Rig Tension - modern IC's

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colinbrown
Posts: 98
Joined: Tue Jun 02, 2009 8:22 pm

Rig Tension - modern IC's

Post by colinbrown » Wed Nov 24, 2010 6:25 pm

We have two new IC's in build up here and would like advice on the position of the shrouds/chainplates. To explain, on the OD we position the chain plates about 330mm ( 13 inches) behind the mast measured along the centreline. This has provided reasonable jib luff tension without massive shroud loads.
Now with the new rules canoe the beam of the canoe is reduced from about 1000mm to 750mm at the chainplates. Should the chainplates be positioned the same distance aft of the mast ie. 330mm, or maintain the same staying angle from the mast to chain plate.?
If the chain plates are moved forward relative to the mast then either we will have reduced forestay tension or need to increase static rig tension by around 25%. Using around 225-250kgs on the shrouds seems excessive.
Anyone able to comment please.

Steve Clark
Posts: 165
Joined: Fri Aug 01, 2003 2:26 pm

Re: Rig Tension - modern IC's

Post by Steve Clark » Wed Nov 24, 2010 7:36 pm

I started with the chain plates about 330 aft of the mast CL.
I felt that while the fore stay tension was fine, the mast was not being supported well athwart ship and successively moved the shrouds forward until they are now about 290mm aft of the mast CL. One advantage of wide chain plates is the ability to make these changes!

The loads on the shrouds and the compression loads on the mast are higher, but not off the chart, I undercooked the mast step compression bulkheads on the first batch of boats, but a second layer of carbon twill has killed it. Along the way, the 12:1 shroud tackles turned into 18:1

SHC
Beatings will continue until morale improves

colinbrown
Posts: 98
Joined: Tue Jun 02, 2009 8:22 pm

Re: Rig Tension - modern IC's

Post by colinbrown » Thu Nov 25, 2010 5:58 pm

Thanks for the reply Steve,I'll start at 300mm or so.
I guess that we, sorry i am a lot weaker than you, as I've been using 24:1 on the shrouds for 20 years ! Using any less hurts the hands !

wee mcp
Posts: 90
Joined: Fri Aug 11, 2006 4:45 pm
Location: United Kingdom

Re: Rig Tension - modern IC's

Post by wee mcp » Sat Nov 27, 2010 10:10 am

The loads on the shrouds and the compression loads on the mast are higher, but not off the chart, I undercooked the mast step compression bulkheads on the first batch of boats, but a second layer of carbon twill has killed it. Along the way, the 12:1 shroud tackles turned into 18:1

SHC
Steve, Could you give some indication of the spec of the support/compression setup for the mast on Second String (GER 78) and how did you fix the forestay fitting into the hull. The photos on SA didn't show this.
Ta, in advance

Ian McP
"Sometimes its a boat, sometimes more of an accident .. It all depends" - Pooh Bear
IC GBR305

Steve Clark
Posts: 165
Joined: Fri Aug 01, 2003 2:26 pm

Re: Rig Tension - modern IC's

Post by Steve Clark » Sun Nov 28, 2010 3:30 am

Using a stress level of 2500 psi for wood, and a guestimated compression load of about 2000 lbs, you something like two square inches of cross section to hold the mast step up with a reasonable factor of safety.
The mast steps loads need to be spread into the skins.
In practice, I put 6mm plywood under the mast step and Tab it to the athwart ship bulkheads. It is more or less 150mm at the mast step and most of 400-500mm by the time it gets to the keel. I count on the bulkheads to keep the aft edge of this web in column, and a 20 x 20 spruce doubler to keep the forward edge in line.

On GER 78 I tabbed a piece of 3mm ply to the keel with a few plies of carbon at +/- 45. The parts that stick through the deck are covered with 6 layers of +/- 45 carbon that I compress ed between some plates with clamps to make it nice and dense. I also bevel the top of the ply so it is pretty thin at the very top but is 100% for he lower 1/2 of the carbon bit.

SHC
Beatings will continue until morale improves

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