Members' Online Book: Sail Handling and Rigging Made Easy, Chapter 23 of 41

Amidships “Preventers”—A Bad Idea That Can Kill

John Published Oct 2, 2018 64 comments Updated October 14, 2018

Back in 2014 I wrote an article arguing that the only right way to rig a boom preventer was from the boom, near the mainsheet bail, to the bow. (I will call these bow preventers for the balance of this article.)

But I still see a surprising number of boats rigging “preventers” from the boom, directly above the toe rail or a little outboard, to a fitting on deck, directly under the boom (in its running position) or slightly forward of that. (I will call these amidships “preventers”*.)

And the Cruising Club of America is still advocating for amidships “preventers”.

So I want to revisit this. To continue reading, please login (top right) or join us.

My Original Thinking

My original thesis was that amidships preventers were a bad idea because using them increased the load on the gear by at least five times the wind force on the sail, thereby dramatically increasing the chances of failure when compared to a bow preventer.

I was wrong.

It’s Much Worse

Thanks to excellent work done by the accident investigation branch of Maritime New Zealand, we now have solid engineering showing that amidships “preventers” are far more heavily loaded, and therefore vulnerable to failure, than even I thought.

And, sadly, this is not just theory. Two people were killed because an amidships “preventer” failed when the yacht Platino was caught aback.

Now at this point I know some will say that:

There were many other contributing factors that led to those fatalities.
Platino is a large boat with much higher loads than my smaller boat.
The loads on a preventer are very low unless the boat gets caught aback.

All that is true, but none of it alters how dangerous amidships preventers are. Here is why:

If the “preventer” had held, those two men would still be alive today.
The loads on amidships preventers, as calculated in the report, will scale by boat size and, since gear strength also scales, the failure risk remains unacceptably high regardless of boat size. And, further, an out-of-control boom can kill even on a 35-foot boat. Also, said boom will be lower on a smaller boat, thereby actually increasing the risk in some cases.
Getting caught aback is far more common and the forces far higher than many people think. In fact, this is exactly the reason we rig a preventer in the first place. See my first article (Further Reading) for details.

Line Load Multiplier

The second point brings me to the key preventer-related take away from the accident report:

As we can see from the above graph and table, the load imposed on an amidships “preventer” is almost 12 times the wind force on the sail when caught aback.

In contrast, said force multiplier on a bow preventer is just a little over two times.

(See the diagram at the top of the post for a graphic representation of the forces being graphed.)

Much, Much Worse

Wait, it gets even worse. To quote from the report:

The graph represents a simplified two dimensional model which does not consider that the padeye on boom was nearly 2 metres above the padeye on the rail. The third dimension would have further complicated (increased) the actual loading.

The key point is that the investigators made no attempt to calculate the added load as a consequence of the amidships “preventer” having a much more extreme downward angle than a bow preventer.

I’m guessing they simply decided that the horizontal force multiplication factor of almost twelve for the amidships preventer was so damning, they really didn’t need to quantify the vertical component. Also, as the investigation stated, adding the vertical component to the calculations is complicated.

Click to enlarge.

And, while I have no pretentions to being able to calculate this added factor, by relating my graphic above to the plan graphic in the report, my guess is that the vertical angle would at least double the load multiplier for the amidships “preventer” from that calculated in the report.

If I’m right, that leaves us with a load multiplying factor on the amidships “preventer” of at least 24!

(It would be great if any of you mechanical engineers could shed more light on this.)

But even if I’m wrong about the factor added by the vertical angle, it simply does not matter. Who cares if the vertical factor increases the horizontal force multiplier by a factor of 1.2, 1.4, 1.6, 2, or some other number? The point is, we know there will be some increase.

The bottom line is that using an “amidships” preventer—the failure of which can (and has) killed—that has a force multiplier factor of something over twelve (the absolute minimum when the vertical component is considered), when rigging a bow preventer would reduce the factor to a little over two, is just poor seamanship.

And even worse yet, by rigging an amidships “preventer” we:

Give the crew a false sense of being safe from the boom and mainsheet, when in fact they are not.
Not only increase the chances of it failing, we also make easing it once aback, to get things under control, far more dangerous.

Bottom line, we need to stop rigging preventers to the amidships area and stop advocating for this dangerous practice.

Practicalities

But what about the practicalities of rigging a bow preventer? We have an article with video on how to make that easy, safe, and quick.

Thank You

A huge thank you to:

Maritime New Zealand for permission to use the graphics from their report.
Member Philip who brought the report to my attention.

Condolences

We at AAC send our condolences to the friends, fellow crew, and family of the two men who died in the accident on Platino.

We also want to make clear that nothing in this article should be construed as criticism of the deceased or their fellow crew. The fact is that amidships “preventers” are commonly used, and even recommended by well-regarded organizations, so their use of one was an understandable mistake.

And, anyway, how many of us who go to sea regularly can claim that we don’t make mistakes? I know I can’t.

Free For One Month

We feel that it’s vital that this information is distributed as widely as possible, so this article will be free for one month. However, reading the bow preventer how-to article requires membership ($US20.00/year).

Comments

If you have questions, or additional thoughts, please leave a comment (membership required). That said, please read my two chapters on preventers under Practicalities and Further Reading first, so we don’t end up covering old ground.