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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 12-09-2006
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Vang/Cunningham

I am wanting to adding both a vang and a cunningham to my boat. Does anyone have a good system for a tackle vang with more that 4:1 purchase? If I could add an attachment point above the bail at the base of the mast I could make my 4:1 vang an 8:1 by adding another block and line, just not sure how to create a secure attachment point???? Does anyone have a good diagram (or source material) to rig the cunningham???

Thanks.
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Old 12-09-2006
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TC,

you got me confused here. As you know a cunningham pulls the sail down and is separate from the vang (boom jack??).

For the cunningham the sail must have another eye maybe 1' foot above the clew.

Is the vang rigid? Do you have a boom jack already?

I have a 8:1 cunningham, and can help you with that. For the vang, mine is just rope, no vang, its a simple boom jack, can help with mine if you need.

Maybe I am not understanding the whole thing. Sorry

HERE:

http://www.harken.com/rigtips/cuninghm.php

Last edited by Giulietta; 12-09-2006 at 03:51 PM.
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Old 12-09-2006
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They make boom vangs in both soft and rigid varieties, with purchases up to 8-to-1 IIRC. I'd look at the Garhauer vangs...they're quite nice, especially the rigid ones, and reasonably priced for essentially semi-custom gear.

I wouldn't use a bail, but a rigid fixture made for boom vangs... it usually wraps about a third of the way around the mast, so that the fasteners are in sheer rather than tension, which makes them much stronger at resisting the pull of the vang. To make a 4:1 vang an 8:1, you could add a block to the bottom attachment point and another to the lower-end of the vang.
This may or may not work well, depending on the design of your current vang.

As for the cunningham, that is a pretty simple thing to add. You can attach the lower end of the cunningham to either the boom or the mast... I prefer using the mast, as it puts less strain on the gooseneck.

IIRC, CS Johnson makes a nice kit that even has a "gauge" tape that you can put on the mast alongside the cunningham to see how much tension you've applied. I would go with at least a 4:1 block on the cunningham, but it really depends on the size of your boat.
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Old 12-09-2006
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G-

Yes, two diferent things. The boat needs both. I have (2) non-ridgid vangs now (not on the boat) both are 4:1 and would like to make at least 8:1. If I add another attachment above the bail (yet to be installed) and attach a line to it that runs up thru a block on the boom bail, then attachs to the vang I think will give enough purchase. Just need the extra attachment point on mast. ???

SD-

Thanks, will check into the cunningham kits.
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Old 12-09-2006
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T34C, If I have you right, I think what you'd end up with is a six to one purchase with the addition of the extra block at the mast. If you end up, somehow, at 8 to one you should be aware that each sheave added should be considered a 10% gain in friction under load. For instance: Your two-fold purchase, with a 4 to 1 mechanical advantage, has a theoretical power of four times the pulling power you apply to it. The actual power is 2.86 times the force applied to it, making your mechanical efficiency 72%. As the number of sheaves in a purchase go up the mechanical efficiency goes down. The easiest to observe illustration of this is your halyard. If your load to hoist on the halyard is 100 lbs the actual force needed is 110 lbs, making the halyard, a single whip block, 90% efficient. If you add a block at the head of your sail, and run the standing part back to the head of the mast, you have a gun tackle with a mechanical advantage of two, but your actual power is 1.67. Instead of taking 50 lbs to hoist the load it takes just under 60lbs. The gun tackle is 83% efficient. So, there is no free lunch.
Modern sailing blocks will yield a coefficient of friction less than the traditional 10% used for calculations, but friction losses still should not be ignored. Anyone who's fairlead his lines through multiple sheaves back to the cockpit will know the feeling of trying to heave in a line that they previously handled with ease on a straight pull.
Back to your attachment point question. Regardless of the number of parts in your purchase the following holds true; if the load on the tackle is 100lbs and you are heaving it in the load at the deck block in the vang case is equal to the 100lbs and the strain you are putting on it. For a 2 to 1 M.A. your load at the block would be 167lbs. I mention this because, when encountering difficulties in heaving in a line, the reaction is often to lead it to a winch. And with the high loads involved that could result in popping a block out of the deck and a head-ache. Make sure the attachment point, whether on boom for cunningham, or mast or deck for vang are secure. This usually calls for through bolting, not screws, and may well be the determining factor in how you rig either of the two.
The cunningham can be rigged to the boom, but you probably won't be happy with that arrangement unless a vang is rigged also. If you have padeyes at the base of your mast either, or both, could be rigged off of there with a block and jamb cleat.
Play around with a snatch block and shackle and you'll come up with the best layout for your boat, before you start drilling holes. Eight to one does seem like a lot of purchase, and will be prone to fouling, but I don't know your boat.
Good luck.
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Old 12-09-2006
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Sail,

