Fwd: [3-7Class] 3.7 Square Top

[mvr]

Here's some info on the various other classes being bandied about.

looks like good advice.

be good if Derek puts his two cents worth in?!

Regards,

maarten 

Begin forwarded message:

From: Russ Evans <russ....@firekart.com>

Subject: Re: [3-7Class] 3.7 Square Top

Date: 7 March 2012 11:35:28 AM NZDT

To: mvr <maar...@slingshot.co.nz>

Hi Maarten

It will certainly look nice! ...and it will make it very um, engaging downwind!

Upwind - I think the rig/sail gurus in the class will  have these points covered but this is what I have pieced together:

0. controlling the twist in the upper leach is the big issue

1. square tops work well on sloop rigs because jib/main slot feeds lower mainsail allowing rig to work with a reasonable amount twist further up

2. with no jib the AOA is the same all the way up the rig meaning you must have much less overall twist to achieve good pointing and low drag

3. less twist = stiffer mast, stiffer battens, higher vang/sheet loads

In the new 18 rigs (putting aside the fact they have jibs) they've apparently moved the both sets of spreader up the mast and gone to straight, very stiff sections to be able to support the heads. 

The musto isn't really near to being a square top. The rig is good in that downhaul can flatten the top of the sail right out, however with no caps the sideways bend at the top makes upper twist hard to control. However the rig was designed over 10 years ago now and things have moved on.

Photo 1 - fully sheeted.  Photo 2 - Eased in gust, too much twist = draggy

The moth rig deals with this a bit better but it has two advantages:

1 - its only 8m2

2 - the camber inducers give a lot of support to the second and third battens down which helps limit mid leach twist

However when it gets windy we still chase our tails:

- more cunno to flatten, which loosens leech

- more kicker to take twist out, which gives more power

- more cunno to flatten....

Two other classes are worth looking at

RS600 - predecessor to 700 with huge roach on 12.2m2 sail - rotating rig with diamond stays 

Devoti D-One - stiff unstayed carbon mast, lots of mainsheet tension and full width traveller

So in summary if I was going to put a square top on a 3.7 I would design it for a very stiff carbon mast perhaps with cap shrouds. I'd have a very powerful cunningham to make sure I could flatten it right off and powerful vang to rein in the upper leech.

Russ

On 7/03/2012, at 9:28 AM, mvr wrote:

Hi Russ,

Farr 3.7 have a AGM coming up and some one has put forward the attached suggestion for a square top.

Wondered what your comment would be, as the musto and moth use that same technology in a fully engineered solution, while the 3.7 would be a late add-on type compromise.

Thanks for your invite for Sat, will come a bit later probably.

Regards,

maarten  

Begin forwarded message:

From: "John Elliott" <jell...@slingshot.co.nz>

Subject: [3-7Class] FW: 3.7 Square Top

Date: 6 March 2012 11:08:27 PM NZDT

To: <3-7c...@googlegroups.com>

Reply-To: jell...@slingshot.co.nz

Hi folks

 

Photos and diagram of square-top sail being developed by Andrew Clarke, I gather...

 

(From Benny Butcher)

 

John E

 

From: Benny Butcher [mailto:bene_...@hotmail.com] 
Sent: Tuesday, 6 March 2012 10:57 p.m.
To: John Elliot
Subject: FW: 3.7 Square Top

 

hi john

dont seem to be able to get these onto google groups etc, are you able to send them out?

cheers benny

---

Date: Tue, 6 Mar 2012 22:50:56 +1300
Subject: 3.7 Square Top
From: andrew....@gmail.com
To: bene_...@hotmail.com

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