Who invented the GNAV??

Was it the 4 tonners or the 14s??

Is this a unique example of SMODs leading development?

Dumb question maybe, but is there any performance difference / issues between a traditional vang and gnav.  What's the upside and downside?

The gnav is up

Vang is down !

Sorry , couldn't resist it

One problem with them is that they are not weighed with the boat meaning that because most open rule classes restrict only hull weight if you have a gnav your boat has a heavier sailing weight than if it does not.

Untill you could make them light enough for the ease of use advantages to outweigh the weight penalty they were not poplular in the development classes.

I think the amount of mast bend will be the same wherever you press - it's the mechanical advantage that changes (it gets easier) but that's the same for any system. It's the old effort-required versus rope-in-the-boat question.

I'm sure someone will tell me I'm talking rubish... and not for the first time

Matt - You're talking rubbish!

The mast bend forces induced by the boom/gnav/vang arrangement have to be higher up with a gnav than with a vang, surely?

But the effect will be the same because the mast is still fixed at the same points - if you supported a mast on a chair at either end and pushed it down in the middle then pushed it down 1/4 along it's length it will form the same shape, it's just easier at the middle. Like I said mechanical advantage but you could get the same effect by adding a block or 2 to your kicker cascade - if there's room.

I agree that strange things might happen if you add lowers into the equation but I think my anaology holds true no matter how many points you constraint the mast at. The vang/gnav will always have the same effect and it's the contstraints that change the bend curve.... although I could just be arguing for arguments sake now - it's been known!

But , whatabout the name of the Int moth with a strut (gnav name not invented then) above.

The bold fella called his boat "muffdiver" groundbreaking too in boat names in 70's ?????????

Love those boat names too... Muffdiver and Shytot. Those Lark boys (and girls) don't have a monopoly on questionable boat and club names then.

Ew! 11 Plus, that brings back memories of a leaky old training boat my club had when I was a kid. No wonder Andy P. moved on to Moths.

No chines .

Looks like a moth I had A long long time ago , had no chines , just a big v sharp at bow flattend a bit a stern , although wide by modern standards , no stability , and bladdy sore on knees , kneeling in a deep veeded cockpit on a run , stability didn't improve with speed .

Wings were a large oval ring attached at Gunwhale and as had no rise hit water quickly. 

got it free with a MK 1 Skol which looked more like a mini nat 12 very wide flared hull no wings , built of rice paper seemingly , look at it wrong way would put a hole in it !

Good fun for the price of a few packs of fags that I had given up.

Not wanting to get the slightest bit techy, but I found something interesting about GNAVs when I put on on my new boat in the summer

Jim C wrote

>>>The Gnav produces the same point push at the gooseneck, but the pull at the heel is replaced by as push higher up the mast.

Actually the push at the gooseneck turns around and becomes a push the otherway so you have to 'connect' the boom to the mast or stump. this assumes you have the slidey bit on the boom not on the mast - Weird though.

Gnavs are great must fix my one...  

Doesn't the 29er and 59er overcome this by having a sliding foot on the boom rather than the lever and pivot that the 49er has?

I would still want to tie the boom to the mast though - that friction thingy can do nasty unexpected things sometimes...

On the 4000 the forces tend to pull the boom backwards and push hard down on the gooseneck (which wears it out quite quickly), and pushes forward on the mast at the gnav attachment point and that is restrained by the lowers.

Here's another consideration.  Enterprises and Scorpions have deck stepped masts and the anchor for the kicker is in the bottom of the boat - this tends to increase the forestay tension - quite useful!

You can't expect a high perfomance boat to be bomb proof and just by looking at a 29er it should be obvious that the middle section of that mast is close to the limit.  You'll find that with any high performance boat its usual to ease the kicker before you go off-wind and its not for speed but to keep the stick in the boat.  It may be that the 29ers Yann mentioned were particularly weak but if you see a mast bending in an "S" shape it will break soon - you simply have to do something about it.  Can I suggest sheet in, ease the kicker and the Cunningham, its part of the skill of sailing.  Incidentally don't let the boom touch the shrouds, if you think about the forces that puts on the mast - levering it up to windward.

