Making a gybe preventer

beancounter

beancounter

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I'm about to fit one to Stargazer . Previous posts I've looked up have been very helpful, apart from one point - how thick/strong should the line from the boom be? I seem to remember a comment somewhere that the system should be capable of giving way if things go really wrong, so too beefy a line would presumably not be a good idea.

What size line do you good folks fit?

Thanks
 
I use one of my spring lines, 14mm three strand. I don't go along with the idea of expecting the preventer to break, others may. Lead the preventer forward to a block attached to a strong point and then back to a cleat at the cockpit so you can release it when required.
 
I made mine for a Sadler 26 with 10 mm double braid. I am really only looking to avoid the accidental gybe resulting from swell or a random gust - not a full powered backed main. I have a line permanently attached from the end of the boom to an elasticated hook at the top of the kicker. The preventer is then clipped onto the line on the boom and runs via a block on the foredeck to a cleat on the rear of the cabin roof so I can release it from the cockpit if I need to gybe in a hurry.
It works well but is a bit messy if you do have to gybe without removing the preventer first.
Morgan
 
The angle made between the boom and the preventer always seems frightenly small, even leading the preventer as far to the front as possible. It seems always as if, however tight the preventer is made, it would not take much backward pressure on the boom to cause enough stretch to let the whole thing come round.

Has never happened to me, just looks as if it might...

(This on a sigma 362)
 
On Spring Fever (26' Albin Vega), we have preventers permanently rigged: - Two lines (8mm 'springy' cords) are secured to the back of the boom on a large snap-shackle; these lead forward to small blocks snap-shackled to the bottom mountings of the rear-lower mast stays, then lead back to the cockpit, when not in use the lines beneath the boom are clipped into two large plastic C clips, which are tap-screwed to the underside of the boom otherwise they're always in the helmsmans way. This system has taken us to the Western Isles and down to the Med via the outside route, proving its worth regularly and in a couple of instances perhaps life-savingly so.
 
If you run it outside the shrouds then a gybe would be impossible. Or am I mistaken? I suppose it depends how far forward you've got your boom.
 
I would'nt want it to break, but I would want to release it very quickly if necessary. I generally use a long warp tied with a very long bowline to the boom (so I can untie it without stretching overboard) going forward to the foredeck cleat and then back to a winch via the cruising chute block. keep it on the self tailing winch so it can be released quickly
 
Our is a similar set up to Morgan's. I followed some advice on a previours forum discussion and it works very well (on our 35'). The preventer itself is a used halyard (12 mm double braid) with chunky snap shackels at either end. It is very easy to swap sides, without going forwards, during a gybe, it can be done in a few seconds as the boom is "stalled" midway over the cockpit. As far as strength and angles are concerned, my impression is that at the beginning of a gybe there is not a huge force pushing the sail back: problems begin when the boom and sail start swinging and accelerate across the cockpit and crash onto the other side. If the reventer is held reasonably taught, this acceleration does not start and you don't have too many problems. Maybe we have just been lucky, but the preventer has surely made for a much more relaxed donwwind sailing: we have it prepared whenever thereis any chance of wind coming anywhere from the back.
 
I just did a quick scale(ish) sketch. I will try it tonight on the boat with tape measure but for now...

If the boom is 30 degrees back of midships (any further forwards and the sail lays hard against the trees) then the preventer is about 9m long. It would only have to be about 9.5m long to have the boom right on the long axis of the boat. Once it gets there its going on round, with the stretch in the "preventor" pulling it round, at least at first. That is only about 7% stretch, isnt it?

Maybe I should carry the boom further out when running and not worry about wear on the mainsail.
 
That is the sort of set-up I was envisaging - a previous thread suggested using a permanently rigged line as far as the kicker, to which the preventer is shackled. Sounds a good scheme to me.
 
Another system is that which I used on Adriana, and is seen a lot on American boats: The boomvang is a tackle, like a mainsheet, with the tail leaving the lower block and running back to the cockpit for cleating. It has a snap shackle at each end, one connecting it to the boom bail, the other to the base of the mast. When running I unclip the vang from the mast and attach it at the toe rail in line with the mast, and crank down on it firmly, cleating the tail back at the cockpit. It has the benefit of pulling the boom down and flattening the main more effectively than when connected at the base of the mast, and it also acts as a preventer. You can have two such tackles set up, one for each gybe, if you prefer. I found this system worked very well and in three years of continuous cruising it never caused concern, even in heavy weather.
 
Many years ago, doing a Yachtmaster course on a Contessa 32, the instructor showed us the technique of using the kicking strap as a preventer. All went well until the navigator (who hadn't seen the 'preventer' being rigged) came up from below, decided that the sail was chafing on the shrouds, and pulled in on the mainsheet. There was a loud bang as the gooseneck casting snapped.

My worry would be the possibility of this happening in a near-gybe. Bearing in mind the lever effect of the boom, what loads are likely on the gooseneck?
 
I think the load on the gooseneck is less with this arrangement than with a line from the end of the boom forward. With the vang going directly down to the deck from the bale to the toerail there is almost no lateral componant tending to push the boom towards the mast. Of course it will be different for different boats, length of boom etc.
If you can find a copy of "The Best of Sail Trim" edited by Charles Mason there is a great picture on the cover of a French racing boat, a big Swan, using the arrangement - and it's a great book.
 
I have always used a 3 strand 12m nylon mooring line or similar as it's readily available, from the boom end to a convienent point forard, deck cleat, or whatever depending on the boat, and back to the cockpit or cabin top so that it can be realeased without going forard.

The loads on it shouldn't be too great at it's only there to prevent the boom swinging in a swell or a small windshift from getting fully into the 'back' of the sail. I don't look at a preventer as holding the boom in a gybe, just helping to ensure that a gybe doesn't happen.
 
Loads on a preventer

Not that big, usually...until you dip the end of the boom in the water, when rolling, at which point you will know all about it.

I prefer to tack downwind; it is probably faster and adds very little to the distance sailed.

Mind you, my boat has a 21 ft boom...
 
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