Chainplate hell

Discussion in 'Sailboats' started by seacap, Aug 4, 2013.

  1. seacap
    Joined: Aug 2010
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    seacap Junior Member

    I have needed to replace the 1/4" 316 SS chainplates on my boat. They appear to have stress cracks as opposed to crevice corrosion. These plates are 25 years old. They are coming off a 28' 15,000lb Bristol Channel Cutter.

    My first shot at doing this was buying flat bar from online metals. They carry aluminum bronze. After drilling and shaping the plates they needed bent to shape. I took them to a local machine shop that does mostly marine work. They broke 3 before giving up. So at that point I either bought more and had the stock Anneled or looked elsewhere. This would have been very expensive.

    I contacted Port Townsend founders, as they were known to cast plates and hardware for the Bristol Channel Cutter. I had in my possession the factory construction manual with detail drawings of the chain plates. I asked if the plates were made to factory specs... "Yes, they were".

    After 2 months the beautifully made chain plates arrived. They were the proper shape. The holes were in all the right places ect. Just like the drawings except...

    The plates were 1/2 inch thick at the top area where the rigging would attach. Now this would not be much of a problem if I was using 1/2 turnbuckles. But I have a 4 year old Dynex Dux rig. The hardware (aluminum deadeyes) are sized for 1/4" chain plates.

    So now I am stuck again.

    I seem to have a few options:
    1) Buy 8 new bronze turnbuckles and fittings for the Dyneema. Cost ~300 each=2400

    2) Buy new aluminum deadeyes of a larger size that would fit over the 1/2 inch bronze. I would use some .02" nylon washers for insulation between the bronze and aluminum. Cost~100 each=800

    Or, have the plates at the attachment point for the rigging machined down to 1/4". Cost, probably a whole lot cheaper than my other options.

    Now my understanding is that manganese bronze is very comparable in strength to SS 316, if not stronger. But the bronze doesn't work harden and fatigue like SS. Or have the problem with crevice corrosion.

    So would I be in just about the same or better position with the bronze cut down to 1/4 as I would have been if I was using 1/4" SS?

    Thanks in advance for any imput or advise. This really has me bummed for a number of reasons.

    Gary
     
  2. Tad
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    Tad Boat Designer

    How about make up short dyneema strops with the deadeye on top and a thimble on the bottom? Use a 1/2" D-shackle in the thimble into the chainplate.
     
  3. pdwiley
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    pdwiley Senior Member

    Machining the plate down should be quick & simple for any even modestly equipped machine shop. It's a simple milling job.

    However I simply don't get that you spent a lot of money & time getting things made to drawings when that drawing was no longer correct for your rig setup. Why didn't you think of this first?

    Secondly, if the simple 1/4" 316 plates had lasted 25 years without failure, why didn't you simply replace them with new ones of the same material, size & shape?

    But then I'm a machinist.... I don't believe in over-complicating a job.

    PDW
     
  4. seacap
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    seacap Junior Member

    Thanks for the response PDW. The drawings worked fine with my rig. The deadeyes I purchased originally were for 1/4 chainplates as per spec. The spec for the wire is 1/4 also, so everything was kosher. 1/2 chainplates would be more suited to 3/8 to 1/2 wire. I don't mind over built, but in this case it just does not work.

    I will probably have the plates machined down to 1/4.

    I don't like or trust stainless in the tropics (where I live), you would be amazed. I would be a happy man without another drop of stainless on my boat.
     
  5. alan white
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    alan white Senior Member

    Just have the top of the chainplates machined down, meaning however much you need to to attach the rigging. The important thing is to taper the transition gently to avoid an abrupt concentration of stress at the transition. This should cost under $100.00 and probably half that. Machining the whole plates would be a huge waste of time. Best would be to taper the top 6" or so so that the top ends up at 1/4" and nevermind milling them parallel since the fittings won't have a problem with that slight taper.
     
  6. pdwiley
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    pdwiley Senior Member

    I'd agree with milling just the top section sufficient to clear the rigging fittings, no more is needed.

    However I'd just do it using something like a ball nosed end mill to avoid a sharp corner transition. Tapering the cut over 6" as suggested is going to be a major PITA to set up and do, I'd quote you 5X the price as an aggravation penalty if you wanted me to do that.

