Rick wrote:... is the knee you are referring to a triangular piece of plywood glassed into the angle where the top of the cabin meets the hull?
Yes, or some generally similar shape. You can create a curve or S-type shape too (on the edge away from the hull) if you like the look. Something generally trapezoidal is what you normally see. Since they're exposed, it would be nice for them to be generally aesthetically pleasing.
Rick wrote:How thick should the plywood be and what length should the sides be for a "reasonably sized" knee?
9mm (3/8") would be more than adequate for your boat. Sizewise, I'd suggest that it extend about 6" athwartships beneath the deck and perhaps 9" down the hull. When glassed in, this would be very strong. When creating the shape of the knee, allow enough bearing surface along the hull and deck so that you can extend tabbing 3" or so onto the knee. In such a small boat with minimal loads, you don't have to go crazy with the tabbing the way one might on a larger boat, particularly if the knees were also to be the main support for the chainplates.
One layer of 15 or 17oz. biax tape, 6" width, would be sufficient for stiffening knees. In your specific boat,I'd add a wider layer of something like 10 oz. cloth over the top if you decided to support your chainplates off the new knees. These laminate schedules are not intended to be thought of as general-purpose quantities for all installations, but for this specific case.
You can fully encapsulate the plywood with one layer of material if you want, but in your case there's no real benefit to doing so.
To secure these knees, once they're cut to fit install them in a bed of thickened epoxy (using cabosil) and then create a broad, easy fillet between the plywood and the hull, and between the plywood and the deck. This will allow your fiberglass to more easily span the gap and also helps spread the loads over a wider area. You can either leave this arrangement till it cures, then wash/sand the fillets and laminate your tabbing over the cured material, or, if you feel up to it, you can glass either directly over the fresh, uncured fillet, or wait several hours and glass over a partially-cured, "green" fillet. It all depends on your comfort level with fiberglass and related work.
Your means of securing the knees to the hull during the curing process may also drive the ultimate way that you glass them in. Clamps or braces might get in the way of a good job, so you may bve forced to wait till the adhesive and fillets cure before glassing.
Sorry that there are so many variables here...but I don't know enough about your boat, or your inclinations to be absolute.
Rick wrote:Do you think i should also glass the deck/hull joint, or stick with the 5200?
Do you have problems specific to the joint, or only related to the cracking caused by the lack of stiffness in way of the chainplates? Your actual hull-deck joint looks like a shoebox from the photos, in which the deck molding fits over the hull a bit (like a shoebox lid) and is then secured. Whether or not you undertake the large job of trying to successfully glass this joint depends on too much information about your boat and more that I don't have.
Rick wrote:Finally, would you recommend securing the chainplates to the new knees or leaving them attached to the hull - maybe with a backing plate?
Other than the cracks in the joint caused by "squeezing" the hull together at deck level, are there any indications that the current chainplates are an issue at all? It'd be more straightforward to leave them as is, and simply stiffen the adjacent areas with the knees. If they seem secure as is, and aren't causing stress cracks in the hull or other signs of weakness or damage, I'd probably choose to leave them be.
It seems to me that the best approach is to thoroughly address the root causes of the problems that you have, but to not get bogged down in relocation or reconfiguration projects that aren't necessary as a part of the actual repairs of the root cause. Projects can grow uncontrollably enough on their own, and don't need human intervention in the form of unnecessary changes.
Somtimes, changes are required, or highly desirable. Other times, changes just mean more work that really has no overall benefit.