vang to depower or power

[Paul Billings]

I have seen numerous times in this forum that the boom vang can be used when closehauled to flatten the mainsail, thereby depowering the sail. This is in contrast to the fact it tightens the leech, reducing twist which powers up the sail. Which is the more dominant effect to increased vang tension?Paul

 

[Don Guillette]

Paul: Sometime, to make the answer clearer, I have to start at the beginning. Starting in the middle only gets me tied up in my underware as your question is not as simple to answer as it sounds.There are only 4 sail trim elements we are dealing with and they are draft depth, draft position, twist and angle of attack. A lot of sailors think there are more elements but there are not. The boom vang controls 2 of the elements. The first is twist. The other sail trim control for twist is the mainsheet. The second element the boom vang controls is draft position. The other sail trim controls for draft position are the cunningham, mainsheet mast bend and outhaul.The boom vangs primary funtion is to remove twist. Twist is a power robber as it spills air out of the top of the sail. So, if your object was to depower the boat, eliminating twist will not accomplish your goal. The boat will go faster as you crank the twist out but you will reach a point where the boat will start to slow down because a really flat sail is not very powerful.If your object is to depower the boat, here is what you would do to the mainsail (the jib is another story) - you would ease out on the mainsheet and lower the traveler making a wider ANGLE OF ATTACK. You would ease the boom vang thus giving more TWIST. You would crank on your outhaul to reduce the DRAFT DEPTH (belly)and lastly you would crank on the cunnigham to move the DRAFT POSITION forward. If you want to power the boat up, just do the opposite.As you can hopefully see, you have to use ALL your sail trim controls to accomplish your objective. Did I answer your question or only serve to confuse you?

 

[Paul Billings]

Don: Thanks for the note. That jibes with the way I was thinking of things. For the most part, increased vang reduces twist, thereby adding power. Your response sparks a couple thoughts, however: 1) You mentioned that to depower "you would ease out on the mainsheet and lower the traveler making a wider ANGLE OF ATTACK" -- I agree, this would depower the sail. This places the boom at position X, let's say. I think an alternative approach would depower *more*: if you raise the traveler, and then ease the mainsheet so the boom still goes to position X, then the main must be twisting more due to a more horizontal angle of pull and less down than the first approach (thus less power). What do you think?2) more a comment about the outhaul when going to windward. I'm sure you know this, but I hear many people talk of "powering through the waves" or "into the wind" with a good draft. "Put the draft in there, you need the power," one fellow told me as onlookers nodded in agreement. (Part of the 75% you talk about perhaps? I'm looking forward to the arrival of your book.) The truth is quite the opposite, however. Upwind performance is far better with a flatter sail, so crank on the outhaul. I assume it is due to less drag, lift being directed more forward (from draft position moving forward), better flow, who knows what else, but empirically I know it is so.Paul

 

[Don Guillette]

Paul: My book goes into much more detail about all your sail trim controls. Space here does not permit a long explanation but I'll give you the short version.Here's the skinny on the traveler. The traveler does not effect the shape of the sail. Here's why - think of your screen door at home and assume it has a pin in the bottom corner that rides in a track on the floor. When you open and close the screen does the shape of the screen change? Obviously it doesn't and that is exactly how the traveler, which only changes the angle of attack, works. In other words, you could pull it over to the 3rd boat away from you and the shape of the sail would not change (that's an exaggeration of course but makes my point). Sooo, if you wanted to depower the boat quickly, just drop the traveler (widens the angle of attack) and ease the mainsheet and boom vang (increases twist) which spills air.On the statement "Put the draft in there, you need power". There are right but the next question is what draft - draft depth or draft position - and what controls do I use to get there and also how much do I apply? Whew, that was a long one!! My book devotes the first chapter to draft depth, draft position , angle of attack and twist. A sailor has to understand those terms in order to understand sail trim. Anyway, the draft position should be at about 50%, which is called draft aft. Draft aft equals power while draft forward (40%)is less power. The draft position should be about 20% to 25%. As the wind pipes up it wants to push the draft aft (beyond 50%) and that is why you need controls to pull it back. The farther past 50% to 55% you let the DP go the less effective it becomes. Hope my explanation didn't confuse anyone. If I did, just drop me a note and I'll explain further.

 

[Ed]

Traveller position

I think paul is suggesting that with the traveller moved up, and mainsheet let out, the boom angle remains the same, but the wind lifts the boom, increasing twist.I don't get the idea that a traveller 3 boats away would not change the shape of the sail. I see that it would not neccesarily change the boom angle, but the boom would lift if the vang was not in play.What am I missing?

