Chain plate sizing.

[Bowdrie]

Originally Posted by RaymondR

It's distribution of the shear forces into the hull material over a large area which defines the load bearing capability of the chain plates and friction between chain plate and hull material is vital to achieving this. The main purpose of the bolts is achieving the friction.

Not at all.
We could take a hull of sufficient thickness that would carry the load without deformation and actually have a pivoting chainplate that uses only one large bolt, zero friction.
If we had an extremely thick fiberglass hull, (such that the amount of glass above the hole would carry the tensile load,) we could just use a giant shackle thru the hole to attach the turnbuckle, also zero friction.
We use long chainplates with several bolts to ensure that there is enough substrate above each, and between each bolt hole so that the aggregate area can withstand the tensile loads, leaving the shear loads to the bolts.
Our "bolts" are basically just "removable rivets", not much different than two steel plates riveted together to take a tensile load.

[team karst]

If the failure is going to be fiberglass ripping out, with metal on both sides, then you will want, what i will call, high clamping force. And the bolt strength is a bit secondary. Calling it friction? Maybe…

The part that needs to be worried with is the substrate. The metal basically just needs to not corrode awAy.

[MicHughV]

This is a complicated subject.

Friction is certainly a component, but I wouldn't count on it, as it is dependent on various factors like how tight the bolt is fastened, the diameter of the bolt, the washer (dia. and thickness) under the bolt (and nut), the material the bolt passes thru', the thickness of the material, the compressibility of the material, the number of bolts, the pattern of the bolt hole locations, the lenght and with of the chain plate, etc.
In addition, the chain plate is relatively smooth and slick as will be sides of the hull it bolted against.
And if you want to get picky, you can factor in the expansion and contraction rate of the metals involved.

This topic is getting slightly out of hand.

[team karst]

Originally Posted by MicHughV

This is a complicated subject.

…..
This topic is getting slightly out of hand.

No way. We haven’t yet called our first Teams meeting.

[MicHughV]

Besides many other skills and talents, I'm also a licensed Professional Engineer, specializing in structural marine engineering for the past 35 years or so....what's your gig ?

[RaymondR]

I have no professional engineering qualifications whatsoever but I broke an awful lot of stuff during my life and was generally curious as to why.

[team karst]

I design electronic stuff for el. substations, once in a while 1GW nuke generation stuff, but that has way slowed down. Replaced by silly 75MW solar fields and wind turbines up to 13MW.

Admittedly, a fair amount of trial and error on my part for the boat. You can design, lets say rigging for expected loads, but how do you estimate transient loads caused by seas or wind gusts, flogging, etc.

[scherzoja]

GILow,

External Chain plates:
Perhaps this information will be helpful.

I did the same thing you are planning. I moved the chain plates external on my Pearson 367. They are in the same fore/aft locations +/- 1" as the original chain plates, except they are now about 4" outboard. I delivered the bronze plate to a machine shop where it was cut into strips, bent, and square holes milled. I drilled the clevis pin myself because I wanted to ensure that the hole was an exact match with the clevis pin diameter in order to have a maximum bearing interface between the pin and the top half of the hole circumference.

I used 3/8" thick silicon bronze for the chain plates and 1/4" thick 316 SS for the backing plates. The backing plates have an additional 1/4" plate welded at about a 90 degree angle and those though bolt to the bulkheads/knees that were the original chain plate attachment points.

Chain plates are fastened with 1/2" silicon bronze carriage bolts. They were custom cut to avoid threads in the bearing areas (chainplate, hull, backing plate). Backing plates are fastened with 1/2" 304 SS bolts. The forward and after lower chain plates are angled to match the angle of the lower shrouds. This caused an issue down below with the backing plates because those angles caused gaps between the bulkheads/knees, which I backfilled with epoxy/1708/colloidal silica.

I chose bronze because I wanted to eliminate the worry about crevice corrosion and I didn't want to polish the SS to the degree necessary effective corrosion resistance.

The chain plates are external to the hull, but run under the rub rail (Duramax 100 series DB-50). I made fiberglass/gel coated wedges for form a ramp for the rub rail to follow over the top of the chain plates. Also, I have two deck scuppers for which I created wedge ramps to create a gap for the small water drainage to dribble under the rub rail. This is my effort to minimize topside staining from any oxidized EPDM rub rail.

I also build-up the area between the chain plate and the hull because there was a ~1/2" tapered to 0" gap between the chain plate and hull. I made this out of polyester resin/1708, sanded to desired shape, then gel coated. This gave the chain plate a consistent mating surface.


Chain plates are sealed with Bed-It Butyl rubber tape.

I followed Skene's specs for clevis pin radii on the chain plate. It's not a constant radius round the top end of the chain plate, the vertical radius is slightly larger than the horizontal radius (i.e. the "offset" in Skene's diagram/specs.

The original chain plates were 1/4" for shrouds and 3/8 for stays. I developed a spreadsheet to help guide me in the design to ensure I had adequate scantlings, depending on the chosen metal, width, thickness, hole diameters. Attached is an early version of that spreadsheet. I had a much more refined version, but that was lost with a hard drive failure (of course, no recent backups, ouch!).

I also created a spreadsheet for Skene's chain plate scantlings to make it a little easier to understand what he wrote. Attached are both spreadsheets.

