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Old 22-08-2016, 05:15   #1
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Vang Preventer: Advise where to position

I have decided the best way to control my boom is with a vang preventer.

Drawing here:

http://www.goodoldboat.com/images/articles/Top-View.gif

Article here:
Good Old Boat - Vang/preventer: article

The application is to control a 20 ft long boom with a 400 square foot sail.

Questions:

#1 Notice in the figure the attachment point on the deck is in front of the attachment point on the boom. Is there a good reason to do this? I would think this would cause a force towards the mast. This would tend to push the base of the mast off a rotating ball joint. For least forces on boom and mast wouldn't having all in line be best?

#2 I intend to lead lines back to my cockpit where they will wrap around two winches. My concern is these are rather big winches from when the vessel had a much larger sail plan. Any thought about how to prevent a crew member from over cranking on winch and damaging boom or ripping sail?

#3 I read where there was potential boom twist caused by a system like this. If I use a rigid eye bolt at the bottom of the boom I can foresee a large force on the gooseneck. If I use some sort of soft shackle around the boom, potentially this could eliminate the twisting moment. Thoughts?

PS I have a pilot house with a very wide free span. This means I would not want a traveler on the pilot house for fear it would lift it. The deck area behind the pilot house is not flat.[IMG]www.goodoldboat.com/images/articles/Top-View.gif[/IMG]
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Old 22-08-2016, 05:44   #2
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Re: Vang Preventer: Advise where to position

Quote:
Originally Posted by pbmaise View Post
I have decided the best way to control my boom is with a vang preventer.

Drawing here:

http://www.goodoldboat.com/images/articles/Top-View.gif

Article here:
Good Old Boat - Vang/preventer: article

The application is to control a 20 ft long boom with a 400 square foot sail.

Questions:

#1 Notice in the figure the attachment point on the deck is in front of the attachment point on the boom. Is there a good reason to do this? I would think this would cause a force towards the mast. This would tend to push the base of the mast off a rotating ball joint. For least forces on boom and mast wouldn't having all in line be best?

#2 I intend to lead lines back to my cockpit where they will wrap around two winches. My concern is these are rather big winches from when the vessel had a much larger sail plan. Any thought about how to prevent a crew member from over cranking on winch and damaging boom or ripping sail?

#3 I read where there was potential boom twist caused by a system like this. If I use a rigid eye bolt at the bottom of the boom I can foresee a large force on the gooseneck. If I use some sort of soft shackle around the boom, potentially this could eliminate the twisting moment. Thoughts?

PS I have a pilot house with a very wide free span. This means I would not want a traveler on the pilot house for fear it would lift it. The deck area behind the pilot house is not flat.[IMG]www.goodoldboat.com/images/articles/Top-View.gif[/IMG]

1. When acting as a preventer on an eased boom, rather than as a Vang, you want the preventer to be pulling the boom towards the bow.

When used as a vang, you have load on both sides of the boom so the twisting moment on the gooseneck should not be very high,

If the attachment points were in line, then as soon as you eased the boom, the pull would be towards the stern.


2. If you are leading to winches, don't use a high multi-purchase system such as shown in the article. That will also mean less line movement required to adjust the system.

3. I would imagine that a soft shackle that can slide around the boom would definitely provide less turning moment on the boom that a fixed attachment point under the boom. Probably a good idea.
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Old 22-08-2016, 05:57   #3
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Re: Vang Preventer: Advise where to position

To better assist you, a bit more information on both the configuration of the rig, as well as what you're attempting to do would be helpful. Because it sounds as if you have a rotating spar? And I'm not certain if you're trying to just rig a Preventer, or a Vang as well as a Preventer?

When you're rigging a Preventer, you usually want to connect it as close to the aft/outboard end of the boom as possible. As it makes for better control of the boom via geometry, as well as mechanical advantage. And you're far less likely to damage the boom, or your Preventer gear. Since the loads on both will be lower, the further outboard the connection point to it is.

On monohulls, unless the boom is grossly over strength, it's not uncommon to damage it, if a Preventer is attached at the same point as the Vang. Particularly if the outboard end of the boom strikes the water when the boat is moving at speed, & the Preventer holds it in place against the water flow.

However even without a high speed immersion, if the boat accidentally gets caught aback with the boom held out by a Preventer attached so far forward, a very large load is placed onto the boom in a location which isn't designed for it. As the load in such circumstances can be, & often is, several times that which would be on the mainsheet were the boom to crash across in an accidental jibe. And a Preventer rigged like this when an accidental jibe happens is one of the most common causes of boom breakages.

