Vang vs. twist - the depowering conundrum

[Brian S]

Ok, so from my small dinghy days, I learned that with boats equipped with a powerful vang, or kicker (upwards of 8:1 or 16:1), you can pull on vang to depower the sail. My understanding is it will bend the mast, curving it forward, which the sail is cut with a forward curve to the luff, thus allowing the sail to fill out its natural shape and have less camber. (Pulling the curved luff straight is what induces camber.)

But then, on a bigger boat, with not necessarily a bendy mast, pulling on vang will tighten the leech, and remove twist from the sail. But allowing the top of the sail to twist off depowers it. So, on a bigger boat, if on point of sail other than reach or run, pulling on vang WON'T depower the sail.

Am I understanding this correctly? If so, it seems that to depower a bigger, fractional rig boat with an adjustable backstay one would want to pull on backstay to get the same mast bend that smaller dinghies achieve with vang, while still allowing the sail to twist off with little to no vang.

Thanks - yeah, I guess I should just pony up for Don's book...

Brian

 

[Don Guillette]

Brian: You're mixing up a few things and I'm not sure where to start. You can watch what I'm about to tell you happen at the comfort of your dock. The primary mainsail sail trim control for TWIST is the boom vang - the secondary control is the mainsheet. Hoist your mainsail with zero tension on the boom vang. Look up the sail and you'll see the top is "twisted off" or "open". To see what I mean, form your right hand as if you were going to salute and then turn your fingers ONLY to the right. That's what a twisted sail looks like. With a twisted sail you're spilling power off the top 1/3 of the sail. Now, on your boat, crank on the boom vang and watch the top of the sail "close" You're now harnessing power through the whole of the sail. Lossen up gradually on the boom vang and watch the sail open up. By twisting off the top of the mainsail you're depowring the boat. BY flatening the sail as much as possible you're also depowering the sail because a flat sail is less powerful due to lack of draft depth, that may sound confusing but it's what happens.

You can't bend or rake the mast with a boom vang. Additionally, the boom vang is one of the sail trim controls that adjusts DRAFT POSITION. Also, raking the mast a lot , not bending the mast, makes the boat think you lifted up the mast and moved it aft and weather helm results. If you had a "bendy" mast you could use it to depower the sail. When you bend the mast you're pulling the fullness out of the middle of the sail and reducing DRAFT DEPTH (belly).

The thing that helps clear up this problem is for mates to determine exactly the function of each sail trim control on the main and jib. Once they understand that they'll be well on your way to understanding sail trim. RichH, Joe from San Diego, Stu j and others won't be calling them for advise but they'll know more that 50% of the sailors worldwide.

Buy the book/chart -- I include a free QUICK REFERENCE that outlines the function of each sail trim control and which way to push or pull it to get the result you desire. Sure beats looking it up yourself!!

 

[Roadking Larry]

Get the book/chart! I got my copy Friday, now I know why I was getting that funny "bubble" in the mainsail and what to do about it.

 

[Jackdaw]

Don,

Brian's original experience as stated is on a small dinghy, like 420s. He's mixed up on several points as you clarify, but on these small boats the vang is absolutely used to bend the mast. Because these boast often have no backstay, it's the primary control to flaten the main and tension the forestay.

420 class tuning guide

http://www.club420.org/PAGES/Library/tuningGuide.htm

 

[Don Guillette]

Jackdaw: Thank you. I have zero experience with small dingy's and did not know that.

 

[RichH]

Many small high tech fractional boats use the vang additionally as a 'mast ram'. The vang is at a low angle attachment to the boom which when the vang tensioned (8:1++ or better) puts a lot of 'forward' force along the boom ...... like pushing an archery bow (flexible mast) forward with your foot/leg (boom).

The 'mast ram' vang bows the mast forward at the bottom sections of the sail - flattens the bottom panels of a sail.
Those additionally with tapered flexible masts and large 'lever arms' at the top of the mast easily flatten the middle and top sections of the sail - simply overtension the backstay by an 8:1 or greater block system - the middle of the mast moves forward and the top moves aft (can be as much as 3+ 'feet' aft) and the mainsail is rendered close to the shape of flat sheet of plywood.

Such planing sailboats can have upwards of 35 to 40 'sailing control' lines .... so that each adjustment can be made from either side of the cockpit when heeled over and no one has to move to operate a control line. Usually the most important lines/controls are those which can bend the mast for fast/quick 'flattening down' - for heeling control and for the best 'speed shape' of the mainsail.
A radically flattened sail is far easier to control to 'blade out' when overpowered, as the 'blade out' will allow the sail to luff instantly and fully along the entire luff section .... and quite predictably too. In contrast, a 'twisted up' sail used to depower will not 'luff' simultaneously along the luff and may go through a radical and unpredictable 'power up' before total luffing occurs. Radical 'power-ups' usually involve and include 'swimming exercises' when sailing the ultralight high speed planing hull boats. When grossly overpowered in such boats one usually avoids 'luffing up' but rather 'ease out and bear off' and "hope to hell that you can keep the boat directly under the mast" as the boat turns into a veritable 'rocketship'. Big difference on boats that dont have a 'hull speed' limitation.

