Go Back   Cruiser Log World Cruising & Sailing Forums > Cruising Forums > General Cruising Forum
Cruiser Wiki

Join Cruiser Log Today

Reply
 
Thread Tools Search this Thread Display Modes
 
Old 05-02-2008, 05:15 PM   #15
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

"Material such as aluminum, copper and magnesium do not show a fatigue limit, therefore they will fail at any stress and number of cycles."

Simple introduction to fatgue and materials...

http://www.sv.vt.edu/classes/MSE2094_NoteB...ly/fatigue.html
__________________

__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 05:22 PM   #16
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

After reading the introduction to fatigue, you can now look at the failed aluminum stock and see that it failed in torsion...not bending. The torsional fatigue crack was initiated at the right side of the shaft and ruptured along maximum shear lines at approximately 45 degrees into the shaft. There IS evidence of combined bending with torsion witnessed by the elongated overlaod zone to the left of the photo.

My guess is that the crack initiated from the bottom of the key-way where you will have a stress riser. Bending took place, but it wasn't root cause. As such, the new tiller arm/quadrant installed by Hanse has equal potential for introducing torsional failure.

IMHO, this is a major design flaw coming from the manufacturer...not the installer of the Edson tiller arm.

Some more good photographic examples of torsional fatigue failure..

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

It is also interesting to note (in the new configuration, second photo) that the hydraulic ram is attached to the bottom side of the quadrant placing frictional wear on the washer being held only by a cotter pin. Unfortunately I had to do the same thing on my install because by boat is 30 yo. and wouldn't accomodate the ram from the top of the tiller arm. There is no excuse for a manufactrer of a new vessel to do this..not good.

I'm waiting for Redbopeep to give her professional opinion on this one
__________________

__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 06:24 PM   #17
Moderator
 
redbopeep's Avatar
 
Join Date: Jun 2007
Home Port: Washington DC
Vessel Name: SV Mahdee
Posts: 3,185
Default

Quote:
Originally Posted by Trim50 View Post
After reading the introduction to fatigue, you can now look at the failed aluminum stock and see that it failed in torsion...not bending. The torsional fatigue crack was initiated at the right side of the shaft and ruptured along maximum shear lines at approximately 45 degrees into the shaft. There IS evidence of combined bending with torsion witnessed by the elongated overlaod zone to the left of the photo.

My guess is that the crack initiated from the bottom of the key-way where you will have a stress riser. Bending took place, but it wasn't root cause. As such, the new tiller arm/quadrant installed by Hanse has equal potential for introducing torsional failure.

IMHO, this is a major design flaw coming from the manufacturer...not the installer of the Edson tiller arm.

Some more good photographic examples of torsional fatigue failure..

http://www.asminternational.org/pdf/spotli...fap0502p011.pdf
The shaft appears to be inadequately sized for the application of a steering load by an autopilot. The fact that they neck "up" to a larger diameter going thru the hull to the high stress point of unsupported shaft right above the rudder blade says to me that they're not being conservative about the top end of the stock. If you size a stock for this unsupported area, you find a pretty large diameter. They could have chosen to keep the larger diameter stock which would have given them a good factor of safety at the steering gear/autopilot loading point.

The use of AL in a high corrosion environment is not ideal. Without the autopilot (or a hydraulic arm for second steering station) loading it up it would likely have lasted though.

I'd have to go back to the report, but I'm assuming from the owners' letter that asymmetric load was applied with single hydraulic arm. I hadn't realized that. I though it was a symmetrically applied load on the autopilot collar. Any asymmetric load over time (whether stainless, AL, bronze, whatever) can eventually get to this point.