Sorry reading this through it sounds a bit patronising, its just that if they made performance boats bomb proof we'd have nothing faster than about PY 1000.

which is why you need to look after the front bolt in the mast step...if you don't the kicker loads make this shear and send your mast through your foredeck...

Did you go straight out and photograph it!. Interesting anyway since again this kicker will increase the forestay tension when used, whereas if it were actually onto the mast it would not.

No I had these photographs on my computer from when  I bought the boat.

Right, getting back to rigs.  Just think in engineering terms.  Many of these skiff type boats have a mast unsupported by a gate at deck level - no wonder they need lowers.  The 29er doesn't, but it has to take all those bending forces fed in by the kicker or equivalent.   The 29er has a big roach on its mainsail requiring quite a lot of force down the leech - usually provided by the kicker in a breeze.  Unfortunately as leach tension increases so does the bending force from the kicker.  One solution is make the lower section thick and heavy like a Laser bottom section, the other solution is ease the kicker when the boom goes out over the side of the boat - which would you prefer? 

I suspect its quite easy to overbend a 29er mast and get creases from the end of the boom to the middle of the sail.  On the Laser 4000 the solution is use lowers to restrain the mast bend, just as well because the 4000 has quite a thin section for its length and power.

Just as an illustration of the affect of the lowers is an incident I had last summer - I broke a lower on a close reach when hitting a wave badly.  The crew got dunked and I eased the main thinking I'd lost the rig.  I was amazed just how much power the boat lost with no restraint on lower mast bend.  In fact I now alter the lowers to suit conditions.  If I have them tight the boat is more powerfull and I can get the crew trapezing in only a few knots of wind however the boat does not accelerate in the gusts and I have to play the mainsheet and I can't do it fast enough on inland waters, so this setting is only used at sea.  Inland its much faster to ease the lowers by half a hole and let the mast breath.  The crew doesn't get out on the wire so much but I don't have to play the mainsheet so much and the boat accelerates in the gusts.  When the wind is stronger still, I let out another half a hole and the sail becomes much less powerfull, and with the mast raked back a bit (equivalent of letting the lowers off more) the boat becomes completely tame at 25 plus knots of wind.  So control of mast bend is really singnificant, however the section is thin and if I didn't let the kicker off just before the windward mark it would be over the side if I let the boom right out.  Futunately if you don't get time to ease it you can always temporarily not ease the mainsheet and that keeps the mast supported until you do get time.

I understand the Star is many times more complicated in keeping the stick in the boat.  I once raced a "Quarter Tonner" on the east coast - that had a rig which would fall over the side at the slightest inattention - but it didn't even though we broached and broached more times than I could count in one race.

If you want a fast boat you have to accept the rig is vulnerable.

Not true.

Kicker is controling the shape of the leach (holding it in) and also bending the mast aft - thus some small amount of increase in forstay tension. 

Let me explain with an example.  The Laser has no forestay and you can tension up the kicker all you like and yet still lift the entire rig out of its socket in that condition.  However try applying the kicker to an Enterprise and then if you can release the forestay - you'll find the rig promptly falls backward out of the boat!

No don't try it on an Enterprise you'll break it but I think you'll get my drift - a kicker anchored to the mast does not increase forestay tension and one anchored to the boat does.

Now here's something to get your head into - what happens when the boom is eased out over the side of the boat?

Ummm.  Yes, the hounds will move down, but also aft. So, shround tension will decrease as the triangle of Shroud(s), mast, shroud/chainplate has changed shape (hounds are nearer the shroud chain plates).  Agreed ?

Thus mast must be bending aft (and comppessing to an extent as it is bending, and also (maybe) moving forward depending if our boat has lowers or not) Agreed ?

Thus, as the mast is bending (Aft and Down) the forstay attachement will be moving down a small amount but aft a too.  Agreed ?

