External Chain Plates

[darmoose]

I have watched this thread for awhile and feel the need to comment only now for my own reasons. I am an engineer and have always adhered to the axiom "form follows function", however, when it comes to the beautiful lines of our Cape Dories I cannot help but strike a blow for asthetics.

I do not know if exterior chainplates are stronger than the original design of our boats. I do know that the original design has stood the test of time in spades, with only the weakness of a poorly selected material to blemish the design.

Removal and replacement of that material while maintaining the original design certainly corrects the problem. Failure to remove the offending material (mild steel) from the hull especially, regardless of your final design plan does not solve the problem, as the steel will continue to corrode in the hull and cause various ongoing problems.(see pictures above)

So, firstly, if one must remove the mild steel, and secondly, if replacing it with an inert material that can never rust (thereby rendering easy and frequent inspections unnecessary) restores your boat to the design and condition it enjoyed on day 1, then thirdly, having accepted the first and second notion, it cannot be easier to design and install a completely new system (while also needing to obliterate any remaining elements of the original design) for no proven added strength, than it is to simply replace the offending mild steel backing plates .

I owned Mystic Rose for twelve years, and without exageration, I can say that I never failed to "glow" when strangers "gushed" at her beauty, and she never failed to solicit those comments wherever we went. I do not know if external chainplates would have dampened those gushers. I feel sure they would not have added to them.

Cape Dories are relatively inexpensive sailboats that enjoy a reputation for strength and beauty disproportional to their cost, no doubt due to great design by Carl Alberg. Were it not for an unfortunate choice of material early on, we would not even be discussing chainplates.

I for one, am loath to mess with Carls design, and would personally hate to see Mystic Rose's beautiful long lines interrupted by external chainplates.

Darrell

PS I believe one must fall in love with a sailboat, just as with a woman. Her lines and asthetics compell us.

[Andy Denmark]

Many years ago I toured the Hatteras Yachts production facility 35 miles from my home. One of the projects underway was a repower on a huge power yacht -- removing two large V-12 Detroit diesels and replacing them with Caterpillars. To do this huge holes were cut through the sides of the boat. I'd guess the dimensions were something like 10 ft long and 6 feet high. The engine removal was accomplished with a fork lift. I asked and was told that this was by far the simplest and most cost effective way to do this without tearing out much of the fine interior joinerwork plus figuring out how to lift this much weight in a severely confined space. Doing the laminate repair to the topsides and resulting cosmetics was the easy part using standard procedures.

This approach started me thinking of how to accomplish a top-down replacement of the mild steel chainplate underpinnings used on many Cape Dories. The entire job would be done through a hole in the deck between the toe rail and some distance along the flat portion of the deck. I'd use bronze (alloy yet determined) in lieu of stainless steel for the metallic portion and would distribute the loading down the inside of the topsides as far as practical. The goal, as Darrell observes, is to minimally disrupt the stock appearance of the finished product. With careful design, and utilizing vacuum bagging (simple one-off molds), high ratio glass to resin molded pieces would fit along the side hull curvature and integrate with a horizontal, flat sub-deck assembly that incorporates a 1/2"bronze plate that would underlie the 3 chain plate fittings. This method uses several separate pieces that are bonded together as they are installed. Only the brnz plate would be in one piece.This method would require some careful measuring and trial and error to make sure everything fitted through the rather long and narrow hole in the deck and still mate up properly. Dry fitting before final assembly would be essential. When everything fitted up it would all have to be properly bonded with a minimum of "inside" work. The easy part would be adding back the decking and finishing everything so it still looked like the boat was built this way.

I think this method will work and probably be stronger than when the boat left the factory. It would be a head scratcher, for sure, but I'll wager it can be done successfully. As with many such endeavors, the second and third attempts will be quicker and probably higher quality than the first shot. If I had a decent shop, tools and a willing owner I might give it a try. I'd have to be about 15 years younger and more willing to revert back to "resineering" yet again, though .............. if someone who has a decent workshop and time, and a boat can be found that's headed for the scrap heap to be used as a trial horse, I'd be willing to devote some time and energy to giving this a shot as a "consultant" to determine the practicality of this fix.

Just some thoughts ...........

[fenixrises]

Hi all,

Make no mistake, Cape Dorys are beautiful boats. The question of internal vs. external chainplates is largely one of esthetics and ease of the work involved. On my CD28 the steel backing plates were already gone when I purchased the boat. To distribute the load of the padeyes to the hull a PO had new internal SS chainplates fabricated and installed. The job was not done very well because of the difficulty in accessing the inside of the boat.

That is why I removed the hanging locker and the head cabinetry. Then I made big access holes in the molded liner to be able to get at the inside of the hull. This was not all that difficult as the cabinettes where not glued together. For me asthetics had to take a backseat to strenghth because of the sailing I planned to do with the boat. And I definitely went overkill on the refit.
But when 1,000 miles from the closest land it is comforting to know that the rig is bullitproof.

Just ask the guy I met in Guam last year whose mast failed just before reaching Midway from the Marshalls. He had to drift sail downwind for 21 days under jury rig.

I would think that when retrofitting any of these boats access is the key. But it can be a daunting task to the uninitiated. As for the size of external chainplates keep in mind that on the previous boats that I built 1/4" X 2" SS flat bar was used for the chainplates. These boats included Westsail 32's and 43's and BCC's.

If you want to do the math Skene's Elements of Yacht Design has a handy table for determining chainplate size. Although I am only guessing I would say that the tensile strenghth of one 1/4" X 2" SS flatbar would enable you to lift any of these boats.
But be aware that many of them used carriage bolts to secure the chainplates to the hull. This is a real no-no as the punch press that cut the square holes in the flat bar also creates stress points at the corners.

These SS chainplates are also subject to aenerobic corrosion on the side bedded to the hull. Something that often shows up now that these boats are approaching the age of about 40 years. Perhaps the reason Larry Pardy made his chainplates from cast bronze.

Anyway SunShine and I are still relaxing in Port Carmen, Cebu Island, Philippines. Will be off for a short visit to points west early next year.

Take care,
Fred

[darmoose]

Fred brings out some interesting information about the "movement" to external chainplates whether on Cape Dories or any other boats, they are quite often "overkill" in regards to size.

When I replaced the bowsprit on Mystic Rose, due to the deteriorated condition of the bow backing plate in the anchor locker, as well as the difficulty with access, I installed a threaded rod to tie the new bowsprit down to the hull. The rod was 1/2" dia. 316 SS, and according to the specs had a stress breaking strength in excess of 16,000 lbs.

The cross sectional area of a 1/2" rod is .19625 sq inches while the cross sectional area of a 1/4"x 2" external chainplate is .5000 sq inches.

So, each external chainplate made of 1/4"x 2" 316 SS has a breaking strength in excess of 40,000 lbs.(almost four times Mystic Roses weight). There are typically six of these.

I wonder what the breaking strength of that little tang at the top of the mast holding the other end of the shroud is???

Darrell

[lewiskeizer]

The chain plates in my new-to-me 1978 CD 28 were showing a lot of corrosion as well as they could be inspected. I bit the bullet and hired an experienced sailor and boat builder to open up the areas and come up with a plan for G-10 or stainless. He preferred stainless butt plates with brackets to the bulkheads and installation of short partial bulkhead for the main shroud. We're documenting the refit with photos. I'll leave the inside hull areas covered with a very large piece I can remove so I can use the area for longer-term canned food and other storage, and I can open it up for inspection and occasional squirts of Corrosion-X. Will post photos of the process when done. Cost in parts and labor about $2500/side. Sticking with Alberg's asthetic--no external chain plates.