I spoke about the 8:1 cunningham not T34, because that is what I have, but my boat is larger than his, and my sail is also "harder". My cunningham attaches to the mast at the base.

T34, a 4:1 Boom jack (assuming the blocks can handle the load) is enough for the boom jack. I have 4:1 into a winch. And don't want a rigis vang, just rope. Try one of the ones you have.
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Old 12-09-2006
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Sailaway- Thanks. Points well taken. The 8:1 by adding 1 block and extra line to the 4:1 tackle came from the Harken catalog??? (wouldn't be the first time they were wrong)

I am not wanting to lead the vang or cunningham to cockpit. Adjustment at the mast without a winch was the goal. (at least to start with) I was basing this off the fact that the mainsail on my boat is about the same size as my last boat and the 4:1 vang tackle just wasn't quite enough. With the mid boom sheeting arangement I have it is hard to move the purchase any farther aft to get better leverage.
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Old 12-09-2006
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To make a 4:1 an 8:1 with adding just one block you need to do a cascading tackle. You add the block to the dead end of the 4:1. Then you have a second line dead tied at the mast, running through the block on the end of the 4:1, then back down to the block at the mast where the original 4:1 ran. That doubles your purchase (less friction, but all purchases assume no friction in the calculation). The comes a point where adding purchase without going to a cascading system actually increases the power needed because of friction.

Charlie
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You can get to 8:1 by adding a block-it just depends on what you have already with your two-fold purchase. If you clap-on a gun tackle, that is two single sheave blocks with the load block floating (get it?), to the hauling part of your two-fold purchase (4:1) you will have an 8:1 mechanical advantage at the hauling part of the gun-tackle. This would be done by rigging your two-fold purchase (known as a Burton tackle) to the boom and to the deck or base of the mast. Now you take the hauling part of the Burton, cut it to proper length, and splice it to the becket of a single sheave block. Now take another block, secure it to the deck or base of mast. (This block should have a becket also, but you could get by without by securing it's line to a padeye.) Run a line through the block at the base of the mast, up to and through the block you have spliced on to the Burton and back down to the becket (or deck) of the block at the mast base. You now have an eight-fold purchase. In essence, you are clapping on a tackle on a tackle.

Obviously, you'll have to lay all this out quayside and get your leads right, but you may actually have some of the fixtures required already in place and that may be the way Harken was perceiving it.

The American Merchant Seaman's Manual, by Cornell Maritime Press, does a decent job of explaining block and tackle as does Knight's Modern Seamanship, by Van Nostrand-Reinhold. Both are worth owning, even if Knight's is edited by a couple of retired Navy officers. Knight's does have the advantage of illustrating the splicing of 2 in 1, Sampson power braid, as well as eight-strand plaited line. BTW, eight strand is overlooked for many uses. It offers superior abrasion resistence, less heat build-up, to other constructions of synthetic line and is far easier to splice than 2 in 1. It's weakness is that it does not reeve well through sheaves. Just so notes from an old rope choker.
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Old 12-09-2006
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SD-

Having trouble locating cunningham kit. Doesn't seem to be a part from Johnson. Any other ideas?
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