Dear Ladies and Gentlemen,
(If you want to cut to the chase and see my Question, look below for
the asterisk *)
I have been wondering about an aerodynamics question for a long
time. I have been too lazy to read for the answer, to proud to ask
sailors for the answer, and too embarrassed to ask aeronautical
engineers for the answer.
Instead going to the source of the theory, I will rather go to the
source of the school of study, trial, and error, ..you, the sailors.
This question is complex in my little mind. But it may in fact be
simple in your simple mind. Just kidding. But really, it may or may
not even be a legitimate query; it could be beyond simplistic for all
I know.
Without further delay, here it is:
As apparent wind speed increases, the sail trim calls for a flatter
sail with the draft position farther forward. As apparent wind angle
decreases, this too calls for the two basic adjustments of draft
position forward and draft depth shallowed.
*What Is the correlation between decreased depth of draft and decreased
percentage of draft position?
(*) (in other words) Are there times when you should decrease draft
depth and decrease draft position in different proportions respectful
to one another, than the usual proportion?
When, where, why?
I get the impression that when I am driving hard and fast on a beam
reach, that I should have my draft position aft while my sail should be
pretty flat. That is if I am going fast.
I also get the impression that when I am sailing close to the wind, in
choppy conditions, with much wind, that I should have a lot of draft in
my sail, with the draft position pretty far forward.
And as usual, if I am going fast in high wind, close to the wind, I
should have the draft position forward while at the same time should
have the draft depth shallow.
Is depth of draft and position of draft always in corresponding
reference with and to one another?
I am under the impression that draft position is a lot more delicate of
a setting than is draft depth. I think that depth is a gross tune issue
while positions is a fine tune issue.
GARY
Gary,
Good questions, I'd try...................................................................................................Decaf!
Perhaps a GPS and some trial and error would objectively give you some answers.
Todd
Alright, I'm going to take a stab at this as I have learned it from numorous people and books I've encountered over the years.
Draft Position Fore and Aft
Putting the draft forward makes the foil less
critical
, meaning that the foil will maintain laminar flow over a wider range of angles of attack. Of course, this is at the cost of additional drag. From what i've been told, pushing the draft forward to about 20-30% makes the foil more forgiving in maintaining flow in choppy conditions. Pushing the draft back to about 50% increases the efficiency and works well if the foil is not disturbed too much.
Depth
The depth of the foil governs the amount of lift that the sail provides. Of course this is also at the cost of drag. The deeper the sail, the more drag. Also, deep foils will not maintain laminar flow at higher air velocities.
Using this information, you can see that there really is no direct correlation between the two. It depends on apparent wind speed and how stabil the foil needs to be (flat vs. choppy water, your ability to sail to a consistent apparent wind). For example, if you are sailng in flat water and a good breeze, you will want the draft at about 40-50% and the depth to be abit shallow.
Of course this also depends on your ability as a sailor. the really good guys are able to have a lower drag, more critical foil and maintain flow when the lesser guys need to have a more forgiving shape. Those are my thoughts, and hopefully one of the good guys will chime in and provide alittle more insight. Good Sailing.
Todd's answer is good. We really do not need more then that to sail fast.
The only addition I would make is that the stability of the sail as an airfoil, even if not so easy to achieve, is very easy to verify using teltales.
While your teltales are flying as recommended by the sailmaker, you can usually forget the position of the curve and focus more on the depth to adjust the much more critical factor, which is the ideal heading x speed balance.
If you want to know more about how an airfoil works, take a look at http://www.monmouth.com/~jsd/how/htm/airfoils.html#toc55 for the Airfoils and Airflow chapter of the online book
See How It Flies
.
Keep it simple,
Luiz
One more thing. The pocket moves back as the wind speed increases, like it or not. So sometimes you are not really moving the pocket forward, but maybe trying to keep it forward as the wind picks up (tightening tapered battens for instance).