Preventer Rigging -- Lessons from the Platino Disaster

[Dockhead]

Originally Posted by sailingharry

I think that the boom loading by the sail can be looked at as a distributed load along the boom by the slugs (perhaps a non-uniform load, heavier at the boom end), and a point load at the clew imparted by the huge loads running down the leach. But, the load along the foot is fairly trivial -- in a loose footed sail, it is easy to pull it toward the boom. The vast majority comes of the force even without a loose foot is at the clew.. . . .

I think that's right. All you have to do is look at a load-path sail, to see the -- load paths:





I guess loose footed or not, the loads are highly concentrated at the corners.


(The drawing cribbed from: https://www.uksailmakers.com/sails-o...ising-mainsail).

[Cpt Pat]

Originally Posted by Yellowtulip

How is yours attached to your deck?
Would you say that a brake is effective in preventing the boom from crossing the cockpit in all circumstances?

My boom brake is rigged to the port/starboard aft shroud chain plates with pin shackles. I can't imagine why it couldn't be rescaled for larger boats.

[cyan]

Originally Posted by wingssail

No, my main sheet attaches to the end of the boom, the same place where I attach the preventer.

My preventer holds the boom forward and down. I place the preventer depending on the point of sail. On a reach it is aft of the shrouds holding the boom down at a good angle. On a run, it is forward of the shrouds, holding the boom also at the best angle. The load on the gooseneck is insignificant.

I don't think that's true when you're backwinded, though. The clew force gets translated to a boom compression force with a nasty leverage-arm multiplier dependent upon your preventer angle. At that point, the boom is applying a HUGE multiplied torque to the gooseneck.

Let's say you have an 880 sqft main downwind in 25 knots (maybe a 55 ft boat) Sampson's formula shows the neighborhood of 3000 lb force at the clew. A backwinded preventer situation could impart more than a 10,000 lb boom compression force, in Dockhead's geometry. With a more acute preventer angle, the force could easily go way beyond that. Which would break first? This preventer line? The Gooseneck toggle? 12 rivets attaching the boom bracket which spans a GAP in a furling mast, suddenly receiving a torque many times what the tack could ever deliver sailing forward?

One has to wonder: Did Selden or Z-Spar mechanical engineers consider this backwinded extreme or simply forward sailing loads? It seems to me this falls into the category of "don't use it like that".

[Cpt Pat]

Originally Posted by Cpt Pat

My boom brake is rigged to the port/starboard aft shroud chain plates with pin shackles. I can't imagine why it couldn't be rescaled for larger boats.

> Would you say that a brake is effective in preventing the boom from crossing the cockpit in all circumstances?

Sorry, I missed this question.. The boom brake isn't set to prevent the boom from swinging across the deck - it merely slows its rate. I don't want a sustained backwinded main, along with the inherent loss of directional control, potential rudder damage, and extreme heel. I'm only concerned about damage/injury caused by an uncontrolled boom swing. The boom brake tension and friction is adjustable - it could be set to restrain the boom from swinging under any circumstances, but that's not it's intended function. It's there to absorb most, but not all, of the gybe energy.

The tension line terminates at a rope clutch directly forward of my sheet winch, which is used for tensioning. The tension affects the resistance of the boom brake friction. If I want the boom to come about unrestrained, I can instantly remove the control produced by the boom brake by opening the tension line rope clutch.

[SailRedemption]

Originally Posted by jmh2002

This may make sense from a cruising point of view but when racing the outhaul (and downhaul, etc - boat dependant) is generally eased when rounding the top mark and heading downwind - it's faster - except in the case of heavy air and wanting to depower the mainsail as much as possible.

That's why I asked, we always ease the outhaul and loosen the Cunningham(downhaul) and ease the main halyard if need be. The same reason sym spinnakers are bellied and not flat shaped. I can understand heavier winds you could tighten everything if it is right at the cusp of having to reef.

[SailRedemption]

Originally Posted by rgleason

My transatlantic friends use a dual port and starboard preventers attached to the end of the boom and stored cleated near the gooseneck. When deployed the preventer is attached with a bowline on a line from the bow back to the cockpit.