Now if you you remember you school maths, the movement aft (on the curve) of the hounds will be much greater than the movement down (on the same curve untill you get past 30 degrees of movement / bend).  Thus, even if we allow a little bit of extra 'down' component for mast compression, I still believe that there will be a larger (Attempted) movement aft than down and so the forestay will be attempt to streached more.   Consider the triangle Deck / Forestay / Mast.  Which will strech or compress most ?

Deck - None (we hope)

Mast - Compresses a little and also bends

Forestay - streaches little, so incresed tension in Forestay

QED.   

Depends on whether the base of the mast is a fixed joint or a pinned joint. 

If the mast falls over without the rigging up, I would say it is a hinged connection.  Therefore if the kicker is attached to the mast - or even to the mast step as on Matt's Contender - its only action is to bend the mast.  The only action on the forestay tension will be as a result of the change in the mast bend.

Only if the kicker is anchored well below the mast step, or if the mast is made to have a fixed base by using lowers or a strut, will there be a direct effect on the forestay tension.

But, when the boom goes out, the kicker tries to bend the mast sideways not fore and aft.  So if it had a fixed base, easing the main would also reduce the forestay tension.  Talk to a Cherub sailor and they will say that the main point of lowers is to stop the mast bending sideways when the main is eased, the strut deals with fore and aft bending.

Firstly, the picture is Mark's Contender, this is mine and the kicker is atached to a flippin big u-bolt through the mast.

Secondly, I might have the answer - assuming I've understood the question: When I step the mast I attach the forstay, heave the mast onto the step, and attach the looseish lowers. Then I attach the halyard to the outhaul and bang on loads of kicker which 'lengthens' the shrouds and lets me pin them in low enough to get the the rig tension I need (I'm quite big), far lower than I could manage just pulling on the trapeze handle.

From this I deduce that pulling on kicker bends the mast shortening the distance between hound and chain plates and also pulls the mast back because of the control lines being lead back (nothing to do with where the cascade it attached).

I think Matt has given us an answer there, but i still think if the anchored point was further back the effect would be increased further as the mast rotates in the mast step as well as bending. It would also be interesting to see the extent of this when using a sail rather than a static rope, as i feel the reason this has not been documented much (well not anywhere i've seen), is because the forces will be neglilible.

If my name ended in Bethwaite i might be able to investigate this with several test boats and rigs and come up with a very firm conclusion, probably with several different types of graph. Unfortunatly i dont have time, money, or expertise so i guess i'll have to leave it up guys, but if anyone does get any figues i would be interested in seeing them.

Hmm, if your mast rotates in the step on a Contender you are (or soon will be) in a bit of bother.

True, what i meant to say was attemping to rotate, i'd hope (for your sake) the forestay would stop the mast falling down.

OK, I thought you meant rotate like a wing mast. You mean the tip rotating about the heel, backwards towards the cockpit, yeah? In which case I agree the bending force would be added to by a rotational one but we're only talking about the forestay being under more tension.

Isn't that where you came in?

Yeah that is what i meant by rotation (communications never been a good point of mine).

So this "rotational" force would surley increase forestay tension, which is about where i came in, I think?

Yes, I believe you're right. Not sure what effect that has on my particular class but the shrouds going slack is a bigger potential problem. Can't remember the process in detail but as the wind increases, and I go out on the wire, the rig depowers itself and I have to come back in - somewhat frustrating hence the high rig tension.

[/QUOTE]

BTW folks kicker can't cntribute that much to forestay tension because its working at a massive mechanical disadvantage due to the length of the levers.[/QUOTE]

Yes to some extent - but you put the rig tension on a 49er with a 5:1 purchase in the boat breaker.  Most kickers that work have some about 16:1 purchase - you can pull quite hard on that...plus if you are looking at hiking classes with typically low rig tension it can be significant.

Yes to some extent - but you put the rig tension on a 49er with a 5:1 purchase in the boat breaker.  Most kickers that work have some about 16:1 purchase - you can pull quite hard on that...plus if you are looking at hiking classes with typically low rig tension it can be significant.