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Old 04-27-2008, 04:54 PM   #1
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Although this happened in 2005, I have just come across the information, and think that it might be worth a discussion here.

Briefly, the 11.25m Hanse sailboat "Megawat" was sailing to Scotland when its rudder failed, snapping off, and proceeded to flood the boat and the boat ultimately sank. I will try to summarize the events as a launching for a discussion, but I encourage you to read the MCIB report to be sure I am not incorrectly reporting some facts. For whatever reason, The Marine Casualty Investigation Board (MCI report does not seem to open up with the published link. However, if you follow this link: http://www.mcib.ie/reports/?-fromsearch=y&...p;-maxrecords=1

you can then click on their "full report download" link and it should download as a .pdf file. It's a large, 4 Mb file. Or you can cut and paste the link to the PDF file and it might download. It just won't download as a link from this forum, or from the Google search link. I have no idea why, it is most peculiar. Here's the link that doesn't work from here, but did for me when I cut and pasted it to a new page: http://www.mcib.ie/download_stream/?id=c30141a7f48d2148

The rudder stock was solid aluminum, and possibly the modifications to the rudder to accomodate an autopilot damaged the tube so that crevice corrosion could occur. The owner speculates that the breaking of the rudder possibly broke the tube off the inside of the hull, allowing massive amounts of water into the boat. Their Zodiac liferaft then failed to inflate, another problem in cascade of problems.

I do not care to second-guess the crew of Megawat in what had to be a terribly frightening mishap, but there are a few questions and comments I'd like to make.

First, do you know how your rudder is attached to your boat? Do you know whether your rudder stock is a stainless steel alloy or aluminum alloy? What type of bottom paint do you use?

In the conclusions, the copper-based bottom paint is stated to pose a danger to the strength of the aluminum rudder stock, though in this instance the crevice corrosion and ultimate breaking of the rudder is blamed on damage to the metal either during manufacture or during attachment of the autopilot to the rudder.

The attachment of the autopilot to the rudder is another concern/question of mine. Our old sailboat has a steering quadrant with a ball pin that is driven by the hydraulic ram. Nothing is attached to the rudder stock. It's all pretty easily inspected, and the cable that runs between the wheel and quadrant is easily inspected for damage or wear from the aft lazarette. We removed the rudder in its entirety several times while we were at anchor, and thus I don't quite understand the "tube inside the boat" allowing water into the boat after the rudder broke. In looking at the drawings, it seems as if perhaps the heavy seas caused the boat to squat introducing water into the bilge, and as the boat filled with water the hole in the hull fell completely below the waterline, introducing water in a huge rush.

From the drawings in the report, it does not appear that the compartment where the rudder comes through the hull is easily accessed. Is that why the water ingress was not discovered until the floorboards were awash, and the reason that the hole was not plugged to stop the water coming in? In other words, loss of the rudder in a heavy seaway almost guaranteed that the boat would sink?

I ask these questions because over the years I've read of too many boats sinking that had not received a mortal blow and hole to its hull, and it seemed that the crew should have been able to stop the water ingress before the boat sank. Thinking back over our experiences, when we were hit by lightning, the first thing we did was check every sea cock to be sure that none had been blown out by the lightning, and that no holes had been blown in our hull. Odd noises were inspected immediately, each time catching a potential problem before it became a real problem.

This isn't to pat ourselves on the back - we made some pretty big mistakes over the years - it's about learning from others' misfortune and cautioning people to learn everything about their boat before they head out of sight of land.

There are many different ways to accomplish something on a boat that the only way for one to protect themselves is to learn about their boat completely, and to play "what-if?"

The failure of the liferaft to inflate deserves its own topic, but for now I can only say that somebody should be very, very worried about their part in the failure of the liferaft.

