Mainsail preventer line

Caladh said:
Will fit one imminently but wondered what size of line I should look at. Yacht is 39’ will fully battened main. Would 10mm line be ok?
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Whatever you use, make sure it doesn't stretch.
Personally I would probably go with a thinner Dyneema line. Maybe 8 mm. There should not be a huge amount of load on a preventer. It really just needs to prevent the boom from starting to swing. If there is a lot of load on it, you have already gone FAR by the lee and really gybed.
Typical setup is to attach it to the end of the boom, run it forward along the boom and make it off somewhere near the gooseneck or vang. When you want to use it, it gets connected to another line (also non-stretch) that runs forward to a block near the bow and back to the cockpit where it gets put on a winch or clutch that can be released under load. Winch is much better.
 
Whilst I agree the main purpose is to prevent the boom gybing before it can gybe, the spec must consider an uncontrolled gybe , and a 10mm line may not be enough to restrain the boom though much depends on the anti-gybe system, knots and hitches in the system, and type of rope you have. Worst case shock load is prob the best spec. 8mm always seems a bit difficult to grab effectively.

There may be case for stretchy nylon to absorb shock.

I have braid on braid nylon 10mm on a 32ft boat, running through a Figure of 8 alloy brake .
 
Caladh said:
Will fit one imminently but wondered what size of line I should look at. Yacht is 39’ will fully battened main. Would 10mm line be ok?
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10 mm braid on braid is what I use on my 39 with a fully battened main. Amply strong enough. As has been said, the point is to prevent the boom to start its swing and build up momentum.
 
bbg said:
Whatever you use, make sure it doesn't stretch.
Personally I would probably go with a thinner Dyneema line. Maybe 8 mm. There should not be a huge amount of load on a preventer.
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Careful with dyneema, our kind of loads it's basically non stretch so you can get massive shock loads very quickly even with a 'not very exciting' gybe. Needs a little stretch in there somewhere. I use abseiling dyneema which has a little bit of give in it, lots more than the likes of SK75 etc.
 
GHA said:
Careful with dyneema, our kind of loads it's basically non stretch so you can get massive shock loads very quickly even with a 'not very exciting' gybe. Needs a little stretch in there somewhere. I use abseiling dyneema which has a little bit of give in it, lots more than the likes of SK75 etc.
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Yeah I was most firmly taught by a number of Ocean sailors that some stretch is necessary to prevent rig damage. Especially gooseneck.
 
duncan99210 said:
I used to use an ex abseil rope, 12mm. I now use a boom brake using 12mm braided polyester. Takes the energy out of the gybe without the hassle of re-rigging the preventer on the opposite tack.
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But on our boat it would be a line you’d have to climb over every time you walked up the side deck. I’d rather rig a proper preventer. When we’re rolling along down wind, I’m confident that there will be NO GYBE of the main almost no matter what a novice crew does.
 
I work on the basis a preventer does more than help stop the boom wobbling around in light airs and chop. For that purpose, any old stuff will do. We want to "prevent" an uncontrolled gybe.

I use hollow braid dyneema permanently spliced, with brummel lock (the lack of which was a critical failure in one Clipper accident) to the boom-end and stored to a piece of bungee at the kicker fitting. Same either side for convenience.

Then attach that to another low-stretch line taken forward to the bow cleat and back to cockpit.

I prefer low-stretch. I pre-tension the preventer using the mainsheet and so I reckon any shock loads are well below what the gooseneck suffers in a controlled gybe. My concern about stretch is that (given aft-swept spreaders and consequent sheeting angle) it doesn't take much movement of the boom to make the preventer angle rather acute and therefore ineffective.

I accept the risk associated with boom end hitting the sea and mitigate this by reefing, if it's that sort of day.
 
My preventer is clipped on the pushpit when not in use, and goes outside everything to a block at the bow just short of the forestay and back to a cleat in the cockpit. In use, it gets clipped to the end of the boom with the mainsheet, It normally lives on the starboard side, so has to be taken round the mast on starboard tack, but the furler line's to port, so I live with it. In use, I keep it taut, so I'm not too bothered about elasticity.

It also doubles as a line to clip onto my mooring if I have to pick up from the cockpit due to high winds. Grab the pickup line, clip on the preventer and let go. I sit a boat length back from the mooring, but I'm safely attached, even if, as once happened, it took me 20 minutes to pull in the line to get the mooring chain over the samson post because I could only gain a foot at a time in lulls.
 
