Har du montert autopiloten din på forsvarlig sted på rorstokken ?

[Enzo]

De er sikkert mange her som husker den episoden der en Hanse 371 sank etter at roret brakk , jeg har funnet en post på det annet forum og klipp/limer litt fra dette da det er litt informativt rundt grunnen til at det brakk.

 

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 (MCIB) 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.

 

Eieren av båten la inn dette innlegget:

 

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

 

 

Og så en link om aluminium http://www.sv.vt.edu/classes/MSE2094_NoteB...ly/fatigue.html

og denne http://www.asminternational.org/pdf/spotli...fap0502p011.pdf

Redigert 8.August.2008 av Enzo (see edit history)

[HaHa]

Interessant!

 

Fra nest siste link:

"The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Material such as aluminum, copper and magnesium do not show a fatigue limit, therefor they will fail at any stress and number of cycles. "

 

Umiddelbart virker jo ikke aluminium som et optimalt materialvalg for en roraksel som blir utsatt for sykliske krefter...

 

Sakset fra MCIB-rapporten:

"The presence of an initial defect leads to rapid growth of a fatigue fracture.

Final failure will occur when the original diameter of the rudderstock is

sufficiently reduced to lead to failure. Aluminium does not have a fatigue

endurance limit unlike steel."

Jeg synes hr Watson avfeier granskningens konklusjoner altfor lett.

Hvis jeg forstår resonnementet til Megawats eier riktig, så utsettes rorstammen for mer fleksing ved autopilotarmen midt mellom øvre og nedre rorlager enn nær et av lagrene. Hvis dette er den avgjørende faktoren, burde jo f.eks. min rorkult montert ca 0.5 m over øverste lager utgjøre en klar fare (se bilde). I virkeligheten vil bøyingen av rorstammen (25 mm syrefast stål) også avgjøres av lengden på kraftarmen (dvs rorkulten på ca 1 m). Videre vil jo ikke skjærkreftene på tvers av rorstammen bli noe mindre med en montering nær et av lagrene, de blir snarere konsentrert nær øvre (eller nedre lager).

 

Noen med mer inngående materialinnsikt som kan kommentere aluminium vs rustfritt stål som materiale til rorstamme?

Redigert 8.August.2008 av HaHa (see edit history)

[Enzo]

Jeg kan ikke si at jeg skjønner problemet helt om det er et rorlager oppe og nede og autopiloten er plassert midt mellom sålenge det ikke er noe slark.

Skulle det gjøre noen forskjell om den var festet høyt oppe under øvre lager eller nede over nedre lager ?

[HaHa]

Selve rorstammen vil bøyes mer med montering av autopilotens kraftam midt mellom lagrene. Men jeg tror ikke dette er grunnen til at Megawats ror brakk.