 

[Don Guillette]

ED: Remember he is closehauled and not on any other point of sail. Reaching and definately running will cause the boom to rise without the vang. What I'm saying is that the traveler does not effect the shape of the sail. Try it on your boat and see what happens. First, don't touch the vang or mainsheet and move the traveler up or down and check the shape of the sail. It should remain the same assuming your sail is properly set for the wind conditions you are sailing in. Next, still closehauled, release the vang and ease the mainsheet and move the traveler up. Nothing should happen to the shape of the sail and the boom should not go up enough to effect the twist. It might go up slightly but I'm not sure because I've never tried to depower a boat by pulling the traveler up. The point I'm trying to make is the way to depower a sailboat quickly (which is the question at hand)is to first drop the traveler and then ease the mainsheet. Pulling the traveler up is backwards and counter productive as it increases the angle of attack and would make the boat go faster.Instead of using two sail trim controls to quickly depower the boat, your only using one and counteracting the easing of the mainsheet by increasing the angle of attack.Your hoping twist will do the job alone, which it won't. I hope I have cleared up any misunderstanding I may have presented. Some of these questions are not easy to answer in a couple of sentences as there are many factors in play.

 

[Ed]

Thanks

I was thinking of reaching, not close hauled. I do understand it is a contrary move to position the traveller higher. I also liked the info about the draft being pulled aft by the wind when pointing up. I'll have to add a cuningham so I can see the effect.

 

[Paul Billings]

Part of the confusion may be that I am actually referring to longer term sail adjustments. By all means, in the immediate case of having to depower NOW, I agree that easing the traveller or main (or both) is the way to go. Consider the case, however, where the wind is just a little too strong and twist will be an adequate adjustment.Don says:"Next, still closehauled, release the vang and ease the mainsheet and move the traveler up. Nothing should happen to the shape of the sail and the boom should not go up enough to effect the twist."If the mainsheet controls twist, then when I ease it, the shape sure better change! Back to basics: the twist increases when you ease the main because the pull is less vertical. (This is my understanding; your book hasn't arrived yet, so I hope it agrees with yours.)Let's say you like your angle of attack, but the main is a little too powerful. I can see simultaneously easing the mainsheet and raising the traveller to dial in increasing amounts of twist while keeping the angle of attack (boom position) constant. The angle of the mainsheet is increasingly more horizontal, and therefore inducing increasingly more twist (== less power).Paul

 

[Don Guillette]

Paul:Here's what I'm trying to say with the statement "release the mainsheet and the vang and then move the traveler and the shape won't change etc". There are two parts to the statement. The first is once you released the sheet and the vang, the mainsail assumed some sort of shape. The second part involves moving the traveler. Moving the traveler will not effect that shape of the sail that occurred when you released the vang and the mainsheet. As always, I suggest that sailors, the next time they are on the water, take a few minutes and try some of the concepts that are discussed here. Fool around with your sail trim controls. Move the traveler up and down and notice what happens to the shape of the sail (nothing) and your speed (it should increase). Release the vang and the mainsheet slowly and watch the top of the sail twist off. Once you see it you won't forget what it looks like. It will then be easy for you to spot on other boats nearby as 50% of the sailors sail with the main twisted all the time!! They would be amazed how much faster their boats would go if they eliminated that twist. Maybe they don't want to go fast - I don't know. Another point - If evertime you moved the traveler the shape changed you'd be constantly messing with the sail. On the older Catalina 30's, which have a curved track traveler, this is a big problem. When you are closehauled and everything is tight and you attempt to move the traveler up, it is actually going downhill on the other side of the track. You have no slack so you have to release the mainsheet, move the traveler and then reset the mainsheet. This is a real pain and the reason I converted to the Garhauer straight track system. Now my traveler works the way it was originally designed to function.

 

[Paul Billings]

I agree, but go one step further

> The first is once you released the sheet and the vang, the mainsail assumed some sort of shape. Exactly, it twists off.> The second part involves moving the traveler. Moving the traveler will not effect that shape of the sail that occurred when you released the vang and the mainsheet.Again, agreed.And the amount of twist is dependent on how much you eased the mainsheet. The more you ease it, the more twist you get. I think we can agree on this as well.It seems like I'm not doing a good job at explaining my point. I realize you are trying to be very clear since the readers treat what you say as gospel. I'm sure they, like I, appreciate the time and energy you spend posting! I also don't want to upset anyone (especially you), so I'll try just once more. (Remember, I'm considering longer term sail adjustments, not gust recovery.)Ultimately, the apparent wind direction will determine the angle of attack required, right? That means the boom must be placed in a specific position (angle of attack). Given that we must maintain boom position and that we desire more twist (vang is already loose), what can be done? Clearly, the mainsheet must be eased since that controls twist. How do we ease the main while maintaining boom position? My answer is to raise the traveller as well.One alternative explaination just occured to me: the mainsheet controls twist and angle of attack. The traveller is angle of attack only. A judicious choice of traveller position can then exactly cancel the change in angle of attack induced by the mainsheet adjustment, leaving a net adjustment of twist only for the combination.As I have heard you say before, the end result is brought about by the *combination* of controls. While the traveller itself does not control twist, it can be used as part of a combination approach to achieve the desired goal.Paul

