Chainplates job and hull strengthening

[Scout 30]

Silicon bronze is a great metal for boats. Of course with any alloy there are variations but I think this might be what you're looking for. I would contact this company & talk to an expert. This should never be confused with silicone bronze which is brown colored sticky **** that comes in a tube & should never be used on a boat.

C65500 High Silicon Bronze "A" - Silicon Bronze - Bronze Alloys - Products | National Bronze & Metals, Inc.

[Julien de Tilly]

Hi all,

Thanks for the good words and the advices.

I am uploading again the pictures. The first three ones show the actual setup while I was installing it. The last one shows its consequences (hull deflexion around the chainplates).

I also think a "mise en contexte" should be appropriate. It's my first boat and I have her since 2012. One of the first big project I made was to replace the chainplates. The original ones were encapsulated in fiberglass and bolted against glassed-in wooden knees. Even if I didn't know anything about it, I knew that the job was due to be done because the wooden knees were literally disintegrating between my fingers when touched them. At the time, I have also noticed flat spots on the hull around the chainplates, but many person at my marina told me it was normal because there was no cracks in the paint or gelcoat, and probably just a matter of hull design (since then, I saw other boats like mine and realized that it really was a problem). So, I thought about it and arrived at the conclusion that it should be better for me just to grind out what was there and to reinstall the new chainplates only on wooden spacers with an external backing plate (see pictures). Of course it was a big mistake. I didn't know that the glassed-in wooden knees were also structural components to stiffen the hull and spread the load. The result is that hull deflection have increased since then, and this why I need to redo it another time.

I've learned by myself that structural questions are not easily answerable. Anyhow, it seems to me the original design wasn't up to the task neither (for both problems caused by encapsulated chainplates and hull deflection). Also, modifications (namely hole drilled through my hull for the mounting bolts) have already been made. This why I think I should try to keep what was good in my first project (having the chainplates not encapsulated and the mounting bolts easily accessible for inspection/replacement if needed) and correct what I made wrong (not having thought of the structural role that the wooden knees were asked to play, and not having considered that my hull needed strengthening in this area). Of course I am not a specialist, but I am planning to work with Dave Gerr "Elements of Boat Strength" for all the calculation, formulas and as a general guide through the whole project. But I appreciate the remarks concerning security - which should always be the first concern - and I'll keep them in mind.

Also, I want to keep an internal installation.

Finally, thanks for suggesting other materials the knees could be made of. I'll think about it

Thanks to all and, again, I wish to have your opinion and hear about your experience.

[Cheechako]

Originally Posted by Rigdaddy

I've seen many monel/stainless chainplates; how common are silicone bronze chainplates? I don't recall having seen them. I'm curious because I will replace most of mine this year.

There are bronze chainplates out there. Often they are cast. Some Noreastern makers used them from the factory etc. But they are a bit unusual.
Bronze flatbar would make excellent chainplates. Might be difficult to get it done depending on size and cost though.
Regarding "the goal is to have metals that do not work harden"... this is not a concern or issue. It takes far more than the pull of the rigging to work harden SS or other metals. It takes actual bending, forging, deformation etc to do so. Also, some work hardening is a benefit and is actually required for some specifications. Most sheet stock for instance is rolled hot , but given a final "cold pass", or rolling, to work harden it to the strength requirement.

[Reefmagnet]

So you changed the design already and are now suffering for it? You should have just replaced the knees and made more effort to protect the wood from future rot. You're not the first to have done this. I know of a HC 38 that has exactly the same problems that you have. The knees do stuff other than just support the chainplates as you have unfortunately discovered.

You can redesign slightly to avoid the encapsulated chain plate. Rather than use expensive materials for the chainplates you can also oversize them out of 316 stainless steel, keeping in mind just about anything you do will survive your ownership period of the vessel.

To get the original alignment back, recut out the original chainplate holes from underneath and rebuild the knees to fit. To get the original angles, use an inclinometer app on your smartphone to measure the rigging angles and then transfer these angles to the chainplate on installation. They won't need to be 100% in alignment, but closer the better.

Here's some photos of a similar rotten knee repair that may help...