There be dragons out there.
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Old 04-27-2008, 11:19 PM   #2
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Originally Posted by JeanneP View Post
The rudder stock was solid aluminum, and possibly the modifications to the rudder to accomodate an autopilot damaged the tube so that crevice corrosion could occur. The owner speculates that the breaking of the rudder possibly broke the tube off the inside of the hull, allowing massive amounts of water into the boat. Their Zodiac liferaft then failed to inflate, another problem in cascade of problems.

From the drawings in the report, it does not appear that the compartment where the rudder comes through the hull is easily accessed. Is that why the water ingress was not discovered until the floorboards were awash, and the reason that the hole was not plugged to stop the water coming in? In other words, loss of the rudder in a heavy seaway almost guaranteed that the boat would sink?

There be dragons out there.
The investigation report for some reason does not deal with how the sea got into the boat - ie what size hole was there?

The Hanse builders might have suggested what would happen if the rudder just fell out?

The owner does cover it briefly in his correspondence :-

hanse_sinking.jpg

When the rudder came away from the boat - did it tear a hole in the boat ?

The rubber boot on the solid stock shaft - what role did it play ?

Was the solid stock shaft running in a tube ? If so was the top of the tube above the water line?

Would a ring around the stock above the lower bearing have stopped the stock from coming out completely?

The sight of a spade rudder, when walking the hardstanding often gives reason to ponder "What if? "

"Do you know all there is to know about your rudder?"
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Old 04-28-2008, 02:18 PM   #3
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was this a spade rudder with no support at the bottom of the rudder? in other words, when the tube or whatever broke there was nothing underneath the rudder to keep it from sinking into davy jones?
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Old 04-28-2008, 03:53 PM   #4
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No, the rudder floated and was recovered days after the sinking. I am under the impression that most if not all rudders will float.
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Old 04-28-2008, 04:28 PM   #5
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...guess, you talk about the Hanse 371, a modern hull with a spade rudder.

Not knowing the details of the construction and being of the opinion that the Hanses are well built boats meeting high safety standards, I always wonder in general, why the stern sections of most yachts with the rudder, all its through hull fittings, engine exhaust and so on (that are all more or less above the water line under normal floating conditions) are not seperated from the rest of the hull with a watertight bulkead?

Yes, it is more expensive for the boatbuilders to install several fittings with seacocks to get the pipes through this bulkead, but in case of flooding, the amount of incoming water will be rather small and the rest of the boat will remain dry!

Loving to look through the nice boats on the boat shows it always irritates me to lift a cusion in an aft cabin and find the rudder stock (and many other holes leading out) just underneath...

I guess, we are not the ones to speculate or criticize about this specific construction but we all are asked to take a thoughtful look into our boats to see, what might happen, if something back there deep down breaks and the water rushes in... and what we could do about it for prevention.

I did and seperated the stern locker from the rest of the inside, which was rather easy, because I had to deal with an aft cocpit and not a modern boat with a spacious aft cabin.

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Old 04-28-2008, 10:40 PM   #6
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Yes, Uwe, part of this discussion was intended to make people think about their own boat, and how to deal with all the crises, minor and major, that can happen.

Not all boats can have a watertight, or at least moderately secure, bulkhead between the rudder and the rest of the boat's bilge. There are a number of vulnerable parts of a boat. The prop shaft is another hole in the hull that can allow a lot of water into the boat quickly. Each through-hull poses a risk as well.

We were instructed to keep a soft pine bung attached to each through-hull so that if something happened to it one could plug the hole quickly with the right size plug. When our prop shaft slipped out of the boat, one of my dish towels became the plug until we could sort things out and get the prop shaft slipped back into the hole. One must know where each of the through hulls is for this to work well.

The point is self-reliance. Peter approaches our passages as if there is no possibility of obtaining assistance, and he plans as best he can so that we should not need assistance. Of course, there is nothing that is perfectly safe in this world, and we who take our small boats out onto the big oceans should know it better than many others.
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Old 04-29-2008, 12:41 AM   #7
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It is common cause that ALL hoses that connect to the outside should have double hose clamps (jubilee clips).