P.S. don't tell the folks who do DTA and predicted fatigue life for aging aircraft that AL doesn't have a fatigue life
__________________
"Do or do not. There is no try." - Yoda

What we're doing - The sailing life aboard and the Schooner Chandlery.

redbopeep is offline   Reply With Quote
Old 05-02-2008, 06:31 PM   #18
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

Quote:
Originally Posted by redbopeep View Post
P.S. don't tell the folks who do DTA and predicted fatigue life for aging aircraft that AL doesn't have a fatigue life
Oh, it can have a predicted fatigue life...it just doesn't have a fatigue limit...therefore fatigue life is not infinite. This is one of the major motivators behind the 787 Carbon Fiber design. Carbon fiber does not fatigue in pure tension and therefore if designed correctly, the aircraft could theoretically fly forever.
__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 06:44 PM   #19
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

Also, if I'm not mistaken, the rudder on the Hanse is not supported from the bottom of the shaft thus loading the rudder post in bending by virtue of design. Why then would they place a diameter reduction high up on the shaft where the bending moment from the rudder is maximum? The bending moment from the hydraulic ram would pale by comparison simply due to the length of the rudder moment arm and the relative location of the tiller arm.

http://hanseyachtsflorida.com/Photos/400/4...er_WEB_GB_C.pdf
__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 06:52 PM   #20
Moderator
 
redbopeep's Avatar
 
Join Date: Jun 2007
Home Port: Washington DC
Vessel Name: SV Mahdee
Posts: 3,185
Default

Quote:
Originally Posted by Trim50 View Post
I'm waiting for Redbopeep to give her professional opinion on this one
Hey, for a professional opinion I'd have to pull out my FEA and DTA programs and start crunching numbers...its been too long and that would be real work! I'd rather opine unprofessionally and just say (as I said before) "the design is weak."

Torsion, yes. The stress riser that exists with a keyway can contribute, but the geometry is that the external load grew the crack and (really) the defect may well have been a dropped tool during installation. The pictures in the report weren't ideal, but I though the defect was too far from the base of the keyway for that to initiate crack growth. Now you're making me want to go back and look at the pics again.

Moving on....your assessment of the way these hydraulic rams are attached is correct--they're not ideal.

Personalizing this, I'm sitting here with our Edson rudder stock collar (that gets bolted up to the worm gear) out of the boat trying to figure out how I'm going to capitalize on its sheer mass for our (symmetric) hydraulic ram installation. I'd prefer not to do what you had to do--but we're headed in that direction from an ease of installation point of view. Further, anything else I might design would require that I take responsibility for the engineering work of the fabricated parts and liability if it failed. Sometimes doing what everyone else is doing keeps one from getting sued...

As an aside, we're actually disconnecting the worm gear and essentially installing hydraulic steering via the autopilot control. A worm gear quick re-connect becomes the backup steering if the hydraulic steering fails. If we chose to, we could accommodate the ram as you would like from the top. However, we'd not be able to have a quick re-connect of the worm gear if we did that. And, a back up tiller would be difficult as well.

Onwards, Trim, I hope you have a good inspection schedule of your autopilot and rudder stock? What are your plans regarding this?

The fact that the owner of the boat completely blows off the corrosion issue is a bit odd, especially since the worst corrosion took place in the unsupported stock area I mentioned earlier. It would have been a problem shortly. I know the accident investigation folks always have an agenda when writing a report and clearly they had one here. The owner exhibits a lack of interest or unawareness of the range of issues surrounding these investigations. The owner appears to have pinned his conclusion of sole cause on the installer rather than convergence of several unsafe acts/conditions which existed to allow the failure to occur.
__________________
"Do or do not. There is no try." - Yoda

What we're doing - The sailing life aboard and the Schooner Chandlery.

redbopeep is offline   Reply With Quote
Old 05-02-2008, 06:56 PM   #21
Moderator
 
redbopeep's Avatar
 
Join Date: Jun 2007
Home Port: Washington DC
Vessel Name: SV Mahdee
Posts: 3,185
Default

Quote:
Originally Posted by Trim50 View Post
Also, if I'm not mistaken, the rudder on the Hanse is not supported from the bottom of the shaft thus loading the rudder post in bending by virtue of design. Why then would they place a diameter reduction high up on the shaft where the bending moment from the rudder is maximum? The bending moment from the hydraulic ram would pale by comparison simply due to the length of the rudder moment arm and the relative location of the tiller arm.

http://hanseyachtsflorida.com/Photos/400/4...er_WEB_GB_C.pdf
Its supported at some point in that "tube" there's a bearing surface. Gotta be. Else, really, really bad design.

