Jib Leech to Main Luff Relationship

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LARRY REEDMAN
Posts: 4
Joined: 30 Apr 2004, 04:49
Location: AUS 422

Jib Leech to Main Luff Relationship

Post by LARRY REEDMAN » 07 Apr 2005, 04:50

Can anybody out there tell me if there is an optimum distance between the leech of the jib and the luff of the main? i.e. the clearance between the end of the jib boom and the leading edge of the mast.
I have varied this distance from 1mm to about 25mm by adjusting the jib boom pivot on the boom and at the deck attachment points. I haven't got any hard data but the boat seems to go better with the boom closer to the mast.
Maybe Lester has a spreadsheet covering this.
All coments welcome .

Regards,
Larry.

Chairman
IOMICA Chairman
Posts: 1197
Joined: 12 Nov 2003, 21:42

Re: Jib Leech to Main Luff Relationship

Post by Chairman » 07 Apr 2005, 12:23

LARRY REEDMAN wrote:Maybe Lester has a spreadsheet covering this.
Hi Larry

Wish I did! I've read two things on this topic: (a) Get the jib as close to the main as possible. (b) When the wind comes up, it might be helpful to move the jib away from main, to "open the slot" without increasing jib sheeting angle.

And, I've observed one relevant item: (c) At the World champs in Omisalj 2001, Gary Cameron came 3rd with a TS-2 in generally light airs, showing boat speed and pointing ability I'd never seen in a TS-2 in such conditions. The only thing I saw different was that his jib leech was so close to the mast that it brushed it as it tacked, and the end of the jib boom was about 1 mm shy of fouling on the mast.
Chairman
IOMICA Executive

cfwahl
Posts: 79
Joined: 23 Nov 2003, 23:01
Location: CAN 62

Post by cfwahl » 10 Apr 2005, 03:32

Think about full-size boats with genoa jibs: it's an advantage aerodynamically to actually have the jib overlap the main; the idea is that the two sails are acting as a single airfoil, with a slot that improves performance of both. Unfortunately, the genoa is the enemy of lightning-fast tacking, and in model yachting that's key.

Most IOM skippers I know try to get the jib leech as close to the mast, and as parallel, as possible (the geometry of the strictly controlled sail rigs seems to have been designed with this sort of thing in mind). The topping lift is what's likely to rub first, especially if your mast has a fair amount of bend. And watch the angle of those spreaders too! Of course when you've got all this set up, then you find that your jib attachment point on the deck isn't in the right place (too much or too little lever arm in concert with the jibstay), so there's another hole in the deck; and so it goes.
Charles Wahl

Steve Landeau
Posts: 256
Joined: 26 Nov 2003, 07:25
Location: USA 12

Post by Steve Landeau » 14 Apr 2005, 04:58

I think there is little aerodynamic difference between a large gap and a small one. As for the jib/main relationship, it either overlaps or it doesn't.
Yes, a genoa makes the main more efficient.
There is certainly a difference when the jib is forward or back, but the difference is the effort balance between the main and jib (further back, inducing weather helm, further forward, inducing lee helm).
Steve Landeau
AMYA 10859
IOM USA 112
Finn USA 112
Cal 25 #548

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