program on bbc iplayer

the science of speed.

   Genoas are popular because they give more area without increasing the length of the spars.  It would probably be more efficient to have the extra area without the overlap.
At one time Scorpions had the jib tacked down behind the shute and this meant more overlap and easier spinnaker deployment.  It didn't last long though the class soon opted for getting the jib out into the fresh air and they reverted to tacking the jib down on the bow even if it meant a more difficult use of the spinnaker.

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redback Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502	Post Options Post Reply Quote redback Report Post Quote Reply Topic: program on bbc iplayer Posted: 23 Feb 09 at 11:17pm
	Genoas are popular because they give more area without increasing the length of the spars. It would probably be more efficient to have the extra area without the overlap. At one time Scorpions had the jib tacked down behind the shute and this meant more overlap and easier spinnaker deployment. It didn't last long though the class soon opted for getting the jib out into the fresh air and they reverted to tacking the jib down on the bow even if it meant a more difficult use of the spinnaker.

I luv Wight Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 28 Jan 05 Location: United Kingdom Online Status: Offline Posts: 628	Post Options Post Reply Quote I luv Wight Report Post Quote Reply Posted: 17 Feb 09 at 9:54pm
	The wingspan of passenger jets is limited by onground docking etc, so there is 'extra' lift compared to the same span without the tips. If the wings were longer, there would also be more lift, less drag. Moth T-foils allowed smaller rudders due to the tip-loss effects, but they have an an increase in wetted area and drag. There is a net benefit due to the T-foil stabilising the boat.
	http://www.bloodaxeboats.co.uk Andy P foiling Int Moth GBR3467 Freedom 21 Codling

tack'ho Send Private Message Find Member's Posts Really should get out more Joined: 08 Feb 06 Location: United Kingdom Online Status: Offline Posts: 1100	Post Options Post Reply Quote tack'ho Report Post Quote Reply Posted: 17 Feb 09 at 6:58pm
	If you watched closely hidden among the simplified science....(seriously though the aerodynamsist actually told a porky pie!) winglets increase lift for a given wing length, they still cause drag though so it's 6 and 2 threes really.
	I might be sailing it, but it's still sh**e!

thenewpete Send Private Message Find Member's Posts Newbie Joined: 31 Jan 09 Location: United Kingdom Online Status: Offline Posts: 24	Post Options Post Reply Quote thenewpete Report Post Quote Reply Posted: 17 Feb 09 at 4:24pm
	theres alot about this in laurie smith and andrew preeces book SAILPOWER the science of speed. Ill try scan it in later and post it.

mike ellis Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 30 Dec 05 Location: United Kingdom Online Status: Offline Posts: 2339	Post Options Post Reply Quote mike ellis Report Post Quote Reply Posted: 17 Feb 09 at 3:56pm
	My 14 has a massive overlapping jib/genoa type thing, but it is a pain to tack with the mast ram in the way, and the foot being about 1 and a half times the length of the gap between the tack and the mast. However it works really well on a reach compared to the other shorter footed jib, but isn't as great upwind when the wind comes up (this might have more to do with overall rig set up than jib shape though).
	600 732, will call it Sticks and Stones when i get round to it. Also International 14, 1318

Noah Send Private Message Find Member's Posts Really should get out more Joined: 29 Dec 04 Location: United Kingdom Online Status: Offline Posts: 611	Post Options Post Reply Quote Noah Report Post Quote Reply Posted: 17 Feb 09 at 11:29am
	Mikey, Thanks for the jib luff vs leach correction - 'twas late & post beer & curry . You might have every-which-way jib sheet adjustment, but mine only goes in and out I seem to recall that I14's used to have overlapping jibs / genoas, but nowadays they don't. Is my memory OK or playing tricks? What's the reasoning behind this? (Just plain faster / self-tacking / rules re sail sizes ???)
	Nick D-Zero 316

Phil eltringham Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 16 Mar 04 Location: England/Hitchin Online Status: Offline Posts: 1105	Post Options Post Reply Quote Phil eltringham Report Post Quote Reply Posted: 16 Feb 09 at 11:53pm
	Given the wind shear and velocity gradient as you move away from the water's surface the tip vortex at the top of the mast is worth more concern than that at the foot. Will expand on this, but its late.
	FLAT IS FAST! Shifts Happen

Mikey 14778 Send Private Message Find Member's Posts Visit Member's Homepage Far too distracted from work Joined: 05 Feb 09 Location: United Kingdom Online Status: Offline Posts: 298	Post Options Post Reply Quote Mikey 14778 Report Post Quote Reply Posted: 16 Feb 09 at 10:54pm
	Ah, good answer Noah, and I agree with you that the jib-on-deck thing is very worthwhile, and much more so than the jib overlap which it affects. But the verticality of the jib leech is an irrelevance, it's the angle of the luff wire which dictates the movement of the jib back/forwards as it goes up/down. And the sheeting angle can be adjusted to suit the change in position, so we needn't worry too much about that either. Of course, the actual back/forwards movement of the jib on a Fireball over the sensible range of jib heights is minimal, and perhaps irrelevant in the real world. I'm just interested to know, all other factors being equal, whether or not a greater overlap is a Good Thing or not.
	Fireball 14778 - 'Cruel and Unusual' Draycote Water, fantastic !

Noah Send Private Message Find Member's Posts Really should get out more Joined: 29 Dec 04 Location: United Kingdom Online Status: Offline Posts: 611	Post Options Post Reply Quote Noah Report Post Quote Reply Posted: 16 Feb 09 at 10:21pm
	Re the 'end-plate' effect of the jib on the deck vs changing the slot by changing the height of the sail the consensus seems to be that the end plate effect has the bigger benefit, given that - in a Fireball at least - the leach seems almost vertical, so raising the sail off the foredeck has a minimal effect on the slot. As with everything, there is a happy medium to be had... Sheeting angle also has to be taken into account in this equation, but that would open a whole new can of worms.
	Nick D-Zero 316

MikeBz Send Private Message Find Member's Posts Really should get out more Joined: 21 Apr 04 Location: United Kingdom Online Status: Offline Posts: 536	Post Options Post Reply Quote MikeBz Report Post Quote Reply Posted: 16 Feb 09 at 9:43pm
	A fatter-headed sail suffers less from tip losses (than a narrow-headed or pointy sail). If you make a fat-headed sail and can stop the top from twisting off excessively then it's harder for pressure to escape around from the high pressure side to the low pressure side - difficult to explain in words and I don't have the wherewithal to draw a diagram. Bethwaite's book explains how the bent-top 18 rig works so well in this respect. Mike