Oh oh.   I think I'm at fault for this, but we're in danger of getting into the aerodynamics theories.  I guess that's natural, because the Myth is based on the general population's understanding of lift, perpetuated by many retellings in casual and technical literature.

David's citation of Arvel Gentry's work is spot-on.  It's funny that after 45 years, many of Arvel's observations about the state of knowledge in sailors about the way sails work are still valid. 

Many phenomena in fluid dynamics are counter-intuitive and in attempts to make them understandable often over-simplify the actual forces at work.  I'm guilty of doing the same, even though I should know better: I taught a variety of sciences for many years, and most of my time was spent helping students unlearn the explanations from previous teachers.  And there were some subjects I taught that out of necessity had simplifications that I'm sure their graduate teachers cussed me for.  Other times  I taught the wrong mechanisms because I simply didn't know better.  There were several moments when in the middle of a third or fourth "textbook" explanation in front of a whole class I'd realize that the book didn't make sense.  Tides were a classic.  Atomic structure, electromagnetism, and gravity were others.

I'm definitely not a fluid dynamics guy, and what I don't know about the subject is tremendous. 

In any case, it's still looking like (perhaps) the mast on the windward side is slower.  Maybe it disrupts the flow (either stream or circulation) and messes with the slow air / high pressure side.  Or maybe it's as simple as the mast sticking out in the laminar flow outside the boundary layer creating drag.  Or maybe something else or a combination of several factors.  Sounds like digital wind tunnel work to me.

 

 

David Tyler wrote:

on the other hand, the mast gets buried in a cambered panel, and doesn't disrupt leeside airflow. 

That doesn't hold for a sail cambered by hinged battens though, like mine. The mast is in the flow, and in my case probaby too far aft to hide in any 'separation bubble'.

I suspect much of the difference in my case is due to problems with the way the sail sets when the mast is to windward of the sail.

Chris

Asmat, that's exactly my understanding of the situation, certainly with regard to flat sails. When there's camber, some more factors come into play. Joddy Chapman found that there was no separation bubble then - but on the other hand, the mast gets buried in a cambered panel, and doesn't disrupt leeside airflow. 

One thing that's always true is that there's a lot of parasitic drag with the mast on the windward side. Which is why I like wingsails, with the mast inside the sail.

It seems to me that this discussion and the anecdotal evidence do seem to support the intuitive concern of a "Bad Tack".  It's just that the "Bad Tack" doesn't appear to be the obvious one.  My guess is that the mast's disturbance of the high pressure airflow (mast on the windward side of the sail) causes enough non-laminar flow to reduce the pressure differential across the two sides of the sail.

I know that when I'm adjusting sail, my natural tendency is to focus on the imaginary flow lines across the leeward side of the sail - it just makes sense to think on that side for some reason.  I almost ignore the windward side since my brain says that I can't effect the shape there anyway - it's just empty air. 

I suspect others do the same, which might explain why sailors naturally think about the sail losing its shape with the mast on the leeward side and not the windward.  But, of course, it's the windward side that provides the driving force, the leeward side is just there to make a low pressure differential. 

If the mast on the windward side really is the Bad Tack, I can think of all sorts of silly things like an asymmetrical mast: foil-like in section on the starboard and round on the port.  Structurally, that really is silly.  But somebody out there might play with spoilers or something of that nature.

On Arion, both with my previous cambered sail and my current "flattish" sail, the boat is noticeably more close winded when the mast is to leeward of the sail.  But then, I am a lazy cruising sailor who chose junk rig because I don't want to tweak things.  Set and forget is my motto.  Where's my cup of tea?  As long as the boat is doing 2 knots I'm happy, unless I need to get into an unlit, rock-bound anchorage before dark.  Then I start my motor! I love the idea that Ketil is scaring the pants off the racing sailors and enjoy reading about his methods and results.  Just as long as he doesn't want me to do it!