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Old 21-06-2011, 10:56   #16
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Re: Reefed main = greater mast breakage risk???

Well he's quite right that a reefed main puts different stresses on the mast column that require design treatment. Fortunately, well designed rigs take this into account.

Rig designers can increase the size of the mast and/or install additional stays. Almost all rigs (except B&G) have forward and after lowers which are about at a 2nd reef. If you look at conservatively rigged blue water boats you'll often see additional intermediate stays above the lowers to keep the mast in column. I once had a friend who added some intermediate stays before a circumnavigation partly because he expected to carry a deep reef much of the time.

It's strongest to to reef such that to headboard is relatively close to one of these attachment points. However most boats - especially older ones - are very conservatively rigged so it's not really a worry. But if I owned a boat with a fancy racing rig designed for minimum weight, I'd get instructions about where to set in a reef.

Look at the picture of Puma. That's a lot of stays.

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Old 21-06-2011, 10:58   #17
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Re: Reefed main = greater mast breakage risk???

Quote:
Originally Posted by hoppy View Post
I'm currently doing a theory course to get some paperwork and the instructor mentioned that reefing the main increased the risk of mast breakage on masthead rig yachts.

His argument was that if you reefed the main as the wind picks up and you maintain the same boat speed, then the reefed sail will be excerting the same amount of force on the mast. When the sail is not reefed, the backstay takes a lot of the force, however when the sail is reefed, the area of mast between the mainsail head and the masthead will take some of the force, bending that section of mast.

Obviously the masts are designed to handle this, but metal fatigue could still happen.

So his suggestion was to leave the main up, reef in the headsail and then when you still have too much sail area, just drop the main.

I'm just wondering what others think about this

It's something I have never thought about but it does make sense
From a structural point of view there is an increased risk to the mast due to the changed loading patten from the sails.

As I see it the sail imparts to the mast 2 point loads (at the gooseneck and at the head board which carries almost all the leech load) and a distributed load along the luff of the sail.

With the main all the way up the headboard load goes immediately thru the mast into shrouds and stays without having to travel along the mast any significant distance. Once you start reefing the headboard load has to go up or down the mast to shrouds to carry the forces down to the hull. The other thing that happens is the luff of the sail provides damping for mast movements, but this damping is decreased as reefing decreases the length of luff attached to the mast. In heavy seas, especially when pounding the head board loads cycles can get very large. This plus reduced damping and a headboard located between shrouds can contribute to rigging and/or mast failures.

The easy steps to alleviate the potential problem include double lower shrouds and having the headboard be within 1' or so of a spreader when deeply reefed (sailmaker may have considered this during sail construction). The more expensive next step is a cutter rig which provides provides better support of the mast and more places on the mast where the headboard is next to shrouds and stays.

In the real world the problems and dangers of an overpowered and/or unbalanced boat outweigh the danger of a reefing related mast failure, so reefing when appropriate is the thing to do.
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Old 21-06-2011, 11:08   #18
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Re: Reefed main = greater mast breakage risk???

I put in the second reef on the main the other day. This was the first time with this boat. I noticed the intermediate stays connect to the mast right where the head of the was.
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Old 21-06-2011, 11:11   #19
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Re: Reefed main = greater mast breakage risk???

Do masts just snap without a stay/fitting breaking first? I've never heard of this... I've always though (assumed) that the only way to break a mast is to lose a supporting stay first.

I thought the purpose of the stays (even extra stays) isn't really about preventing the mast from breaking, they are the supporting structure that is taking all the load (or at least the majority of it) in the first place. The mast is just there to hang the sail on...the stays are the load bearing structure.
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Old 21-06-2011, 11:22   #20
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Re: Reefed main = greater mast breakage risk???

Quote:
Originally Posted by callmecrazy View Post
Do masts just snap without a stay/fitting breaking first? I've never heard of this... I've always though (assumed) that the only way to break a mast is to lose a supporting stay first.

I thought the purpose of the stays (even extra stays) isn't really about preventing the mast from breaking, they are the supporting structure that is taking all the load (or at least the majority of it) in the first place. The mast is just there to hang the sail on...the stays are the load bearing structure.
The mast is necessary to carry compression loads, the rigging can only carry tension loads. The are other rare alternatives such as the bi-pod mast of the Amoco Procyon (http://books.google.com/books?id=sP7...rocyon&f=false)

I can't say for sure, but I believe that it would be possible to break the mast without a rigging failure. Let's say the mast is a single spreader and the main has a 3rd reef so the headboard doesn't line up with any shrouds or stays, the boat is sailed very hard in pounding conditions so the mast pumps fore and aft a lot and it only had single lowers. Contributing issues would be undersized mast section, long fatigue history, pounding seas in resonance with the mast pumping, and rigging set up to maximize the compression loads in the mast.

