Preventers

[Fr J Hackett]

First video is great with some excellent explanations.

 

[garymalmgren]

RE, The Mariner"s first video.
38 minutes,
"You cannot rig mid-ship preventers and call yourself a serious sailor"

I guess that rules Pip out. Fact be known these are brakes, not preventers.

My dinky preventer.
Gary

 

[Laser310]

I guess that rules Pip out.

but when, in the video, she does rig a preventer, it is from the boom end to the bow.

i don't much like that boom brake; it looks as if one could put enough wraps to stop the boom moving and possibly break the boom when using the vang attachment, which is quite far forward.

 

[Fr J Hackett]

but when, in the video, she does rig a preventer, it is from the boom end to the bow.

i don't much like that boom brake; it looks as if one could put enough wraps to stop the boom moving and possibly break the boom when using the vang attachment, which is quite far forward.

When using a boom brake you need to use enough wraps to slow and control the boom but not enough to prevent it moving which as you say could lead to the boom breaking. The Walder one I have experience of gives recommendations in its instructions BUT it isn't a preventer.

 

[Laser310]

As I mentioned above, in racing we are sometimes required to demonstrate to an inspector a good preventer.

Usually this is from the boom end to the bow.

but on modern boats with asymmetrical spinnakers, it's not often that the boom is eased enough that we feel we need to actually use it - we are pretty far from the main getting by the lee.

 

[Dockhead]

When I bought my boat, like most sailors I checked the control systems and realised that there were no preventers. That is not to say that the previous owner never rigged something up but It was something that I had to consider. There are various methods of laying out control lines and devices that use friction to stop the boom accelerating violently. After a lot of thought I rejected all the pre described systems focussing on what I wanted to achieve which was to safely get the boom into a position where I could control it with the mainsheet and mainsheet traveller. I achieve this by setting a block on the gunwale further aft with control lines running from the boom to the cockpit This allows the boom to move freely for about 30 to 45deg before it is held by the restrictor and at this point it is under very little load as the wind has not yet caught the sail. In this position I can now take control with the mainsheet and Traveller. My Preventers live most of their life hung on the guard rails ready to be clipped on when needed.

Like others, I'm having a big of trouble visualizing this. Do you mean you aren't restraining the boom at all from forward? Maybe I'm just not seeing it, but that doesn't sound good to me. Why?

1. As others have said, if the boom has scope to move, it will be able to build up momentum which will greatly increase the force on it. It doesn't need to move all the way across the cockpit for this to be a factor.

2. What is the angle of the line to the boom which you are expecting to restrain it? This is critically important, because the angle magnifies the force, up to many orders of magnitude. The closer the attachment point is to being under the boom, as opposed to being ahead of it -- so the angle calculated by horizontal vs. vertical distance between boom and attachment point -- the greater the magnification of force.

That is why best practice is considered to take the preventer all the way to the bow, which improves this angle, and why preventers taken somewhere midships are considered extremely bad practice.

The definitive resource on this question is the Platino accident report, which you can find here: https://safety4sea.com/wp-content/u...-NZ-Platino-Accident-Final-Report-2018_07.pdf. No one who has not studied this carefully, can consider himself up to speed on preventers.

The accident in this case, which killed two sailors, resulted from improper rigging of a preventer, in this case near midships rather than right out to the bow. The accident report gives all the physics and math to show why.

I am in the camp, along with several others posting in this thread, who rig their preventers right out to the bow, and who put them on hard to reduce motion as much as possible. My preventers are Dyneema to reduce motion further. As someone else commented -- if the boom can't move, it can't develop a lot of force. The force of the sail being taken aback is manageable so long as you don't have momentum of a swinging boom to deal with.

Note, however, that there is yet another problem with preventers, not solved by taking them out to the bow -- if you have aft-swept spreaders, as I do, then the boom may not be far enough out to get a reasonable angle. Which has an additional bad effect of loading up the gooseneck. On my boat (YMMV) the angle is OK when running off deep, but not good enough on a slightly broad reach with the boom in further, but not in far enough to not need a preventer. In bad weather in such a posture, I use a jockey pole to change the angle of the preventer.

 

[Dockhead]

Key graphics from the Platino accident report:

 

[Fr J Hackett]

All show and discussed in the first video of Sandy's post #20

 

[Laser310]

Note, however, that there is yet another problem with preventers, not solved by taking them out to the bow -- if you have aft-swept spreaders, as I do, then the boom may not be far enough out to get a reasonable angle.

not only that.., but because the main won't go very far out.., you are more likely to get the main by the lee than on a boat with inline spreaders where the boom will go out to 90 deg.

the problem is worse when cruising or delivering, because the spinnaker may not be up, and the tendency is to try and soak down to a deeper course if your destination is down wind. If the spinnaker is up, and you are sailing the right TWA for it, the risk of sailing too deep and gybing is greatly reduced.

trying to sail wing on wing with highly swept spreaders can really put you at risk of an accidental gybe.

it can be a big issue on some catamarans where the shrouds can be very far aft. The easy solution there is to sail downwind without the main, and with a symmetrical spinnaker with sheets and tack lines to both clews. each bow gets one of the tack lines and you trim the sheet on one side and the tack line on the other side - depending on which tack you want to be on. It's very stable, and very safe.

