Sailing Techniques

Boom Vang

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Boom Vang

Postby sea_bee » Tue Sep 25, 2012 9:51 pm

Let's get this new forum started!

I am a long way from launching, with only one hull of my Sarah built at this stage, but it doesn't stop me looking at photos of completed boats. I have noticed a lack of boom vang on all the Easy cat photos that I have carefully studied.

In sailing monohulls over the decades, I have found the vang a necessary tool for keeping a flat mainsail on a reach in heavy winds. On my first little boat, without the vang, it would round up uncontrollably.

I must confess to never having sailed a bigger cat than a Hobie 16, so I have no idea whether the practicality is different on a big cat, but I have read a lot about the need to "dump" the main in gusts. Without a vang, when you dump the main, won't it belly up and get too full?

So why have I never seen a photo of a boom vang?

Chris
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Re: Boom Vang

Postby Corley » Wed Sep 26, 2012 5:45 am

Vangs have generally fallen out of favor on multihulls due to mainsheet travellers being fitted on most modern multi's to retain sail shape when the main is eased. I'm not sure how the Sarah's mainsheet is configured though. Here is an article by Richard Woods that discusses some of his experiences in downwind sailing he is quite a fan of boom vangs and also goosewinging on his multi. Unfortunately the diagrams have not translated well to a .pdf but the text is ok.

http://www.multihull.com.au/site/www/pdf/pdfs/spinnakers.pdf

On a reach you will find the traveller (if fitted) an effective tool for depowering let the boom out on the traveller and flatten the main it works well.

Actually found a better copy of the article on Richards website. I'm a bit dubious about his thoughts on vangs actually as the situation he notes would not necessarily have been prevented by a boom vang because there is a limitation to how far you can let the main out on a fractionally rigged multi better to use a boom preventer if your concerned about a gybe occuring all standing imo. The consensus on sailing downwind is not shared by everyone some cruisers think that sailing square downwind is the way to go you really have to check your VMG to establish whether tacking downwind offers benefits on your boat and under what conditions.

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Re: Boom Vang

Postby Smooth Cruiser » Thu Sep 27, 2012 9:50 am

Agree - in a multi the width of available traveller really is more effective in allowing you control of the shape of the sail than a vang is. In a mono where the traveller, even when fully let out, doesn't always change the boom position much - you have no real way of holding the boom down when the sail is eased except by using a vang.

On a multi though you will find you often let the main down the traveller a metre or so when shy reaching while still keeping the sheet fairly tight on. This keeps the boom down and the sail flat. As you go more off the wind or want to belly the sail out you can then trim by easing the sheet.

So on a multi the boom angle from side to side, and hence the attack angle between the wind and the sail, is really contralled with the traveller while the boom angle up and down and hence the amount of twist and belly (or the "fullness") in the sail is controlled with the sheet. The width of the boat and hence width of the traveller track gives you this extra control and a vang isn't really required anymore. Having a vang on hard can make it harder to dump the main in a hurry and on a multi this can be a big concern so normally they are just left off.
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Re: Boom Vang

Postby Darren » Thu Sep 27, 2012 10:11 am

Any thoughts on the reverse vang thing that the FreeFlow 46 "Kiesha" has http://www.freeflowcatamarans.com/FF46.aspx
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Re: Boom Vang

Postby Corley » Sat Sep 29, 2012 10:12 am

I'll ask at the next club meeting one of the members has a Cirrostratus trimaran with the same boom system. The review of "Kiesha" in Multihull World does not mention any problems with the system.
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Re: Boom Vang

Postby Corley » Tue Oct 02, 2012 8:30 am

I asked Phil on the Boatdesign forums what he thought of the wishbone boom on his Robin Chamberlain design catamaran Kankama, he likes it, here is his reply.

Gday Corley

I love my wishbone and was happy to see Keisha fitted with a wishbone/boom hybrid. Interestingly Stanton used to own Songlines - the cat that got me onto wishbones.

As the original poster says - every normal mono has a vang so why don't cats. I think it is because beach cats don't and that the cabin gets in the way on larger cats. Add to that the extra loads of non heeling and a stable platform and it gets too hard for the engineers.

Kankama has had a wishbone for all her 12 years. In that time I have rebuilt her cockpit, added a new cabin, changed the steering twice, built a back deck and am in the process of building new dinghy davits. I would change the boom if needed but the wishbone is wonderful.

You get fingertip control of the main - no winch needed, you save thousands of dollars - no travellers and winches, no boom and gooseneck, increase crew safety - no boom and no traveller near fingers and you go faster. So you go faster, save thousands and its easier - should be an easy sell but it's not.

A wishbone makes downwind sailing faster as you can ease out the main fully and the sail swings out like a door with no twist. The sail is protected from chafe by the wishbone. Upwind you get less twist and so keep speed up too.

A couple of my friends came out sailing with me before they ordered their rigs. They liked the wishbone but felt it was too way out to put on their boats but for me the advantages are obvious and stark. You need a different rig setup. The three stay a side no spreader rig is required - this saves even more money in mast hardware. The lowers should be about 30 percent of the distance up the mast to handle the wishbone's forward thrust. I almost sold Kankama years ago but that meant I would have had to got a cat without a wishbone and I couldn't do that.

cheers

Phil

Here is a link to a youtube video of Phil giving a tour through his boat some neat ideas integrated there.

http://www.youtube.com/watch?v=XEiIQn6Ikls
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Re: Boom Vang

Postby Corley » Tue Oct 23, 2012 9:05 pm

The article on the Nathan Stanton freeflow design Keshi is available in the multihull.com.au .pdf articles free if anyone is interested in reading it.

http://www.multihull.com.au/site/www/pdf/pdfs/freeflow.pdf
Multihull news and my projects http://trimaranproject.blogspot.com.au/
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Re: Boom Vang

Postby sea_bee » Thu Oct 25, 2012 3:36 pm

I have been most interested to read the reports on the wishbone booms in those links, Corley.

They sound like an innovation worth considering, especially on a multi-hull.

Do you reckon the reports are a bit "one-eyed"? If they are really that good, why aren't they more widely in use? Just the conservative nature of the builders?
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Re: Boom Vang

Postby Corley » Fri Oct 26, 2012 9:50 am

I think conservatism does play a role but also consider there are a lot of cruiser/racers out there. By that I mean there are a lot of cruising boats with moderate performance that have scaled down racing rigs. The racing rigs obviously provide the best efficiency but it's not necessarily the most user friendly method for a cruiser. I'd say the wishbone is somewhere in between in efficiency and well ahead of the very short travellers used on some production catamarans that have limited adjustment(The Lightwave 38 comes to mind). You have a bit of extra drag from the stays and the wishbone itself and attached flow to the mainsail would be a little more interrupted by the fixed mast but I cant see any really large negatives in terms of performance.
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Re: Boom Vang

Postby 44c » Fri Mar 01, 2013 2:27 pm

Must cat's don't have vangs because they simply don't need them. You can adequately control boom lift by using the traveller and mainsheet. Vangs introduce huge loads to the mast, boom and gooseneck. It's actually not overly rare for monohull's booms to fold under the load of dipping a boom in the sea against the vang tension.
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