Expectation is that only unsupported area exists outside of hull just on top of the blade.
__________________
"Do or do not. There is no try." - Yoda

What we're doing - The sailing life aboard and the Schooner Chandlery.

redbopeep is offline   Reply With Quote
Old 05-02-2008, 07:02 PM   #22
Moderator
 
redbopeep's Avatar
 
Join Date: Jun 2007
Home Port: Washington DC
Vessel Name: SV Mahdee
Posts: 3,185
Default

Loading of hydraulic ram is localized, equal and opposite the rudder loading which is spread, by this point, through the full stock diameter. Strain is very high locally with the autopilot hydraulic ram asymmetry. Quick crack growth. No diversion path for stresses locally applied like that.
__________________
"Do or do not. There is no try." - Yoda

What we're doing - The sailing life aboard and the Schooner Chandlery.

redbopeep is offline   Reply With Quote
Old 05-02-2008, 07:08 PM   #23
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

Quote:
Originally Posted by redbopeep View Post
Onwards, Trim, I hope you have a good inspection schedule of your autopilot and rudder stock? What are your plans regarding this?
I have a full keel vessel with a barndoor rudder supported from top and bottom on a 2.25" diameter stainless steel post thru-hull above water and two keyways on my bronze tiller arm. My weakest link is the ram mounting to the tiller arm from below as described above.

Also, inspection is difficult since I shoe-horned a genset in below the cockpit in front of the steerage. I'm sure I will eventually regret installing it there simply due to maintainance of the genset alone. I do plan to buy loads on solar, so the genset will be backup power only.
__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 07:14 PM   #24
Admiral
 
Trim50's Avatar
 
Join Date: Feb 2006
Home Port: Who cares really...
Vessel Name: T
Posts: 1,215
Send a message via Yahoo to Trim50
Default

Quote:
Originally Posted by redbopeep View Post
Its supported at some point in that "tube" there's a bearing surface. Gotta be. Else, really, really bad design.

Expectation is that only unsupported area exists outside of hull just on top of the blade.
Yep...most likely, but it would have to be one hellova bearing design not to transfer any bending load to the upper bearing.

I'm like'n my full keel barndoor rudder more all the time.
__________________
[
Trim50 is offline   Reply With Quote
Old 05-02-2008, 07:44 PM   #25
Moderator
 
redbopeep's Avatar
 
Join Date: Jun 2007
Home Port: Washington DC
Vessel Name: SV Mahdee
Posts: 3,185
Default

speaking of barn doors...John, who's working with my hubby, Dave, on the boat is calling our rudder the "Snoopy" rudder because he thinks its going to look like snoopy riding his doghouse and shooting at the Red Baron

Its huge with trailing edge sticking way back there. Keel hung, of course. Thanks for the compliment on our blog regarding the gudgeon casting. Its (gudgeon) installed now, but no pics on the site yet. the rudder is about 2.25" thick on the trailing edge with a bronze strap around its trailing edge. John wanted to feather it down and we had a big discussion about how I wanted turbulence at the trailing edge to help with the turn. He just said that my Snoopy rudder has plenty of beef to turn the boat with out extra turbulence...
__________________

__________________
"Do or do not. There is no try." - Yoda

What we're doing - The sailing life aboard and the Schooner Chandlery.

redbopeep is offline   Reply With Quote
Reply

Tags
rudder


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rudder Failure Far Far From Land Trim50 General Cruising Forum 11 06-30-2014 06:13 PM
Us Stock Market Having Cardiac Arrest Trim50 The Poop Deck 66 10-03-2011 01:53 PM
How Can I Stop My Boat From Sinking? Exiles General Cruising Forum 5 02-11-2011 07:33 PM
Boat Abandoned After Rudder Failure, Arc JeanneP General Cruising Forum 19 11-30-2009 11:12 PM

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

RV & Travel Trailer Communities

Our RV & Travel Trailer sites encompasses virtually all types of Recreational Vehicles, from brand-specific to general RV communities.

» More about our RV Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities


All times are GMT. The time now is 03:43 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.0