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Jon Neely 02-04-2014 16:24

Advice on external chainplate bolt placement
I am going to bounce this question off a few local riggers but I wanted to ask the community as well. I am currently in the process of fabricating new bronze chainplate for my HC33 due to the originals are SS and are embedded into the caprail and hull. I have purchased the the 3/8'' stock and have cut it to length. My biggest question as of now is bolt placement and size of bolt. Its hard to find any hard facts on sheer strength of 3/8'' or 1/2'' hex silicon bronze bolts. My rig currently has 3/8'' (breaking strength 17000#) outer shrouds and lower and runners are 5/16'' (12000#). My second questions is how many bolts to use and is there a secret method of spacing? After walking the docks here in Berkeley I got a good idea on external chainplate boats it it mostly went into 3 categories. Most half have 4-5 bolts equally spaced all the way to the cap rail bend. The next set of boats have 3-4 closely spaced bolts under the rub rail, a bend with a gap of a good 6-12 inches depending on the size of the boat and then a bolt just under the cap rail. Then the last set is the same as #2 but without the bolt under the cap rail. After talking to Pete up at Port Townsend Foundry he made it sound like bolt placement is not a big deal except for that last bolt right under the cap rail. Now if someone could please explain to me why that is the case, and maybe it is worth another call or email to Pete. Shannon and I are going to walk the docks agin tonight, Ill try to snap pictures of each design incase I didn't explain well enough. Any Help is welcome!



a64pilot 02-04-2014 17:10

Re: Advice on external chainplate bolt placement
Your talking shear loads, and 5 or 6 .5" bolts in shear would have an astronomically high shear strength, I believe way in excess to the breaking strength of the shroud it's attached to. Be interesting ot say what more knowledgeable people will say

psk125 02-04-2014 17:54

Re: Advice on external chainplate bolt placement
A lot of times the placement of the bolts has something to do with what's inside the hull and what's being bolted through. Plank-on-frame boats will have bolts where they cross frames, for example. Fiberglass boats may locate theirs where they cross stringers. Your existing embedded chainplates are probably also bolted in somehow. Engineers like the belt AND suspenders approach, and HC is not known for being under-engineered. You may want to replicate your existing setup exactly to avoid problems like having them pull out, or break from being loaded at an unplanned angle.

Jon Neely 02-04-2014 18:19

Re: Advice on external chainplate bolt placement
Indeed, shear. I heav what you are saying PSK. From the info I have gathered from other HC33 owners that have dug out their chainplates. The SS chainplate I see coming threw the top of the cap rail is strait until it hits the main curvature of the boat under the Lip on the side of the hull. There It goes into an "S" shape where it continues down the hull another 2-3 feet. My Idea so far is to use the very thick hull (2'' Where I plan to drill) and the existing chainplates as my backing plates. As we did our second walk it seems pretty clear that the main load bearing bolts are where the most strength is in the hull. What I ponder about is why did I remember Pete make such a big deal about that last bolt under the cap rail. Ill shoot him an email. Would 3 1/2'' silicon bonze bots be ample? Im guessing they would. Here are some examples on the dock:
This one is only held by 2 Bolts! ^^^^^
Again 2 Bolts

Nicholson58 02-04-2014 18:20

Re: Advice on external chainplate bolt placement
The shear strength of the bolt is far less important than the material and the location & spread of the bolts. There is probably nothing where you will anchor the bolts that is strong enough to shear a bolt - AND - you should not plan to stress them to more than 10% or so of yield for this type of use The real measure of strength is to spread the loads evenly over enough of the vessel that the loads 'disappear' into the structure. Unless you are building a go-fast ultra-light throw-away racer, you don't want the vessel or structure stressed anywhere near yield.

Lojanica 02-04-2014 18:58

Re: Advice on external chainplate bolt placement
The "last bolt" near the cap rail/ rub rail is "important" because that's where the flex will most likely occur. You really only need 2 bolts. Top and bottom with the top bolt most important. The others just reduce the load on those two bolts. Also the bolts may need to distribute the load as the material through bolted may need reduced loads and not the chainplate per se. If the chainplates start flexing at the top then the metal can change its characteristic and "work hardening" or "stress corrosion" could occur, etc.

transmitterdan 02-04-2014 19:03

Re: Advice on external chainplate bolt placement

I would think long and hard about this before finalizing. The bolt placement isn't the biggest concern in my view. One place you might go for advice is Brion Toss. They have some experience with this boat and others like it. You can stop right here if you like as I have no where near their knowledge.

Ok, you couldn't help yourself so here goes...

The chain plate is the linkage between the mast and the keel. This connection transfers the righting moments which can be substantially more than the normal loads. When the boat is exactly at the limit of positive stability the load in the chainplate will be maximized. For your boat that happens when the masthead is a few feet under water. That's precisely when you don't want to find out the chainplate placement and mounting doesn't work. Incorrect placement of the chain plate can flex the hull in weird and wonderful ways. That's why chainplates are normally tied to a major bulkhead or purpose built knee designed to spread the load and efficiently transfer it to the keel without forcing the hull to flex too much.

The bolts will not be in shear. Multiple bolts are there to apply sufficient pressure to prevent the plate from slipping. The friction of the plate against the mounting surface should hold the plate from moving under worst case loads. Bolts in parallel can't share the load well unless one bolt begins to yield allowing another to begin load sharing but by then it's too late. If the bolts are so loose such that the plate slips even a little then the only thing holding the plate is one bolt in shear and the rig will be in grave danger.

I would trust a known rigger such as Brion Toss or a NA such as Perry to answer your question. But I would not trust just looking at what somebody else thought was ok.

