Chain plates

[kingfish]

Hello all,
I am changing out my weak u-bolt chain plates to 15" straps going down the side of the hull. Dose anyone have any imput on the strenght of the type of chain plates?
Cheers
D

[Richhh]

What boat?
Whats your intended sailing venue? lake, coastal, offshore ... makes a big difference in chainplate size.
WHY are you changing the plates? etc. etc. etc.

Changing from inboard to outboard plates is not as easy as it looks from a structural/materials engineering point of view, especially if the new plates will need to be 'bent' - which can vastly decrease the fatigue resistance of the metal. Chainplates should probably be engineered for fatigue resistance rather than for ductile failure - if this is a 'serious' boat. There are many other design factors that can seriously affect the service life and integrity of c-plates.
A bit more of info as to boat type, model, etc. etc. might help to produce and answer.

Otherwise, chainplates for coastal sailing should be calculated for less than 30000 psi max. design stress using 90000 psi (or greater) UTS/UYS material ... and there should be NO sharp corners, notches, sharp bends, surface irregularities or weldments on the plates. The plates should be scrupulously mechanically polished to a 'bright mirror' finish. AND the plates should be attached/bolted using a precise torque wrench.

Boat designers usually dont understand fatigue and how to design for it ... thus most OEM chainplates are usually 'too light' and they usually fail by cracking, embrittlement, etc. --- all a sudden catastrophic failure mode.

;-)

[kingfish]

The boat is a nantuckett clipper 32. She will be used for serious offshore work, The chain plate matierial is 1/4 318 stainless, 15" long with 4 1/2" stainless bolts with 1/4 stainless backing plates. This boat has already circumnavigated with the standard u- bolts.
The shrouds are attached to the deck just inside the toe rail so they will only be moved out around 1"-2". Saying that brings me to my question for you, Why do they have to be pollished?
Thanks
D

[CSY Man]

Polishing is for corrosion resistance.
I had my chainplates electro-polished...So far, so good.

[Richhh]

Polishing is for fatigue resistance. Microscopic fatigue cracks start to form just about any time a heavy load is applied. Polishing lessens the rate at which the cracks form. Once the cracks form, the material is vulnerable for crevice etc. corrosion. Two types of desctructions going on at the same time. CSYman is right - once you obtain a high mechanical mirror polish, electopolishing can be done to further smooth the surface for additional fatigue resistance (and corrosion resistance). The smoother the surface the lesser chance of fatigue microcracks forming.

If you plan on going offshore ... go the whole distance and dont skimp on the chainplates. If you can, use 'beefier' plates to get the normal applied stress level down further... then polish.

[GordMay]

On Polishing Stainless, from ‘SparTalk’ (Brion Toss Yacht Riggers):

< quote >
The "easiest" method to polish stainless steel is to use a right angle air tool and 3-M Rollok (sp?) sanding discs then moving to the 3-M abrasive pads and then buffing wheels.

For welds start with 36 grit discs then 60, and 80. Practice your method of using the outer diameters of the discs so as to not cause unwanted damage to areas adjacent to the welds. Move then through the coarse, medium, and then fine abrasive pads. The 2 inch diameter pads work best on tight places and work everywhere else also. Use the abrasive pads to work through removing the marks left by the bending dies used to form your piece.

Buffing takes a lot of horsepower. Use the black sticks for compound designed for stainless. Use 3-M (there are a few other brands as well) fiber wheels (they look like a plastic version of wire wheels and cups) which are impregnated with a fine abrasive, to get inside the welds that you cannot dress down with the fiber pads if the welds have not been left "proud" of the adjacent virgin stainless. This removes mostly the coloration left by the welding process which needs passivation to keep from showing rust and discoloration. A better method of "electrocleaning" without having to use nasty combinations of Nitric acid is to use a 25-30 Volt power supply current limited to about 1 Amp *. Attach the negative lead of the supply to a sponge soaked with Oxalic acid (a common benign acid found in Rhubarb, for example, and most easily obtained as FSR...Fiberglass Stain Remover in boat stores). Attach the positive lead to the stainless. Slowly work the gel-impregnated sponge around the welds and watch the discoloration dissappear. This works well for cleaning rust off of stainless. Grease and oil will interfere with this process.

If you allow too much heat buildup to occur when dwelling with a buffer wheel on the part you will discolor the stainless and need to either electroclean it with the power supply or use an abrasive pad again before buffing again.
< end quote >

* Can anyone comment on this DIY method of electrocleaning ?

See also, the previous discussion at:
http://www.cruisersforum.com/showthr...&threadid=1705

[Richhh]

electropolishing after mechanical mirror polishing will enhance corrosion resistance but enhancement of corrosion resistance can be done effectively with citric acid etc. passivation. If the part needs to be 'elegant' as well as passivated, then both citric passivation followed by electropolish is probably best.

[Alan Wheeler]

The acid is removing the more easily corrodable metals from the surface. The ones that protect the steel are also harder to remove, so the surface becomes "rich" in the chromes etc. The more corrodable steel and it's oxide is etched away.

