Draft Depth & Draft Position

[Don Guillette]

Warren: My book, THE SAIL TRIM USERS GUIDE, goes into much more detail regarding draft depth and draft position and space here only permits the Readers Digest version.

Both of these elements control power, acceleration and drag and are therefore very important elements of sail trim. The easiest one to understand is draft depth (belly) and it is measured as a percentage. The percentage is determined by the length of a straight line from the boom to the deepest part of the sail. That is not the scientific definition but the plain English definition. Determine the length of the boom (say 10’ – 10’ is easy to calculate) and if the length of the line is 2’ then you have draft depth of 20%. If you want to be scientific, instead of measuring the boom you would measure the chord line. The outhaul is the draft depth sail trim control.

Draft position is a bit more difficult to understand and the vast majority of sailors don’t bother with it or would not even know it if they saw it, but every time you adjust your boom vang, Cunningham, mainsheet or outhaul you are messing with it. The definition of draft position is the point along the chord line where the maximum depth falls. What’s that mean? Well, you identify the deepest part of the draft depth (belly) and draw an imaginary line from top to bottom through that point of the sail. Where that line falls on the chord line is called the draft position. What you are looking for in plain English is to determine the middle of the foot of your sail and mark that position on your boom. That point is called 50%. Scientifically, it is a bit more technical but the above gets you in the ballpark.

Draft position of 50% is a good average position. The problem is as the wind increases it pushes the draft position aft and your sail trim controls are used to move it back to 50% or 45% which is more forgiving. Conversely, less wind causes it to move forward and you must use your sail trim controls to move it aft..

As I mentioned, most sailors don’t bother with draft position and have never seen it. It is hard to visualize the first time but once you recognize it you’ll see it every time but whether you know it or not the draft position of your main and jib is effecting the efficiency of your boat. When you finally see it for the first time and by messing with your sail trim controls you can actually see it move forward and aft.

 

[Warren Milberg]

Thanks, Don. I have typically adjusted depth using the outhaul when in light air, and position using the Cunningham when I it's blowing like stink and I want a really flat main.

 

[Don Guillette]

Warren: It is really a balancing act and here's why. When you adjust ANGLE OF ATTACK your primary sail trim control is the traveler. Angle of attack is all the traveler adjusts. When you adjust TWIST your primary sail trim control is the boom vang. This is the start of the DRAFT POSITION adjustment problem. When you adjust DRAFT DEPTH (belly) your primary sail trim control is the outhaul. We now have made the DRAFT POSITION sail trim adjustment a bit more difficult. There are 4 sail trim control for DRAFT POSITION and they are the boom vang, Cunningham (halyard), mainsheet (which also adjusts angle of attack and twist) and the outhaul.

So here's what happens. A mate cranks on the boom vang to reduce twist. He has just made the first step in moving the draft position forward thus slowing the boat down. Next he cranks on the outhaul to reduce draft depth. He has just completed the second step in moving the draft position forward and further slowing down the boat and all he was trying to do was adjust two of the 4 elements.

The most important step in learning sail trim is to understand completely what each sail trim control for the main and jib is adjsuting. I've said this a millions times and that is a mate has to have an understanding of draft depth, draft position, twist and angle of attack or sail trim for the main and jib will never make any sense.

I devote the first chapter of my book, THE SAIL TRIM USERS GUIDE, that subject and advise readers not to go any further until they understand it. With each SAIL TRIM CHART I include FREE a QUICK REFERENCE which identifies what controls for the main and jib adjust draft depth, draft position, twist and angle of attack and what happens when you push or pull each one of them.

A lot of times at sail trim seminars I've conducted and even short presentation I put on for Yacht or sailing clubs I run into guys that understand these element (which I quickly deduce from their questions) and I really have to jack my mind in gear because these guys know their onions about sail trim.

 

[Sam Salter]

Don,
I don't understand how the boom vang and outhaul move draft position.

The vang is tightening or slackening the leech - twist.
The outhaul is "lengthening" or "reducing" the foot - draft depth.

The halyard / Cunningham by pulling diagonally against the warp and weft of a crosscut mainsail will pull the draft position forward and conversely allow it to slide back or be blown back when slackened.

