Preventers

[Dockhead]

Agree with all of the above.
Interestingly, I recently read one of John Kretschmer's books and he advocates for a degree of elasticity in a preventer. He generally knows what he is doing but on this occasion I find myself disagreeing with him.

John goes pretty deep into the technical reasons why you do not want elasticity in the preventer.

Besides the forces generated by allowing momentum to be generated, as the preventer stretches, the angle gets worse.

John also shows how you can end up with the preventer wrapped around a shroud, threatening to bring the rig down.

I find all this very convincing, but I have always felt that you want them to be as strong as possible and with no stretch. I have used dyneema for my preventers for ages.

 

[NormanS]

Maybe not fatal, but it's only 3/4 as good as attaching it to the boom-end. Much better than mid-boom, however.

You'll put less stress on boom and gooseneck, if you take it all the way to the end.

Well maybe I sail differently, but when I rig my preventer, it is solely as prevention, which means that the boom doesn't get started to move. There is never more tension on the preventer than that applied by myself. I find it particularly beneficial when running downwind when the seas are bigger than justified by the wind strength, when a roll might induce the boom to swing over. The boat, as designed, has the mainsheet attachment about 3/4 along the boom, and of course the load from the four part mainsheet, can be a lot, but the relatively heavy boom section is more than adequate.

 

[Dockhead]

Well maybe I sail differently, but when I rig my preventer, it is solely as prevention, which means that the boom doesn't get started to move. There is never more tension on the preventer than that applied by myself. I find it particularly beneficial when running downwind when the seas are bigger than justified by the wind strength, when a roll might induce the boom to swing over. The boat, as designed, has the mainsheet attachment about 3/4 along the boom, and of course the load from the four part mainsheet, can be a lot, but the relatively heavy boom section is more than adequate.

Well, that's a bit like saying -- the way I drive my car, the seatbelts are never tensioned enough to lock the inertial reels.

The preventer is designed to save you in case something non-normal happens and the main gets backwinded. It needs to the strong enough for that. Not just for your usual calm sailing.

In 16 years with my present boat and tens of thousands of miles, I've never once tested the strength of my preventer system.

But just last December I was doing a difficult passage around Hatteras along the U.S. East coast, on a Discovery 67, and -- not on on my watch! -- we got sideways in very rough seas, despite the best efforts of the pilot, the preventer broke, and we had a horrendous jibe, fortunately no one killed, and the boom survived somehow, against odds. So for me, it's not a merely theoretical question.

3/4 is vastly better than 1/2 and maybe is fine. Maybe.

 

[dunedin]

Re attaching to the boom end, my mainsheet is attached about 3/4 of the way along the boom, and the preventer is attached to the same point. Anybody see anything wrong?

As ever ...... it depends.

What generally breaks the boom is a mid boom preventer PLUS dipping the boom end in the sea. Not a gybe oer se.
So depends on boom height above water, and how big waves you are sailing in. Not recommended for an Atlantic crossing but probably ok for coastal sailing.

 

[Dockhead]

As ever ...... it depends.

What generally breaks the boom is a mid boom preventer PLUS dipping the boom end in the sea. Not a gybe oer se.
So depends on boom height above water, and how big waves you are sailing in. Not recommended for an Atlantic crossing but probably ok for coastal sailing.

Yeah, but dipping the boom you're trucked in any case. God forbid. My worst nightmare. Almost.

In really big sea conditions I, personally, get rid of the mainsail and sheet the boom in amidships. Just too horrifying a prospect.

 

[Roberto]

I d like to understand how could a major disaster happen with this geometry.
m = mast
c --> loose footed mainsail clew
e-b --> mainsheet, attached by three spaced blocks about 2/3 3/4 of the boom, angle about 90deg when out
b-d --> preventer, same angle as the sheet 90deg, brought back to cockpit
The boom is quite high, or anyway should it touch the water conditions would be such that boom breakage would be the least worry. Also, it is sufficiently high not to represent a danger to people in the cockpit.
Normal sailing, bending moment on the boom at 'b' is boom perpendicular component of force at 'c' times bc, inside rig design parameters (or the boom would have broken already).
Fully backwinded mainsail, preventer 'bd' comes into action, the new bending moment just changes due to change in apparent wind speed, again inside design parameters as absolutely the same as the mainsheet with the same higher apparent wind, the boom would have snapped earlier by mainsheet action alone during normal same wind speed sailing; otherwise nothing specific happens apart from possibly stiff momentary boat movements, heeling and ooooh aaaah. In both cases the vertical component (of mainsheet or preventer) is taken care of by the vang, the preventer is relatively vertical so it helps reduce the vang load, otherwise the two just mainly provide the horizontal component. Everything looks balanced and inside working parameters.
Now change to preventer 'ac', backwinded mainsail. No more bending moment on the boom at 'b', good though not strictly necessary. A lot more tension on the preventer and on point 'c', the famous 'angle' which is a lot narrower, then compression load on the gooseneck, both higher the narrower the 'cam' angle, by definition higher than the former case with perpendicular ropes. Elasticity in the preventer may have it moving little/a lot, or not, open to choice.
I can t see any reasons to change from the first to the second system, where more forces are added and general loads increase overall.