In general I would think rigging failure is the most likely cause of mast failure, but I wouldn't rule out buckling as a possiblity.
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Old 21-06-2011, 12:38   #21
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Re: Reefed main = greater mast breakage risk???

..but isn't the question"what is the difference between Reefed Main+Headsail versus Main alone unreefed."

Otherwise,Seeing as I don't have any wires on my rig,I can't resist..

The halyard tension on the headboard and where it is means little because it's compression via halyard is always on the masthead (from the clew attachment point) however deeply reefed.The sideloads are lowered and that's about it..


The number of stays reflects the desire to keep the column in line and in perfect compression by shortening free column lengths VERSUS the strength of the column per wall thickness,diameter,etc..Weaker or taller columns need more stays...(ie lighter,taller).It has nothing to do with where the headboard is reefing.

The failure,in the end result,should be all-at-once.:failure by wire breaking windward,fore and aft(not leeward) and mast crushed to powder at the same time.The real failures are compression columns out of column (no intermediate backstays for instance-a weakness in wired rigs) and therefore tension failures and shearing of rivets etc etc etc

I'd reef.
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Old 21-06-2011, 13:06   #22
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Re: Reefed main = greater mast breakage risk???

No problem at all on a Searunner. When we reef the main, the top of the sail is now at the upper ends of the staysail stay and running backs. Essentially, it is a fixed masthead rig here too. Reef more... the lower third of the mast is held rigid with the lower swept backs and baby stay.

Besides the mast base, the rig is held rigid in three other places! It is a well thought out rig, and allows one to keep the sail plan balanced at all times.

In winds to the upper 30's, and into the 40s, it sails best by striking the main entirely, and sailing with the staysail alone. With the mast being in the middle of the boat, she balances perfectly this way.

These two "action shots" were in the mid to upper 30s, but not quite time to lower the main. (Seas were 13-15', and we were HARD to windward with a coral reef 200' to leeward!)

Each rig is different. Look at yours and determine how well supported it is when reefed.

Regards,
Mark
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Old 21-06-2011, 13:34   #23
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Re: Reefed main = greater mast breakage risk???

Quote:
Originally Posted by HappySeagull View Post
..but isn't the question"what is the difference between Reefed Main+Headsail versus Main alone unreefed."

Otherwise,Seeing as I don't have any wires on my rig,I can't resist..

The halyard tension on the headboard and where it is means little because it's compression via halyard is always on the masthead (from the clew attachment point) however deeply reefed.The sideloads are lowered and that's about it..


The number of stays reflects the desire to keep the column in line and in perfect compression by shortening free column lengths VERSUS the strength of the column per wall thickness,diameter,etc..Weaker or taller columns need more stays...(ie lighter,taller).It has nothing to do with where the headboard is reefing.

The failure,in the end result,should be all-at-once.:failure by wire breaking windward,fore and aft(not leeward) and mast crushed to powder at the same time.The real failures are compression columns out of column (no intermediate backstays for instance-a weakness in wired rigs) and therefore tension failures and shearing of rivets etc etc etc

I'd reef.
I believe you are incorrect in your understanding of the failure mode, the location of the headboard is important because it carries leech loads which are in 2 directions, down which is resisted by the halyard, and perpendicular to the mast which is resisted by shrouds and stays if the headboard is near them or by bending of the mast if not. This combined with mast pumping caused by pounding into heavy seas combined with decreased damping due to less luff length attached to the mast can push the mast out of column leading to a buckling failure. Alternatively if a shroud or diagonal fail then the mast fails in bending. It is unlikely that there would be a crushing/compression failure of the mast unless it had a reduced section.

That said the risks of other problems associated with heavy weather increase faster for most boats than problems tied to reefing the main, so I agree that reefing the main is the answer unless you have a really skinny mast then you should have had a discussion with a navel architect about what to do long before you went out in such weather.
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Old 21-06-2011, 13:52   #24
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Re: Reefed main = greater mast breakage risk???

I read the purpose of the class, but what kind of class is it? Hopefully after class you can give an explanation every can understand. I personally am in the B.S. crowd......i2f
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Old 21-06-2011, 14:16   #25
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Re: Reefed main = greater mast breakage risk???

Quote:
Originally Posted by hoppy View Post
I'm currently doing a theory course to get some paperwork and the instructor mentioned that reefing the main increased the risk of mast breakage on masthead rig yachts.

His argument was that if you reefed the main as the wind picks up and you maintain the same boat speed, then the reefed sail will be excerting the same amount of force on the mast. When the sail is not reefed, the backstay takes a lot of the force, however when the sail is reefed, the area of mast between the mainsail head and the masthead will take some of the force, bending that section of mast.