I always think of these controls as being virtually independent of one another.
Or are you saying that ANY change of ANY control will affect draft position as your messing with a 3 dimensional object - not sure that's the right way to say that! Sort of like the butterfly flapping its wings in Japan, creating a storm in California.

sam

 

[Don Guillette]

Sam; I don't know what to tell you but I'll give it a shot. The mainsail and jib are 3 dimensional objects. Pushing or pulling on any part of the object changes a good part of the object. Try it sitting in front of your computer with a handkerchief. The luff, the foot and the leech are adjusted by different sail trim controls. The PRIMARY sail trim control for the leech is the halyard or Cunningham. For the foot it is the outhaul and for the leech it is the mainsail. There are also secondary controls. The outhaul adjusts only the bottom 1/3 of the main. Other controls are necessary to adjust the other 2/3 of the sail.

The PRIMARY TWIST control is the boom vang and the secondary control is the mainsheet. ANGLE OF ATTACK is primarily adjusted by the traveler with the secondary control being the main sheet. The primary control for DRAFT DEPTH is the outhaul and the secondary control is mast bend (if you have a bendy mast).

So it is not just one control for each element - draft depth, draft position, twist and angle of attack) you're adjusting.

There are 8 primary sail trim controls for the and 6 primary sail trim controls for the jib that are adjusting 4 elements (draft depth, draft position, twist and angle of attack). In other words, there are more butterflys than you might think.

Each of the 4 elements of sail trim are important but in order of importance draft position is last because the vast majority of sailors would not recognize it if it hit them in the head. If beginners to intermedaite can master the other 3 then they are way ahead of the game.

Anyway, back to your question - how would you explain how all the sail trim controls for the main and jib work to adjust the 4 element of sail trim? Incidentally, I'm not trying to be a wise guy. You sound like you know what your talking about and I'm interested in your version so I can incorporate it into mine because I don't want to be confusing beginners to intermedaite who have it tough enough trying to figure out how to sail a silly sail boat.

 

[Don Guillette]

Joe: Thank you. Luff = Cunningham/halyard and leech = mainsheet. Call it a senior moment!!

 

[Joe]

Joe: Thank you. Luff = Cunningham/halyard and leech = mainsheet. Call it a senior moment!!

Yep.... Don, I know exactly what you mean. Thanks for all you do here. Joe

 

[Sam Salter]

Here's My Version.

OK – here’s how I think about it and do it! I’m not a racer, just an anal, fast (…ish) cruiser.

Once I’m out and sailing (…say beating to weather) I start at the front:

Set draft position in the Genoa using the Jib halyard. (…about 35% to 45%)
Set Genoa car position so that all tell tales are flying and break about the same time if I luff up or bear off.
Genoa should be about equally off the shrouds at the foot and the spreader. (…I’ve only got one)

Set draft position in the main using the main halyard. (…about 45% to 50%)
Set draft depth using outhaul.
Bring 2nd to top batten parallel to boom using boom vang.
Set angle of attack with traveler so that all leach tell tales are flying. (With my boat – C&C 26 – I’ve usually got a fisherman’s reef (…a bubble) all the way up the luff when the tell tales are flying.

Check speed and heel angle.

Do it all again – hopefully speed has increased and heel reduced.

Continue tweeking - ad nauseum and use speed and heel as a guide. Keep to about 15degs heel and speed as high as you can get it! Set windward Genoa car to same position as the working car.

Depower Genoa by moving car back to twist off leech at the top and flatten the foot.
Depower lower part of mainsail by reducing draft using outhaul and slackening vang to twist off the top. Use traveler to change angle of attack without altering sail shape.

I haven’t got an adjustable backstay so can’t bend the mast (…masthead IOR rig; most of the power in the Genoa)

Once I’m going, speed is the criteria – if a “tweek” raises speed it’s a keeper; if not, got back to the last setting and try something else. I try to imagine the wind stream lines when I look at the sails and make changes accordingly, not just randomly pull a piece if string. That part is the seat of the pants stuff and is hard to explain.

I’ve been sailing off and on for 50 years. But it wasn’t until I took an advanced sail trim course about 4 years ago that I really got this stuff. Some of it I’d not seen in books or it was difficult to put it all together when I got out on the water. 2 days on a Whylie 52 with every sail trim control imaginable and it all made sense. My biggest revelation was when the instructor had us put up all the sail. We were sailing at say 6 or 7 knots and heeling at 29 degs. We then did what I tried to explain above. Every time we changed a setting the boat would stand up a little and the speed would increase. We ended up at 16 degs of heel and somewhere around10 or 11 knots. (…I can’t remember the exact speed; something like that)

Not sure if that’s what you wanted from me – I feel better now!
Great column Don – keep it up!
Sam