Re: jib track blocks

[Al Pritchard <al.pritchard@xxxxxxxxxx>]

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
>
> <PA170004.JPG>
> <PA170005.JPG>
> This is the Swift Solo mailing list.  For unsubscribe instructions,
> visit here: http://catzooks.com/swift-solo/