 

[Rick Macdonald]

Vang to Depower Or Power (nit pick)

Nobody picked up on the words below (the writers clearly know what they meant) but I'd like to clarify them just in case any reader was confused...The phrase "ease out on the mainsheet and lower the traveler making a wider ANGLE OF ATTACK" is unfortunate. The angle between the sail and the centerline of the boat becomes wider but of course the angle of attack is the angle between the relative wind and the sail, which is _decreased_ by the amount you let the sail out.

 

[Bob]

Don, you said

in three consecutive sentences, the following:"Anyway, the draft position should be about 50%, which is called draft aft. Draft aft equals power while draft forward (40%) is less power. The draft position should be about 20% to 25%."Is this, in its entirety, what you wanted to say? If so, am I the only one confused by it?

 

[Don Guillette]

Vang To Depower

Bob; Wow, I was throwing to manny numbers around in that message!! Sorry about the confusion. I read the messages I write a couple of time because I don't want to mislead people but I sure missed that one. What I ment to say was the draft position should be in the 50% to 55% range. A draft position of 20% to 25% would be impossable to obtain.I'm glad to see some sailors are reading the messages and keeping me honest and on my toes.

 

[Don Guillette]

Rick: Your explanation is correct. I know mates have trouble visualizing angle of attack because they tell me so during seminars I present. They just shake their heads because what I telling them appears to be opposite of what they are thinking. They hear Increase/decrease - and think what is he talking about? It is simple to explain with a diagram and with a Pond Model sail boat I use for demonstration purposes. They get it in seconds.I'm going to give it a try without those two props. Lets see what happens and I hope I don't confuse folks even more.Assume your a bird flying above a closehauled boat and looking down the mast. Here's what you see. The apparent wind is coming into the side of the boat at a 45 degree angle off the bow. To show the direction of the apparent wind assume a piece of wood is sticking over the side of the boat. Instead of using the sail as the other factor involved, lets use the boom and assume a long piece of wood is attached to the front of the boom and sticking out over the side of boat along side the one for the apparent wind. Looking down from above you can see an ANGLE formed between the two pieces of wood. For folks who have my book please go to page 18 to see the diagram.The apparent wind wooden piece is not going to move but the boom wooden piece will move. Remember your looking at this from above. Soo, as you ease the traveler down, the boom wooden piece moves closer to the apparent wood piece or the angle gets smaller (decreases). As you pull the traveler up, the wooden boom piece moves away from apparent wood piece and the angle gets larger (increases). I'm almost sorry I opened up this can of worms called angle of attack but unfortunately it is a sailing term you have to deal with if you want to become a proficient sail trimmer.

 

[Don Guillette]

Paul Billings "on one step further". You give me too much credit on my stuff being gospel. I'm just a common seaman who has spent a lot of time to try to figure out exactly what is happening to my sails. After I gathered all the infomation I spent hours upon hours testing the principles on my boat. Also, I have to keep the explanations as simple as possable or I'll confuse myself!! An example is the discussion on Chord. I know what it actually is but it is too hard for the average beginner to intermediate to see so I change it a bit and make it easier for them to understand. It is hard to do a complete explanation in this medium because space is too short and sometimes one or two words changes or adds to the answer. Sometimes an extra word involves using another sail trim control. If we were sitting in the YC bar or at your breakfast table we'd have this straightened out in nothing flat. Your right that the apparent wind direction and additionally WIND SPEED determines the angle of attack and the boom has a specific position on all points of sail and wind conditions.There is a point at which no more twist can be induced into the main or jib. You can find that on your boat by experimenting and noting where that point is. So once you get to that point there ain't no more. How you "fine tune" to get the sail where you want it may be different from me but the advantage you have over the average sailor is you know what you want to achieve and have a pretty good idea how to get there.My last point is there are 4 elements we are dealing with and they are draft depth, draft position, twist and angle of attack for the main and jib and all the sail trim controls that GOD put on the boat must be worked together on each point of sail and wind condition to achieve a result of 1005 efficiency.Next time I get to Hawaii, maybe we can meet and have a few beers on Wakaki Beach and talk about sail trim!!

 

[Paul Billings]

first round's on me

Aww, you're too modest! I do appreciate all your comments. Let me know when you're on this side of the pond!Paul