boom preventers, swept back spreaders,physics junkies

[RON20406]

Boom preventers are a necessary complement to safe sailing for the offshore sailor. There is, however, a debate that rages among sailors that should be based upon physical laws rather than ease or personal preference. Theoretically, a vessel with swept back spreaders could use a mid-boom attachment point to a block mounted aft of the shrouds that would end at a terminus in the cockpit and would probably avoid breaking the boom on a broach or a roll over since the boom is not perpendicular to the mast on a dead run unlike a vessel with traditional spreaders that would allow the boom to swing to a point perpendicular to the mast and would surely suffer damage on a roll/broach. The question is: Is there a benefit to end boom attachment on a vessel with swept back spreaders or does the mid boom attachment suffice and the question is moot? Best, Ron

 

[RichH]

The advantage of end boom 'preventers' is that it only needs to be 'half as strong' as a mid-boom preventer to do the same structurally 'loaded' functional job. Force and distance = applied 'moment' or Archemedian 'law of levers'.

In accordance to structural 'beam formulas' .... when the boom/sail 'goes underwater' the end boom arrangement will be the 'lesser cause' of the boom breaking ... because the 'reaction loads' are at the 'ends' of the beam (boom) instead of being 'in the middle' ('moment' or applied Load times distance) ... BTW also a structural consideration advantage for a 'loose footed' mainsail with respect to loading TO the boom. The disadvantage of mid boom preventers is it 'enhances' buckling failure of the boom - if the boom is being 'side loaded' (boom preventer attachment position) as well as and IF the boom is increasing its compression along it's 'long axis' by the event

 

[alexco38]

We just use a figure 8 repelling device use by rescue workers to slow the boom, not to prevent it, from coming about.

Wichard sells a "marine" priced solution.

IMHO, a preventor is more dangerous as it prevent the boom from coming about. Under such heavy load, you just can't release it in time to avoid broaching.

 

[RON20406]

The advantage of end boom 'preventers' is that it only needs to be 'half as strong' as a mid-boom preventer to do the same structurally 'loaded' functional job. Force and distance = applied 'moment' or Archemedian 'law of levers'.

In accordance to structural 'beam formulas' .... when the boom/sail 'goes underwater' the end boom arrangement will be the 'lesser cause' of the boom breaking ... because the 'reaction loads' are at the 'ends' of the beam (boom) instead of being 'in the middle' ('moment' or applied Load times distance) ... BTW also a structural consideration advantage for a 'loose footed' mainsail with respect to loading TO the boom. The disadvantage of mid boom preventers is it 'enhances' buckling failure of the boom - if the boom is being 'side loaded' (boom preventer attachment position) as well as and IF the boom is increasing its compression along it's 'long axis' by the event

Rich, would the blocks have to be mounted on the bow/foredeck as is commonly done with end boom attachments, or could they still be mounted aft of the chainplate/shrouds for a cleaner , less cluttered application? Ron

 

[RichH]

If you use a figure-8 repelling device you eventually will wind up with a fully jammed knot. Suggest that you change your Figure-8 to a "Rescue-8" .... a figure 8 'with ears'.
Example: http://www.google.com/products/cata...=1&ie=UTF-8&tbm=shop&cid=17909086352849763829

Setting up any preventer in a LOCKED configuration is an invitation to disaster ... IMHO all such devices should be set up to SLOW DOWN a gybe to 'acceptable' speeds, otherwise one can easily 'clear' the mast and rigging if it all 'goes wrong'.

 

[Bill Roosa]

I'm interested in how many folks actually have ever had an accidental jibe while the boom is "all the way out". My experience is that is pretty hard to let happen unless you are totally not paying attention or have a sudden wind shift.
My suspicion is that accidental jibes only happen with the boom partially out in which case you don’t really get broach with boom dragging in the water.

 

[RichH]

Can easily happen in 'confused and lump' following seas (such as when a 'storm' passes your position with changing wind direction and changing wave patterns, etc.), especially when the boat's roll period immerses the boom end into the water .... usually at night when one cant see whats coming and thus cant SEE fast enough to react. Usually happens when the helmsman is below cat-napping or is on the 'crapper', etc. Have done it twice, once with major damage. <grin>

 

[Rick D]

I'm interested in how many folks actually have ever had an accidental jibe while the boom is "all the way out".

Four times on four boats, all the result of helmsman inattention to wind direction. You are IMHO correct that it is rare otherwise. I do rig a preventer in offshore races, but mostly to stabilize the mainsail more than to handle an unintentional jibe. My crew has managed to backwind it a couple of times though.

 

[alexco38]

Yes I meant a rescue 8 device. Thanks.

Generally speaking, sailing dead downwind is slow. Broad reach and gibe is the way to go. You don't have to leave the rum line too much.