[pjfsail]

Kingfish, at some point I need to replace the chainplates on my 33' Yorktown, Olympian. I would like to know some of the details of your replacement project. I recently looked at a similar boat that had the chainplates replaced and I thought the backing on the inside of the hull was a bit on the light side. Can you tell me how you are going about making decisions on size of the chainplates, what stainless steel you are using, and how you are going to back them up on the interior of the hull? Thanks for your input.

[delmarrey]

If I may, I will touch on backing plates, seeing how I've installed my share.

There are several reasons for backing plates and determining which one is in relevance to the type of hull and the function of the deck/outer fitting.

The first and main reason is to keep the fastener from being pulled thru the hull/deck. Most FG and wood hulls have a soft inner core or will acquire one after a period of use and moisture saturation. Metal hulls I will discuss later.

As well a backing plate (bp) keeps the hull from being crushed by the fastener by spreading the pressure out more equally, providing the bp is sufficient thickness for its type of material.

Second, is it aids in the sealing process to keep the moisture from passing thru the hull/deck. Especially in aluminum and steel hulls where the fastener maybe tight but has been moving around with out notice. This is a common problem on metal boats.
Whereas, in a FG or wood boats it (adheres) becomes part of the material which adds to the security of the fastener.

A few pointers:

Always try to use SS backing plates unless it's a metal hull/deck. Then you'll want to use the same metal as the hull.
Or on a wood hull/deck using bronze fasteners, then of course, use bronze plate. Bronze thru hull fittings is a good example. Use bronze plate or washers. With plastic fittings, I prefer SS.

Make sure the thickness of the bp is sufficient. If the plate crushes when tightening the fastener that may be due to a couple things. On FG it could be the core is soft. With balsawood it will be. And the best remedy for that is by re-coring the spot with epoxy filler.
My route is to drill out the outer hole 1/32" oversize then stick in a Allen wrench attached to a drill motor. Then spinning the balsa core out for at least a half an inch and vacuum out the old wood. Then refilling with epoxy filler and re-drill to the proper size. This stiffens the deck/hull? And also ads in keeping the core dry if it ever happens to leak, which it will in time (no doubt).
As for wood, if it crushes it maybe ply that is wet or solid that is rotting. You better check it out. Any backing plate that crushes will eventually become loose.

The bigger the better! But don't get crazy The main thing it to keep the deck/hull from flexing under load. If you see your deck/hull fittings moving/ rising when the load is on, chances are that you'll need a stronger bp. On boats with thin hulls there maybe no relief, just keep the fasteners from getting pulled thru.

My general rule for a 40' boat it 1/4" thickness and 200% X the base of the fitting+, for rigging fittings. And just person judgment for anything else as long as the bp doesn't bend under a tightened fastener.

One more thing! IMHO I despise aluminum bp's, except under alum. Pedestals or other alum. fittings with alum. fasteners. Once water starts to seep thru a fastener the alum. starts to oxidize and will swell up to twice it's thickness, crushing the deck/hull leaving a divot behind, that needs to be filled.

Track and travelers are usually made of alum. But the fasteners are almost always SS. So, SS backing plates would not make any difference as far as corrosion/electrolysis. A new track is cheap compared to the labor involved in repairing a deck/hull. As well SS backing plates can be reused, just wire brush them off and home acid treat them. They’ll be like new again.

On my boat I DO carry a batt. op. jigsaw and couple 1/8" X 18" sq. aluminum plates with a hole in the middle, just in case I ever get holed, or whatever. I guess it goes back to my "Damage Controls" days in the Navy riding on the PBR"s and Swift's.

I'm sure I've missed something here! So, others will probably have thier input as well.

[pjfsail]

Delmarrey, thanks for your detailed explanation of backing plates. I have a couple of questions. You said,
"My general rule for a 40' boat it 1/4" thickness and 200% X the base of the fitting+, for rigging fittings. And just person judgment for anything else as long as the bp doesn't bend under a tightened fastener."

1. I'm not clear what you mean by, "1/4" thickness and 200% X the base of the fitting+, for rigging fittings?" Would you elaborate please?

2. In making new chainplates is the above information to be applied?

3. When you make and install backing plates for chain plates would you make individual plates for each through bolt - like in your photo? Or would you make a mirror image of the chainplate or does it make any difference which method you use?

4.And, would you make a statement as to the size of the through bolts?

Thanks again.

[delmarrey]

In the pictures above the thru bolts were 5/16" (8 mm) so I went with 1/4" plate. 1/4" is good for 3/8" bolts as well, but when you start sizing up the bolts it's better the size up the plate, or start adding washers (fender washers first). The reason for multipule bp's in the picture is alignment. It's very hard to get all those holes to line up thru a 1" deck and then thru another piece. also it was EZ'er to install and access with all the overhead gear in the way. Plus, remnant SS plate can be bought by the pound.
FYI I'm a Machinist/Toolmaker so machinery is EZ access for me.

