Chainplate Knee Replacement and Material

Hey Folks - Longtime reader and first time poster here - I'm not sure if this question should be posted here or in Boat design, but here goes! I've been restoring a 39ft Frers custom sloop, his first commission after leaving S&S in 1969. Hull design is very similar to an S&S Swan 40, though lighter displacement at 16,300lbs, and with 670sqft of sail area. The boat and sistership were built by Frans Maas in just three months to be ready for the Admiral's cup series, and many of the period-acceptable construction methods that were employed at the time have now become issues.

I'm currently considering the chainplates, which consist of a 5mm stainless gusset and 8mm welded lug, glassed into the hull via three long welded tabs. The area of the hull has had no visible moisture damage, though water has leaked down the lug (which penetrates through the deck) for decades. The deck rotted in this area from water intrusion, though the hull shelf appears quite solid. My concern is the inability to inspect where the chainplate pierces the deck and shelf, and that they're over 50 years old and have never been serviced or replaced. A few incisions in the shelf adjacent to the lugs have revealed minor staining of the metal, with some stubborn areas that don't appear to clean up with scrubbing - crevice corrosion? Fatigue? Can't say without removing, and I don't want to trust them as they are.

In anticipation of long distance sailing and bringing peace of mind, I'd like to address the old chainplate lugs in one of two ways. The first option would be to cut the existing lug at the hull flange, and drill the 5mm plate to receive new chainplate lugs. I don't look forward to drilling 1/2" holes through the stainless gusset, but it would be the simplest way to install new lugs - the gusset and glassed tabbing is dry w/ no delamination or rust evident.

The more thorough option is to remove the stainless frames entirely and glass in new composite knees in their place, through which a standard stainless chainplate lug would be through bolted. In this case, I've drawn the knee to be longer and wider for greater surface area against the hull. What to use as a base material for the knee? G10, Plywood w/ epoxy anulus for each bolt, Coosa? According to the blueprints, the hull is solid glass with extra laminations in the way of the chainplates.

I'm not an engineer or designer, but prefer to work along the adage of "nothing too strong ever broke". I'm looking for input from engineers or designers on the forum as to the validity of either of these solutions, as well as preference on base/core material for the composite knee. I'm also very happy to pay an engineer or designer for their time and assistance if anyone wants to message me directly.

Thank you in advance!
-Eric

Quest of Paget, Login • Instagram https://www.instagram.com/sv_questofpaget/

 

Too strong can be an issue by creating stress risers. A too rigid structure member may cause other parts to fail. Why don't you send German an email and ask about it. He designed it after all.

 

Thank you! That's quite true - I don't want to make the chainplate knees too stiff in relation to the rest of the hull. Interestingly, Frans Maas didn't build the chainplates quite the way Frers designed, at least not according to the construction plans that I have. I should have noted that I have attempted to reach Frers as well.

 

I just had a look at your Instagram photos - Quest is simply gorgeous (I have a soft spot for all S & S designs, and their relatives), and you are doing a wonderful job with the restoration.
I had a look for a photo or two of the chainplates taken from down below, but couldn't see any - if it is possible to photograph them easily, could you post some photos of them for reference please?

 

Where are the photos of the boat?

 

Thank you - It's been a lot of work so far, still plenty to go, but at least the deck and cabin are structurally solid now. A big improvement over hopping from deck beam to deck beam!

This photo shows the aft lower chainplate, starboard side. It's the only section that doesn't have subsequent layers of interior paint applied to it as it's usually hidden by mahogany ceilings. You can make out the middle and bottom tabs glassed into place, and the upper tab is hidden by the trim, but is just below the hull flange. I've also included a scan of the original construction drawing drawn in '69.

One thing I like about this design is that any water coming down the lug drips off the end where it is welded to the gusset, instead of down into the glass. Perhaps I should be, but I'm not actually concerned much with the structure within the glass.
-Eric

 

If you're not concerned about the structure in the glass, simply cut of the lug and weld a new one to the knee. A good welder can do it in situ, even with that beam there.
Second option would be to take it out and have a new one made. Either stainless, bronze or titanium, as you wish. Bronze and titanium don't have crevice corrosion problems.
Both options will require a deck repair with a generous epoxy plug, sufficient clearance for sealant and a top cover plate.
Third option is to replace with a composite chainplate. G10 is convenient to use as a core, saves some lamination time over ply or foam. The holes and uni bonding from your drawing are not necessary, just sufficient tabbing. The advantage is the absence of hull penetrations, the deck is epoxied to the chainplate.

Do you know what the original chainplate arrangement was as drawn by Frers? Just out of curiosity, since the present one worked fine for 50 years.

 

I'm not a designer at all, but have some opinions for whatever they are worth. Take it with a grain of salt but figured that i should provide some insights.

Neighbors on a Cooper 37 just went through the process of removing the chainplates in the same fashion that you have. Their stainless plates ran down beyond the knees and into a giant glassed in stainless tab. They did this exact same thing that you proposed, but their knee's were rotted so had to rebuild them. If your core material is structurally sound, I dont know that i'd do anything other than plug the holes and rotate the plates to fit on how you are proposing. I would definitely make damn sure the core material is protected from any moisture by glassing in some G10 tubing for the bolt holes. The biggest reason i suggest this route is for serviceability. I dont know how they are fastened to the leading edge of the knee, but it seems like through-bolting would be the better option(in the event the coring material becomes compromised in any way).