 

[Dockhead]

not only that.., but because the main won't go very far out.., you are more likely to get the main by the lee than on a boat with inline spreaders where the boom will go out to 90 deg.

the problem is worse when cruising or delivering, because the spinnaker may not be up, and the tendency is to try and soak down to a deeper course if your destination is down wind. If the spinnaker is up, and you are sailing the right TWA for it, the risk of sailing too deep and gybing is greatly reduced.

trying to sail wing on wing with highly swept spreaders can really put you at risk of an accidental gybe.

it can be a big issue on some catamarans where the shrouds can be very far aft. The easy solution there is to sail downwind without the main, and with a symmetrical spinnaker with sheets and tack lines to both clews. each bow gets one of the tack lines and you trim the sheet on one side and the tack line on the other side - depending on which tack you want to be on. It's very stable, and very safe.

Agree entirely. This is certainly a big drawback of aft-swept spreaders.

 

[Supertramp]

I have changed how I do preventers as a result of previous threads and the various videos so that the line runs right forward from the boom end. I like Laser 310 #29 spinnaker suggestion and on a dead run I use twin jibs (monohull) and lose the main if gybing is a worry.

I do however still use a line taken less far forward when reaching or close reaching in rolly seas and in lighter winds to stop the boom crashing about if the sail is not staying full. But that is not a preventer and is not subject to gybe loads. A line right forward doesn't work when the boom is not well out.

I have no mainsheet track, just fixed padeyes, and when gybing I use a small course alteration to ensure I don't accidentally gybe while bringing the main in and then gybe the sail and set up on the new lee side. Which seemed to be what the OP was using his preventer line for initially. It might be useful but as others have said is not the best way to rig a true gybe preventer.

I learnt early that gybing was a thing to be scared of and avoided if possible. Having experienced several crash gybes racing I can see why and in my cruising life such dramas are best avoided.

 

[Laser310]

I do however still use a line taken less far forward when reaching or close reaching in rolly seas and in lighter winds to stop the boom crashing about if the sail is not staying full.

a topping lift can be very helpful in that situation.

an advantage of using a topping lift - along with other lines, as you have described - is that you can still leave the sheet pretty loose and the boom high enough that the leech tension is not greater than you actually want, but the boom is still stabilized. Having a good bit of twist in the main is often pretty fast in those conditions.

many boats are not rigged with topping lifts these days...

Racing, I have used an available halyard as a topping lift to stabilize the boom - even if it's fractional. It looks odd, and you have to bring it to the other side if you tack, but it can be fast in the light and sloppy.

 

[RunAgroundHard]

Attainable Adventure Cruising aka www.morganscloud.com (or ACC) has just completed series of articles comparing gybe preventers, where a contributor used current force calculating software to analysis the loads. There is no doubt, that a midship preventer is a waste of time and ineffective compared to a boom end protector. It produces twice the lateral force of an end boom preventer and causes significant bending of the boom at the mid point, with the gooseneck and mainsheet effectively pinning the two other end points. The vang point is even worse, at 2.7 times the wind force. Just don't use the mid boom point as a preventer space. Also don't secure the gybe preventer from boom end or mid point directly to the mid ship area as the tension force can be increased more than a factor of 4. Pip Hare is using a device to slow an accidental gybe, not prevent it, but if adjusted wrongly, could become a a preventer.

ACC is a subscription service, so you need to pay to read the three in depth articles.

I use a line (permanently rigged) from my boom end, coiled and stored at the gooseneck when not in use but still connected to the boom end, rigged to run through a snatch block on the bow fairlead (hole in the gunwale in my case). The end of the line runs back to the mishap cleat. The midships cleat is not the most convenient place but it is strong and a cleat at the centre cockpit area would be better. Maybe a project for the future.

My point is don't use the mid ship cleat directly secured to the boom end or mid boom point and never use the mid boom point for a preventer. The arithmetic and calculations demonstrate that folly of that.

 

[Metalicmike]

We have a situation where every one is correct in their own scenarios. Sailing down wind in heavy seas is completely different to those preferring to sail in light airs. The fact is that how we sail and the conditions that we are happy to sail in dictate the decisions that we make. I prefer to use a spinnaker for downwind sailing rigged with a sock than I can control from the cockpit and if the wind picks up I use my genoa only. I don't like wing on wing, I sail within my comfort zone and set the boat up to avoid leaving the cockpit, if things get too hairy I have an engine of which I inspect regularly. So to sum up, I avoid situations where an uncontrolled Jibe can occur and certainly avoid conditions where such a jibe is so violent.

