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Preventors do I need one

Oct 22, 2014
13,959
CAL 35 Cruiser moored EVERETT WA
Simply put a "preventor" is a line running from the boom to the boat. It is set to hold the boom out on the set side, restricting the forces, while sailing, from moving the boom across the boat.

One of the first questions that pops into my mind:
  1. Do I need one and why?
It depends.
  1. On the type of sailing you do,
  2. the way you rig your boat,
  3. the conditions you sail in
  4. and will an unplanned movement of the boom across the boat endanger the boat or crew.
What type of conditions are you talking about?
  1. Your on a downwind run in moderate breezes. You have the boom out to port and a jib or spinnaker out to starboard. The crew is young and enjoying the sun. They are staning up and moving about the boat. Some of the crew are new to sailing. NOW would be a prudent time to rig a preventor - or start warning your crew about the possibility of an unplanned gybe and the havoc it can cause should you get struck by the boom.
  2. A sudden unplanned swing of the boom may cause damage to the boat.
  3. Or you are out on 'Big Water', perhaps running in big waves. The conditions in this image and video...

Dipping the boom.JPG


Keeping the boom in place with a preventor may be a security issue. The fear is the boom might break as it dips into the wave.

Winter sailing in 'Big Water' can raise all sorts of new challenges. How do you rig a preventor. I prefer to rig mine on a long line from the end of the boom to a bow cleat. Running the line through a block and bringing the line back to the cockpit where I can control the boom position by adjusting the preventor and the mainsheet. This serves to fix the boom in place. The long line also provides a bit of a shock absorber for the boom.

Winter sailing offers fun new challenges that ask the sailor to adapt.

Have fun out there.:)
 
Mar 26, 2011
2,737
Corsair F-24 MK I Deale, MD
I find them most useful when running wing-and-wing. Often getting the genoa just right requires running slightly by the lee, and accidental jibes are not always violent in moderate conditions, but they are annoying (not on the F-24--on my last cruising boat).

Running wing-and-wing does not get a lot of respect, in part because all the racers seem to reach and on the ocean the rolling can suck. But often it is MUCH smoother and actually faster for slower cats and most non-planing boats. It just feels slow. Also good in rivers. Perfect for a cruiser on a breezy day that doesn't want to tear-ass along but that does need to get down wind.



I don't find them useful with a cruising chute, just more spaghetti. The wind is generally well to one side, and assuming something went very wrong, having the boom trapped on the wrong side could be even worse.

Mid-boom to stanchion base is a good way to break something in a blow. The angles are terrible and the forces huge. Not my opinion, the opinion of the world. Boom end to bow, with a release. For wing-and-wing, you can just bring the tail to the primary winch.

In big waves you can rig the boom a little high, since it will be reefed anyway. Safer too. Just leave the tack a foot or two above the gooseneck and secure with a cunningham.
 
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Likes: jssailem

capta

.
Jun 4, 2009
4,133
Pearson 530 Admiralty Bay, Bequia SVG
We run our preventer, not to be confused with a vang, from the boom end to the forward chock on the same side of the bow and then to a docking cleat. I use Dacron, not nylon.
 
Jul 1, 2010
794
Seaward 25, Catalina 350 Erie, Pa
On our Seaward 25, I run a single line from the cockpit up to a block at a stanchion base, back to a climbing figure 8 on the boom, and back up to an anchor point on the opposite side stanchion (furthest one up toward the bow). That way, I can tighten it up from a single point in the cockpit and tie it off at a stern cleat. It won't stop an unintended jibe, but slows it way down, and does keep the main where I want it for wing on wing sailing.

I was going to do the same thing on our Catalina 350 but never really felt the need. Maybe because of the longer traveler on that boat, not really sure. In higher winds, we're usually running on a reefed genoa alone anyway.
 
Nov 8, 2007
1,365
Hunter 27_75-84 Sandusky Harbor Marina, Ohio
We use a preventer on any long downwind cruising leg, to ensure against an accidental gybe. I undo the snap shackle at the bottom end of our Vang, then attach it to the toerail downwind, ahead of the mast. The only danger is that dipping the end of the boom at speed could bend the boom. So, if we are running in high seas (more than 4 footers) we rig the snap shackle at the top end of the Vang to the boom end. Or, we don’t use a preventer, and watch for a surprise gybe.

I‘m remembering one wing on wing run down the Detroit River on a northeast wind with our Asymetric, and main. Spectacular!
 