If the Rubber Boot on the Hanse 371's rudder system was designed to keep water out of the boat,

then 1. the Boat Builder should have doubled up with clips (clamps) on the boot ; and

2. the owner/skipper should have checked and added the necessary extra clips to conform to the code for offshore boats.

The photo of the boot on the sister yacht to Megawatt does not show clips/clamps at the lower end of the boot, but it does show a single clamp at the top.

Rubber_Boot.jpg

The report does say that the crew saw the rudder floating - and it was subsequently found.

Most rudders would disappear for ever.

Richard
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Old 04-30-2008, 05:01 PM   #8
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Random thoughts on this--

Thank you, J, for pointing this accident out. It makes me feel much, much, MUCH better about the money we just spent to have a new 2.5" diameter 10 ft long bronze rudder stock (with tricky aperture) made for our boat. And we did replace a rudder that many people would have simply kept using. I decided to have replacement made because the old bronze rudder stock was a bit pink on the surface (pink=corroded) and to turn it down getting rid of the pink would take away more than 10% of the diameter. My own calc's showed that a 2" stock would be sufficient for our boat but since we intend to put a hydraulic autopilot on, we wanted the beefier 2.5" stock to remain in place. Ours is a keel hung rudder with a pin at the bottom that fits into a gudgeon at the bottom of the keel, so a break in the stock wouldn't likely end up with the rudder lost. However, we didn't want to be stranded far from home with a cracked rudder stock. Having been thru the process of replacing this particular stock, I am now amazed at how difficult it was to find a shop capable of doing the work and willing to do it! I sent a request for quote with detailed drawings to 11 different shops (10 in the US, 1 in Canada) capable of doing the work in stainless steel or bronze--I only received a quote from two because the particular rudder stock (with lovely prop aperture shape but hard to fabricate) was such a difficult project. It took almost 8 months to (try to) get quotes and then 6 weeks for the shop who made the shaft to fit it into their schedule (busy with US Navy ships and other larger projects). I would have hated to be in a foreign port and trying to deal with this particular problem. My worries about the corrosion were focused on fatigue crack growth much as that which was described in the safety board report of the Megawat incident.

I spent 6 years working as a mechanical engineer for US Department of Transportation (DOT), my work included transportation accident investigation; as such I worked closely with our National Transportation Safety Board (NTS. One of my responsibilities was oversight of federally funded research and the rail transportation industry use of damage tolerance analysis (DTA) of metallic structures and pressure vessels used in rail transportation. DTA is used to predict the fatigue life of a structural element.

I've read and contributed to numerous reports such as this one. The story is always the same: defect is present from fabrication or impact during operational use; defect grows into a crack that reaches a critical crack length and then failure occurs. Such is the story of fatigue failures. Unfortunately, sometimes there is little that can be done to prevent these failures. Inspection becomes key.

The things we can do to avoid being the victim of a runaway crack? First, ensure the manufacture and repair of critical structural elements of your boat are done by competent shops. Find out if your part employs appropriate materials for the application. Either hire a surveyor/naval architect/marine engineer or do the following yourself if you're of a technical nature: Inspect the work. Know the standards the part was built to meet; if the part is different than the original then understand the differences and what they mean to the structural integrity of the part. Have a qualified non destructive testing (NDT) professional do further inspection if this is a repair or replacement part (ultrasound, acoustic emission, enhanced visual, x-ray...), find out what a reasonable inspection period is for the critical parts and make sure that appropriate inspections or replacement of the part takes place in a timely manner. Don't simply wait for failure to happen.

The addition of a rudder stock -mounted collar for the autopilot is going to introduce loading different than the loading expected from hand steering the boat. Especially if it is a hydraulic pilot with fast response times. If you, like us, plan to add an autopilot to your rudder stock, you should go back to the designer/mfr of the boat to make sure the collar design is appropriate for your particular stock. The turn-down from 88 mm to 50 mm on this particular stock (probably to allow smaller standard steering components to be employed) is something I would not have wanted to see on a rudder stock as it increases stresses in that portion of the stock.