 

[Stu Jackson]

Going back to Brian's OP, "bigger" boats usually don't have bendy masts, even if fractionally rigged. We often describe our masts as "telephone poles" so that backstay adjusters are used to tension forestays and have little to no effect on the shape of the mainsail. On our boat, vang flattens the mainsail and depowers (flattens) it.

Don's advice to try it out and see what happens on the boat he has, rather than to attempt to transfer small boat experiences (as Rich mentioned) is well taken.

 

[Jackdaw]

Going back to Brian's OP, "bigger" boats usually don't have bendy masts, even if fractionally rigged. We often describe our masts as "telephone poles" so that backstay adjusters are used to tension forestays and have little to no effect on the shape of the mainsail. On our boat, vang flattens the mainsail and depowers (flattens) it.

Don's advice to try it out and see what happens on the boat he has, rather than to attempt to transfer small boat experiences (as Rich mentioned) is well taken.

Stu, well, the OP WAS asking about fractional rigs, and with the exception of Hunter's B&R rig, they are all bendy. I know of no one with a fractional stick that would call their mast a telephone pole, like a masthead owner would do. We bend the mast of our 367 with backstay, vang, and mainsheet over half a foot. With the exception of the RAM (or GNAV) vang, all of Rich's comments about sail shaping are universal for fractional boats. But in the end you are right of course, it's important to correctly apply any knowledge to your current boat.

 

[RichH]

In a high tech sense and including mast head rigs ... virtually all masts can be 'bowed' to affect better performance. Mast head rigs 'should' have a definite amount of minimum 'pre-bend' to meet the normal 'design cut' of the luff profile of even plain vanilla mainsails. The B&R is usually set up with significant prebend.
In any rig, for extreme performance one can simply and temporarily exceed the 'normal' limits of pre-bending or pre-bowing ... but shouldnt be leaving the rig so over-tensioned for long periods of time as fatigue and extremely short service life of the rigging will result .... however to 'win' races, its done all the time by those who dominate racing fleets. Its probably the 'best' way to 'flatten' and/or to obtain a high speed & less aggressive heeling 'flattened' shape. Dont include prebend in just about any mast and you will have a slow/sluggish, cranky boat that 'prefers' to 'heel over'; a 'straight up' non pre-bent mast will simply be carrying 'way too much' draft and a draft that wont respond all that well to increased halyard and outhaul tension when you do attempt to set the correct amount of draft for the day's wind/wave conditions. ALL masts should be 'pre-bowed'.

The 'plain vanilla' amount of pre-bow for masthead rigs is:
3/4" forward pre-bow for single spreader rigs; 1/2" pre-bow for each set of spreaders on multi spreader rigs. Makes a big difference in draft control ..... and safety, especially in the higher wind ranges.

 

[RichH]

I think that most sailors simply are unaware of 'how' to set up the amount of draft in the sail. Most cruising sailors probably raise the mainsail and simply dont go any further than that; the outhaul is simply 'tied off' and forgotten for the entire season.

How to set the amount of draft (really should be done at least daily to set up for the current wind and wave conditions or when those conditiions change):
•Go onto a hard beat to windward.
• Let the boat 'settle down' to is maximum speedometer reading via sail trim adjustment - tweak the trim, etc.
• Then, adjust/tension the outhaul to get the MAXIMUM amount of speed out of the boat while 'pointing' and while watching the speedo.
• Then, if you need to depower down to your accustomed comfort level ...
A. further increase the outhaul tension;
B. increase the halyard tension while watching to be sure that the leach shape is either slightly 'pointing towards the leeward side or aligned parallel with the boats centerline' ..... and NEVER 'hooked up' or slightly towards towards the windward side'. The more halyard tension applied, the more 'open' the leach (less draft or 'curved shape at the aft end of the sail).
• Additionally, and very important when depowering - do NOT apply too much tension to the mainsheet ... will also cause the leech to 'hook up' towards the weather side and will cause massive increase in overall draft (heeling) .... when adjusting the mainsheet, always LOOK at the leech to be sure its not 'hooked up' creating large amounts of artificial overall draft to occur in the mainsail.
Allowing the leech to hook-up to weather is like putting 'the flaps down' on an airplane - only good for landings and takeoffs !!!!!!!

;-)

 

[RichH]

Clarification on preceding post.

Increasing halyard tension, besides shifting the point at where the maximum draft occurs - fore or aft - in the mainsail, it also 'opens' the leech or causes the leech to 'point more towards leewards' ... also reducing 'overall draft'.
Raising a mainsail to 'just up' and without proper luff/boltrope tension results in a 'hooked up' leech ..... artificially increases overall draft.

Since dacron sails are 'stretchy' you can never make just one 'adjustment' in order to 'shape' such a sail. This is because when you adjust the tension along one side of stretchy sail, the longest side opposite of where you changed the 'stretch' will also react and that 'other side' usually has to be corrected because you changed the tension in the original side .....

