Curved Traveller

Sailing World posted a photo of an America's Cup boat's traveler. It appeared to be curved with the centre of radius somewhere up the mast. To me, this would allow the boom to lift when the traveler was outboard. Any comments?

What shape is your traveler? Straight and flat? Curved horizontally? Curved to follow the cabintop?

I'd like to believe that there is a trickle-down from AC racing technology. What can we re-apply?

 

Link?

At full speed the sail on these yachts will always be close to the centreline, and that is also the situation when they presumably want a flat sail for low drag. The sail is sheeted wide for acceleration after manoeuvres, at which moments they would want more body in the sail. It seems neat to achieve this effect with a single control, reducing the need to adjust the sheet as they adjust the traveller.

This is pure supposition on my part of course.

My traveller is straight, but also much shorter than an AC one.

 

Nothing new about curved travellers. I have a 1972 NZ built Cherub with one.

 

The traveler is curved in the vertical plane. Scan down the article . . .

The Flight Controller's Perspective https://www.sailingworld.com/racing/american-magic-insight-with-andrew-campbell/

 

Vertically curved travellers are pretty ancient; Tasars have had them since the mid '70s and they weren't new then. Gggguest's is probably vertically curved.

 

I am intrigued. What is the principle? And why are they not in the mainstream?

 

Curved travelers are popular enough to be considered mainstream. The increased cost of bending the track may keep them off of low budget craft.

One common curve uses the mast as the center of curvature. This keeps the sheet length equal throughout the full length of track.

Other tracks follow the decks curve. This allows for a very solid connection berries track and deck. It also requires that the sheet be shortened as the boom is pulled in.

Occasionally a track is up turned. This allows the boom to raise as it travels from center line to close hauled. Very useful in gusty conditions by decreasing leach tension as the boom swings out.

 

This is getting extremely interesting. Blueknarr's explanation is clear and concise. Thanks!

So follow along. A straight traveler could allow the boom to lift if the traveler was a chord to the arc scribed by the swing of the boom. The sheeting point of the boom would get closer to the traveler as the boom swung outboard. There would be more sheet available to allow the boom to lift. The traveler would be forward of the sheet at the centreline, but the sheet could be directly over the traveler at the outboard end of the traveler. Right? That would accomplish the same effect as a vertically curved traveler.

A multi-purchase mainsheet would exaggerate the boom lift as the straight traveler was eased outboard. And, mast rake would decrease the boom lift.

This analysis requires a spreadsheet.

The significant assumption is that the boom SHOULD LIFT as the traveler is eased. Is that absolutely true?

 

So much depends on the exact geometry of the boom and traveler.
Mast rake and traveler actions are independent of each other.
When the clew is over the traveler, the sheet line usually has greater influence on the leach tension than the vang. But if the clew is outboard of the traveler then the vang controls leach tension.
IF THE TRAVELER IS AFT OF THE BOOM'S SHEETING
And if greater leach tension is desired at the end of the traveler. Then bend the traveler's tips aft/downwards.
If less leach tension is desired then bring the traveler's tips forward/upward (tip farther from boom than mid point)

IF THE TRAVELER IS FORWARD OF THE BOOM'S SHEETING

The opposite geometry is required.

The difference in length from the boom sheeting at mid track vs the ends will have the same effect on leach tension regardless of how many purchase loops involved.

Easing both leach tension and sheeting angle is useful when de/recovering in gusty conditions and a straight course is desired.
BUT
If changing course from close hauled to reaching, then tightening the leach while easing the boom is usually called for

Edit
Only the horizontal bend changes with sheeting orientation. Any vertical bend will have the same effect forward or aft of boom sheeting.
Sometimes horizontal curvature is used to counter act vertical curvature.

The above is only true when the sheet remains constant and the traveler's controls are used to adjust sail angle of attack. Changing sheet length negates traveler curvature.

 

Slight uptick at the ends, horizontal curve determined by mast/ boom radius. Sheet, outhaul, & traveller had to be adjusted constantly, along with vang, although the full width traveller took care of 85% of things vang related, although it was always handy jibing, along with cunningham: adjustments to wind shear are hardly ever a one line affair, since boat speed and heel change how the sails are going through the apparent wind. My $.02.

 

The simple answer is probably that there is no simple answer and no absolutes. I've seen convex vertical curves, concave vertical curves, horizontal curves centred on the mast, horizontal curves centred forward of the mast... Individual owners will have their own theories/concepts and set their boat up accordingly.

 

I'm still pondering this and what I've discovered is that I really don't know the function of the boom vang, other than when sailing downwind. Please enlighten me. When and how do you tweak the vang?

 

For clarity I am ignoring that some vangs have either hydraulic or spring capacity to provide an upward force on the boom. And will focus on the vang's contraction and downward pull.


You already use the vang to control leach tension while reaching or running. This is the vang's job. Open the leach to allow more twist in the mainsail. More air is spilled from the top, depowering it.

Pull on the the vang to tighten the leach, reducing twist and increase the potential power in the main.

This twist control is also desirable while close hauled.

Now go to your boat .

Notice how the vang's attached to the base of the mast and 1/4 to 1/3 back on the boom. This gives the vang equal downward pull on the boom regardless of the booms angle to the boat's centerline.

Now look at your mainsheet and boom. Swing the boom as far as it will go. When the mainsheet is pulled it reduces the booms angle the the boat's centerline. But also notice the there is also a vertical difference between the boom and traveler. PULLING ON THE MAINSHEET PRODUCES A DOWNWARD FORCE AS WELL AS THE INWARD FORCE.
When the boom is well outboard the mainsheet's downward force is insufficient to tighten the leach and the vang is a more efficient leach control.

Now center the boom and observe the direction of pull that the sheet applies. Close hauled the mainsheet applies nearly all of its force downward on the boom and very little laterally. In this position the mainsheet has more leverage than the vang and therefore controls leach tension

 

There is an optimal amount of twist for any wind speed and the vang is used to arrive at this point.Tell tales on the leach will aid in establishing this setting.It is well established that the surface of the water creates drag on the wind travelling across it and this alters the apparent wind speed over the height of the rig.In boats that don't reef the twist can be increased to permit the head of the sail to feather and thus reduce heeling moment.

One other reason why curved travellers aren't seen more often is that a chandler would be unlikely to stock a range of hardware with the correct curvature for all the different types of boat that might be equipped with them.They can be curved either vertically or horizontally or simply left straight and straight eases the retailers capital tied up in stock that won't be moving.Those of us that sail dinghies with adequate vanging don't actually need a traveller and it frees up space not to have one.It also saves weight and cost.

 

Depends on the mainsheet system. Windward sheeting cars, and bridles, for instance make for minimal leech tension from the mainsheet.