From a read of the incident report, nothing really jumps out as being the sort of thing that one would say "Ah-ha! THAT'S the problem!" Instead, it appears to be business as usual in the recreational marine industry: so-so materials in use, so-so design/construction/finish, so-so inspection, so-so quality of repair, limited inspection/testing/verification of quality going into the boats. In my opinion, recreational boat owners and builders are far too cavalier about ensuring the structural integrity of recreational boats.

Finding 5.3.1 thru 5.3.9 indicate in a foggy way indicate that the material selected and fabrication level of finish were thought to be not appropriate for the rudder stock. This is quite open ended in the way it is presented. Further, the report doesn't place this finding in the context of giving us the proper comparison of successful use of the same material in other rudder stocks (with better finish or the same).

Finding 5.3.4 indicates a very small defect of 0.3 mm started the fatigue crack. The author of the report (in 5.3.4-5.3.6) gives us a speculative look at how the defect came into being. This defect is so small, it could have happened with the drop of a tool onto the stock.

I personally have issues with the change in diameter of the stock from 88 mm to 50 mm but the investigators did not approach this design element as part of the problem. Probably because--

The author clearly doesn't like the fit and finish of this rudder stock, he may not like the manufacturer either--he points out that the manufacturer doesn't tell the owner in the owner's manual not to paint AL with CU bottom paint...Why point out what should have been obvious to the owner and the maintainer of this boat--the person who painted the boat should have know to isolate the AL with an epoxy primer from the CU bottom paint--or simply to not use such a bottom paint on it. If it was the mfr who painted the rudder stock, well, then thats a problem--but I didn't see that in the report during my quick read of it.

Onwards, the corrosion of the rudder stock between the hull and the blade didn't appear to contribute to the failure but would have become a high risk area and likely problem in the near future without appropriate inspection/repair. Thus it is pointed out by the report.

With the focus the investigators have on the paint, I am surprised they do not further discuss a recommendation about enhanced communication between paint mfr/retailer and owner/applicator/maintainer.

Someone in this thread asked about waterline vs height of tube top---The top of the rudder stock tube IS above the waterline. Looks like about 370 mm or so per drawing Appendix 7.1. This is a little "short" but given the height of the cockpit sole, it appears to be about as high as possible given the boat design. A watertight bulkhead would be useful, but is frequently almost impossible to implement. A watertight box up to the level of the cockpit sole would have been possible (such a box instead of a tube was frequently used in the construction of wooden boats in the past)

The stock SHOULD have had a watertight system in place much like a prop shaft has. Yet, what we see in the sister-ship is a bit wimpy for the application. Point of reference is my own rudder tube (extends about 18" above waterline) which has a packing nut exactly as a prop shaft would have) .

Those are my "random thoughts" on this particular incident. This kind of defect could be present on many, many rudder stocks out there. The same with the corrosion and the growth of a fatigue crack. There's just nothing that looks like "if we fix this, no one will have this problem."

I would have liked to have seen very strong recommendations for periodic inspection of the rudder stock around the autopilot collar including the use of appropriate NDT and removal of the collar for that inspection.

Regards all!
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Old 04-30-2008, 05:26 PM   #9
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@redbopeep



Thank you for taking the time! I'm sure we have all learned a great deal from this.
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Old 04-30-2008, 06:37 PM   #10
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Some will disagree with me, but I find the whole idea of using aluminum of any grade in salt water completely ludicrous. The only thing protecting aluminum from corrosion is corrosion.
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Old 04-30-2008, 07:50 PM   #11
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Some will disagree with me, but I find the whole idea of using aluminum of any grade in salt water completely ludicrous. The only thing protecting aluminum from corrosion is corrosion.
Hey, you're talking to the girl who didn't even want to use stainless steel on her boat (I'm jaded, I've seen too many fatigue failures and too much crevice corrosion) much less aluminum!

