Thanks for clearing this up guys. I understand the concept a little better
now.
Ron Medlin
#1829 "Bash"
Ron,
Terry's statement about the Vang was both Sailtrim related and
Mechanically related to the effort of making the boat move through the water.
Sailtrim-wise we have seen significant improvements on the main in cloth
and construction over the past several years, but ultimately any sail built for
the entire windspan will twist open as the loads become excessive. The Vang
helps induce bend in the lower section of the mast, flattening the bottom
portion of the main. In this way it assists in depowering the main. The vang
does help control some of the twist, but these are not conditions you want a
hooked leech in, so a properly designed main should not hook to windward as the
breeze gets above 15kt even if you use lots of vang. This is part of the
delicate design game.
Mechanically, Terry was referring to the constant
need to play the main. Using lots of vang allows the driver to alleviate
leech-load from the mainsheet so that the driver can more easily "play" the
mainsheet particularly in waves. If the mainsheet is so loaded up that the
driver can not ease the sail in a puff, than most of that puff is waisted
sliding sideways rather than accellerating forward.
Sailtrim in heavy
air has to be set up for control first and foremost. This is why you wonder how
the good guys can keep the boat so flat in the windy stuff even with the genoa
up.
Good Luck Ron
Will Crump
Twist is a fairly complicated concept. There are volumes written about it,
but even if you comprehend the concept it is sometimes hard to match the
appropriate amount of twist to the appropriate condition.
In light air
the weight of the boom is usually enough to pull too much twist out of the top
of the main. Since the backstay is off the mainsail is as deep as it ever will
be so it doesn’t take much to stall the top tell tail.
In moderate air
there is enough wind to lift the boom so you can use your traveler and mainsheet
to keep the boom on center line and the top batten on the verge of stall.
In heavy air the backstay is on tight so most of the shape is pulled out
of the main. Dacron mainsail material is fairly elastic so the leech also tends
to blow open. With the main so flat it takes a lot more pressure on the
mainsheet to pull twist out of the main. Eventually we can’t pull the
appropriate twist out of the main because the traveler is so short. So the next
device is the vang. The vang on a J/24 is very powerful and the boom is very
strong so vang sheeting is the best way to control twist in heavy air.
I had an interesting experience the other weekend. I was crewing for another
j24 in a 15-20 knot class regatta. The person I was sailing with had his main at
max vang on the upwind legs. I have also read an article by Terry Hutchinson
where he says in heavy air the more vang he uses the faster he goes. I don't
understand this. All of the books I have read suggest to twist off the leech
when a boat is overpowered. What are the reasons for further closing the leech
on a boat that is already overpowered?
Ron Medlin
#1829 "Bash"