If you use Bronze, it will look like crap as soon as water hits it, so consider that if you go that route.

Another thing to consider is the length of the upper shroud chain plate. I should have made it about 3" or 4" longer than the lowers. That would have looked a little better rather than all the same length. With all the same length, the lowers appear longer because of their inward angle.

Attached are pictures of the wedges, the chain plates just prior to installation (polished), chain plates after a few month installed, Skene's table of scantlings, and a few pictures of the internal backing plates.



Polishing:
For a mirror finish, I start with 50 grit on angle grinder to remove the mill scale. Be really careful because you can easily gouge the surface. Next, I use a random orbital sander, starting with 40 grit and increase grits, ending at 400 (40, 60, 80, 120, 220)

With the orbital, for each grit, I first use it in 'rotation' mode, then switch to random orbit.

When finished with random orbital sander, I hand sand, beginning at 320 and ending at 2000 (320, 400, 600, 1000, 2000).

I've experimented with no lube, cutting oil, paint thinner, water/soapy water when hand sanding. There seemed to be no difference among the lubes, but dry was definitely not the way to go. You need something to carry away the metal particles.

After sanding, I polish with Black, White, then Green compounds, using my lathe with a chuck to hold the polishing wheels. Polishing flat surfaces with polishing wheel (see post #13) chucked in a rotating device is not the best, but that is what I had. It would be better to have polishing disks because the wheels can leave variations in the quality of the final surface.

Polishing flat stainless steel is hard work, but necessary to maximize corrosion protection.

After polishing, I clean with paint thinner then it gets a hot water/citric acid bath to passivate the surface.

Lastly, brief soak with cold water and baking soda to neutralize the acid.

I do this only for SS that is exterior or visible down below. I did not polish the backing plates to this extent. Those are in dry environment and behind cabinetry.

That's why I chose bronze for chain plates.

[Bowdrie]

Originally Posted by MicHughV

Besides many other skills and talents, I'm also a licensed Professional Engineer, specializing in structural marine engineering for the past 35 years or so....what's your gig ?

Good for you.
I hope your more knowledgeable than the PE, (who while I was installing a refrigeration system,) told me with a straight face that ice boxes didn't need insulation on the lids because heat rises and needed a way to escape.

On the subject at hand, I will give 2 scenarios, one that I've observed many times, the other hypothetical.
Observed.
Many older wooden boats have internal chainplates, (this was seen as a higher standard of construction for yachts, as opposed to a "fishing schooner", for example).
In such construction the chainplates came thru a caulked slot on deck and were secured by flat head/fin head/carriage bolts, that were recessed into the planking and plugged over.
On the inside there was backing blocking to span the plank seams, to which the chainplates lay against, many/much of the time without any form of adhesive beyond a layer of Dolphinite.
If you just keep cranking on the nuts you never acheive a very high clamping force, you just pull the bolt head deeper into the wood planking.
Result: next to zero "friction", tension loads all taken in shear by the bolts.
Hypothetical.
Let's suppose your chainplate uses 4 bolts.
Now, let's suppose we could just clamp the chainplate to the hull with 4 special C-clamps, each applying a compressive load equal to what their respective bolt would.
Now we would have a chainplate that was held in place by, (in some words,) the "friction" forces generated by the clamps.
Would you trust that to hold?
Theory vs Reality?

[scherzoja]

Two attachments failed to upload. Here they are.

[MicHughV]

It's never been my intention now, nor in past, to bring up my qualifications on any particular topic....rather, I try to explain, in the most practical way I can think of, how things work, that I'm particularly familiar with to someone that may not have the technical background that I do.

I've come to learn that there is a number of extremely smart and inventive people on this forum that forward an opinion or thought, based on their particular experience. It doesn't take a degree or otherwise, to put forward an idea, fix or solution, based on their personal experience, often better than a "professional opinion".

I just chime in on occasion, where I see a need for a technical explanation to better illustrate the topic at hand.

When I was a youngster, 20-something, I set out to see the world on a boat I built with my own two hands. I had numerous "professionals" guide me, often whenever they saw something amiss, far more often than I willing to admit too.

So, in return, I'm returning the favor.

[Bowdrie]

Originally Posted by MicHughV

It's never been my intention now, nor in past, to bring up my qualifications on any particular topic...

Please, no MicHughV.
If you or anyone else reading these posts think that I was trying to disparage knowledge gained by formal education or developed techniques, then I have no qualms about apologizing to all, and I do so now.
I've been able to glean much from these forums on many aspects of boats/sailing.
As an "old guy" sometimes it's hard to learn/accept "new tricks", but I'm working on it.

[MicHughV]

no harm done

[GILow]

Hmm, it all got very technical while I was working. (Back in paid employment for a while to pay for all of this.)

For the record, I've already got quotes to have these cut. With water jets of all thing. Who'd have guessed you can cut stainless steel with water?

Just lining up my options and seeing how each option plays into what is getting to be a complex moving puzzle of paid work, boat selling and boat refitting.

Thank you to all who have contributed.

[RaymondR]

[QUOTE=GILow;3677841]Who'd have guessed you can cut stainless steel with water?
[QUOTE]

Yep, they cut all sorts of things with it. it only takes 30,000-40,000 psi. They introduce grit into it for some materials.