Re: datum point again?

["Greg Ryan" <gregoryrryan@xxxxxxxxxxx>]

Dave, check with the deck layout plan, but you will see the deck layout plan 
measurements are from the center of station 0 and this is printed on the 
plan in the small print at station 0. Don't be confused with the class 
measurement rules where the measurements are referenced to the center of 
station 14 which is the aft measurement point (AMP). Also when you are 
building your mast you might also be momentarily confused, as the 
measurements in the plans are from the top of the tube and the class rules 
for overall length and placing bands etc reference the bottom of the tennon 
or the bottom of the mast base. Hope that is all clear, just requires a bit 
of math here and there.
As they say, measure twice, cut once.

As for the jib track, it can be mounted as originally designed between the 
wing tank sides as shown in the manual. Most boats (and mine) now have the 
track mounted on the top of the wing for ease of attachment. The geometry of 
the  track position with the forestay is important, so I tried to mount it 
as close to the original position (but on the wing top) as possible. 
Theoretically, I guess, it should move back a little, if it is moved up at 
all. But you cannot move it back much as its quite near the front of the 
mast.  For me, fitting it therefore involved shortening each end to just 
about a little over 2 inches over the wingtop and a little filing on the 
back edge to allow it to sit flush on the wing top when set at the correct 
angle.  I set the angle by measuring 36 inches straight up from where my 
forestay fitting would be or approximately 40 1/2 inches vertically from the 
fore measurement point at station 0with my tape measure and sighting through 
the central two holes in the track to this point. Adjust the angle untill 
the sight holes are perfectly aligned with the view from the point on the 
tape measure.

Others had told me that if you bolt it down tight it will dent into the 
cedar so I clamped it in place with a sheet of Mylar under each end, put 
some glass/carbon under the ends and made a dam of tape around each end to 
hold in a puddle of epoxy/silica, thereby making a shaped support saddle 
that I subsequently sanded to a suitable beveled shape. You might notice 
that the ends of the track are close to being over station 6 (just a bit in 
front) so if you put some backing reinforcement under there that reaches 
back to 6, just remember you will need to accommodate the bonding flanges of 
station 6 around that reinforcement when you glue the bulkhead flanges to 
the underside of the deck. Its an area that gets stepped on and worried so 
remember if you are using T nuts make sure they are secured so that they 
cant fall out if they get loosened. I remember having to get nuts in a 
similar position on my 49er after removing the bolts only to hear the nuts 
drop into the boat, what a pain, and the 49er has no portholes.
Greg


----- Original Message ----- 
From: "Dave Lindsey" <lindsey@xxxxxxxxxxx>
To: <swiftsolo@xxxxxxxxxxxx>
Sent: Wednesday, July 05, 2006 10:52 PM
Subject: datum point again?


> Where is our datum point? Is it a station zero  which is the same point
> where the bow piece connects. I ask this because I have lost my reference
> mark and I'm putting various "stuff on the deck".
>
> Oh, and the jib track. Obviously it has to be cut shorter. How are people
> attaching this to the deck? How short? Perhaps this a R.F.M. answer?
>
> Dave Lindsey
> CAN 046
> British Columbia, Canada
>
>
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