Chainplates

rescuesailor · Post by **rescuesailor** » Fri Mar 14, 2008 10:24 pm

Hi Again. I have a new question concerning chainplates. I am rebuilding a TrueNorth 34. This is a chinese boat built in the 70's. The hull is solid glass that is over one and half inches thick at the turn of the bilge. It is close to 3/4 of an inch at the top. However, the old chainplates were simply a piece of half inch SS welded perpendicular to another plate that was placed under the deck. They were not attached to the bulkheads or the hull. That's all that held up a 50' mast. When I bought the boat the tension on the shrouds was pulling the deck up. I have decided to attach the the main and the rear chainplate to the hull. The main chainplate takes two shrouds while the front and rear handle a single shroud that are attached just below the lower spreader. This move only requires a couple of inches outboard. My question is how long should the chainplates be? I was thinking 24 inches. This would make the main chainplate 3 3/4 inches wide 24 inches long and 1/2 inch thick. What do you think?

deckhand · Post by **deckhand** » Sat Mar 15, 2008 1:41 am

Are you planning on making the chainplates external or keeping them internal but further out?

Otherwise, I think it sounds solid.

Post by **Tim** » Sat Mar 15, 2008 7:10 am

That sounds abundantly sufficient.

rescuesailor · Post by **rescuesailor** » Sat Mar 15, 2008 12:01 pm

The main and rear chainlates will be external. The forward plate will be located in the middle of the deck and I have added a bulkhead to attach them to. I was thinking the 24 inches might be too much but I feel safe with that. The cost is a little frightening however.

Hirilondë · Post by **Hirilondë** » Sat Mar 15, 2008 3:58 pm

Stainless steel 1/2" thick? I don't think the Little Harbor 75 we take care of has 'em that thick. Maybe close, but for a 34 ft. boat it would seem over-kill.

CharlieJ · Post by **CharlieJ** » Sat Mar 15, 2008 4:36 pm

I agree- I think that's way overkill. My chain plates on my 35 foot trimaran were 1 1/2 wide by 1/4 thick stainless. PLENTY of strength there.

Multihulls load rigging far more than monohulls do, size for size, which is why they carry heavier rigging.

Just be sure you drill the holes correctly- SLIGHTLY over sized, but only VERY slightly. There should be no wobble between the clevis pins and the hole- just a smooth slide fit.

rescuesailor · Post by **rescuesailor** » Sat Mar 15, 2008 9:15 pm

The old chainplates were 1/2 inch thick. I was trying to keep it the same. I would love to reduce the thickness as it will save a bundle. I have no idea about the load allthough my main is huge. The old shrouds and stays are 1/4 material. The boom measures 20 feet, the main has a huge roach. The boat has both a boomkin and a bowsprite. Overall length is 42 feet. The boat is 34 on deck. Where can I find information on loads and strength of the rig? This is a deck stepped mast. I don't want to under build it.

Rachel · Post by **Rachel** » Sat Mar 15, 2008 9:35 pm

Side note: True North 34 - I buddy-boated with one for awhile and was totally taken with the built in "library chairs" in the saloon. Do you have those?

Okay, on topic: Doesn't Skene's have forumulas for chainplates?

Edited to say "Aha, I found my reference, and yes, Skene's Elements of Yacht Design does have this information. I found this out because I once made a query in a "chainplates" thread on another board wherein I asked if there was a way to figure out how large you'd need to make bronze chainplates, if you were replacing stainless steel (I thought they'd need to be larger but wasn't sure).

Robert Gainer (whom I've asked to please drop in here sometime because I think the place would suit him), answered me with this (I don't think he'd mind if I quote him here):

"The easiest may to design a bronze chainplate is to use the cookbook approach and one example is in Skene’s Elements of Yacht Design. The table is on page 188 in the 8th edition. The design is based on a factor of safety of 2.5 and a strain of 4/3 of the breaking strength of the appropriate wire. It’s interesting to note that the size for stainless and silicon bronze is the same and only aluminum (Alloy 5086-H32) is different in that it is beefier. Any good library will have a copy or I can post the size for a given wire. All the best, Robert Gainer."

I would double-check this, of course, but I would think you'd want to find some sort of way to calculate the size, whether this one or another. The ones you mentioned did sound awfully beefy.

Rachel

Shoalcove · Post by **Shoalcove** » Sat Mar 15, 2008 9:44 pm

I replaced my chainplates (Cheoy Lee35, #15,000) with 5/16 SS. Recommended by Bob Perry himself. I guess if you have the bucks the 1/2 will let you sleep like a baby. I'm no engineer, so I can't offer advice beyond that.
Best regards, David

rescuesailor · Post by **rescuesailor** » Sun Mar 16, 2008 9:36 pm

Rachel,

Please tell me more. What is the size needed for 1/4 wire? I would really like to order the material soon so I can make the needed fiberglass modifications to accomodate the chainplates.

Rachel · Post by **Rachel** » Sun Mar 16, 2008 10:13 pm

Well my query happened to include the fact of 1/4" wire, and below is the response I got. The thing is, part of me gets why a given wire size calls for "x" size chainplate on ANY boat (breaking strength of wire, I think); but another part can't quite get my mind around that fact, and wants to consider boat design, angles, etc.