 

[NormanS]

Yeah, but dipping the boom you're trucked in any case. God forbid. My worst nightmare. Almost.

In really big sea conditions I, personally, get rid of the mainsail and sheet the boom in amidships. Just too horrifying a prospect.

Long before it gets to that stage, my mainsail is furled into the mainmast, and I'm running with all or part of the Genoa. In bad conditions I much prefer being pulled along by a headsail, than being pushed by the main or mizzen. Ours is a deck saloon, so the boom is slightly higher than with a boat with a coach roof, so the chances of dipping the end of the boom are reduced.

 

[nigel1]

Roberto,
Your sketch indicates a very short boom in relation to the beam of the boat.

 

[Sandy]

3. Should not use elastic cordage.

I wonder what John means by 'elastic cordage'?

Meanwhile, I've found another YouTube video on the use and abuse of preventors. Another one that has some elasticity in the system.

 

[dunedin]

Yeah, but dipping the boom you're trucked in any case. God forbid. My worst nightmare. Almost.

In really big sea conditions I, personally, get rid of the mainsail and sheet the boom in amidships. Just too horrifying a prospect.

Lots of boats dip the boom ends in minor broaches etc and the water flow simply swings the boom in a bit. So rarely a big issue.
But with a preventer on to the middle of the boom, the boom just folds - common to see boats arriving at other side of ARC (or NARC) with half a boom.

 

[Roberto]

Roberto,
Your sketch indicates a very short boom in relation to the beam of the boat.

Yes that s what I have, dimensions and angles are correct give or take 10deg, it s a former typical IOR style sailplan, relatively small mainsail with high AR --> short-ish boom, production boat so there may be thousands like that; 40ft hull my boom is about 4m. An additional advantage comes with reefing (I have added a reefed mainsail, brown): the clew is progressively brought forward so the bending moment at 'b' decreases, if/when the reefed clew arrives at 'b' the bending stress disappears so nearing a perfect world, the aft section of the boom becomes structurally useless (apart from ropes et), in this case adding a preventer from boom end recreates a bending moment so basically looking for trouble.



Many modern boats rigs are quite different, booms often 1-1.5m longer, I see them often over the head of the helmsman at the stern, in those cases of course geometry (and vectors decomposition) is quite different and an end boom preventer is a lot better as drawings in the previous messages show. It s also better for the mainsheet to be attached near the end of the boom, in particular with fat roach/square top mainsail which when reefed (if ever by racing boats) will keep the reef clew very near the boom end, the more all the loads (sail, mainsheet, preventer) are concentrated in one place the better.
Just to say that all this preventer doomsday if not bow to end of boom may not apply everywhere.

ps Next time on the boat I ll take a photograph.

 

[Dockhead]

Roberto,
Your sketch indicates a very short boom in relation to the beam of the boat.

Indeed. Never seen such a short boom.

Also, the boom in this drawing is still trimmed in as if for a close to beam reach -- still far from where you really need a preventer.

Lastly, the drawing shows the geometry only in the horizontal plane. But the angle in the vertical plane is just as important.

 

[Dockhead]

I wonder what John means by 'elastic cordage'?

Nylon.

 

[Daydream believer]

Nylon.

As tights, or with suspenders. I prefer the latter

 

[Sandy]

Nylon.

Sorry, as usual I am being a pedantic engineer and am aware of the properties of nylon, I'd really like to understand what John meant by the statement. Please note I gave up with his website years ago as, to me, he comes across as 'word of god' and really does not engage in debate.

Both Chris and John in the videos I've posted state that there should be some elasticity in the system in order to reduce the system failing 'explosively'. I am just interested why John has come to a different conclusion and what that is based on.

If John means that there should be 0%-5% elasticity in the system, I understand that dyneema stretches up to 5%, then that makes no sense as shock loads would come on instantly and that tends to break things. One reason why climbing rope stretches far more than anything we use on boats.

 

[Supertramp]

Re attaching to the boom end, my mainsheet is attached about 3/4 of the way along the boom, and the preventer is attached to the same point. Anybody see anything wrong?

Having the same set up and a furling main which puts the sail load well down the boom near the mainsheet attachment if reefed I have used the mainsheet point for a preventer in the past.

But having read several good posts on here I now appreciate that the extra length from using the boom end (to bow) when the boom is fully out significantly reduces the forces on the preventer line if I gybe.

In lighter conditions and when the boom is not fully out (reaching) I still use a line from the toerail to mainsheet point and back to the midships cleat to stop slatting in lighter winds.

 

[Supertramp]

Long before it gets to that stage, my mainsail is furled into the mainmast, and I'm running with all or part of the Genoa. In bad conditions I much prefer being pulled along by a headsail, than being pushed by the main or mizzen. Ours is a deck saloon, so the boom is slightly higher than with a boat with a coach roof, so the chances of dipping the end of the boom are reduced.

Exactly - there is a huge difference between cruising where choices are made around safety and comfort and sailing a performance yacht hard. Much better to have a rig with multiple sail choices.

