengths? Many thanks for your continued patience with newbie questions!

Peter

On a stiff masted boat (my old laser 2 fun) all it does is make the sail less twisted, giving more power (ie shedding less power off the top of the sail).  Though twist is useful if there is more wind high up (eg when the wind is light).

... I'll include mee self in that also, before anyone else says it!

For a boat like a 600, I'd imagine that you need to progressively pull on more and more until the wind is so high that you're still not holding it without ragging the main, and then start to ease it a bit to allow the leech to open up a bit.Definitely ease it for windy bear-aways and downwind sailing, otherwise you're going to be playing with a lot more power than you want. 

Probably worth bearing in mind that I haven't done much proper dinghy sailing for about 2 years (doing jib trim on keelboats, so I don't mess about with the main a lot), so what I've said might not necessarily be the best way to do it, although hopefully it is a good start.There are plenty of people on here with far more knowledge than me! 

Sounds like you are doing extremely well to me.

Apparent wind is easy. Open the car window when driving at 40mph on a windless day and stick your arm out (when safe to do so of course). The gale you feel will be apparent wind.

Drive directly away from a 40mph gale at 40mph and you will (theoretically) feel no wind at all on an arm held out of the window. Thats' apparent wind too.

Drive at 40mph with a 90-degree 40mph cross wind, hold up your burgee and it will suggest the wind is coming at 45% from your bow, being a composite of the true cross-wind and the head-wind created by the vehicles motion. The composite is the apparent wind of course.

The Gnav does bend the mast around a higher point, hence the lowers are where the gnav joins the mast rather than at the goose neck.  But the effect is the same.

PS: no lowers on a 600, I'd never noticed!  Also a 29er.  Wonder why.  

And it misses how wind induced sideways bend contributes to that....an often missed and vital part of rig tune in many classes......so a coaching tip......do the ball thing.  Then hang a selection of different weights on the mast tip and see wgat happens.

There is a difference. Vang pushes the boom down and forward. Gnav pushes the boom down and back. Both apply same direction bending moment to the mast, but applied at different heights. Replacing Gnav with Vang, or vice versa would therefore give slightly different result

Indeed, and that becomes significant insofar as you can resist the mast bending forward at the upper attachment point of the gnav using lower shrouds, but it is much harder to find a satisfactory way of stopping it bending back at the gooseneck.  You could use a baby stay from the foredeck to the gooseneck, but your jib won't thank you for it, and probably your original foredeck won't appreciate the loading either.  Thus the mast bend (and so leech tension) is only partially controlled.

For that reason I'd pick a conventional kicker over a gnav every time if the crew wasn't desperately short on space. I have a gnav on the V3k and my crew loves it even if I don't, but on the Laser 3k, I found I could do a lot just by moving the kicker attachment point on the boom forward, closer to the mast, and uprating the purchase to 16:1.  The boom was up to it (or maybe I was just lucky!).

Why a big boat like the Alto, carbon mast and all,  has a gnav I can't imagine.  Extra friction and extra cost that could be better put to a carbon boom that would be made more feasible by use of a conventional kicker.

Like THIS.    Hmm, maybe.   

Certainly the point of force bending the mast changes (as I said in previous post) but the boom isn't actually pushed BACK just DOWN (the GNAV strut is fixed on the mast and slides on the boom). 

Possibly the mast IS pulled back at the gooseneck, though I've never noticed it.  I guess the bit from gnav to deck is short and stiff so relatively unbending.  The main mast bending action comes from pulling down on the leach of the main sail anyway and that doesn't affect the bottom part of the mast so much. 

Got my crew trapped between the kicker, centreboard case and side tank this afternoon during a hasty gybe and ensuing swim - so I quite appreciate the ergonomic advantages of the GNAV even on a big boat like an Alto; on the V3000 it's a godsend when double handing.

On the other hand - the GNAV certainly has more friction, you can only feed the sail up one side of the boom when raising the main and it does make the bottom corner of the sail look messy on one tack.

PS the Farr 3.7 manages the trick of a rotating mast with lowers:- as the mast rotates, the leeward lowers are wrapped around the mast and tensioned, hooking the top of the mast to windward.  Neat trick!

Diagrams reflect my understanding of what goes on.  Whether you regard it as the sail puling up on the boom end or the boom end pulling down on the sail is immaterial.  Either way, using the gnav the mast is being bent by a pull at the gooseneck and a push at the upper gnav attachment point, instead of at the mast-foot and gooseneck, respectively, when using a vang.

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

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!

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.

What about an unstayed mast?

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? 

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.

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