Boatguy30 02-04-2014 19:10

Re: Advice on external chainplate bolt placement
The bolts take the load not magic fiction between the hull the metal. What is the surface area of a 5/8" clevis pin? What is the difference in strength between bronze and stainless? Do the math on what area you need in bronze to equal that 5/8" ss
pin. Double it, bolt them on.

Saltyhog 04-04-2014 14:47

Re: Advice on external chainplate bolt placement

Originally Posted by Boatguy30 (Post 1508242)
The bolts take the load not magic fiction between the hull the metal. What is the surface area of a 5/8" clevis pin? What is the difference in strength between bronze and stainless? Do the math on what area you need in bronze to equal that 5/8" ss
pin. Double it, bolt them on.

I'm afraid you are wrong. This type of joint is know as a slip-critical-joint. If it were not for the friction between the plate and hull (or bulkhead or structural knee) The fasteners would tear through the members they are attached to. A slip-critical-joint has far more strength in shear than the bolts holding it together as it distributes the load throughout the joint and not just at the fasteners. If it were not for the friction in this joint, you could not bother to tighten the nuts on the bolts.
To the OP: I agree with Dan on all counts. Consult a pro rigger. Brion Toss is a good choice if he's local. He'll likely open your eyes to a host of other considerations to your project you had not even thought to ask about.

Stumble 04-04-2014 16:36

Re: Advice on external chainplate bolt placement
The only advice I ever give on moving to external chainplates is this....

Unless you have the advice of the NA who designed you boat, a competent NA to redesign your boat, or a professional rigger (Brion Toss as an example) not just the local splices then you are replacing the knowledge and experience of a professional who makes a living doing this with your own. It's foolhardy and asking for trouble unless you know exactly what you are doing.

It's easy to figure out how thick the bolts need to be, but when you drill thru the hull, how much residual strength is left in the hull? What effect does having the chainplates outboard of their designed placement have on the rig?

I don't like external chainplates anyway since I can't see the advantage in replacing one hole in the deck with five holes in the hull, but that's a different issue.

Absent professional advice you are potentially weakening the most critical structure on the boat. I can't recommend it.

Terra Nova 04-04-2014 16:49

Re: Advice on external chainplate bolt placement
hog--good post.

OP--why are you trying skimp to save $ on a few bolts? It isn't the shear strength of the bolts that limit the load carrying ability. Larger bolts have more surface area resisting tear-out.

Try this--stick a toothpick through a piece of paper and pull sideways. That rip in your paper is what each of the bolts is trying to do to your hull's fiberglass. Each bolt you add helps each other bolt. The chainplate and backing plate sandwich the hull between them, each bolt adding resistance to pull-out.

2-1/2" or 3" apart is not too close together; 5-6 bolts in each plate. Make the plates so they spread the load over as large an area as practicable.

roverhi 04-04-2014 17:11

Re: Advice on external chainplate bolt placement
4 3/8" bronze bolts spaced 3" or so apart should do you just fine.

External chainplates are better than through the deck chainplates. No problems with the chainplates leaking into the core as you have with through the deck chainplates. Still have crevice corrosion possibles where the chainplates meet the hull if not well caulked.

Jon Neely 04-04-2014 22:20

Re: Advice on external chainplate bolt placement
Defiantly not trying to skimp on bolts du to price, but here is what Im working with. On the HC33 we have a the hull, then a raised part of the hull (sure it has a name but I don't know it) then that goes into the teak bulwark then cap rail. I would like to located the main bolts on this raised piece of glass. From what other owners tell me this is where the main stringer is and where a stainless cross member is that hold all three chainplates together when they where laying them up in the hull. I would like to locate however many bolts necessary here vs getting into the teak bulwark because the bulwark is hallow meaning I would have to drill and inspection hole on the inside, build it up to the same thickness as the inside glass with G10 or fiberglass tube(1-2") then glass or seal it up with an exposed nut. If it is safe and I can get away with all the bolts under the bulwark. I really don't care how big the bolts are or how many I need to use. This Fiberglass strip is 8'' wide. I picked up some purple heart to be a spacer under the chainplate between the the bulwark. Thanks for all the responses. I am compiling an email right now to Mr.Toss.

Terra Nova 04-04-2014 23:39

Re: Advice on external chainplate bolt placement

You'd like to move the chain plates outboard and have them peek over the top of the non-structural bulwarks? And limit their mounting and length so they don't go below the raised shear band?

Nicholson58 05-04-2014 05:47

Re: Advice on external chainplate bolt placement

Originally Posted by Terra Nova (Post 1509984)

You'd like to move the chain plates outboard and have them peek over the top of the non-structural bulwarks? And limit their mounting and length so they don't go below the raised shear band?

I am beginning to get a better picture on this. First, if you are not a structures guy with some capability to calculate the loads you are making me nervous.

TRANSMITTERDAN is offering a proper understanding of the forces. Tera Nova's comment about loading the thwart in both lifting (tension) and bending will surely put you at risk.

If you insist on moving shrouds outboard from an existing through deck location to outboard you will need to consider the long cantilever of the forces over the raised thwart. The thwart on our boat is several inches thick and solid glass, well capable of securing part of the loads. You need to be sure yours is likewise strong. Your thwarts appear quite tall. You might need to make the part of the strap above the deck very thick or build out structure to prevent the thwart from being loaded at all.

You will need to locate the shrouds essentially where they are now, presuming that they are currently anchored on a bulkhead or other purpose built structure. This will permit you to make accommodations inside to transfer the loads to the necessary structure. You might also consider backing plates, either flat or shaped to fasten to the internal structure and the bolts.

To get a simplistic grasp on the order of magnitude of the forces, consider your boat on the hard, on its side, mast horizontal and held down only by the mast. If you have an idea of the masses, you can even calculate what these forces could be.

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