As far as the amount of mechanical advantage is concerned, I think it is
somewhat a matter of personal preference. My old boat, a 14 ft racing
dinghy called a Sesame, had a 3:1 vang which took a lot of strength to
operate in a breeze.
I have a different consideration as regards vangs on the P-15: I've
experimented a little with off-center vang tackle to pull the boom down
when sailing off the wind. This type of gear, if you are not familiar
with it, attachs to the deck or cabin top (on the Potter) and pulls
straight down, rather than at the 45 to 60 degree angle that a mast
anchored vang pulls at. The obvious disadvantage is that the vang must
be disconnected in order to jibe or tack (though it also acts as a
preventer against unwanted jibes).
My observation is that the P-15 boom is very light and flexible and may
not be up to the task of handling a 4:1 or higher purchase unless it is
used very delicately. In addition, conventional vangs pull the boom into
the mast with as much or more force as they pull down. This places a lot
of compression force on the gooseneck, another not-very-robust part on
the P-15. Indeed, some competition sailors will use the compression
force of the vang to windward to intentionally bend the mast, thereby
changing the shape of the sail.
The standard mainsheet arrangement on the P-15, with the mid-boom
purchase does a fair job of keeping the boom down in my experience, so I
guess I'd like to question whether one should install a vang at all on a
P-15 with the standard mainsheet. I know some of the folks who have
switched to end-boom sheeting have subsequently added a vang with good
results, but I'm curious as to how that compares with the original
set-up. Can anybody help me with this?
Dave Kautz
P-15 #1632 "Tilly Lucy"