Roller Furling Gear
A month or so ago, one of my sailing friend couples had quite
an adventure on a breezy day. The wind
was up enough that they had tucked a reef in their mainsail and had rolled up
their jib part way using their CDI furler.
All was well until the furling line parted and the jib sail fully
deployed. Now there was too much sail
area, the boat began heeling excessively, and became almost
uncontrollable. Eventually, with the
help of another sailor who jumped aboard, the jib was rolled up by hand… the
mainsail doused… and my friends motored back to the marina.
What had happened???
Obviously, the furling line had chafed and under load had parted at the
chafe point. But, what were the forces
at work??? Interestingly, modern furling
gear is generally pretty good at rolling up the sail completely. This effort is usually straight forward and
doesn’t require a lot of effort.
However, if the sail is “roller reefed” (rolled up partially in strong
winds) much of the wind’s energy is now divided between the furling line and
the loaded jib sheet. And, think about
it…… the furling line on most small cruisers (22 ft. to 28 ft. in length) is
probably one-quarter inch diameter line.
The jib sheets, on-the-other-hand, are probably 3/8 or 7/16 inch
line. A chafed furling line is much
weaker than a whole line.
How much energy is transmitted? If the load is equally divided between the
furling line and the sheet we can use a formula.
Sheet load
in lb. = 0.00431 X sail area in sq. ft. X wind velocity in knots squared
So, if you have a 150 sq. ft. jib sail roller furled 33% and
the wind gust hits 30 knots how much load do the lines “feel”? 0.00431 X 100 X 25 squared… this equals a
line loading of about 390 pounds of load.
Normally, with a good furling line this should be “no problem”. The breaking strength of the furling line is
probably between 1,600 and 2,700 lbs.
But, the breaking strength falls off rapidly with chafe. So, be vigilant and replace any furling lines
that show signs of chafe.
After a new furling line had been installed, my friends
observed that the sail did not roll up easily.
There seemed to be a considerable amount of resistance as the furling
line was pulled. The called CDI and the
tech there advised that the furling drum might have a lot of dirt in it that
was causing excess friction with the mechanism.
The next weekend I helped lift the drum/extrusion and sail up so that we
could get a look into the drum housing and the nylon baring that the furling
mechanism rests on. The bottom of the drum
housing was packed with dirt and what looked to be fine gray dust (maybe left
over from Mt. St. Helens’ eruption). We
used water to clean the drum housing thoroughly and the furler works like new
now.
So, if your boat is equipped with roller furling - - 1)
inspect the furling line frequently for chafe; and 2) periodically clean the
furling mechanism to eliminate dirt and excessive friction.
