[C320-list] What the reefing shuttle block in the boom is for (was "Outhaul lost in boom")

[Scott Thompson]

I'm catching up a bit late on this, and have changed the subject line.

Larry, Bruce and others:

It is NOT TRUE that the internal shuttle block on the "single line" 
reefing system is there to give you additional purchase.  Since there 
are actually two lines in this system, each turned 180 degrees at the 
two ends of the internal shuttle block, it is a 2:2 system inside the 
boom, and has purchase equivalent to a single straight through line (1:1 
inside the boom).

Because of this, many have speculated that the shuttle block serves no 
purpose and just adds friction due to the extra sheaves, and have 
removed the shuttle block, replacing it with a single straight through 
line.  In my mind this is a mistake.  The internal shuttle block serves 
two purposes other than additional purchase.

1.  Despite the extra blocks, it actually reduces total friction in the 
system relative to a system that uses a single straight through line. 
This is very counterintuitive, but is true.  The savings come because 
with the shuttle block system less line needs to be pulled through the 
forward (tack) cringle -- a very high friction point in the system -- in 
order to get any particular degree of reefing.

Of course you can overcome that friction at the tack cringle by 
installing blocks on the sail itself.  However it's my experience that 
this is not necessary if you use the internal shuttle block.  (Of course 
doing both will reduce friction the most.)

2.  Because the internal shuttle block system actually uses separate 
lines at the tack and clew it can be reeved in a manner that permits 
removing the reef lines from the sail without having to unreeve the end 
of the line going to the cockpit, and without removing any lines going 
through the boom.  On my boat I can untie a single bowline at each end 
of the boom to unreeve the reef lines from the mainsail.  I do not 
believe this is possible without having two separate lines, as with the 
internal shuttle block arrangement.

p.s. to Larry:  Yes there is less stretch with two lines inside the boom 
than with one, but there is also twice as much line to stretch.  For any 
given load on the reef line (measured at the cockpit, ignoring friction 
for the moment) you will have the same load on the line through the tack 
and the line through the clew with either system, since the purchase is 
the same.  But there possibly will be a bit more stretch with the 
shuttle block due to the additional length of line used.

Scott Thompson
Surprise, #653

bruceheyman at cox.net wrote:
> Larry,
> I believe the blocks are there to give you purchase.
> Bruce
> Somerset 671 SoCal
> Sent via BlackBerry by AT&T
> 
> -----Original Message-----
> From: "Larry Frank" <WindSwept at stx.rr.com>
> Date: Wed, 16 Jun 2010 06:14:16 
> To: <C320-List at Catalina320.com>
> Subject: Re: [C320-list] Outhaul lost in boom
> 
> I have often wondered the reason for the internal block.  It does mean that
> there is twice as much line holding the tack and clew down and with two
> lines there will be less stretch than one.  Any stretch in the reefing lines
> during a gust will allow the tack and clew to raise a little making the sail
> draft deeper adding to the heeling forces, just what you do not want to
> happen at this point.
> 
> You could accomplish a reduction in stretch by using a higher tech low
> stretch line too.
> 
> Larry
> Windswept C320 #246 
> 
> -----Original Message-----
> From: c320-list-bounces at lists.catalina320.com
> [mailto:c320-list-bounces at lists.catalina320.com] On Behalf Of MICHAEL COLE
> Sent: Wednesday, June 16, 2010 12:00 AM
> To: Catalina320
> Subject: [C320-list] Outhaul lost in boom
> 
> Peter I took that internal block out some years ago when putting in a 2nd
> reefing line. I've never missed it . Both reefing lines work perfectly well
> without it
>                Mike Cole     "Mio Dio" # 421
> 

-- 
Scott Thompson
Surprise, #653