Hint: There are always TWO adjustments required when 'shaping' dacron sails; the first adjustment to get the shape you want and the second adjustment to correct the reaction of change in tension to the 'opposite' side.

Sail 'shaping' all starts here: Repeat -- How to correctly 'raise' a boltroped dacron mainsail: http://forums.sbo.sailboatowners.com/showthread.php?t=120970 - see post #1
Correctly raising a boltroped dacron mainsail will usually correctly 'set' the leech shape which also helps control the overall amount of 'draft'.

 

[Brian S]

Thanks, folks, this has basically confirmed what I was thinking.

It's tough when you're a puddle sailer, because the wind is always changing, either shifting, or puffing. This weekend, I will try to play around with my downhaul some, because I have a sliding gooseneck, so I hoist the main, and do all luff tensioning with the downhaul. I will see if I can determine speed changes with different downhaul amounts.

It's further unfortunate that my boat doesn't have a traveller, and has end boom sheeting. I think it's more likely for me to apply too much mainsheet, and hook the leech while closehauled, in an attempt to get the boom as close to amidships for pointing. My main leech tell tails seem to fly better with the end of the boom just inwards of the stern corner of the boat, without the main hauled in as much as possible.

Yes, I will be getting Don's book and chart!

On my GP14 dinghy forum, people often buy old GP14s to use for cruising, and are advised to get old racing sails to use, the GP14 being designed to race and cruise. You wouldn't want a beat up racing engine knocking around in your family sedan, you'd rather have a nicely built engine. So you'd rather not have stretched and worn racing sails while cruising, you'd rather have good quality cruising cut sails. Further to this, if the sails are like the motor, sail trim is like the transmission. You wouldn't drive your manual transmission at 25mph in 5th gear, or 75mph in 3rd - so you should trim your sails properly while cruising. Again, as RichH demonstrates, poor sail trim and control settings leads to baggy, weather helm-y, heel-y sailing...

Brian

 

[25yearslater]

Thanks Don and Rich. I just got back in after a few hours tinkering and adjusting to allow the main and jib to twist off about 2/3s of the way up. It works along with flattening the main with the outhaul as the wind was above 15 again. The result was amazing in that the heel was much different (less) without any appreciable decrease in speed. The chop was about 3ft and powering through was a snap.

 

[JoeSmith]

I recommend Don's book, it breaks down sail trim topics in a logical, easy to follow and thorough way. It identifies the exact aspects about the sail that are controlled, then identifies the controls for each aspect to be controlled, and then teaches how to use each control and the effect it has.
It's the best primer I've come across for sail trim.

 

[michellepinto]

1) If your mast curves, the sail becomes full and carries more power.
2) If you want to depower the sail, pull the cunning ham of your sail, the kicker only controls he leech.
3) Kicker is not used to bent the mast - backstay, chocks etc are used to control the mast bend. Any bend achieved through pulling the kicker is not intentionally to bend the mast.
Hope this helps and clarifies your doubts

 

[Scott T-Bird]

It's ok to talk about pre-bend for a masthead rig, but I question how it is actually achieved. It seems to me that it's only possible to affect pre-bend if the masthead rig has forward and rear lower shrouds. My boat has only one set of lower shrouds, so I assume it isn't possible to affect mast pre-bend with rig tuning. If I'm wrong, I would like to know how it is done.

 

[Jackdaw]

It's ok to talk about pre-bend for a masthead rig, but I question how it is actually achieved. It seems to me that it's only possible to affect pre-bend if the masthead rig has forward and rear lower shrouds. My boat has only one set of lower shrouds, so I assume it isn't possible to affect mast pre-bend with rig tuning. If I'm wrong, I would like to know how it is done.

Scott,

Thats kinda true.... that rig setup allows only for static adjustment of the rig; like at the dock.

Most racy boats with masthead rigs have pure inline shrouds (like yours), but have a baby-stay that can be adjusted under sail to bend the rig while underway.

 

[Scott T-Bird]

Ah! I see ... there are many ways to occupy a crew of 6 even on a pocket cruising racer!

Out of curiosity, I have a Kenyon MORC 4060 section (deck-stepped) ... I wonder how much bend can be produced with a baby stay? The length of mast is 33'.

Would the baby stay be run to a fitting on the deck so that the baby stay basically parallels the forestay or would it be run to the stem fitting? I could see how a bend could be produced with tension on the baby stay and the adjustable backstay! Is the bend basically dialed in with the baby stay after each tack? Or I suppose the head sail could pass in front of the baby stay without touching the stay.

 

[Jackdaw]

Ah! I see ... there are many ways to occupy a crew of 6 even on a pocket cruising racer!

Yep. But that has changed. Over the last 15 years or so, masthead rigs have basically disappeared from high performance sailboats. C&C was probably the last hold-out, but they too have gone to fractional rigs; 7/8ths or 9/10ths.