Just go and look at some real racers. They seldom push the boom out that far.

On jibing, it is prudent to sheet-in the main first.

 

[RichH]

But some times you are forced DDW because of 'passes' through 'narrows' or there is 'land getting in the way', ship traffic, etc. and/or are already at max. hull speed (similar to VNE on aircraft). Sure we dont purposely sail DDW if we want a 'fast' passage but sometimes we HAVE TO and thats when we break booms, etc.

 

[Benny17441]

That is a question for which there is no absolute answer at this time. I do believe it could be adequately addressed for practical purposes by a logical application of generally accepted principles. My personal preference is marred by my desire in this matter to use the safest method and that is to attach the preventer to booms end. More of an angle and surface to dissipate forces by compression along the whole length. For those looking for an empirical absolute answer corresponding to calculations derived from physical laws, good luck and good debating. Booms are not designed to handle comparatively large forces on their horizontal axis. Where you place those forces and at which angles does come to bear on their ability to resist reaching the limit. The good thing about this is that we can all test our convictions on our own booms.

 

[RON20406]

For those of us who sail offshore, sailing DDW is not only necessary, but practical as well as described by Rich and Rick in the above comments. It is a skill needed to complement a successful passage if you desire to sail ,rather than motor ,in less than ideal conditions. However, I would still like an answer as to where is the best placement for blocks if one uses end boom attachment--at the bow/foredeck or aft of the chainplates for better sheet leading? This is where the "physics junkies" should excel based upon dynamics vs. personal opinion. Best, Ron

 

[mitiempo]

A preventer rigged from the end of the boom, forward to a block on the toerail and then aft to the cockpit and a cleat works well. The cleat should be dedicated to the preventer and one should be rigged on each side.

 

[RichH]

For those of us who sail offshore, sailing DDW is not only necessary, but practical as well as described by Rich and Rick in the above comments. It is a skill needed to complement a successful passage if you desire to sail ,rather than motor ,in less than ideal conditions. However, I would still like an answer as to where is the best placement for blocks if one uses end boom attachment--at the bow/foredeck or aft of the chainplates for better sheet leading? This is where the "physics junkies" should excel based upon dynamics vs. personal opinion. Best, Ron

Simple trigonometry will dictate that the 'more to perpendicular' the attachment will produce the least amount of INDUCED (multiplied as a lever, etc.) forces to the boom .... attach to the bow/stem produces the least multiplication of (lever) forces into the boom as its the 'best' to-perpendicular you can get on a boat.

 

[Warren Milberg]

I was surprised to read about how many times Chichester broached while setting huge sails, often poled out headsails of 600+ sq ft, and then went below to sleep. He often awoke to the thunder clap of a 25 ft whisker pole breaking after a total 180 -deg wind shift .... Stuff happens.

 

[Rick D]

I use a block & tackle (Garhauer) with the tail led back to the cockpit for fast release and the foot attached to a stanchion base with a rubber mooring snubber to take up the impact if the boom dipped into the water which has happened a couple of times (when lucky enough to have that much wind!). So, I attach it to one of the boom mainsheet bails which is a bit over 60%. Certainly not ideal for strength, but safer to rig and de-rig IMHO since there is usually some jibing needed. (I don't just use the rig DDW).

 

[RON20406]

Simple trigonometry will dictate that the 'more to perpendicular' the attachment will produce the least amount of INDUCED (multiplied as a lever, etc.) forces to the boom .... attach to the bow/stem produces the least multiplication of (lever) forces into the boom as its the 'best' to-perpendicular you can get on a boat.

This is why the dynamics of sailing are so interesting. Thanks Rich for the answer in "physical" terms and thank you to everyone for your feedback. Galileo is smiling. Good luck and good sailing, Ron

 

[RobertLangdirect]

"sudden wind shift."

I think that was the case last time I ducked.

 

[HukilauMike]

If you use a figure-8 repelling device you eventually will wind up with a fully jammed knot. Suggest that you change your Figure-8 to a "Rescue-8" .... a figure 8 'with ears'.
Example: http://www.google.com/products/cata...=1&ie=UTF-8&tbm=shop&cid=17909086352849763829

Setting up any preventer in a LOCKED configuration is an invitation to disaster ... IMHO all such devices should be set up to SLOW DOWN a gybe to 'acceptable' speeds, otherwise one can easily 'clear' the mast and rigging if it all 'goes wrong'.

Rich, do you have a picture or diagram how this is set up?

 

[stephen Penny]

Install a preventer such as mitiempo proposes as I did on my short boom Hunter 31 lead aft through a cam cleat or install a Walder, Scott Boomlock, Wichard or other boom brake. I use a French Walder boom brake and it works for me. It operates as a preventer or boom break depending on how much tension is on it. Attached is photo of the Walder boom brake.