please remove me from this list
-----Original Message-----
From: Al Pritchard <al.pritchard@xxxxxxxxxx>
To: swiftsolo@xxxxxxxxxxxx
Sent: Mon, 17 Oct 2005 20:12:55 -0400
Subject: Re: jib track blocks
Thought Id way in with my $0.02.....
Keep in mind that I have not seen the Swift Rigged yet. So.... Large grain of
salt.
1st, what happens when we rase the jib clew:
Moving the jib track higher, and in essence the clew higher will cause the jib
leech to become more hollow. (I don't know how much luff round is cut into the
sail, or the degree of sag that is built into the leech. But it will shift it
from its designed optimal configuration.) Because we don't have a back stay, we
can only use the forstay to adjust the camber of the jib. Higher forstay
tensions will lessen the amount of forstay sag, and thus removing depth from
the jib. (Not to mention the affect on the main) And you do not want a
completely flat forstay. (Flat, is not fast in this instance.)
By moving the clew higher we increase the draft of the jib, and increase the
tendency for the laminar flow to detach sooner form the jib. Or, we will cause
the main to luff, or back wind by increasing the flow, and thus pressure on the
leeward side of the main.
2nd, why it so cool that we have a jib track:
The jib track allows us to place more control over the fairlead position. The
fairlead (floating block on the jib track) allows us to control 2 aspects of
the jib. a) athwartship, controlling the trim angle, and b) fore and aft,
controlling the tension along the foot and leech.
So, now that we know what we have done... Isn't education grand....
We have 2 potential answers:
1) Have your jib re-cut to remove luff round in proportion to the change in
position of the jib track....
or
2) Shorten the amount of line between the faired and jib clew. This should be
shortened proportionately to maintain the same position and force on the clew
as before the change of position of the jib track.
The reality is that 1/2 inch is a significant amount of change. (Take out one
of the plan sheets, and hang it from the door jam. Now rase the bottom corner
1/2 inch. Yep, that much. Now think about your jib.
I will admit that I am in the same boat. And like all of you will be thinking
about this over the next couple of months. The simple answer may be, we have
Bram (our link to the sail loft) change the amount of jib luff cut into the
sail. TO account for the change in position of the jib track.
To address Gregs response earlier, regarding moving the jib higher up the
forstay. This will also have an affect on the interaction between the jib and
main. We afectavly move the center of effort higher, and cause the larger
amount of pressure to fall higher on the main. (This is where we can start
talking about changing the cut of the main. See how this can rapidly become a
never ending cycle)
The reality. At some point we have to stop playing with numbers, and build a
boat. (loose quote from Wind, the movie).
We should keep talking about this, as its something that will be affecting the
class. And I think that moving the jib track up above the wing is the right
thing to do. Creating a more reparable item, and keeping the bits away from the
sock.
Please remember that I am no expert in sail design, so keep that grain of salt
handy when you think about this.
Al Pritchard
Swift Solo - Anastasia (#069) <--- I know some don't want to refer to them as
numbers, but in this case.... :-)
al.pritchard@xxxxxxxxxx
+1 732 895 4021
On Oct 17, 2005, at 3:57 PM, BDally6107@xxxxxxx wrote:
From: "Mark White" <Mark@xxxxxxxxxxxxxxxxxxxxxx>
Date: October 17, 2005 3:43:33 PM EDT
To: "Bram Dally" <BDally6107@xxxxxxx>
Subject: jib track blocks
Bram,
Do you think this is all right? I don't want to create any problems for myself
down the line.
Sincerely,
Mark
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