 

[Dockhead]

Attainable Adventure Cruising aka www.morganscloud.com (or ACC) has just completed series of articles comparing gybe preventers, where a contributor used current force calculating software to analysis the loads. There is no doubt, that a midship preventer is a waste of time and ineffective compared to a boom end protector. It produces twice the lateral force of an end boom preventer and causes significant bending of the boom at the mid point, with the gooseneck and mainsheet effectively pinning the two other end points. The vang point is even worse, at 2.7 times the wind force. Just don't use the mid boom point as a preventer space. Also don't secure the gybe preventer from boom end or mid point directly to the mid ship area as the tension force can be increased more than a factor of 4. Pip Hare is using a device to slow an accidental gybe, not prevent it, but if adjusted wrongly, could become a a preventer.

ACC is a subscription service, so you need to pay to read the three in depth articles.

I use a line (permanently rigged) from my boom end, coiled and stored at the gooseneck when not in use but still connected to the boom end, rigged to run through a snatch block on the bow fairlead (hole in the gunwale in my case). The end of the line runs back to the mishap cleat. The midships cleat is not the most convenient place but it is strong and a cleat at the centre cockpit area would be better. Maybe a project for the future.

My point is don't use the mid ship cleat directly secured to the boom end or mid boom point and never use the mid boom point for a preventer. The arithmetic and calculations demonstrate that folly of that.

This is the best resource on preventers I've ever seen; this and the Platino accident report should be required reading.

Basic summary points from the extremely well-researched and engineering-literate series of articles:

1. Preventers should always be rigged to boom-end, never mid-boom.
2. Should always be run as far forward as possible; never midships.
3. Should not use elastic cordage.
4. Should be strong enough based on calculations (a spreadsheet is provided).
5. Should be fixed to an appropriately strong fixing, and never a mainsheet bail or outhaul car.
6. Turning point at the bow should consider doubling of the load for a 180 degree turn (bring across and back the other side if this is a problem).
7. Should be set hard with no slack and some pre-tension.
8. Should be used from a beam reach and anything deeper.

All definitively proven with math and engineering.

In addition, John doesn't like boom brakes, and argues persuasively against them.

 

[Dockhead]

We have a situation where every one is correct in their own scenarios. Sailing down wind in heavy seas is completely different to those preferring to sail in light airs. The fact is that how we sail and the conditions that we are happy to sail in dictate the decisions that we make. I prefer to use a spinnaker for downwind sailing rigged with a sock than I can control from the cockpit and if the wind picks up I use my genoa only. I don't like wing on wing, I sail within my comfort zone and set the boat up to avoid leaving the cockpit, if things get too hairy I have an engine of which I inspect regularly. So to sum up, I avoid situations where an uncontrolled Jibe can occur and certainly avoid conditions where such a jibe is so violent.

All true, and avoiding risk of uncontrolled jibes is good seamanship.

But if you do use a preventer, it makes sense to considering the engineering of them, which does not change according to the mood or taste of the sailor, but is governed by the laws of physics.

 

[Sea Change]

This is the best resource on preventers I've ever seen; this and the Platino accident report should be required reading.

Basic summary points from the extremely well-researched and engineering-literate series of articles:

1. Preventers should always be rigged to boom-end, never mid-boom.
2. Should always be run as far forward as possible; never midships.
3. Should not use elastic cordage.
4. Should be strong enough based on calculations (a spreadsheet is provided).
5. Should be fixed to an appropriately strong fixing, and never a mainsheet bail or outhaul car.
6. Turning point at the bow should consider doubling of the load for a 180 degree turn (bring across and back the other side if this is a problem).
7. Should be set hard with no slack and some pre-tension.
8. Should be used from a beam reach and anything deeper.

All definitively proven with math and engineering.

In addition, John doesn't like boom brakes, and argues persuasively against them.

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.

 

[dunedin]

True but the damage happens on the windward side after the boom has crossed the cockpit

Not sure I understand that statement - the injuries and fatalities generally occur AS the boom crosses the cockpit, not “after” as you suggest. Hence why most of us keep the preventer tight and rigged to the bow to prevent the boom sweeping the cockpit.

And separately you said that stopping the boom slatting is not necessary for you as “then you hoist the spinnaker and sheet in the mainsheet”, which again I struggle to understand.
Unless sailingna fast multihull or planning / foiling race boat, most cruisers tend to go best quite deep downwind, and don’t bring the apparent wind forward of the beam like very fast racers with asymmetrics. Therefore not clear why you would sheet in the mainsheet when setting a spinnaker?

 

[NormanS]

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?

 

[Dockhead]

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?

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.