The % rate is taking the size of the base of the fitting and multiplying it times 200%. I'm not an engineer or have found this formula anywhere. But it works well for me.
e.g. 2" sq. deckfitting = 4 sq. in. X 2 = 8 sq. in. which = 2.83. so, I round it up to 3" squares, which works well. If the deck were to flex inside then I'd go 400%

For me, the plate thickness seems to be around 66%+ of the bolt size.

As for the chainplates I, IMHO, would tend to goto a thicker plate, and one that covers all the bolts in one piece. As well, cover as much of the inside of the hull up to 500% as you can, adhereing it to the inside with a 3M 5200 or eqiv. Some people say 5200 is hard to get off. but I run a piano wire behind sawing the adhesive apart, much like a car windshield.

Anything that takes a looot of stress (winches, padeyes, chainplates etc.) I'll upgrade. Basically I have experimented with the gear on my boat , by taking it out in the rough, and found this formula to work pretty good. Yours could be differant. I'm just telling my story here. Plus, I have a nack for this kind of stuff, working in the metal trades for 35 years. These are just MY opions, not science. Every boat is differant and needs to be treated as so!
Hope this helps!



Standing By!!!.........................._/)

[GordMay]

Delmarrey:

Thanks for a well conceived and written tutorial on Backing Plates.
I like your rules of thumb for backing plate sizes:

1. Plate thickness 1/4" minimum, up to 3/8" diameter fastening, and increasing in thickness to maintain 66% minimum of bolt diameter (ie: 1/2" thick for 3/4" Bolt) for larger diameter bolts.

2. Plate area 200% (“rounded up”) of hardware base area approximates my own habit of using:
~ Backing Plate dimensions (length x width) of 150% of hardware base dimensions (about 225% of area) - except for Chainplates, which are a “special”* case.
3. Note Del’s emphasis on gouging then backfilling the core with epoxy (a compression post for the sandwich skins).

* Chainplates (and their “backers” or “gussets”) require a little more “engineering”, due to the variety of configurations (load transfer mechanisms).

I too prefer continuous backing plates, to individual “washer” plates. As Del pointed out, this is not always practical, so individual plates must be substituted. I’ve often compromised by using multiple two-hole plates, especially at critical loading points such as: ends, higher loads (ie: stanchions on toe rails, or habitual sheet block locations), or directional changes.

Gord

[pjfsail]

Delmarry and Gord, what an education. Thanks to both of you. My background is 20 years construction mechanic working mostly on Caterpillar equipment and 10 years as a cabinet maker. I learned to sail at Kittery Point, Maine in 1974. I have a complete cabinet shop at the back of my home in a covered patio. So, I'm handy at doing most things on and in boats. I now have more questions.

1. How does one go about engineering the size of the chainplates?


2. The photo in Delmarrey's last post shows the deck with a hole sawn piece drilled out of the deck and set to one side. How are you going to complete the process so the fitting can be reinstalled? Will you fill up this area with thickened epoxy or can a plug made out of marine plywood be epoxied in with thickened epoxy?

[delmarrey]

The big problem with the padeye in the latest picture is that the previous racer sailors didn't really care for the boat. But just moved fittings around at will without any concern for the future of the boat.

Here the padeye had been replaced/moved 2-3 times without proper sealing or reinforcement so the water got in and rotted the balsa, which allowed the deck to crush.

After cutting out the plug I cleaned & sealed the edges of the pocket with epoxy/filler. Then ground down the edges to a nice long taper. I cut out a solid plug from marine ply and coated it with epoxy resin. I mixed up some more epoxy/filler and glued in the plug to the inner section of the hole filling around the edges. After curing I cleaned it off with the grinder and started lying in the fiberglass cloth using polyester resin.

They (whoever they are?) say you CAN put epoxy over polyester but NOT polyester over epoxy.
WELL, I've run experiments doing both here in the yard. And it's just as strong one way as the other. Although, I would not build a boat that way, just repairs!!!
Below is part of a piece that has been sitting out in the weather for the past three years. Once in a while I'll give it a good smack with a hammer. As you can see, it's still bonded. I've cut on it, beat on it, pried on it and it stands up.
It'll be around long after I'm gone!



Anyway, I start out with smaller pieces of FG just to cover the wood and then make them bigger each time I lay in a new one. Until the last one I add some wax to the polyester so it all cures. One reason I used polyester rather then epoxy, is that, it cures faster then epoxy. I'm able to start finish sanding on this in 3-4 hours. Whereas with epoxy, I'd have to let it sit over night. Anything below the waterline, always use epoxy!

After the top section is done, I went below and filled the remaining holes with epoxy/filler, and ground smooth.

It's was a good thing that this fitting was close to the deck edge. Or, I would have had to dig out all the rotten wood core and fill. If you ever have to re-core anywhere, remember that both sides of the deck/hull have to be bonded to the new core. Otherwise, when the boat is twisting and bending in the swells, the un-bonded area will start a chain reaction and the whole area could loose the bond (delaminate).

Over & standing by..............................._/)