Vangs are better setup seperately from Preventers, & used for trimming the sail. Not so much for fixing the boom in place when sailing deeper wind angles. And the following is a common sense based guess. But if you have a rotating spar, then a rigid Vang may be the best way to go. One which is connected to the mast near it's base. That way the Vang wouldn't be acting to try & move the base of the spar off of it's rotator. And it would move with the spar & boom. Though hopefully someone who's rigged such will chime in on this.
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Old 22-08-2016, 07:03   #4
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Re: Vang Preventer: Advise where to position

Quote:
Originally Posted by UNCIVILIZED View Post
To better assist you, a bit more information on both the configuration of the rig, as well as what you're attempting to do would be helpful. Because it sounds as if you have a rotating spar? And I'm not certain if you're trying to just rig a Preventer, or a Vang as well as a Preventer?

When you're rigging a Preventer, you usually want to connect it as close to the aft/outboard end of the boom as possible. As it makes for better control of the boom via geometry, as well as mechanical advantage. And you're far less likely to damage the boom, or your Preventer gear. Since the loads on both will be lower, the further outboard the connection point to it is.

On monohulls, unless the boom is grossly over strength, it's not uncommon to damage it, if a Preventer is attached at the same point as the Vang. Particularly if the outboard end of the boom strikes the water when the boat is moving at speed, & the Preventer holds it in place against the water flow.

However even without a high speed immersion, if the boat accidentally gets caught aback with the boom held out by a Preventer attached so far forward, a very large load is placed onto the boom in a location which isn't designed for it. As the load in such circumstances can be, & often is, several times that which would be on the mainsheet were the boom to crash across in an accidental jibe. And a Preventer rigged like this when an accidental jibe happens is one of the most common causes of boom breakages.

Vangs are better setup seperately from Preventers, & used for trimming the sail. Not so much for fixing the boom in place when sailing deeper wind angles. And the following is a common sense based guess. But if you have a rotating spar, then a rigid Vang may be the best way to go. One which is connected to the mast near it's base. That way the Vang wouldn't be acting to try & move the base of the spar off of it's rotator. And it would move with the spar & boom. Though hopefully someone who's rigged such will chime in on this.
1. Yes I have a rotating mast. The attachment of a rotating mast is primarily in compression and so that is why I am concerned by lateral forces caused by this running rigging.

2. Essentially I understand this is called a two sheet control method. Here is a picture of attachment at the end of the boom.

http://www.boatdesign.net/forums/att...raveller-2.jpg

The vessel above has a very high boom and no pilot house in the way of the lines.

3. What I am striving for is more of a midboom control as seen in this photo.

http://bwsailing.com/bw/wp-content/u...er-300x250.jpg

Here a single sheet appears to control the position of the boom both horizontally and vertically. Although I don't see how it can accomplish this feat.

What I like about this control is the simplicity and load is directed to the base of the mast instead of the deck.

What I don't like is the fact there is no preventer. Or perhaps the two fixed lines in this design act like a preventer.
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Old 22-08-2016, 07:19   #5
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Re: Vang Preventer: Advise where to position

Quote:
Originally Posted by pbmaise View Post
3. What I am striving for is more of a midboom control as seen in this photo.

http://bwsailing.com/bw/wp-content/u...er-300x250.jpg

Here a single sheet appears to control the position of the boom both horizontally and vertically. Although I don't see how it can accomplish this feat.
The rigid vang controls the boom vertically, the sheet only controls the boom horizontally.
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Old 22-08-2016, 07:46   #6
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Re: Vang Preventer: Advise where to position

On some boats there's the Vang option that some of the 12m's used. Where there's a hemispherical track bolted to the deck, with a traveler car on it. And an adjustable vang, vertically in between the car & the underside of the boom.
Also, needs be, you can add a track & car to the boom's underside, to compensate for percieved boom length changes as the boom swings out/pivots.

They use similar setups on some smaller classes of boats as well.
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Old 22-08-2016, 07:54   #7
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Re: Vang Preventer: Advise where to position

Ideally, a "Preventer" (to prevent an uncontrolled jybe) should run from as near the outboard end of the boom as possible so that, in the event the yacht rolls down and one pokes the "prevented" boom in the water, the preventer carries load caused by the flow of water across the boom/foot of the sail. A mid-boom connection allows the load to bend the boom about the point of connection, which it is not designed to do, and has caused more than a few broken booms. The preventer should pass through a turning block as far forward of the mast as practical on a given boat to keep the angle between the boom and the preventer as great as possible to minimize induced compression in the boom.

The function of the vang is to minimize vertical lift in the boom to control that shape of the leach of the sail. A double vang led to the rails rather than the mast base can accomplish that but cannot effectively function as a preventer without risk to the boom in the manner mentioned above.

FWIW...
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Old 22-08-2016, 08:38   #8
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Re: Vang Preventer: Advise where to position

Decades ago I rigged the boom vang as a preventer for an offshore trip. The autopilot had a failure, the boat jibed, the boom from the vang to the mast didn't jibe, the rest of the boom did. I then had a boom with a 180 degree bend in it while on the way to Hawaii.

The boat had end boom sheeting, so probably was not designed for the bending loads of mid boom sheeting. The load point being at the vang made it even worse as a bad decision.