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Likes: jon hansen
May 25, 2012
3,317
john alden caravelle 42 sturgeon bay, wis
having control of your rig is wise. preventing uncontrolled jibes is obvious. you can kill people with a flying boom.

unlike all you guys, i have the original roller reefing boom set up on the 66' alden. no vang connected to the base of the mast. i have an eye bolt that screws into a plate on the side of my deck about a foot in from the toe rail. to vang the sail down i use a 4 to 1 block and tackle system that attaches to a wide strap that goes over the boom about 6' out from the goose neck. this old system doubles as a preventer. same on the mizzen. as the wind increases the first sail down is the mizzen when off the wind. getting the rig to weathervane better. the main mast is farther forward than a sloop rig on the same vessel. my center of effort is father forward sailing off the wind.
on displacement boats, Bethwaite teaches that it is faster to keep the sails reaching until such time that the winds will have you at hull speed in a run. speed = control! as mother nature pushes harder, i try to push harder back. i find the ride is better. obviously, one will have to back off when overwhelmed.
flying a spinnaker is my favorite thing to do. i have a very strong vessel with a very strong rig and a 1 1/2 oz. chicken chute that will take it all. when it's blowing 40, your rolling 10, the wind on deck is 30. let'er rip. the gear can easily handle the power.
a jibe with a uncontrolled boom will kill. not good.
i much prefer to sail with a crew. i do enjoy the opportunities that a manned vessel offers. 'MORE SAIL'

in lighter winds as well, with a crew working the deck the booms must be controlled.

i'm a safety geek. :cool:
 
Dec 25, 2000
4,675
Hunter Passage 42 Shelter Bay, WA
Always use one when downwind sailing, wing on wing. Snap shackle on each end of a double braid line; one end to the end of the boom, the other to the toe rail forward. I keep it stowed on the underside of the boom between uses.
 
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Likes: jon hansen
Jul 27, 2011
4,240
Bavaria 38E Alamitos Bay
I’m presently using a boom brake (Gyb’Easy). When I’ve used a preventer on the Bavaria, the line was attached at mid-boom, run f’wd to a turning block at the bow cleat, then aft to my spinnaker cockpit winch. But that’s a lot of “trouble” to rig and use. So in light conditions, it is stopped off directly to the mid-ship cleat.

Thus far, I like the boom brake better, especially if single handing. It’s a different idea. Instead of preventing an uncontrolled gybe it brakes it so the boom does not slam. Although the boom can move fast initially, it is braked at the end of the sheet length. The tension has to be adjusted properly, however. When single-handing, I can gybe the boat w/o having to control the boom by sheeting. Rather like using a self-tacking jib. I have not gybed the boat in wind stronger than about 15 kt apparent on a broad reach where, with the brake attached, the boom is allowed to swing over on to the new tack. (Still a relatively new piece of gear.) So, unlike with the preventer the boom WILL cross the midline and it can be moving fast depending on the tension adjustment. So, heads must always stay low, etc.:thumbup:
 
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Likes: jssailem
Oct 24, 2010
2,309
Hunter 30 Everett, WA
in most cases, If wing on wing, I put our traveler all the way out, which shortens the sheet. This way an accidental gybe only brings the boom partway across. It lessens the force during a jibe. In heavier air, I would rig a preventer or more likely go to a single headsail.
 
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dLj

.
Mar 23, 2017
1,358
Hunter 30 Snug Harbor, Lake Champlain
This conversation is another reason why I like a solid top rail instead of lifelines. That gives you a location to tie off all sorts of things, including preventers.

dj
 
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Likes: Will Gilmore
Oct 22, 2014
13,959
CAL 35 Cruiser moored EVERETT WA
That gives you a location to tie off all sorts of things, including preventers.
Yes it does. Assessing the strength of the connection (tie off) is paramount. Can it handle the forces that may be placed against the connection?

My opinion is that a preventor attached to the extended boom exerts forces that can destroy many of the connection points or the boom depending how it is set and what causes the force to occur. My worse case concern is a dip of the boom into the ocean under sail and strong wind. The forces are sudden and significant.

The selection of the bow cleat is purposeful.
  1. I know the cleat is properly back plated and is as strong as I can make it (well beyond a padeye or stanchion bases).
  2. The attachment point at the boom end is the strongest option on the boom.
  3. The long line and the angle from the boom end to the bow gives the best use of the line to buffer the forces and reduce the point forces at the connection points.
  4. I use a strong soft shackle to attach the block to the bow cleat. This is a quick and solid attachment.
  5. The block is a large enough block to handle the anticipated loads.
As this is a system there are fuses in the system that may fail. This is a designed structure. Points of failure provide ways to manage the risks associated.

These points of failure (fuses) are the block that turns the line from the boom at the bow. The line used. The connection of the line on the boom.

