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atlanticcruiser 11-15-2010 08:10 PM

Chain plate help
 
We're searching for a good 45' bluewater cruiser and the Liberty is on our short list. the layout, quality and offshore capability make her our ideal boat. One concern we have is the chain plate design. The chain plates appear to be built into the hull and there doesn't seem to be any access. how do you get comfortable with the integrity of the plates when you can't see them? Any info would be greatly appreciated!

BigZ 11-15-2010 08:41 PM

Can't offer firsthand experience, but this thread was interesting:

Chain plate a go-go - Sailing Anarchy Forums

Pretty scary when you see some of the pics. There was some talk on the thread about x-rays to detect deterioration.
Good luck.

copacabana 11-16-2010 05:28 AM

Hello Atlantic cruiser and welcome to Sailnet!

Can the chainplates be moved outboard and fixed to the outside of the hull? On a cruising boat that doesn't point too high to begin with, moving them outboard can offer a number of advantages. While you lose a bit of the sheeting angle, you gain in other areas. You can make very strong and inexpensive chainplates out of stock SS (even make a reserve set while you're at it). They are easy to inspect and re-bed when necessary. They tend not to have corrosion issues because they are exposed to air and freshwater rinses. They free up your side decks, making movement on board safer and easier.

Just an idea ....

atlanticcruiser 11-17-2010 07:10 PM

Thanks for the posts. I like the idea of the xray, very interesting article. I believe the cost of the xray a small price to pay for peace of mind. Moving the chain plates outboard seems to be a great idea and would take it under consideration if the situation arises.

Atlanticcruiser

RichH 11-17-2010 08:05 PM

Quote:

Originally Posted by atlanticcruiser (Post 667153)
how do you get comfortable with the integrity of the plates when you can't see them? Any info would be greatly appreciated!

Chainplates should be 'visualized' for fatigue cracking either by 'dye penetrant' techniques or by 'magnetic flux' analysis via specialized equipment. Therefore the plates must be removed to inspect. Xray is only 'incidental' in such analysis as it cannot define the small micro-cracks, etc. that are a stark 'forewarning' of imminent catastrophic failure. Cyclically loaded 300 series stainless steel that goes beyond 1 million load cycles of loading at beyond 30% of maximum design strength .... will usually be 'ready' for sudden catastrophic brittle/fatigue failure .... well beyond 'worn out' and 'exhausted/tired' metal'.

Those chainplates 'must' be removed and visualized, etc. ... for 'safety sake'.

wkeenan78 12-17-2010 03:09 PM

Aluminum Chainplates?
 
I have a Starratt & Jenks 45' sloop with the chainplates fiberglassed into the hull. I dont trust them and will be fabricating and installing external ones. I have found a few pieces of stainless on the boat that had very small rust lines on them, when I tested them with a lever, they snapped very easy on the line. I want to make new ones out of 7075 T6 Aluminum alloy .75" X 2.5" flat bar. The 7075 T6 alloy is not your average aluminum with a yield strength of over 53,000 lbs. Thoughts and input would be greatly helpful.

sailingdog 12-17-2010 03:26 PM

IMHO, you'd be better off with 316L stainless steel for chainplates. First, IIRC aluminum is far less fatigue resistant. Second, aluminum is less noble and you'd probably be using stainless steel fasteners with them. Using stainless steel and aluminum together with fairly large stainless steel fasteners is generally a bad idea, since the galvanic corrosion issues are far more likely because of the size of the fasteners.

Quote:

Originally Posted by wkeenan78 (Post 678224)
I have a Starratt & Jenks 45' sloop with the chainplates fiberglassed into the hull. I dont trust them and will be fabricating and installing external ones. I have found a few pieces of stainless on the boat that had very small rust lines on them, when I tested them with a lever, they snapped very easy on the line. I want to make new ones out of 7075 T6 Aluminum alloy .75" X 2.5" flat bar. The 7075 T6 alloy is not your average aluminum with a yield strength of over 53,000 lbs. Thoughts and input would be greatly helpful.


RichH 12-17-2010 09:09 PM

Aluminum7075 T6 in contact with halides (NaCl > 0.5% wt.) similar to seawater has a very low nominal "Fatigue Endurance Limit" .... about 15,000 psi. Chainplates and other components that are subject to repetitive cyclic stress loads will typically fail in fatigue/brittle failure mode and not in 'ductile failure'.

The nominal Fatigue Endurance Limit FEL for 7075 T6, depending on post treatment, can be as high as 25-30,000 psi; but, when in contact with chlorides, etc. can be 1/2 that value 12-15ksi. Assuming a design Safety Factor for this boat at FS=3 and a UTS of 60ksi, UYS of 53ksi, FEL of 15ksi ... ... ... ... I'd be selecting stainless 316 for the plates and would be mechanical mirror polishing (followed by electropolishing if possible) the 316 to retard both fatigue and crevice corrosion potential and would be selecting 30ksi as the FEL for that material.

sailingdog 12-17-2010 09:56 PM

Rich—

316L... ;) agree with the electropolishing though.

wkeenan78 12-17-2010 10:16 PM

So stainless it is, 316L sounds to be the bees knees for this application. Can I also purchase 316L hardware? What size hardware? What length should I make the chainplates? What size stainless stock do I use to make them out of? How far apart do I drill the mounting holes? Can I use aluminum for backing plates or should I use stainless for that too?

Mabey I need to go buy a book on rigging... I still cant figure out how much tension to adjust the shrouds to either. I'm not a racer, just a cruiser. Thank you so much for all your help. Any answers you can give are much appreciated!


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