I would have been very disappointed to not be able to use bronze for our rudder stock. As it was, our old one lasted in good shape (unpainted by the way) for approximately 70 years until something suddenly happened that the previous owner of our boat still doesn't understand. He suspects that perhaps a boat in the neighboring slip had a current leak since a couple other boats also ended up with eaten up zincs and pink props at the same time ...the owner of our boat was out of town for for months at a time when this happened ...he only expected that the hull would be totally filthy since he does his own bottom cleaning. Instead he came back to completely eaten away zincs and pink, pink, pink bronze prop, rudder stock, and misc hardware at the stern. He was getting ready to haul the boat for the rebuild, so he didn't track down the problem, just chalked it up to another of our typical "boat owner" issues that we all face.

I would never consider using aluminum in a rudder stock unless for a "throwaway" boat purpose built for a short racing life. Even in that application, there are better materials.

I bring up inspection and early replacement because it seems that far too many mariners don't fully inspect things nor do they actually have an idea of the expected life of the hardware on their boat. Such info is not readily available or discussed by manufacturers. Probably a liability thing.
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Old 05-02-2008, 02:58 AM   #12
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Hi,

I got this letter from the owner Philip Watson in which he further clarifies the reasons for the stock(shaft) failure. He also sent me some decent photographs - here are 3 (the photos in the govn. report were/are useless!)

Hello Richard,

It is nearly 3 years since Megawat sank, and hundreds of thousands of words have been written surrounding our mishap. Most have been speculative and largely uninformed. Even the UK yachting press failed to grasp the real cause of our sinking. One French yachting magazine got it absolutely right and had diagrams to explain. The report from our government body was absolutely full of waffle and the geniuses that wrote it couldn't even understand the obvious - lots of rubbish about copper in the antifouling etc etc. none of which was relevant. They are so stupid they couldn't even understand that there was little or no gas in the liferaft cylinder, and that is why it failed to inflate.

Here it is :- the autopilot (which we had fitted locally by a professional firm) was attached to the stock by a tiller (which was a genuine Lewmar part) very close to the end of the taper near the middle of the shaft....i.e. about halfway between the top & bottom bearing

Because that tiller was about a foot below the top bearing, each time the tiller was pushed or pulled the shaft was not only twisted but also bent - result after 15,000 sea miles was fatigue and then catastrophic failure.

We worked it out immediately after speaking to the builders that one should NEVER connect a tiller to a shaft except exactly beside the top (or bottom) bearing. If our autopilot fitter had contacted Hanse they would have offered one of their quadrants with two attachment points which clamp right beside the top of the shaft.

Easy to be wise afterwards, but I am now, and that's why I confidently ordered another Hanse 371 and have had a few great seasons in her - you can see in the pic. attached the Hanse quadrant with attachment points for manual and autopilot. Obviously Hansebacker had the factory-fitted Autopilot!

I don't suppose we'll ever stop people making up their own story - this one just runs and runs.

Cheers,

Philip Watson

Hanse_371_Rudder_007_3.jpg

HansebackerR_shaft.JPG

06.JPG
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Old 05-02-2008, 04:30 PM   #13
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Aluminum is NOT the right material for a rudder shaft. I don't care where you place the tiller or quadrant. For such a critical element of a vessel which is always being cyclically fatigued, why on earth would you use a material that does not have infinite fatigue life????

If weight is so important for Hanse, why not use carbon fiber or Titanium?
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Old 05-02-2008, 04:55 PM   #14
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I sailed in a few fast coast guard cutters. The entire hull was aluminium but the propellers, propeller axles and rudder stock were stainless steel. That might just say something in support of what Trim50 wrote.

Aye // Stephen
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