Main trim on Stinger has probably the most important effect on speed. What I try to do in subsequent paragraphs is discuss some simple ideas to try and help us maximize speed and minimize confusion while we optimize main trim. Most of what I am going to say here is also in the North Sails trim guide. I really do recommend that you get one of these and read it as many times as I have read it.
The power of any sail is determined by its shape. Its shape is determined by the amount of twist it carries, its attack angle to the wind, and the position and depth of its draft. You have to think of a wing relative to all this but because our sails are soft the shape is much harder to optimize. I suppose everyone who has ever looked at a sail understands the effect of sail shape but, like me at times, may not know what to do to about it. Every sail control effects more then one aspect of sail trim at any given time, but the shape and affect we want is pretty clear in most cases.
Here is my view of what each sail control does
Primary functions:
The
main sheet is the primary controller of the main's twist
The
traveler sets the main's angle of attack to the wind.
The
backstay simultaneously controls main and jib shape (fat or flat) and headstay sag.
The
cunningham sets the main's draft position relative to the luff.
The
vang sets the depth of the bottom third of the main.
The
outhaul also sets the depth of the bottom third of the main.
It should be clear from these descriptions that use of these controls can produce some pretty dramatic effects on the shape of the main. Generally when the winds are light a fat main is desirable. When the winds are strong a skinny main is the order of the day. Clearly in all other cases, trim settings are somewhere between these two extremes. In the first case the vang, outhaul, cunningham, and backstay should be loose with the traveler up high with the main sheet set so the boom was on or slightly above center. In latter case the vang, outhaul, cunningham, backstay and main sheet are on hard but the traveler low. Clearly in the middle case, setting the main shape for optimum speed requires trim for a target shape somewhere between these two extremes. From my perspective, this is the hard part. Nevertheless, because a 105 is not starved for power in moderate winds we seem to do this pretty well.

As, an example, this shot shows the looseness of the sails in light air. Traveler is high and sheet is loose. Its worth noting that the jib sheet is also loose resulting in a highly twisted jib. Speed by sailing the boat fat is the issue here.
In contrast to light air the main here is very flat. This means the backstay and main sheet are on hard and the traveler is low
Secondary functions:
Each of the aforementioned controls has more then one effect on main shape. For now, let's not worry to much about that. Let's just be aware that each can have an effect on more then one aspect of sail shape.
Sail Trim in Various Conditions
OK, let's consider how to set these nasty things under various conditions. I'm going to take just a bit different tack then one reads in most of the sail trim guides.
As we race around the course we generally find ourselves in one of three states. We need to
accelerate, we need to
point, and we need to obtain
optimum speed. To some degree each of these states requires slightly different main trim. A 105 is much faster slightly off then on the wind so to accelerate we need to foot off with the nose down and in that case we need lots of twist so the main must be a bit loose. When we need to point we need a narrow tack angle and not so much twist so the main has to be in and the traveler high. Unfortunately, what each of the terms loose, narrow, low and high mean is different depending on the wind conditions.
Let's see if we can set some rules of thumb for the kinds of wind conditions we usually find ourselves in.
Wind speed 0-10 knots
At the start we will always need to accelerate as we head toward the line. We want to hit the line with as much speed as possible. In these light air conditions, the traveler should be high (narrow attack angle) and the main a bit loose (lots of twist) . Here our objective is speed not point.
Once we get off the line we would like to optimize point so the main should come in a bit and we should be patient until we get a feel for what the boat is going to do. This action is really affected by sea state. In flat water less twist produces more point. In waves or chop, more twist produces more power and so maintains or increases speed. Generally a high traveler setting with the boom on or slightly over the center line is best in these conditions. With these settings the bottom baton should point directly at the backstay.
The general rule of thumb in 0-10 knots is to have the main full, no backstay, traveler up at or above the windward sheet edge, tight mainsheet, no vang, and wrinkles in the luff (light cunningham at most), and loose outhaul.
This is what the main should look like in 10 knots or less. Its a bit hard to see, but the luff has wrinkles. Its loose!. Note that the draft is roughly in the middle. Its certainly not all that forward so the cunningham must be off. The tack angle must be narrow and consequently the traveler must be high. I would bet that the outhaul is also loose.
Wind speed 10-15
Going for speed at the start, the traveler should be as high as possible without too much rudder drag. High here can be as much as 10 inches lower then the setting in 0-10 knots. It should still be above center but can be close to center line. We will not be starved for power but speed and acceleration will still be controlled by the main sheet.
As we get over the line, we will usually have enough speed to point as high as we can. Note that the settings remain the same conceptually, they are just not as high as they were when the wind was light. A General rule of thumb is to trim the sheet so that the top batten will stall about 50% of the time. Boom should be on centerline or maybe no more then 4 inches below.
The general rule of thumb in 10-15 knots is to have the sails a bit shallow, light to moderate backstay, boom as close to center as possible to perhaps 4 inches low, snug but not hard vang. no wrinkles in the luff and tight outhaul.
Wind speed 15-20+
We will have lots of power in these wind conditions. Coming up to speed is just a matter of main sheet adjustments. Here the traveler will be low at close to center or at 20 knots very likely below.
The general rule of thumb in 15-20 knots is to have a flat main, hard backstay, boom 6 inches below center. vang on hard. full on cunningham. and full outhaul as well.
A WORD OF CAUTION. WHEN THE VANG IS ON HARD IT NEEDS TO BE RELEASE AS WE ROUND THE WINDWARD MARK. If it is not released the result could be rig damage, sail damage, or both.
I would hope that someone would read this but I am uncomfortable telling someone this directly. I can usually visualize what I want in the sail, but I cannot do this while I'm at the helm.