 

[Laser310]

I d like to understand how could a major disaster happen with this geometry.
m = mast
c --> loose footed mainsail clew
e-b --> mainsheet, attached by three spaced blocks about 2/3 3/4 of the boom, angle about 90deg when out
b-d --> preventer, same angle as the sheet 90deg, brought back to cockpit
The boom is quite high, or anyway should it touch the water conditions would be such that boom breakage would be the least worry. Also, it is sufficiently high not to represent a danger to people in the cockpit.
Normal sailing, bending moment on the boom at 'b' is boom perpendicular component of force at 'c' times bc, inside rig design parameters (or the boom would have broken already).
Fully backwinded mainsail, preventer 'bd' comes into action, the new bending moment just changes due to change in apparent wind speed, again inside design parameters as absolutely the same as the mainsheet with the same higher apparent wind, the boom would have snapped earlier by mainsheet action alone during normal same wind speed sailing; otherwise nothing specific happens apart from possibly stiff momentary boat movements, heeling and ooooh aaaah. In both cases the vertical component (of mainsheet or preventer) is taken care of by the vang, the preventer is relatively vertical so it helps reduce the vang load, otherwise the two just mainly provide the horizontal component. Everything looks balanced and inside working parameters.
Now change to preventer 'ac', backwinded mainsail. No more bending moment on the boom at 'b', good though not strictly necessary. A lot more tension on the preventer and on point 'c', the famous 'angle' which is a lot narrower, then compression load on the gooseneck, both higher the narrower the 'cam' angle, by definition higher than the former case with perpendicular ropes. Elasticity in the preventer may have it moving little/a lot, or not, open to choice.
I can t see any reasons to change from the first to the second system, where more forces are added and general loads increase overall.

View attachment 192530

If you are attaching your preventer to the place where the main sheet attaches, then, even if it's a bit forward, presumably the boom is engineered to take the loads there.

i guess what i don't know is whether you have drawn the whole boom in your figure, or whether you have drawn just to where the main sheet attaches. I know you have said the boat has a short boom, but is it end-boom sheeting?

modern boats often have end-boom sheeting, and the boom is not really designed to take a bending moment forward.

But there is also another potential problem; the loads on the preventer itself

the further forward one attaches the preventer, the greater the potential loads.., irrespective of where the main sheet attaches.

so the next question is: is your preventer and all its attachments strong enough to take the load generated by that geometry? Your main sheet might be 4:1, or 8:1, is built with strong fittings, and is designed to handle the load..., whereas the preventer is probably a single line..., what are the fittings?

as shown above, the further out on the boom you can get the preventer attached, the lower the loads on that system, and the difference can be dramatic.

 

[Sea Change]

Both Chris and John in the videos I've posted state that there should be some elasticity in the system in order to reduce the system failing 'explosively'. I am just interested why John has come to a different conclusion and what that is based on.

If John means that there should be 0%-5% elasticity in the system, I understand that dyneema stretches up to 5%, then that makes no sense as shock loads would come on instantly and that tends to break things. One reason why climbing rope stretches far more than anything we use on boats.

You can argue it both ways I think.
I prefer a tight, low stretch preventer. In my experience when the main backwinds, the boat will pivot and round up, rather than anything catastrophic happening. But maybe on really large boats this isn't a viable strategy?

 

[Dockhead]

Sorry, as usual I am being a pedantic engineer and am aware of the properties of nylon, I'd really like to understand what John meant by the statement. Please note I gave up with his website years ago as, to me, he comes across as 'word of god' and really does not engage in debate.

Both Chris and John in the videos I've posted state that there should be some elasticity in the system in order to reduce the system failing 'explosively'. I am just interested why John has come to a different conclusion and what that is based on.

If John means that there should be 0%-5% elasticity in the system, I understand that dyneema stretches up to 5%, then that makes no sense as shock loads would come on instantly and that tends to break things. One reason why climbing rope stretches far more than anything we use on boats.

I have a great deal of respect for John and feel like paying for his resource is a good investment, for me. I don't consider him to be infallible, but I can't find any fault with his reasoning on preventers.

Even before reading John I always thought dyneema was the right material for preventers. In this application they are not entirely inelastic as they are so long the working length of mine is more than 30 meters.

30 meters of nylon would allow quite a bit of boom moving, and I think you want to immobilize the boom as much as possible to prevent momentum building up. The boom swinging around can also allow it to backwind in some cases where it might not, otherwise.

I don't see how you would get an "explosive" failure of this system other than by dipping the boom in the water, and in that case I don't think stretch in the preventer is going to save you.

You can argue it both ways I think.
I prefer a tight, low stretch preventer. In my experience when the main backwinds, the boat will pivot and round up, rather than anything catastrophic happening. But maybe on really large boats this isn't a viable strategy?

I agree. There aren't shock loads as long as the boom is not allowed to start moving around. At least not in my experience. The sail backwinding is much less violent if the boom is fixed in place -- the sail moves over but is caught quickly. I don't think this changes with size. The boat's rounding up further softens this, but I don't think is a critical part of this.