Draft Depth

[Don Guillette]

Mates: Draft depth is the second element of what I refer to as the "4 elements of sail trim". Yesterday, we discussed the easiest one to understand, which is angle of attack. Draft depth is the second easiest to understand. As usual, I am going to keep the explantion as simple as possable. Complicated theories, linear coefficients and complicated material are not my bag.One problem we will have is that space on this forum does not provide enough room to really get into the subject so I can only deal with the highlights. If your interested in the "whole enchilida" or want to get the complete picture of sail trim all at once, take a look at my book "The Sail Trim Users Guide" plus the "Sail Trim Chart", which is sold in the ships store of this site. To understand draft depth you have to understand what a "chord line" is. The definition is "the measurement of a sail front to back or horizontally". Technically, what I'm going to do with this chord line is not totally correct but it is good enough to get you in the mainsail trim ball park.Draft depth is the amount of belly you put in the sail. What you do is visualize a straight line drawn from the tack to the clew of the sail- use the boom or the foot of the sail as reference. From that line you measure the distance to the deepest part of the sail. The amount of draft depth is expressed as a percent. The question is, a percent of what?This is where we are going to cheat a bit. Measure the foot of your mainsail. Assume it is 10'. Say the distance from the boom or foot of the sail to the deepest part of the belly of the main is 2' then the draft depth (belly)would be expressed as 20%. If it was 3', it would be 30%. So if the skipper calls for draft depth of 25%, he want the mainsheet trimer to induce 2 1/2'of belly into the sail.Why do we even care about draft depth? The reason is power. Power is increased as the sail is made deeper. It is the same reason a pilot uses flaps to land or take off. More flaps equals more power. Same with a sail.Now that you know what draft depth is, how much should you apply? The answer is - as much as the sail will take. It should be in the 20% to 30% range but the sail will tell you when you've gone too far. Experiment and go wild with it. See what your boat can take and mentally note the setting. Physically note the settings on your outhaul.The primary control for draft depth in the mainsail is the outhaul and secondarily mast bend but unless you have a "J" boat you can not bend the mast in the true definition of mast bend. When these mast bend guys are describing is actually a mast rake. Mast rake does not take the fullness out of the middle of the mainsail.The jib sail controls for draft depth are the fairleads, halyard and the jib sheet plus our old friend "mast bend".Any questions on draft depth? Next topic is draft position and then twist.

 

[David Hoch]

Draft

What indicators will tell when the draft is too deep with a given set of conditions?If I understand your comments, Don - you ease the outhaul, increasing draft until... what?

 

[Alan]

Don

I have read several times that you said that the masts on everything but J Boats cannot be bent effectively. I have to strongly disagree with that statement. On certain boats like the Cat 30, this is true because the section is short and fat. However, there are more than a few boats, mine for example(L35.5) that has a particularly tall mast with a small section. I can easily put 17" of bend in my mast. Fractional rigs can almost always put a large bend in due to the location of the hounds.

 

[Cliff]

You say that that to induce more draft is to increase power like on a air plane wing when landing and taking off. Not sure I agree with how you have this worded. On a sail we need more power in light air and so we do increase the draft of the sail for more power. But when the air speed comes up we decrease the amount of draft in the sail for less power and to make the boat faster and stay on it's feet. A sail that has say 3 inches of draft when it is blowing 20 knots will be healing the boat and makeing the boat go slow compared to cranking on the out haul for a flat sail in these condition's. When the wind starts to blow cranking on the out haul is one of the things we do first.Also bending the telephone pole mast on a Catalina 30 I would agree is not possible. But bending the mast on my Mast Head H 28.5 does us allot of good. We are able to bend it back 8 inches or so. We can take all the slack out of our headstay and we can flatten out our mainsail also. Flattening out our mainsail by moving the draft of the main sail forward keep's the boat on it's feet and faster. I do agree it is not as bendy as a J 30 but it is not a Frac rig like on all J boat's. We also move the draft of our head sail by the Halyard tension more than anything else. We then use the sheet tension and sheet block location to control our angle of attack.Cliff

 

[Stu Jackson]

Your

Donyourandyou're (you are)are two different words

 

[Arlyn Stewart]

your or you're

either yer your or you're works for me

 

[Alan]

Bob

"You're right", but there are no grades in this course. Sail Trim knowledge is far more important. Spelling, punctuation, and grammar don't count.

 

[Don Guillette]

Alan & Arlyn: Thank you for your support. I went to Catholic school in the late 40's and 50's and I "knew why God created me" and all my prays but I was not too good with spelling, nouns and verbs. Math was not also not of my strong points.

 

[Don Guillette]

David: Sorry I missed your question. I got distracted by my grammer and spelling lesson and then I could not locate your post. I remembered it but could not find it.Anyway, good question but it depends on a number of things. First, if you have a bolt rope setup there is only so far you can go anyway. If you have a loose footed main, you could go a long way. In fact, you could go past the point of diminished return.In most cases, 2' of belly is probably the max you would ever need sailing to windward in the full range of moderate wind. Remember draft depth is only part of the puzzel. In other words, draft position and twist must be set correctly for the wind speed you are sailing in. If they are not set correctly you can mess with the outhaul all day long and you will not get 100% efficiency.That is nice to say, but how does one know the correct settings? Easy, spend $15 bucks and buy my "Sail Trim Chart" where I've given you the settings for all the sail trim controls for the main and jib for every point of sail and wind condition. If you want to know WHY you are making a particular sail trim adjustment, spend $18.50 and buy my book, "The Sail Trim Users Guide".How would you know when you have gone to far with your outhaul? The sails will tell you. They will stall. Before They stall, the telltails (I assume you have telltails - shame if you don't)will flop all over the place. Next time your out sailing, play with the outhaul. Ease it out and haul it back and see what happens. When you get the feel of it be sure to make your settings so you can go back to them.