At one time before I explored the issues associated with the use of a preventor, I simply used the vang attached midships with a snap shackle to a padeye. It managed to hold the boom out against a gybe. As I sailed in heavy water and strong winds the set up started to concern me. I watched it working on the boat systems.

We each have to consider our boat and the conditions we are using the boat in. For many a bow preventor is way beyond the conditions that affect their boat use.

Presenting the information here is a way to share varying ideas and to enable all of us with a bit of fellow sailor knowledge the we can have available should our situation and conditions require.

Fair winds and enjoyable sailing to all.
 

dLj

.
Mar 23, 2017
1,358
Hunter 30 Snug Harbor, Lake Champlain
Yes it does. Assessing the strength of the connection (tie off) is paramount. Can it handle the forces that may be placed against the connection?
Working with tubular structures is quite interesting. Here's a blurb from a company that does these kinds of structures:

A Lighter, Stronger Alternative
Structural strength can come from many places—design, materials, manufacturing. But it can also come from basic physics. For example, the strength and simplicity derived from the circle over its square and rectangular counterparts.

While traditional structural shapes (square and rectangle) have a history of strength and a place in low-profile or low-voltage situations, their complexity and bulk can be unwieldy. However, tubular structures (circular) provide the same strength with significantly less weight, fewer bolts and lower costs.

Our designers and engineers have determined that tubular structures are especially beneficial for dead ends, those final structures that hold the largest loads. Tubular termination and dead-end structures also offer:
  • Up to 50% lighter weight
  • Generally simpler connections with fewer joints
  • Higher stiffness per pound
  • High torsional rigidity
  • No weak axis bracing
  • Fewer foundations and reduced number of members
  • Faster erection and assembly
  • Easier access
  • Reduces forces from wind
  • More flexibility in design
dj
 
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Likes: Will Gilmore
Oct 22, 2014
13,959
CAL 35 Cruiser moored EVERETT WA
I like the idea of a solid surround on a boat. Especially if going to the open sea. Watching the 2008 Global Challenge video of the deck hands being swept off the deck and washed against the life lines is eye opening. It can happen in a flash of a second. Not a place for the weak of heart.

It is not the strength of the tubular structure, it is the way the structure is attached to an FRP boat. I know my lifelines while light in weight and strong as a total system, gain that strength from the length of the lifeline and the multiple attachments to the deck. The attachment of a preventor to the base of one stanchion does not have the strength of the system. The attachment to the base of a solid set of rails is better than a lifeline stanchion as it is using the strength of the solid rails.

Without a solid rail system, the best option on my boat is the bow cleat.
 

dLj

.
Mar 23, 2017
1,358
Hunter 30 Snug Harbor, Lake Champlain
Yes it does. Assessing the strength of the connection (tie off) is paramount. Can it handle the forces that may be placed against the connection?
Now, let's take a quick overview of the concerns you've stated. You are worried about pulling out the fixation point. So, with a welded solid tubular hand rail instead of a life-line, you are creating a structure where a number of the attachment points exist. All of your stanchions are involved. If you take the bases of your stanchions and put backing plates under each, a practice often considered the better way to mount your stanchions, you have a solid point of contact for each stanchion. Now, when you then connect them all through a welded tubular top structure, you are now allowing each stanchion to have loading transmitted through it. Of course, the stanchions closest to the point of connection on that top rail will have a larger part of the load applied, but there will be loading transmitted to other stanchions nearby, and in any case you are transmitting load through at least two of those points of connection, in fact more, but for simplicity, we'll say two.

Now, you have the strength of the tube itself (something jackdaw pointed out in another thread here) and there you are looking to have the wall thickness of the tubing designed to withstand the loading you are designing for. This is pretty easy to do. Think about crane booms. Those are high strength structures made from tubular materials. There are many examples of tubular structures used in high strength applications. The top rail is an example where this type of structure is very well applied. I used to own a blue water boat built this way, I can't begin to tell you the added utility, strength, security that it adds to being on a sailboat going off-shore. I could just tie off to other boats in harbor using that rail - it gives you a continuous rail for attachment of lines, bumpers, whatever...

dj
 
Oct 22, 2014
13,959
CAL 35 Cruiser moored EVERETT WA
I would rather attach a fender to a solid tubular system than a wire lifeline. I would rather attach the fender to to padeye with backing plate rather than the stanchion base. Even the connection of a chainplate would be a stronger connection than a stanchion.

Many boaters seeking a quick and easy connection, the life lines are used with special clips. I do not favor such as the lifelines are not a convenience they are a safety fixture. I prefer to leave them as such.