I guess it's like designing a ground tackle system, wherein once you know the strength of one of your main desired (or unchangeable) components, you just size the rest of it so as not to create a "weak link."

Still, please don't take this as gospel, without double-checking, since I don't know enough to endorse it for sure.

*****************

"1/4” 1X19 wire chainplate design for silicon bronze from 8th edition of Skene’s Elements of Yacht Design. Based on 1/2" pin and 1/4” wire.
1) Headstay and backstay tangs are 5/16” thick; chainplates for shrouds are 1/4” thick.
2) Width in both cases is 1 3/8”
3) Center of hole for pin is 13/16” from the end, and the end is round with an 11/16” radius."

*****************

Note: There is nothing about length of the plates; I suppose that would depend on your installation factors.

Second note: If I'm reading your original post correctly, you have one chainplate that holds both the cap shrouds (uppers) and the (aft?) lowers. I would guess that that would change things, because you would then have the breaking strength of two wires, and since you tension each one to within (say) 10% of its breaking strength, that would be twice the tension. I don't know whether you would just double the width of the plate, or whether the change in stress would be exponential. I would guess the former, but...?

Rachel

rescuesailor · Post by **rescuesailor** » Sun Mar 16, 2008 10:59 pm

Thanks Rachel,

Your right about he rigging configuration. I have thought about seperating the two, but it makes since to maintain one chainplate since they are so close together. Seems like I am back to where I was. 1/2 inch is large but I would rather be overbuilt than under. Thanks for all the input.

David

Rachel · Post by **Rachel** » Sun Mar 16, 2008 11:56 pm

I meant twice as wide, not twice as thick, i.e. like you had two same chainplates side-by-side and then just merged them into one.

At least that's how I interpret it.

R.

feetup · Post by **feetup** » Mon Mar 17, 2008 10:44 am

If you treat the shrouds as though they were rods of the same diameter and calculate the cross sectional area of the total number of shouds to each chain plate you will come up with and area that you can use to determine the cross sectional area of the chain plates at the most highly loaded point, which will be at the area around the clevis pin holes. For example, if you have one 5/16 dia wire you will have a cross sectional area of .0767 sqare inches. Because the wire is wrought by pulling it through succesively smaller dies the grain structure is ideal for linear strain whereas a chainplate will come from a piece of plate, or flat bar where the grain structure is linear and does not follow around the clevis hole. If the strain is in line with the chain plate (as it should be) it will be trying to shear the small area above the clevis hole out of the plate directly in line with the grain. as a consequence you must use a larger safety factor, X2 or greater. Simply put, the area of the small section of chain plate that surounds the clevis pin should be at least twice the cross sectional area of the shroud, or in our example .1534 square inches. As you can see, a 1/4 inch thick chain plate with 1/4 inch of material remaining above the hole would not suffice, requiring almost 5/8 inch of material. A 3/8 inch chain plate with 1/2 inch of material left would be more than adequate. This is basic engineering, not based on actual practice. It is amaizing how little has sufficed in so many cases. I look at chainplates and shrouds to be one of those little areas where too much strenth is just enough, considering the consequence of a failure.
The other important factor with chainplates is to ensure that the pull of the shrouds is directly in line with the plane of the chainplate. If the pull of the shroud flexes the chain plate, especially at a stress riser like the top bolt hole it won't take long before stress induced cracking begins. Stainless steel is rather prone to work hardening, also, with stainless, stress corrosion cracking is likely as well due to the chlorides in the salt water.
Although not quite as strong initially 316 stainless is far better than 304 due to it's higher nickel, lower chromium content, and the addition of 2-3% molybdenum. If there is welding involved, insist on 316L because of its lower carbon content. There are more exotic stainless steels that are even better for marine use but thier cost does not offset the improvement over 316/L.

rescuesailor · Post by **rescuesailor** » Mon Mar 17, 2008 8:43 pm

Feetup,

I was really hoping to get an answer like yours. While I have no formal training in engineering I know the value of good math. To make sure I read your response correctly let me try to paraphrase, 3/8 316SS with a minimum of 1/2 above the holes for the clevis pins will provide the given strength for the two 1/4 wires on the main shrouds. This would mean the centerline for a 1/2inch clevis pin hole would be located 3/4 from the top of the chainplate. The chainplate should be bent so that the pull is in line with the shroud. I hope I have this right!

feetup · Post by **feetup** » Tue Mar 18, 2008 10:06 am

David;

You've got it. Do both the shrouds come from the same clevis pin? It would work if they do but it would be better if they pulled from their own pins symetricaly about the center line. The other end of the problem is the fastening to the hull but you sound like you intend to do that with the same attention to detail.
As an aside, if you put a bit of effort into bringing the finished chain plates to a reasonal degree of polish you will REDUCE the likelyhood of pitting corrosion on the hidden side of the metal. Besides they take on that Hinkley look. Just give them time after polish to passivate (form an oxide layer) before you slather them with sealant and install. 24 hours is plenty, or you can speed it along by giving them a wipe with a reasonably strong acid solution for a short time then rinsing them with clean water. The acid used to passivate stainless commercially is a mixture of nitric and hydrofluoric acids, but the muriatic acid used to clean masonary will work, and you can buy it at most hardware stores.

Let us know how it goes

Feetup