I am rebuilding my Pacific International Marine - Pacific 41 - which is incredibly similar to the Swan's. I am in the process of doing the rigging/chainplates which has lead me to find that 2 of my lower knees no longer have any core material thanks to some genius who had the deck plates fastened with 3" machine screws that went directly through the deck and into the knee coring.

This weekend i am going to be fabricating my new chainplates and in June I am planning to start the work of cutting them open and rebuilding them. I am planning to leave the "face" of my knee in which the chainplate bolts onto in place, while cutting out the backing plate side. This will allow me to epoxy in new plywood coring and build up an entire new face. I preliminary numbers shows 16 layers of 17oz biax should give me a thickness thats slightly thicker than existing material. Tabbing on the hull side will be 8-10 inches wide with a 3 inch below the knee(space constrained)

 

I have a German Frets designed boat with 10 chainplates like this-- if you ate still interested in this discussion -- lets talk about how the F&C 44' changed out out chainplates and went with a more modern & manageable maintenance model chain plate and backbone...

Cheers,

Skipper T
S/Y Explorar Conmigo

 

I am certainly interested in the new chainplates on your F & C 44 - could you post some more details please, and a photo or two of the boat as well?

 

Some photos of the boat....

 

Photos of what the Original German Frers Designed Chain Plates looked like... Mind you-- they lasted 42 years without failing... Over built-- and built to be abused for sure... Out with the old... In with the new....

 

Here are the G10 angled chainplate knees that I made out of 1/4" sheet G10--just for the FORM... Then on the hull side, I ground out a minimum diameter of 18" (some places larger depending on if I had the room or the bite) all around where the SS chainplate knees went-- and layed 8-10 layers of structural glass to build it back up into ONE SOLID sheet against the hull... (West Systems Epoxy layups) I do not have the finished photos of my chainplates yet-- as I am in the middle of re-enforcing bulkheads ATM-- and then will paint inside of the hull, chainplates included. Then-- the future of the actual chainplates will comprise of simple 2" x 1/2 x 32 316L Bars dropped down through the deck as original design...followed by 6qty 1/2" 2205 Bolts going through-- spreading the load over a 25" area...

 

Send me your digits if you would like a quick convo-- this is an old post so you probably already resolved this issue?

Cheers,

Skipper T

 

Hello everyone,

In thinking about how so many questions go unresolved on forums, I wanted to share the outcome and finished product as it related to my original question about chainplate replacement two years ago.

First, I was able to have a an email discussion with Frers yacht design, and ultimately received a technical drawing for how to build, laminate, and install new chainplates for Quest. Though the firm mostly deals with superyachts now, German Frers personally responded to my inquiries and concerns, and shared a few anecdotes of sailing on Quest and Wizard with the owners in Bermuda. With dimensions and a lamination schedule in hand for new knees, I got to work tearing out the old stainless frames and cleaning up the hull for new work.

I used a mix of an angle grinder with a diamond blade and an oscillating tool to cut around the glassed stainless tabs, and then used a heavy chisel to pry the whole frame loose. It was clear that the chainplate frames were installed after the hull and deck were constructed - They were set into thickened resin, then had about 1/4" of glass laid over them - the secondary bond of the polyester resin broke free from the hull with some careful prying, but I observed zero prior delamination.

After grinding, roughing up the glass, and cleaning the hull, I made patterns for knees that fit the hull curvature, and laminated new knees of Sapele plywood and multiple layers of 1708 glass set in epoxy to the specs in Frers' plan. The knees were set onto the hull with thickened epoxy resin, and a heavy filet applied to each side. They were glassed into the hull using layers of 18oz biaxial with epoxy per Frers' plan, with the lower shroud knees set to an angle to match that of the shrouds. I have general fabricating background, and manufactured the new chainplate lugs out of 3/8" 316ss to Frers' plan, making them slightly longer to account for the deck and hull shelf thickness not considered on the plan. These were polished and passivated prior to installation with 5/8" and 1/2" 316ss bolts, with a matching 1/4" thick 316ss plate on the opposite side of the knee. Above deck, the only difference is that the chainplate lugs are substantially larger, and run abeam instead of fore and aft. Below deck, the knees and lugs are also much larger than the original stainless gusset, and I painted and left the knees exposed in the interior for easy inspection.

The whole project took me about 6 weeks working part time on it, and then a couple more weeks to fit the interior back together around the new knees. I'm glad to have the project done, and have confidence in the structure of the new chainplates. Did the stainless steel fracture and crumble upon removal like so many scary pictures seen online? Nope! The eyes were worn oblong from 60 years of use, and I'm sure would not pass destructive testing in a lab. The lugs showed no cracks, rusty pin holes, or other deterioration where they passed through the deck, and there was very little to no rust on the tabs glassed into the hull. With that said, there was no way to see that without removing them, so I have no regrets about doing this.

Thanks again for the thoughts and suggestions posted earlier - Hopefully the photos and description of the work can help others too.