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Scooby_simon
Really should get out more Joined: 02 Apr 04 Location: United Kingdom Online Status: Offline Posts: 2415 |
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Topic: GNAV... Posted: 05 Feb 05 at 12:16am |
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. |
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redback
Really should get out more Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
Post Options Quote Reply Posted: 05 Feb 05 at 4:05pm |
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? |
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Twin Poles
Newbie Joined: 06 Nov 04 Location: United Kingdom Online Status: Offline Posts: 33 |
Post Options Quote Reply Posted: 05 Feb 05 at 8:52pm |
What scooby simon is saying is also true (if not really relevant), as the kicker will bend the mast backwards therefore increasing forestay tenson, no matter where it is attched. But yes a kicker achored to the hull will increase forestay tenson more than kicker anchored to the mast.
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redback
Really should get out more Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
Post Options Quote Reply Posted: 06 Feb 05 at 12:06am |
Sorry fellas I think you'll actually have to take some measurements. If anything the kicker decreases forestay tension (provided its achored to the mast). Here's how. When you bend the mast you shorten the distance from the hounds to the bottom and if that distance is less then the rig will have less tension - agreed?
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Garry
Really should get out more Joined: 18 Apr 04 Location: United Kingdom Online Status: Offline Posts: 536 |
Post Options Quote Reply Posted: 06 Feb 05 at 12:51pm |
Someone needs to talk to a spar maker. It is unlikely you'll be able to measure the changes accurately enough to determine what's happening with a standard rig tension gauge. Don't forget the mast acts as a spring with moments about its foot, the shroud / forestay attachment points and the gooseneck / strut / chocks. If you tighten the kicker the mast bends, the sail changes shape and all the forces will adjust to obtain an equillibrium. It seems reasonable to assume the forestay/shroud tensions will remain similar with the mast bending to accommodate changes in kicker tension. If the mast was assumed rigid doing the calculations on the moments would be relatively straigt forward and the application of kicker would increas forestay tension regardless of where the kicker was anchored. However, the mast bends. If it bends about the forestay attachment on the mast then forestay tension will not change. If it is forced to bend somewhere else bacause of a strut / chocks / gooseneck then the forestay and shroud tensions will also change, but I wouldn't want to predict how without having a model. Unfortunately, I studied control and instrumentation and a rigid beam is as far as my mechanics went (even if I could remember the theory).
Gael could ask Seldon masts to do an article on mast mechanics? |
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Garry
Lark 2252, Contender 298 www.cuckoos.eclipse.co.uk |
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Scooby_simon
Really should get out more Joined: 02 Apr 04 Location: United Kingdom Online Status: Offline Posts: 2415 |
Post Options Quote Reply Posted: 06 Feb 05 at 3:12pm |
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.
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redback
Really should get out more Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
Post Options Quote Reply Posted: 06 Feb 05 at 6:14pm |
We'll have to measure it to setle this one.
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Garry
Really should get out more Joined: 18 Apr 04 Location: United Kingdom Online Status: Offline Posts: 536 |
Post Options Quote Reply Posted: 06 Feb 05 at 7:57pm |
Anyone want a price for me to instrument their mast and fit loadcells to their shrouds and stays? Bet the mastmakers have already done it...
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Garry
Lark 2252, Contender 298 www.cuckoos.eclipse.co.uk |
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redback
Really should get out more Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
Post Options Quote Reply Posted: 06 Feb 05 at 9:31pm |
No but I'll get a gauge and fiddle with my kicker and tell you the results.
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Blobby
Really should get out more Joined: 07 May 04 Location: Australia Online Status: Offline Posts: 779 |
Post Options Quote Reply Posted: 07 Feb 05 at 1:16am |
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. |
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