Monday, February 14, 2011

Vang Design

I first saw the lever style vang on a forum after a 12 ft skiff had trialled the setup. Shortly after i saw it on some international 14's. The advantages that i saw immediately were the increase in cockpit space for the crew over a conventional vang system and (for my boat particularly) the clearance for the centreboard to be raised downwind.


Image 1: 12ft skiff design, i think it was built around 2006
Image 2: i14 design, possibly built around 2003
Images from (http://forums.sailinganarchy.com/index.php?showtopic=31443)

I began to look at this design concept for my older boat "Heart Shaped Box" and began a basic force analysis to compare a conventional setup to a lever style and the loads involved. This was fairly basic and going over it now i can see that there are several errors but i believe it represents a reasonable comparison between the two systems. The assumptions that i made were a 50kg load along the leech of the main sail and i only considered the lever vang forces parallel to the boom (not accounting for misalignment, which I'll explain later)




The results of this analysis was that an axial load on the purchase system needed for each system was as follows at 50kg of leech load.

Conventional = 1805 N (184 kg)
Lever = 1345 N (137 kg)

This gives a 25% reduction in the load at the rope. Meaning that where a 16:1 purchase was used before a 12:1 system could now be used for the same power. Having less purchase means less excess rope in the boat when the vang is sheeted on hard giving less rope to tangle in the boat.

While i was looking at this system a cherub at Lane Cove 12ft skiff club "Ajax" trialed the lever vang systems as they had played with the system on a 12ft skiff. After discussing with them to mentioned that you need to be wary of the sheeting angle at the tip of the lever as it can place a vertical load on the goose neck and this needs to be understood to make sure it isn't neglected.

To understand this lifting force you need to see that the boom/lever member is pivoting about the goose neck and the point which you are applying to load is the end of the lever. When this system is pivoting the tip of the lever will trace an arc with the centre point being the goose neck. If the load at the tip of the lever isn't applied tangent to this arc then the force can be broken into a vertical and a horizontal force. As this misalignment gets larger the vertical component starts to grow, but is still fairly small.

The next this i looked at the was the clearance on the centreboard as on my older boat the centre case was fairly close to the mast meaning that there was not a lot of room between the vang and centreboard. It showed i could get a significant increase in the clearance.


When it came to fitting out my new boat i looked at how i would actually set up the system. In order for the vang to work properly the leech tension should remain the same as the boom is eased through its travel. Since the boom is pivoted about the goose neck if the ropes on the end of the lever is pivoted about a similar position on the mast then the leech tension should remain the same. I was first concerned about the bending this would place on the lower mast and how this would effect the mast bend further up the rig. So i looked at options for having the pulley systems attached to the hull rather than the mast. The image below shows different loci that i had traced from different ways of wrapping the ropes around the mast to approximate an arc from the mast. The blue lines show that the system followed an arc about the centre of the mast until one of the ropes stopped touching the side of the mast, at which point it started tracing an ellipse from the pulley on the hull and the mast. I felt that this would approximate the arc enough to be a practical system.

These images below show the setup that i used. The lever was made from carbon over a polystyrene foam core with a section of windsurfer mast as the main structure inside. This was don't to make it as stiff as possible as flex in the boom or lever results in a loss of leech tension when the boom moves from dead centre to slightly eased as the pulley's at the boom tip stop taking the vertical load of the leech.


The purchase i went with was a 3:1 followed by a 2:1 around the mast, giving a 6:1 all up on the lever tip. Although this was lower than first calculated it was decided to try it as the old vang system was rather easy.

Conclusions:
After sailing with this setup for a little while now there are several changes I plan to make. First of all, we are struggling to get the height out of the boat that others have and I'm starting to feel that this is due to the leech on our mainsail. The vang is slightly underpowered meaning that in the stronger breezes it is hard to get a good trim on the leech.

Secondly the way the vang wraps around the mast gives some deviation from an arc so i will move the pickup point to the back of the mast. As a small deviation at the lever tip is multiplied by four at the tip of boom (ratio of lengths) so 2 mm at the lever = 8 mm at boom tip, which will reduce the leech tension quite a bit.

1. Power: I will increase the purchase of the system to 10:1 0r 12:1 from the 6:1 i originally had.

2. Pick up points and locus: i will move the pick up to the back of tha mast and if mast bend is excessive i will place a strap/strop to the hull forward of the mast.

Other than this the vang shows promise, giving good clearance to the centreboard and more room for the crew. The lever i added to my boom is on the heavy side (about 1 kg), as it was placed on the boom two days before the nationals and i didn't want it to fail. So improvements could definitely be made there.


just a few more photos

4 comments:

  1. Hi Adam,

    I ended up buying Dragon Slayers Inc from Adelaide, and in talking to Dale Geddes the subject of this style of boom vang came up and he suggested they don't measure, and checking the rules "Spars must pass through a 105mm Ring when stripped of all fittings". What was the feedback from the measurers at the Nationals.. is the vertical leg considered a "Fitting".

    Gimme More had this vang as well. Surely it must be acceptable.

    Having now had a couple of sails on a Cherub, I can see a big advantage to crew space in this conversion.

    Ron Garner

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    Replies
    1. I wouldn't be too sure about the legality of anything on Gimme More. Only because I know the owners.

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  2. Hi Ron,

    Nice one,

    Yes there is some debate over this issue at the moment. At the nationals it was not protested, yet I was the only boat supporting this style of vang at the Adelaide nationals and didn't place particularly highly in the reggata.

    At the moment there are two schools of through.

    1. That the lever is considered a fitting and therefore exempt from the rule, for example fixed spreaders are a fitting which are glued onto the rig. Though that is also a potential grey area.

    2. That since it is glued/laminated to the boom it is therefore not a fitting and illegal.

    Since this has not actually been formally protested or discussed it is unclear whether the vang is legal or not.

    Although the next school of thought is that it is quite simple the make this type of vang into a removable fitting. Either by bolting a lever to the boom or attaching a strut and wire/specta brace arrangement (a brace quite close to the goose neck would still improve cockpit room).

    I would suggest that if you want to place one on your boat, you look at making it so it can be remove to be sure that it is legal, in the event that it is protested against at the next nationals.

    There is another cherub using a similarly unusual vang set up on "pocket rocket" that used to sail at Belmont and Lane cove that could also be considered illegal if it was not seen as a fitting.

    Hope this helps
    Cheers Adam

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