It was a Cal 34 with a 14 foot boom.
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Old 22-08-2016, 11:07   #9
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Re: Vang Preventer: Advise where to position

I much prefer a rigid vang for safety as well as sail control.

As for a preventer I have used successfully a "Walder Boom Brake" and also on my W32 a rubber snubber designed for this purpose. They really control the jibe to a slow one..
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Old 22-08-2016, 11:52   #10
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Re: Vang Preventer: Advise where to position

Have you done much reading on this topic in older posts here? Such as this one for example Preventer Rigging?
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Old 22-08-2016, 12:56   #11
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Re: Vang Preventer: Advise where to position

Quote:
Originally Posted by svHyLyte View Post
Ideally, a "Preventer" (to prevent an uncontrolled jybe) should run from as near the outboard end of the boom as possible so that, in the event the yacht rolls down and one pokes the "prevented" boom in the water, the preventer carries load caused by the flow of water across the boom/foot of the sail. A mid-boom connection allows the load to bend the boom about the point of connection, which it is not designed to do, and has caused more than a few broken booms. The preventer should pass through a turning block as far forward of the mast as practical on a given boat to keep the angle between the boom and the preventer as great as possible to minimize induced compression in the boom.

The function of the vang is to minimize vertical lift in the boom to control that shape of the leach of the sail. A double vang led to the rails rather than the mast base can accomplish that but cannot effectively function as a preventer without risk to the boom in the manner mentioned above.

FWIW...
Aft end of the boom, led as far forward as possible. That is critical.
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Old 22-08-2016, 13:09   #12
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Re: Vang Preventer: Advise where to position

A rotating mast introduces a a completely different set of directional forces at the gooseneck. Does your boat have a positive mast rotator assembly to control mast bend ?
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Old 22-08-2016, 13:33   #13
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Re: Vang Preventer: Advise where to position

Have twin vangs at about 2/3rds of the way out on a 15' long boom. The deck attachment point is an old padeye left over from some forgotten piece of sail trim gear. To get the vang as far aft as possible and still clear the dodger the boom mounting point is forward of the deck mounting point. When the boom is centered, the vang makes a about a 45 degree angle at the boom attachment point. Not a big thing as this angle is quite a bit less when the boom is swung out. The deck mounting point is centered in the walkway which I have questioned. There is considerable upward pull on the deck fitting and can feel it lift the deck when the mainsail slats. May bight the bullet and move it outboard so the upward load will be taken more by the hull and much less by the deck. One advantage of being inboard, the vang doesn't impinge on the lifelines till the boom is swung quite a ways out do don't have to move the vang inboard/outboard of the lifelines.

The vang is a 4 part tackle led to the cockpit. The tail is secured by some light weight Spinlock PXR cam cleats. Went with these small cleats as a possible weak link should the boom dip in the water and possibly release the tension. Just a theory that has never been tested. It takes considerable strength to vang the sail down and need to use a winch to get ideal flattening of the sail in trade wind strength breezes. Might think about going with 6-1 or more tackle if you've got a long boom/bigger boat.

The vang system worked fine on a moderate trade wind sail to Hawaii. One big plus at the dock/anchor is the boom is firmly held in position with both vangs cinched up. Can position the boom outboard if it needs to be out of the way or firmly centered for a secure handhold. Have no illusions about the possible danger to the boom should a roll of the boat seriously dip the boom in the water. Fortunately boat is not a roller and never came close to getting the boom end wet. Have forgotten to release the vang occasionally when jibing/tacking and it's really nice to be able to ease the boom across with the vang.

Never seriously considered a mast to boom vang. The boom is quite low to the deck and long so the resulting acute angle at the boom attachment point would have seriously high loading. The PO had installed a 4 part tackle secured by a two fastener padeye at the mast. The padeye fasteners pulled out of the mast in the first windy weather encountered.
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Old 22-08-2016, 14:45   #14
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Re: Vang Preventer: Advise where to position

This is the kind that leads to boom breaking up.

Unless you have VEEEEEERy strong boom. Or else if you just sail inshore.

And if you put the ends on the winches, you render the winches useless for other purposes.

This works but you want to look at your deck and gear to find if you can perhaps improve on the plan.

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Old 22-08-2016, 15:53   #15
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Re: Vang Preventer: Advise where to position

have used a diy boom brake for over 20 years/its become sailing habit to loosen the boom brake clutch on port side then pull the rope through the starboard clutch making sure to lock the other clutch back in and vice versa/we are much smaller boat with boom roller furling the brake take off points are either side of the main sheet truneon on the end of the boom then through turning blocks on either front cabin corners the cabin front is 3 '' reinforced huon pine bulkhead then back to the clutches either side of the cabin roof in the cockpit /works well/ the load back against the mast is equalled by twin inner forestays that pull on the lowers and apply pressure back on the mast base
short handed with low boom wen things go wrong the boom brake is a life saver.
we use double braid lines on the boom break of slightly lower capacity than the fittings (all ronstan )
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