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GNAV v's Kicker

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neilw View Drop Down
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Post Options Post Options   Quote neilw Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 08 at 10:37pm

are you talking about the type of traveller on the Finn - also used by OKs and FDs. If so here's one on my Finn

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MikeBz View Drop Down
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Post Options Post Options   Quote MikeBz Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 08 at 10:57pm
Originally posted by Medway Maniac

Originally posted by MikeBz

A conventional kicker produces a forward thrust at the gooseneck which is proportional to the amount of kicker tension (thus proportional to the leech tension) which induces bend (there may be strut(s) or lower(s) to counteract it).  The kicker pulls back at the heel of the mast with a similarly proportional force, but because it's at the heel it has no effect.   So put simply, more kicker equals more bend trying to be induced at the gooseneck.

A gnav swaps these 2 forces over - the forward thrust is induced further up the mast where the hardware meets the mast.   This is countered by the boom being thrust backwards with a similarly proportional force.  In this case the backward force on the boom (which presumably is bolted to the gooseneck in some way otherwise it would just fall off) will to some extent mitigate the forward force at the top of the lever.  In practice this 'push-me-pull-me' act some way up from the heel of the mast will be trying to induce an S-bend - imagine you've got a long whippy cane, both ends clamped, and then you get hold of it with both hands and pull with one hand whilst you push with the other.

I would have thought from the above that the gnav would induce less bend, albeit subject to positioning of struts/lowers/gooseneck/heel etc.  That's an intuitive guess rather than an engineering/mathematical proof - although it wouldn't take too much of an engineer to come up with the proof (or otherwise).

Agreed, Mike, up to the point where you say "the gnav would induce less bend".

With a conventional kicker you can use a ram or lower shrouds to effectively lock the gooseneck in position, so that the only effect of the kicker is to apply leach tension. With a gnav, sure you can lock the position of the attachment point of the gnav on the mast in a similar manner, but to prevent bend you also need a system to stop the gooseneck moving aft, inducing bend.

Well to be fair I did say "subject to positioning of struts/lowers/gooseneck/heel". But you're right, I didn't consider the inversion that you'll get below the gnav if you have lowers at that point preventing the mast from going forward. As you say that inversion will cause some forward bend above the lowers, but IMO it will be very little - go back to my example of grabbing the mast with both hands and doing the push/pull thing with say 12-18" between the 2 points.  Without lowers or strut I still think the gnav will induce less bend than a conventional kicker on a keel-stepped mast (hence a fair distance between the heel and the gooseneck).

Mike

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Post Options Post Options   Quote Merlinboy Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 08 at 11:16pm
Originally posted by neilw

are you talking about the type of traveller on the Finn - also used by OKs and FDs. If so here's one on my Finn

 

Fook me i want one!!! 

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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 12:04am
Originally posted by neilw

are you talking about the type of traveller on the Finn

*That's* not a wide traveller... *This* is a wide traveller...


(apologies for the photo credit YY)

Personally I'm no fan of the push kicker* because it messes up one of the more effective bits of sail, puts bend in the mast where I really don't want it, and needs extra long lowers (=more drag) to try and stop it doing so.

As someone who's always sailed forward hand I don't really buy the space thing.


*I refuse to use the vile V "word"

Edited by JimC
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craiggo View Drop Down
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 8:16am
Perfect illustration Jim,

Come on then Graeme get one of those on your Alto, it'll give Jumanji something to smash his shins into !!

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Iain C View Drop Down
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Post Options Post Options   Quote Iain C Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 8:24am

I too am a big fan of the temple vang.  Nice and simple, more room for the crew, and all the purchases can be hidden up the side of the centreboard case.  However I could only really see it being light, strong and practical on a carbon boom, I guess you could weld one up in alloy but I would not give it long and it will probably weigh a ton.  If made in carbon, it must be done right, as in VERY strong, and at just the correct angle, as it's easy to get it wrong and go "block to block", especially if you have a raking rig.

You can just see one here on this 12.  The white part of the mast up to goosneck height is not mast, it's actually part of the hull, it's called a stump and it's completley devoid of any support such as lowers, and has not broken yet despite our best crashing efforts!  The vertical member near the gooseneck is in massive compression and the member sloping back is in tension although obviously does not have to be as strong as long as it's well attached to the boom.



Edited by Iain C
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Post Options Post Options   Quote Jack Sparrow Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 8:24am
Originally posted by tgruitt

If someone can tell me how to put a picture here on a mac, I have a great pic
which explains all!


Use Firefox as a browser. Seems to be good for me.

I will report back on my 'temple vangs' strength when I finally get the thing on the water. But I am sure Aardvark Issues will vouch for it's strength, it was made by his sister company - Aardspars. ( or should I say farther company?)
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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 9:03am
Originally posted by Iain C




I must just say, is it me or is that a fabulous spinnaker, dunno what it is
about the shot, the lovely curve of the luff or the radial clew but that has
to be the nicest spinnaker I believe I've ever noticed.

Makes ours a joke by comparison.

Anyway back on topic, I think the guy developing the Alto would totally
freak if we asked about a traveller like that, but I can see the benefit, we
also have this big hoop thing in the middle of the boat like the rs400 yet
it's sheeted aft. I did ponder the thought of an aft traveller running over
and above the stick tiller thing so the damn sheets don't keep wrapping
themselves either around the stick handle or the corner of the stern at
the most inopportune moment.

We've already changed so much, I worry he'll lose what patience he has,
he's concerned about the class not becoming like the 505's and pricing
and complicating itself out of the market.

I can fully sympathise, given my own confusion even a few months ago I
would have looked at that Finn and the brain would have short circuited
and I'd have had to walk away in frustration simply at the lack of
understanding of what all the ropes do. Now here I am actively looking to
complicate things further against all my prejudices regarding such
complications, races are after all won and lost more on sailing tactics and
ninety percent of that on the start, than any complicated setting or
adjustments has always been my opinion. You can lose more places
fiddling with an adjustable outhaul or downhaul than you'll ever regain up
a beat or down a reach for the difference such adjustments make.

But it's such a delightful temptation with a boat where you don't run
quite the risk of falling in, having a fiddle and the trauma of that distance
race has changed my view - I think.

Edited by G.R.F.
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craiggo View Drop Down
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 10:05am
Graeme, get rid of the hoop, as you say if you have a bridled mainsheet onto the transom which is then led along the boom and down into the cockpit there is no need for a hoop.

Regarding the scorpion kickers system I mentioned previously I cant find any photos but here is a sketch (sorry for the lack of artistry its an MS Paint special), effectively a temple vang mounted from the foredeck rather than the boom. wire from prodder to cockpit sole/bulkhead junction ensures that prodder sees no bending only compression. THe prodder is only pinned to the back of the foredeck to allow it to rotate laterally with the boom.



Edited by craiggo
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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Sep 08 at 10:27am
Note that I'd never use a traveller these days: the old girl only has it because that's how she was built and its also structural. The transom bridle is my preferred solution. I can envisage using a *kicker* on a radiused track, as some leadmines do, but there are formidable engineering challenges. One solution for the bridle/transom corner thing is a bit of shock cord as per 29er/49er.

Temple Vang *and* a stump Iain! I guess I can see why, but I'm not comvinced. Although I'm still in two minds about the temple vang thing anyway: I'd like to see an actual engineering analysis of where the loads go...
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