The aluminum plate has the advantage of being nearly flush with the cabin top, so no acorn nuts sticking out. This is kind of important for a 6' guy in a 6' cabin. It is easy to manage the corrosion problem with some Duralac and since the thing will be dry now the problem shouldn't recur anyway. The core I'm using under the clutch is crazy high density and wouldn't compress under even very high loading. To make it 110% bulletproof I could drill and fill too. I may have to do a test with a scrap piece to see exactly what happens with this stuff under the high compression of a nut and bolt. Then if it stops raining I could get back up there and fair this out and put it back together.

Below is a side view of this high density core (the dark stuff next to the green Divinycell), it already has two layers of glass on the top and bottom surfaces. Add to that two layers of 20 oz. on top of this core and 10 oz. added to the inner skin before adding this core.

Here is a side view of the plate that was under these rope clutches. These things are worth their weight not only for the strength they provide the clutches, but they will save my head from serious damage. Remember these clutches are right over the galley and the ice box, both locations are highly frequented by myself and other people of similar or taller height. Acorn nuts are not enough protection from injuries to your head. This one is 3/8" thick total and the tapered nuts are flush to the surface. So even if your head is sliding along the overhead you are not likely to hurt yourself on the plate or the tapered nuts holding the clutch.



I suspect that someone had this thing made.

I'm a tall guy, too and have the scars to prove it!!. Do you think countersinking the nuts (in this case) into a thin stainless plate would work. The plate may have to be just a bit thicker than 1/8 to accommodate the countersink correctly i suppose. That aluminum looks like 3/8"?

My deck seems ok, But I am going to put plates under the winches and maybe the clutches after reading your thread. Mainly, i want to get 'em off and pot the holes if they haven't been done.

This plate is 3/8" thick aluminum. It would seem like something similar could be done in SS I'm just not sure if 1/8" would be the right thickness for the bevel to work out correctly. Time to call in the engineers. Also the aluminum would have been fine if there had been some Duralac or Lancote used on them and no steady source of water pouring through the bolts. I plan to reuse these plates, the corrosion on the one plate while substantial is not enough to structurally impact it and it is on the inside surface.

The deck in this area seemed fine until I pulled the clutch off. The seriously damaged balsa was all directly below the clutch. The wet balsa also didn't extend very far out from the clutch. The area under the secondary winch was wet, but not rotted. While I had the high density core to use, I think some epoxy coated plywood would have worked pretty well in this area as well. Plywood could handle the compressive loads and the shear loads much better than the balsa. It is important to also realize that these clutches and the loading they apply is dynamic and not static, so the entire thing is wanting to work around some, again making the sealant work to or beyond its limits.

I also think I will take one more step before I put this all back together. I am going to cut a piece of oak the width of the clutch, bevel an edge on it and place it as a stop block in front of the clutch and glass it in. The idea being to provide something for the clutch to bear against beside just the surface area of the bolts going through the glass. What I don't want happening in the future is the shear forces making the sealant around the bolts to fail around these through bolts. This is probably major overkill, but I hate leaks.

After your deck is done, the bolt should be torqued enough so that the clamping force will not allow the clutch to move. Bolts are not meant to be used in a shear situation. They are meant to clamp (why a bolt should never be used as a clevis pin). If you need a block to keep the clutch from moving it's because your bolts aren't tight enough or your deck isn't strong enough to not deflect. I have no doubt your deck will be strong. Your bolts should hold if they are torqued properly.

I look at potting through deck holes as not only waterproofing the core but also providing an extremely solid bolting core. Something that will not compress. I use silica as my filler for these holes to provide the strongest bolting core. I am not shy about hogging out the core to be quite oversized. I've even considered installing stainless thick-wall tubes into the deck to give a clamping core. Haven't gone that far yet.

my $.02

These bolts are inherently in both shear and compressive loading, but it isn't the bolt that will fail in this application, it is the skin that has to resist the shear and lateral bearing forces. Hogging out holes large enough and filling them with something non compressive solves the compressive and much of the bearing area issue if the filled hole is large enough and very well bonded to the upper skin.