What is the best way and place to attach to the boom?
On my 32' boat 12.6' boom, could I use two wichard eyes on either side through bolted? How far from the end of the boom? 1/3?

What is the most reliable way to attach the bow line to the boom preventer?

Since hardware can fail, is it easier/better to just run the bowline around one of the 1.25" vertical bow pulpit stanchion bases which is welded to the bowsprit main support?

Would a 3/4" rubber snubber used for moorings help reduce shock or would it get in the way?

Would it be possible to rig a good boom brake at the bow that would go to port/starboard preventers?

Reading these comments, I have begun to consider the loads on my traveler and one or two important shackles in the system, which have held for 38 years, but if broken would become a real hazard.

In a few days I'll be rigging our boat for cruising and will take pictures of our setup. Ours is similar to your transatlantic friends.

[wingssail]

Originally Posted by cyan

I don't think that's true when you're backwinded, though. The clew force gets translated to a boom compression force with a nasty leverage-arm multiplier dependent upon your preventer angle. At that point, the boom is applying a HUGE multiplied torque to the gooseneck.

Let's say you have an 880 sqft main downwind in 25 knots (maybe a 55 ft boat) Sampson's formula shows the neighborhood of 3000 lb force at the clew. A backwinded preventer situation could impart more than a 10,000 lb boom compression force, in Dockhead's geometry. With a more acute preventer angle, the force could easily go way beyond that. Which would break first? This preventer line? The Gooseneck toggle? 12 rivets attaching the boom bracket which spans a GAP in a furling mast, suddenly receiving a torque many times what the tack could ever deliver sailing forward?

One has to wonder: Did Selden or Z-Spar mechanical engineers consider this backwinded extreme or simply forward sailing loads? It seems to me this falls into the category of "don't use it like that".

Interesting points, and I see the math in it. However, I have been in a gybe situation where the prevented mainsail held the boat flat down, probably 85 degrees, for about 15 minutes while I struggled with the helm and trying to release the preventer. Judy got on deck pretty quickly and together we released the preventer (jammed in a cleat!, my error) and the boom swung across and that was that (other than a torn mainsail). The wind strength was about 27 knots.

Maybe being an old IOR two tonner my boat was built to tolerate very high loads, these boats load up instead of going fast. Anyhow, we've had very few breakages due to loads such as are suggested here in a gybe with a preventer. Our preventer was a simple 6mm polyester line, and it did not break. The preventer was led through, of all things, a stanchion base, and it did not break. Yet the backwinded main held the boat flat!

BTW, we did not go backwards, we just stayed totally down, rudder mostly out of the water, not going anywhere. I could not reach the engine controls because I was holding the tiller under my chin, otherwise I'd have started the engine and motored back onto my proper course, gybing back.

But even on bigger boats the equipment should be enough to take these loads.

Keep things tight to avoid the shocks.

Put the preventer on a winch to allow easier releasing.

Avoid gybes.

[Montanan]

Maybe I am missing a major point, but I don't see any major issue with just using a boom brake to allow for a controlled gybe to occur if and when the wind shifts and / or the boat's hull direction gets turned about due to sea conditions. Becoming backwinded and held down are not the type of conditions that I desire to allow to happen. What's the big deal with just letting the boat to desire to pursue a different course in a controlled manner, instead of fighting the wind or worse dipping the sail? A preventer does not prevent the change in the wind angle, it just limits the boom's motion to be held in an adverse direction, and yes keeps one head from being knocked and or the boom slamming into the rigging, but a brake avoids such hazards and allows for proper positioning of the sail relative to the wind. Don't fight the breeze, accommodate for it. Obviously, the brake system that is deployed must be designed to handle the energy levels that it will need to dissipate, no different than designing the braking schemes for terrestrial vehicles, be those friction, back pressure or regenerative loading.

[Don C L]

Originally Posted by Cpt Pat

My boom brake is rigged to the port/starboard aft shroud chain plates with pin shackles. I can't imagine why it couldn't be rescaled for larger boats.

