[Public-list] Deck Beam Repair Question - What is the load

[Don Campbell]

    Some of you may remember that the Aussies had an America's Cup boat break up
and sink on the West Coast about 11 years ago now, during the Louis Vuitton
races. The boat broke in two amidships and sank in less than 3 minutes. Sailing
World did a story on that sinking and as I recall, the loading on that hull was
calculated using the moments of the boat length.
    What most of us tend to forget is that the forces must balance and the
resultant is 0. We really have a big, inverted "w" effect on a boat with the
deck beam pushing up, mast pushing down, stays pulling up, and boat hull holding
down. Thus whatever force is downward on the mast and upward by the deck beam,
or upward on the stay and downward by the hull  must be equal and opposite.  The
principles of leverage enter the picture because the stays are not at the base
of the mast . Thus the pull, particularly from the fore and aft stays must also
include the moment of the distance along the deck from the mast to the
chainplate. The forestay tension is multiplied by 10.5 feet and the backstay
tension is multiplied by19 feet to determine the force on the hull and hence the
deck beam. Similarly, sidestay tension is multiplied by 4 feet.
    I have the feeling that the force to multiply by 4 as suggested earlier in
this chain is much larger than you have determined thus far.
    Sailing World calculated the force at over 500,000 lbs on the Aussie hull
when she broke when fore and aftstay tension was at 20,000 lbs and thus mast
compression due to fore and aftstays only,  was 40,000 lbs. (Sailing World May
1995). Weather conditions at the time were average windspeed at 15.89 knots,
gusts to 21 knots and waves just over 5 feet. Three other boats were damaged in
the same conditions: Stars and Stripes lost her mainsail, Mighty Mary cracked
her forward ring frame and delaminated her hull and the French boat lost her
mast when a spreader endfitting failed.
    It would appear as if the calculated compression on the mast is the
arithmetic sum of the tension in all stays. Thus if all stays average 1200 lbs,
that would get to 9600 lbs of mast compression before shock loading. A factor of
4 gets to required support of 38,000 lbs without consideration for the span
underneath without support. With consideration for shock loading and unsupported
moment  on the deck beam,  50,000 lbs does not seem unreasonable. Rankin's
reinforcement has not failed yet, so far as we know.
    As an aside, I have never seen any advantage to having a rig so tight it
forced the mast through the deck. I have been much more a proponent of some of
the dinghy rigs that have totally relaxed rigging and that then develop sail
shape and change rake with the controls available on the boat. While dinghies
can bend masts, not us! Pointing ability while in the speed groove is something
we all need to tune for our own boat because of the wide variability in control
equipment.
    What we do to hull to deck joints and chainplate knees is another whole
story if we reconsider the effect of the moments on aluminum deck rivets and
1/4" bolts.
Don # 528


 1115394536.0