Re: [Amel Yacht Owners] Genoa sheet broke under sail - looking for recommendations for replacement


Alexandre Uster von Baar
 

Well, looks like I got ripped off one more time…
Thanks for the info Bill.



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On Thu, 8/10/17, greatketch@yahoo.com [amelyachtowners] <amelyachtowners@yahoogroups.com> wrote:

Subject: Re: [Amel Yacht Owners] Genoa sheet broke under sail - looking for recommendations for replacement
To: amelyachtowners@yahoogroups.com
Date: Thursday, August 10, 2017, 8:13 AM


 









This is WAY more information on jib sheets than any
sane person could want...  but we are sailors, so by
definition our sanity is suspect!
Let's refresh our memory about
what the original factory spec for Super Maramu Jib sheets
was from Amel:
16mm (~5/8
inch) polyester double braid.  Nothing fancy.  Amel used
low-stretch exotics on their boats where it mattered, and
avoided them where it did not.  
Line of this type, at best, has a
typical breaking strength of 16,000 lbs, and 41 meters of it
will cost about $350.  There is no need for the strength
(and cost!) of dyneema in this application.  The reason to
spend money on exotics like dyneema is they allow you to
downsize the line, keep the strength and lose the weight, OR
keep the size and reduce stretch to a bare minimum.
 Neither is something we really care about in this
application.
Just to
put us in the ballpark for what we need, Harken has
published a formula for estimating the genoa sheet load
based on sail size and wind strength.  
SL = SA x V2 x 0.00431
   SL = Sheet load in
pounds   SA = Sail area in square
feet   V = Wind speed in
knots.
For a 700
square foot genoa, fully out, in 40 knots of wind, the
predicted sheet loads are about 4800 pounds.  50 knots
bumps this up to 7500 pounds, and I very much doubt the sail
itself is strong enough for much past
that.
It is
important to understand that jib sheets are not (usually)
selected based on breaking strength.  They are selected for
ease in handling, and for most boats, anything that is big
enough to handle comfortably is way, way, more than strong
enough.
Certainly if
you want to use something bigger than this because it is
comfortable to handle, go ahead--it's not a huge deal.
But understand that bigger lines do have a cost other than
just money.  The extra weight means the sail take more wind
to "fly" and it will cut the boat's light air
performance.  
Anytime a line breaks in the middle,
we can assume there was a reason other than simple load.
 It might have chaffed or damaged in some way, there might
be a manufacturing defect, it might have come tight against
a sharp edge.  I can say this with confidence, because even
the best tied bowline will reduce the strength of the line
by at least 40%, and will ALWAYS be the weak point.  With
exotics (dyneema, spectra, etc) the knots are an even
tougher problem.  The lines are "slippery" and
don't take to the crushing loads in a knot well at all.
 To keep anywhere near the full strength, they need to be
spliced.  If you tie a knot, it's likely you don't
have any more real world strength than you would have had
with a polyester double braid!
For what it is worth, I do not use a
bowline on my jib sheets, rather I use a single length, and
larks head the line to the clew of the sail.  It is not any
stronger, but it never hangs up on the shrouds on a tack.
 You can't always do this with exotic lines because
they can be slippery enough they slide under heavy
load.
Bill
KinneySM160, HarmonieBar
Harbor, Maine.

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