{Stus-List} Rod Rigging Tension - TUNING GUIDE

[Gregory Cutter]

Onno - Just blindly following some rig settings without first tuning the
rig is not a good idea. Actually, the settings you listed seem a bit high
to me, but it does depend on the wind strength, etc. In any event, here's
my rig tuning guide that I've posted on the list many times (the last was
March). Martin L. requested a repost too.

This tuning guide is somewhat generic and depends a bit on the exact boat.
Have fun, Greg

Assumptions: Multiple spreaders (or shrouds...upper, intermediate and
lower), adjustable backstay, continuous rigging (not discontinuous, that
is, it starts and stops at spreaders; doesn’t really change the procedures,
just harder to do), and all rigging is loose (just put the mast back in for
example).

1. Make sure no adjustable rigging is tight, including vang, mainsheet,
babystay, and backstay (runners if you have them are off), and mast is
centered in the collar.
2. The TOP of the mast must be placed in the center of the boat.
Conventional wisdom says use the halyard and the shroud chainplates as
reference points, but the halyard may stretch and the chainplates may not
be symmetrical! So, take a tape measure and measure from the forestay
chainplate back to the shrouds (where you'll check the mast centering); if
not the same distance (withing a 1/4", then measure a fixed distance from
the forestay to the toe rail adjacent to the shrouds and mark it; this will
be  your reference point. Now, buy a long steel tape measure (60-75') that
you fully hoist to measure centering to chain plates or reference points)
and adjust upper shrouds until it's in the middle.
3. Now, do rake, but the value depends on the boat, usually somewhere
between 6-10”, measured from the back of the mast at the gooseneck to the
halyard hanging free with a 10 lb. (or so) weight on the end. It is vital
while measuring rake that no one be on the boat and that either the sails,
etc are off or are piled in the center over the keel. Rake is adjusted by
the backstay and forestay turnbuckles., but make sure the mast partners
(blocks in collar) are fixed in place.
The issue of prebend is a tricky one, but if you start playing with it, you
may mess up the trim of your mainsail that was cut with the pre-exiting
prebend. Most C&C's need little if any prebend and with in line
shrouds-spreaders-mast, the only way to change it is with the mast partners
and maybe mast step position (e.g., ram forward in collar to induce some
prebend). Remember that prebend is fixed; your babystay induces adjustable
bend. My recommendation: don't screw with the prebend unless your mast is
"inverted"..bows backwards when you're all done tuning which will cause it
to "go out of column" and collapse!
4. OK, so you've got the top of the mast in the center and the angle (rake)
about right; now start tightening and adjusting athwardships (side to
side). Start at the top and tighten the two uppers the SAME amount (because
it’s in the center). I suggest that you take a Shapie pen and mark a big
line on your turnbuckles to serve as the reference point for counting
turns; have someone else with you count too. Tighten the uppers so that you
get a very low low bass note...maybe 5-6 turns past hand tight (where  you
were when you were centering. KEEP NOTES on the # of turns for each shroud
at this point on; you'll need to refer to them later).
5. Sight up the mast track...which way and how much is it bending
athwartships from a straight line? OK, so tighen the intemediate a few
turns less than you did the upper (4?) on one side, then tighten the other
to get it straight in the upper 1/2 of the mast. If you have discontinuous
rigging, someone has to go up the mast to do this at your turnbuckle at the
lower spreader. OK, adjust back and forth, but keep the total # of turns on
any turnbuckle less than that of the upper ones; this may mean that you
have to loosen one to achieve a straight line.
6. Right, now do the same procedure with the lowers. If you have double
lowers, so you have to adjust 2 the same on one side, although you can
induce a little prebend with the front lower...wait on this till the end.
7. Now, all of this adjustment should end up with a straight (athwartships)
mast. Now, you need to adjust the forestay tension and this depends on the
amount of adjustment your backstay has. In a light air area, you want a
pretty loose forestay, but a lot of adjustment in the backstay. Keep in
mind that assuming the rake is correct (it better be!), then tighten the
backstay and forestay roughly equivalent to tighten the forestay (if you
only adjust one, you change the rake). I think the forestay should be
pretty "floppy"..moves side to side when you grab it and shake it about a
foot. Then when you tighten the backstay adjuster, all this flop goes away
and it is pretty solid. If you don't have enough backstay adjustability,
then tighten the forestay as above a bit more.
8. OK, everything should be statically correct, so let's see what the
adjustments do. First have a helper tighten the backstay while you sight up
the mast track. The only thing you should see is a little bend fore and
aft, but no change athwarships; if it does move sideways a bit, tighten all
shrouds one turn. The forestay should be TIGHT. Now ease the backstay to
about 1/2-3/4 full and tighten the babystay; the middle of the mast should
bow forward (again helper does this while you watch), but not  sideways.

9. Go sailing in 10-12 kts...close reach (not beating, not beam) and few
tacks with sightings before any adjustments. If you did the dock tune
right, the adjustments will be minor. ASSUME that the top of the mast is
fixed, but verify by seeing if on a close reach in 10-12kts if the uppers
are loose; if they are, first tighten both of them equivalently. Now, the
only thing that can change tack to tack are the intermediates and lowers,
and these are what you adjust after tacking and sighting up the mast;  only
do 1/2 turn adjustments but make sure you don't tighten the intermediates
or lowers more than the uppers (recall you kept notes on # of turns on
each?). Now that you've got this right, try adjusting backstay, babystay,
etc as before to observe their operation and make sure they don't affect
your side to side tuning...shouldn't. ALSO, make sure that there is NO aft
bend to the mast when you're done.
10. Now that you're done, go on a beat (close hauled) and see how loose the
leeward rig is. For these boats in about 10-12, I'd say they can get soft,
but NO flooping around. Tighten everything equivalently to fix if it is.,
but I'd err on the loose side for most areas. If it's going to be a windy
race, then tighten everything one turn.
Now, pin the rig. I use split rings rather than cotter pins are they're
easy to remove even with out tools and facilitate adjusting. If you have a
Loos gauge for rod, I'd now make the measurements and log them (facilitates
retuning); ideally, the tension should be the same on each shroud port and
starboard, but if the rig is straight, then don't change the tension just
to make them the same.





Greg Cutter
Professor
Department of Ocean, Earth, and Atmospheric
Sciences
Old Dominion University
4600 Elkhorn Ave.
Norfolk, VA 23529-0276 USA
(757) 683-4929 - office
(757) 683-5303 - fax
Internet: gcutter at odu.edu
Web Site:
http://www.odu.edu/sci/oceanography/people/faculty/cutter/index.htm