Sent from my SGP521 using Cruisers Sailing Forum mobile app

[reed1v]

Usually one just sisters the old "knee"/bulkhead/chain support since the overall hull stresses are calculated with those positions in mind. Try not to make any "hard" spots when you attach whatever your doing to the hull. Flange the attachment.

[Julien de Tilly]

Thanks Reefmagnet. So do you think I should avoid the use of external backing plates (as they were not there in the original design)? My concern about it was that they can create again hard spots on the hull (as reed1V pointed out on his last post).

[Reefmagnet]

Originally Posted by Julien de Tilly

Thanks Reefmagnet. So do you think I should avoid the use of external backing plates (as they were not there in the original design)? My concern about it was that they can create again hard spots on the hull (as reed1V pointed out on his last post).

I wouldn't worry too much about creating hard spots on the hull with knees. Liners and bulkheads are a different story. On my boat the construction method was to layer about 1/8" fibreglass both sides of a piece of 1/2" ply sheet and then cut the knees from the sheet before fully tabbing them in. This method had no affect on my hull in 30+ years. Another, smaller boat I owned had the ply knees glued to the hull and then fully fibreglassed over, again without any seemingly detrimental affects. If there are genuine concerns, the risk of hard spots is eliminated by the use of a thin strip of softish material like high density foam paced between the hull and the knee if you are really concerned due to thickness of the hull.

Having said all that, your original problem (like mine) was that the ply had been damaged by water. Just replicate the original construction and work on removing the source of the ingress of water that lead to rot (hint - chainplate holes, butyl). My only concern is if your indented hull has fatigue cracking where it has distorted. It may be difficult to get back into shape and may require some grinding repair and added internal layers to strengthen. In any case, just reshape the new knees to suit the profile.

[Julien de Tilly]

Thanks again, but my concern is not so much about the knees themselves than the outside backing plates I still considered to install. Because my chainplates are parallel to the hull, I don't see any other ways to have permanent access to the mounting blots than to bolt the chainplates through the wooden knees and the hull and using an exterior backing plates (like I have done the first time). But I am afraid that such a design will put again all the load on a little portion of the hull. Even if I strengthen the hull and if the knees are glued and glassed on the hull, I am not sure if it will be sufficient to avoid that problem (because all the load will come again from the outside backing plate). The other option I consider is to rebuilt everything like it was originally done, but this will have the (bad) consequence of hiding one end of the bolts behind the wooden knee, making them practically inaccessible for inspection/replacement without grinding out everything else (which problem I however consider could be not so much a concern if I am making sure that no water gets in).

[Reefmagnet]

Originally Posted by Julien de Tilly

Thanks again, but my concern is not so much about the knees themselves than the outside backing plates I still considered to install. Because my chainplates are parallel to the hull, I don't see any other ways to have permanent access to the mounting blots than to bolt the chainplates through the wooden knees and the hull and using an exterior backing plates (like I have done the first time). But I am afraid that such a design will put again all the load on a little portion of the hull. Even if I strengthen the hull and if the knees are glued and glassed on the hull, I am not sure if it will be sufficient to avoid that problem (because all the load will come again from the outside backing plate). The other option I consider is to rebuilt everything like it was originally done, but this will have the (bad) consequence of hiding one end of the bolts behind the wooden knee, making them practically inaccessible for inspection/replacement without grinding out everything else (which problem I however consider could be not so much a concern if I am making sure that no water gets in).

I assumed your chainplates were originally mounted to the knees which are installed perpendicular to the hull. This wasn't the case? Again, best thing is to return to the original design. Attempting to sandwich a curved fibreglass hull between two possibly stout metal plates can cause all sorts of issues if not done properly. Keep in mind your boat is an old boat and the original setup lasted until recently? If you replicate the design, it will last as long again which probably exceeds the life of the rest of the vessel. If you can modify your plates for easy maintenance and inspection without undo hassle all very good. In fact you'll find more than a few examples of people that have done just that on the internet if you can devise an appropriate google search.

One other thing that hasn't been mentioned. Did you make sure you haven't had your rigging set too tight. To some looking at your pictures, they might say this was the case or at least contributed.