I use a 12mm polyester ex No3 Reef Braid on Braid. It runs from the dedicated fitting on the boom end cap, tied with a bowline, through a snatch block on the bow, down to a midship cleat. It works for me and is simple and quick to rig. Boat is mast head rigged 41’ Bermudian rigged, fully battened main. The purpose of the bowline at the boom end is to prevent any wear on the cast fitting as I have seen shackles cut a slot into these grey cast fittings; other knots would be better from a strength perspective.
 
I have a heavyish 35 foot sloop. I use a 10mm preventer of halyard braid on braid clipped as needed to the boom end and run through a block at the bow and back to the winch. On two Atlantic crossings, one singlehanded, heavy conditions and no doubt my poor management resulted in occasional gybes against the tightened preventer. Really no problem to retrieve the situation after this happened and no feeling that damage was likely even in winds of 45kt. I would be confident using the 10mm line. Using thinner Dyneema doesn't seem a good idea to me - not enough give and I also like to be able to hand haul on the line when setting up.
 
I tried this method and didn't like the permanent clutter on the boom or the opportunity for a joining knot or shackle hooking up on the shrouds. I now simply sheet into boom for long enough to be able to clip on the preventer and let out the sail to its normal position. In my opinion it is important to tension up the preventer with the winch and then harden in a bit on the mainsheet too.
 
Caladh said:
All great info from all of you. If I run a line from the boom end to the goose neck (bungeed to store) and then attach to a preventer how do you attach the two lines together when needed?
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Anyway you like. I have an eye spliced into the dyneema; I use a snap-shackle to secure it to the line led forward (which is thicker dyneema ex-sheet for handling)
 
Ps one advantage of using thin dyneema as the "permanent" bit is that it doesn't dangle down under its own weight and garrot you. As clutter goes, it's minimal
 
Caladh said:
All great info from all of you. If I run a line from the boom end to the goose neck (bungeed to store) and then attach to a preventer how do you attach the two lines together when needed?
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I have a soft eye in the 12mm line that goes along the boom from the outboard end. From this I have 1 metre of thin line (about 6mm) that allows me to make it off to a small cleat on the side of the boom near the gooseneck. So easily undone without leaning along the boom. The eye of the 6mm extension is through the eye of the main boom line.

When I need the preventer, I undo the little line from the cleat and take the line/s down to the rail where I have previously clipped the snap shackle on the end of my long preventer line . I take the line off the guard rail and clip to the soft eye of the line coming from the end of the boom.

I have a line on each side of the boom.
If this does not make sense I can find a photo...

TudorSailor
 
If you sail a crewed boat, with watches etc, make sure everyone knows when the preventer is actually rigged, and how.
Should muscled crew begin to enthusiastically winch in a well loaded mainsheet, say with fresh wind, there is all sort of risk, from snapping the line to breaking the boom in two.
 
bbg said:
... It really just needs to prevent the boom from starting to swing. If there is a lot of load on it, you have already gone FAR by the lee and really gybed...
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This is mostly true but I have had the main taken aback by a rather sudden (to put it mildly) blast from somewhere I wasn't expecting and am pleased to say that the boom stayed nicely in place and the rudder had sufficient authority to defeat the gybe and bring the wind back to the correct side of the main. So I think the line should be strong enough to take that: it was F8 (but less apparent of course) on a rather big fully battened main with a decent roach on a 42' boat. I used 12mm braid-on-braid polyester, but probably for no better reason than I had it to hand (ex spinnaker halyard).

To calculate the force: I have never not been able to pull in the mainsheet. I can exert 70kg force maybe and have an 6:1 purchase. That's 6 x 70 = 420kg. Looking at 12mm bread-on-braid its BS is about 3200kg, so plenty of margin for knots etc.
 
Stemar said:
My preventer is clipped on the pushpit when not in use, and goes outside everything to a block at the bow just short of the forestay and back to a cleat in the cockpit. In use, it gets clipped to the end of the boom with the mainsheet, It normally lives on the starboard side, .
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I do the same except I have a preventer on each side of the boat with a hook on it so that when I gybe, I can slacken the preventer, haul the main in, disconnect the old preventer, clip on the new preventer, complete the gybe and then set everything up without leaving the cockpit. The preventer is also used to keep the boom out at anchor to minimise shade on the solar panels on top of my wheelhouse/bimini
 
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