Putting a block in front of the clutch is just another way to provide bearing area from the clutch to the skin. The high density core material I used and the very heavy skin I have applied is probably more than adequate as is, but that would mean the lateral bearing area would still be primarily against the exterior skin around the bolts. In my mind the question is what is the simplest way to get a massive increase in lateral bearing area where it does the most good and that would be a small block glassed in front of the clutch. In the hogged out hole approach it would be necessary to also glass over the plug to completely bond it to the surface skin, otherwise you are relying on the size of the plug bearing against the skin and balsa. If the filled plug is pulled laterally hard enough it can crack away from the exterior skin. It probably would still hold, but you are back to a leaky situation.

I just think the steps at this point to add a small block is simpler and stronger than boring new large holes, filling them and then glassing back over them. In the case of the winches I will probably go the large bored holes and glass over route. I do think glassing back over the filled plug is important in these kinds of applications.

It occurs to me that the load case on these bolts for the clutches and winches is somewhat different than on many other elements on these composite hulls. There are a great many cases where the simple drill and fill approach is totally adequate for the situation, but there are two places that I think need a more advanced approach, clutches and the winches come quickly to mind. These items have very large lateral forces that are trying to pull the bolts sideways. Drilling and filling provides the required solution for the compression that comes with bolting and screws, but the bolt even with a large barrel of epoxy around it is then resisting the lateral force at the joint with the outside skin and the bearing against the balsa core, which is crazy soft in this load direction. The bolt and barrel of epoxy will try to essentially rotate or pivot about the opening of the inner skin (unless it too tries to fail). The critical junction in this arrangement is the bond between the epoxy barrel and the outer skin. The epoxy barrel won't compress, but it will try to pivot and likely crack the bond between the epoxy barrel and the skin. The larger the epoxy barrel the stronger it would be, but adding a layer of glass over the barrel and the skin would go a very long way to sharing the lateral load of the barrel to the skin.

Please excuse my drawing below

The high density material I used is Cousa Bluewater 20. Here is a link to Cousa's website and also the test data on this material:

Cousa Composites

This is a perfect type of material to use under highly loaded areas like the clutches, winches. I also used it around the exhaust, but mainly because it was flat and easily templated and installed.

Stainless 18-8 bolts yield a typical minimum of 65000 PSI, about 10000 PSI more than a grade 2 steel bolt.
https://www.boltdepot.com/fastener-information/Materials-and-Grades/Bolt-Grade-Chart.aspx

At a suggested torque of 66 in-lbs (~5.5 ft-lbs) the clamping force of a 1/4"-20 grade 2 bolt is 1320 lb.
https://spaenaur.com/pdf/sectionD/D48.pdf

I'm not engineery enough to interpret the Cousa test data to know if it will crush under 1320 lb of clamping force. However it does appear some balsa grades may be more resistant. https://www.corelitecomposites.com/...heets/BALSASUD%20Core%20Data%20Sheet.pdf

me thinks g10 or plywood anywhere there is a compressive load would be a good idea. G10 may be better in case water ingress is possible-- which it *always* is, right?

Ok, here is another way to look at it. I just went out to the shop and found 1/4" 20 carriage head bolts drilled through the Cousa and some new balsa I had (I have no idea what grade it is, other than it appears very similar to all the other balsa I have seen in my boat) and got the good torque wrench I have out. The Cousa almost got to 10 ft. lbs. (the lowest level this wrench can do) before the carriage head started to pull through the material. The balsa on the other hand didn't come anywhere close to tripping the wrench before the carriage head was pulling through. If the surface area under the carriage head were larger for sure the torque or compressive values would have gone higher for both material tests, but the results would still have favored the Cousa board.

What this crazy simple test doesn't tell you anything about is what the compressive strength of either material would be with the same amount of added FRP on both surfaces and with a proper sized washer to spread the load out on both surfaces. But if the bare material has higher compressive strength, then it would seem reasonable to me that with the same amount of added glass on both samples the results would still be essentially the same with just higher numbers.

I am not an engeneer either, just a hands on kind of guy.