Obviously the masts are designed to handle this, but metal fatigue could still happen.
There are several flaws in your instructor’s proposition. I believe that the mistakes are based on erroneous assumptions s/he made. Some were already addressed above; I’ll (further) comment on two.

First, even if at constant wind speed the boat speed is the same after reefing, that does not necessarily mean the same forces are in play. It is quite likely that the boat is in better balance and the forces throughout are reduced. For example, both heel and weather helm (as previously mentioned) are probably significantly reduced by reefing the mainsail, which result in a decrease in resistance (at that same boat speed). Therefore, although the same speed is reached, the driving force (on the sails and in particular the mast) has been reduced to match the reduced drag. Furthermore, other forces on the mast that don’t directly contribute to speed (such as transverse / heeling and perhaps compression) have also been reduced. Conclusion is same boat speed does not equal same force(s) on mast.

Second, “that section of mast” was already taking forces. The mast is a beam (or column) with limited support – mast base, partners (if keel stepped), fore/aft stays, shrouds and spreaders. All sections of the mast are always reacting to the various forces and reactions throughout the mast. So reefing will change the forces/reactions in “that section of mast,” but it’s not going from zero to some massive number; it will change, increasing in some areas and decreasing in others, but as you said, the mast is designed for it.


Quote:
So his suggestion was to leave the main up, reef in the headsail and then when you still have too much sail area, just drop the main.

I'm just wondering what others think about this

It's something I have never thought about but it does make sense
I think it is wrong and suggest you look elsewhere for your paperwork. And no, it does not make sense.

Question: how would you apply your instructor’s proposition to a free-standing mast?
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Old 21-06-2011, 14:19   #26
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Re: Reefed main = greater mast breakage risk???

You guys with the "he's dangerous" attitude are totally wrong. He is not advocating something dangerous instead he is actually offering an over cautious approach.

His solution is not "don't reef", but it is to have a cutter or inner forestay along with running backstays to share the load when the main is reefed in.

Basically he is a BWB sailor who thinks all bendy boat sailors are going to die... or at least be dismasted

Those of you who are on the BWB side of the fence should embrace this idea, and you probably have by the way your boat is rigged.
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Old 21-06-2011, 14:22   #27
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Re: Reefed main = greater mast breakage risk???

Quote:
I believe you are incorrect in your understanding of the failure mode, the location of the headboard is important because it carries leech loads which are in 2 directions, down which is resisted by the halyard, and perpendicular to the mast which is resisted by shrouds and stays if the headboard is near them or by bending of the mast if not. This combined with mast pumping caused by pounding into heavy seas combined with decreased damping due to less luff length attached to the mast can push the mast out of column leading to a buckling failure. Alternatively if a shroud or diagonal fail then the mast fails in bending. It is unlikely that there would be a crushing/compression failure of the mast unless it had a reduced section.
A1+

out of column loads increase when the main is reefed, not to mention wind pressure is a function of the square of wind speed

hence a reefed main places more force on the upper centre of the mast. An area that aside from running back stays, tends not to be well supported,


But the designer has taken athat all into account. The advice not to reef is not good advice.

Another significant risk is actually motoring with little or no headsail and the mast pumping in a seaway.

Dave
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Old 21-06-2011, 14:42   #28
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Re: Reefed main = greater mast breakage risk???

FWIW,

A few years ago (1996) we were having a new stick built for Insatiable I. The original mast was a big Proctor spar with single spreaders, single lowers plus baby stay, I dimension nearly 50 feet. The first proposed design was also a single spreader rig utilizing the same chainplates, and we signed a contract. A few days later the engineer ran a detailed stress analysis on the new spar and it flunked! He said that if one chose to use a deeply reefed main AND a full genoa the mast became unstable. The fix was to use the same extrusion and double spreaders. The firm, Sparmaster of the Brisbane area in Queensland (sadly no longer in business) did the redesign and built the spar... at the original price! Great bunch of guys, and a superior product, but I digress...

Note that they didn't suggest that I never use that sailplan, but rather made the mast strong enough to deal with the situation.

So, I'm inclined to agree in theory with the "instructor" but disagree with his risk analysis and his suggested cure... they're just silly IMO.

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Old 21-06-2011, 14:45   #29
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Re: Reefed main = greater mast breakage risk???

This instructor is dangerous and not welcome on my boat!
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Old 21-06-2011, 16:04   #30
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Re: Reefed main = greater mast breakage risk???

Don't believe I've heard of a mast breaking off a reefed boat, except at the base of the mast. Taking a load off the sails takes the load off the mast.
I should be in real trouble when the wind picks up. My main comes down completely and run on jib and mizzen.
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