I've seen that, and on smaller boats I think you can get away with it, but I personally wouldn't choose that. Just me and my sensibilities maybe, but a separate chainplate, with the correct angle employed, is the preferred option IMO. It can be, as you know, one heck of a shock load on that fitting, if someone has let the boat get overcanvassed as the wind piped up.

[transmitterdan]

I have enjoyed the discussion and appreciate all the points of view. It has reinforced for me that all boats are sufficiently different that no one method is “the best”.

[a64pilot]

Originally Posted by Dockhead

I think that's right. All you have to do is look at a load-path sail, to see the -- load paths:


Attachment 201160


I guess loose footed or not, the loads are highly concentrated at the corners.


(The drawing cribbed from: https://www.uksailmakers.com/sails-o...ising-mainsail).

I don’t believe it’s the corners, rather it’s the attachment points, I’d assume just by looking that is a head sail.

[a64pilot]

Originally Posted by Montanan

Maybe I am missing a major point, but I don't see any major issue with just using a boom brake to allow for a controlled gybe to occur if and when the wind shifts and / or the boat's hull direction gets turned about due to sea conditions. Becoming backwinded and held down are not the type of conditions that I desire to allow to happen. What's the big deal with just letting the boat to desire to pursue a different course in a controlled manner, instead of fighting the wind or worse dipping the sail? A preventer does not prevent the change in the wind angle, it just limits the boom's motion to be held in an adverse direction, and yes keeps one head from being knocked and or the boom slamming into the rigging, but a brake avoids such hazards and allows for proper positioning of the sail relative to the wind. Don't fight the breeze, accommodate for it. Obviously, the brake system that is deployed must be designed to handle the energy levels that it will need to dissipate, no different than designing the braking schemes for terrestrial vehicles, be those friction, back pressure or regenerative loading.

I believe some just don’t like brakes.
They didn’t grow up with them, don’t see the need, and may not trust them.
That and apparently BIG brakes aren’t made. My opinion on that is that there isn’t enough of a market to make money is all, not that physics has a practicable limit on how big a brake can be.

However having limited experience, I cannot comment with certainty.


I would have expected factory installed, Engineered systems on high end boats though. I wonder why there isn’t?

[cyan]

Originally Posted by a64pilot

That and apparently BIG brakes aren’t made

Walder has a drum brake for a main up to 110 sqm, (1184 sqft) which should be good into the 60 ft yacht range with 20mm line. Beyond that, not sure. I saw a Swan 115 with some funky round contraption, but maybe it was just main sheet rigging?

[transmitterdan]

Originally Posted by a64pilot

I would have expected factory installed, Engineered systems on high end boats though. I wonder why there isn’t?

Our boat has a factory preventer rigged. It is always deployed and ready. The main weakness is the attachment to the boom is not adequate but easily remedied by making two lashings or high modulus soft shackles around the boom.

The Valiant factory preventer gets some criticism but I have analyzed it and find it has a suitable safety factor. I haven’t heard of a failure.

[Yellowtulip]

Originally Posted by Cpt Pat

> Would you say that a brake is effective in preventing the boom from crossing the cockpit in all circumstances?

Sorry, I missed this question.. The boom brake isn't set to prevent the boom from swinging across the deck - it merely slows its rate. I don't want a sustained backwinded main, along with the inherent loss of directional control, potential rudder damage, and extreme heel. I'm only concerned about damage/injury caused by an uncontrolled boom swing. The boom brake tension and friction is adjustable - it could be set to restrain the boom from swinging under any circumstances, but that's not it's intended function. It's there to absorb most, but not all, of the gybe energy.

The tension line terminates at a rope clutch directly forward of my sheet winch, which is used for tensioning. The tension affects the resistance of the boom brake friction. If I want the boom to come about unrestrained, I can instantly remove the control produced by the boom brake by opening the tension line rope clutch.

Thanks for that.
This has been a very interesting thought-provoking and useful thread.