Posted by katorpus (katorpus@…>)
I’m no fluid dynamics engineer either…but…
The laminar flow on BOTH sides of the sail with a sleeved sail is
going to be better than what’s experienced with a single sided sail
attached to a track at the midpoint of the backside of the mast.
While the frontal area of the mast is no different, the sleeved sail
provides a smoother curve beginning at the tangent of the mast at
which it first departs from the mast wall…on both sides of the
mast.
The “wake turbulence” of the air flow behind the points on the mast
wall which fall at 90 degrees from the wind direction will interrupt
the flow in any wind conditions.
The mast is about 10" in diameter at the point of intersection with
the boom. With the wind forward of the beam, the futher off the wind
you are sailing, the further back the disturbance of airflow
resulting from the “obstruction” of the mast (on the windward side).
The inverse holds for the leeward side of the sail.
The mast narrows over the next 48 or so feet, so it’s less evident
higher up, but the same thing is going on all the way up (to some
extent or the other), but SOME portion of the leading edge of the
sail is contributing absolutely nothing in the way of propulsion at
all times.
Trimming the halyard and outhaul tension can alleviate this
somewhat, but there’s a LITTLE bit of luff always.
I’ve read that airflow is disturbed downwind of a fixed object for a
distance equal to its height above the surrounding terrain. I’m not
sure that I buy this. If it were true, the “apparent windspeed” 6
feet behind a solid 6 foot fence would be the same as that 6 feet
upwind of it. I don’t think it is (at ground level). For that
matter, if that holds true, then you shouldn’t experience ANY
blanketing by passing to leeward of another sailboat by a distance
equal to the height of his mast. I KNOW that’s not the case.
If this hypothesis holds, then the tracked sail is losing as much
as 8" of the effective area at leading edge of the sail (down to
about 2" at the head) to turbulence.
This is MORE than compensated for by the additional sail area
available in the fully battened tracked sail with a roach. There’s
no reason to NOT roach the sail. (No backstays, remember?). Lots
more drive down low where you need it, large bites can be taken out
with reefs. Win Win Win.
As I said, I’m no expert. Just my thoughts here.