    Cost will be dependent on where you are. I don't do work for 3rd parties but suffice to say, if you wanted something like this done for $100 and you wanted 6" tapers from 1/2" to 1/4" I'd tell you to go away - and I have the machinery to do this work so I do know what's involved.

    Cheapest way is a single pass with a ball nosed end mill to get a nice radius then another pass with an end mill of appropriate diameter to clean out the excess material. Doing 1 side only is faster than doing 2 sides but won't look 'symmetrical' if you care about that.

    PDW
     
  7. michael pierzga
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    michael pierzga Senior Member

    Nothing wrong with stainless. Its cheap, available and effective. Ive never had a problem. There are critical stainless fasteners and components all over a modern boat.

    Most if not all boats are poorly maintained. Chainplates, stem fittings, mast steps, turnbuckles.... are considered part of the standing rigging. The correct maintenance schedule has the mast unstepped and all standing rigging inspected , repaired or replaced every 6 to 8 years.


    With correct maintenance you will never have a problem
     
  8. pdwiley
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    pdwiley Senior Member

    Yeah but there's no point arguing about it with someone who's already spent probably 50X what they needed to for some 316L flat bar, which they could have replaced every 10 years. After all the original set lasted 25 years without actual failure, just some potential stress crack issues.

    This is why a couple machine shops I know *love* boat owners, and why I try never to have anything to do with their projects.

    PDW
     
  9. michael pierzga
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    michael pierzga Senior Member

    People get carried away, thinking they need the best materials.

    Always educational to look at commercial workboat build standards. No money wasted...best bang for the buck, long service life.
     
  10. alan white
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    alan white Senior Member

    It would be a hassle (and time) to do the long taper. I thought at first, as you said, a short transition made by the cutter itself. Then I added the "best" way would be to do a long taper.
    A lot of rig "machining" needn't be too exacting. In this case, even an .015 tolerance would satisfy the need to fit the 1/4" rigging. Err on the big side and you're actually better off. I've made lots of fittings out of aluminum using a table saw and a drill press. You can save huge bucks fashioning your own fittings. I've made a lot of tangs and parts for later welding.
    I'd love to have a milling machine but can't afford one.
     
  11. pdwiley
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    pdwiley Senior Member

    Bigger is generally cheaper when it comes to mills.

    Actually thinking on this if I *had* to do it with minimal tooling I'd just use an angle grinder with a thin cutoff wheel followed by a flap wheel to clean it up a little. It's not a job calling for great precision anyway.

    PDW
     
  12. pdwiley
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    pdwiley Senior Member

    Yeah - I came from a oceanographic & fisheries R&D background. If we needed it, we'd buy it. Mostly we used industrial stuff because it works.

    Shackles are a classic example. Yachts are loaded with nice shiny stainless ones, not one of which (generally) carry any SWL stamp. Fishing boats are loaded with galvanised shackles, all of which have a stamped SWL *and* cost about 1/3 of the stainless ones. But they're not 'pretty'.

    PDW
     
  13. seacap
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    seacap Junior Member

    Ok folks here's the deal.
    Talked to one of 2 machine shops here. They wanted 100/hr open ended with no estimate. Like PDW they didn't sound like they wanted to do it. The chain plate has 3-4 bends in it and would be difficult according to the machinist.

    I have attached the factory drawings and a photo of the suspected stress cracks.
    Had to do this in 2 posts.

    I have ordered larger sized deadeyes that will fit over the 1/2" chainplates. So it will be an easy job at this point. Except for all the woodwork that has to be torn out to do the install. That is one of the reasons I wanted bronze so I won't have to do this again. It's a huge job. If the CP's had been made of bronze originally we would not be having this conversation.

    I want to thank everyone for there imput.
     

    Attached Files:

  14. seacap
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    seacap Junior Member

    Here is the photo of stress cracks.
     

    Attached Files:


  15. motorbike
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    motorbike Senior Member

    Could be a sunk cost fallacy in action here. Youve spent the money already on the chainplates- its gone and your not getting it back. Why throw more money at it? Go and buy some 316 flat bar of the right dimensions and solve all problems. I made my own chainplates up out of 10 mm flat bar, using an angle grinder to radiius corners, a cutoff wheel, a few drill bits and a flapper disc and various grades of sandpaper to polish it all up.

    Pulling out woodwork, buying new deadeyes etc just sounds like uneccessary money and work!
     
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