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Old 28-06-2013, 19:06   #1
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Soggy Knees, New Idea?

I have an idea! But is it a good one? Has anyone done this or know of someone who did?
Last winter I drilled some test holes in my knees (boat knees) and found varying degrees of moisture. Some dry, some moist, some soaked. Even the worst one still seems fine once dry but of course, they're only getting dry in the holes.
So I was thinking I'd use penetrating epoxy but now I'm starting to shy away from that idea as I doubt that it would penetrate completely.
So my new idea is to drill into the knees from deck side with a long auger bit. Then using a long chisel, remove as much of the plywood, whether good or not. Insert either wood doweling or plastic tubes through the bolt holes and then pour in fibreglass resin and tamp in glass matting, making sure to get all air pockets out. Let set, drill out wood dowels or plastic tubes, apply gelcoat and done! The end result would be a solid fibreglass knee.
Sounds like a lot of work but I think that it would be stronger, better and actually less work than some of the other knee replacement projects that I've looked at online and there would be no more wood core to rot or even get wet.

So whad'ya think, is it so crazy it just might work?

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Old 29-06-2013, 07:17   #2
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Re: Soggy Knees, new idea?

Definitely crazy. What will keep the resin from going all over your headliner or the cabin sole? You'd have to build a mold around the knee first. There are a whole lotta holes in this theory. The end result would be extremely brittle and weak as well, even if you pulled it off. Don't do it! Fix those knees properly by removing them and replacing them with something better. And then try to stay on top of chain plate bedding! You should probably check those too...
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Old 29-06-2013, 07:23   #3
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Re: Soggy Knees, new idea?

Well I would plug the holes that I have already made in the knees. Why would they be brittle? If brittleness were a problem in fibreglass, wouldn't that be a problem for solid glass hulls? They are subject to a lot of flexion.
It is my understanding (I'm no expert, that's why I'm asking), that the plywood core is merely a mould to build the knee around. Wasn't it the CS that was built with solid fibreglass knees?

Edit: Found this from the Valiant 42... "Chainplates – Stainless steel with backing plates through bolted to solid fiberglass structural knees."
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Old 29-06-2013, 08:14   #4
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Re: Soggy Knees, New Idea?

Hi:

Fiberglass gets its strength from LONG strands in a more or less straight line. It gets its strength from the glass, not the resin, so more glass is good, less resin is good. The problems with your idea are that you will never get much glass tamped in, and it it will not be in more or less straight lines. It will be little balled up clumps. It will be full of voids. It will only be marginally stronger than plain resin, which isn't very strong.

One of my plywood knees looked like it had a crack in it. Or it might have been a surface flaw in the top ply that was there forever - I could not see a problem on the other side. To be conservative, I laminated glass cloth with epoxy on each side to reinforce it. If your wood isn't rotted, but merely wet, perhaps get it dry and reinforce as I did.

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Old 29-06-2013, 08:27   #5
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Re: Soggy Knees, New Idea?

I see what you're saying and was thinking chopped glass tamped in, comparing that again with a solid f/g hull. But I can see that long strands would be better and I'm envisioning (always easier than actually doing, lol) pouring resin into the hollow knee and brushing the sides of the existing f/g generously, then using a long thin tool, pushing the fibreglass strands into the knee. My knees are 18" deep, so I would cut the first strips 36" long and push in the centre so that the strips would adhere to the sides of the cavity. Continue with that process until the cavity was full as possible of wetted out f/g matting.
My existing f/g is still bonded to the hull and strong. I would only be adding to the strength.
My aversion to drying the existing cores is not because I don't think it will necessarily work but because there's no way to know for sure if it has.

Thanks for the input, I don't mean to seem like I'm arguing about it, I just like the process of discussion. I am simply trying to learn and do appreciate everyone's knowledge and time.
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Old 29-06-2013, 10:23   #6
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Re: Soggy Knees, New Idea?

Sir, I'm a bit confused by your description of the knee. Is it primarily a ply knee with a thin layer of glass over, or is it a lot of glass wrapped around a thinner bit of ply?

The Yankee 30 that I owned years ago had knees similar to the latter description, and I wouldn't have worried about the ply disappearing completely in them!

Give us a more detailed description, please, complete with dimensions.

Cheers,

Jim
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Old 29-06-2013, 10:44   #7
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Re: Soggy Knees, New Idea?

Ok, the knees are, as you no doubt know, triangular shaped braces between the bottom side of the deck and the inside of the hull. They are 12" along the top edge where they meet the deck and 24" along the vertical side that is tabbed to the hull.
They are 2" thick and along the long diagonal edge (the hypotenuse) the f/g appears to be about 3/8" thick. I'm assuming that the plywood core would be 3/4" plywood and that the faces or fore and aft sides of the knees are much thicker f/g.
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Old 29-06-2013, 10:52   #8
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Re: Soggy Knees, New Idea?

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Originally Posted by Sir Rondo Normal View Post
Ok, the knees are, as you no doubt know, triangular shaped braces between the bottom side of the deck and the inside of the hull. They are 12" along the top edge where they meet the deck and 24" along the vertical side that is tabbed to the hull.
They are 2" thick and along the long diagonal edge (the hypotenuse) the f/g appears to be about 3/8" thick. I'm assuming that the plywood core would be 3/4" plywood and that the faces or fore and aft sides of the knees are much thicker f/g.
Well, if the knee is two inches thick, and the ply is 3/4 inch then the f/g must be on the order of 5/8, not 3/8 thick. And, if so, most of the strength is in the glass and the ply does not matter very much.

Have you ever observed any deflection of the knees under sailing loads?

I think perhaps you have a non-problem!

Cheers,

Jim
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Old 29-06-2013, 11:03   #9
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Re: Soggy Knees, New Idea?

Ya for sure, as I said above, the fore and aft sides of the knees would likely be much thicker than the inside edge due to the tabbing. I agree with you, that the strength of the knee is in fact in the f/g and that at this point, we don't have a problem but I'm thinking that the problem arises when the wet plywood eventually rots and leaves a void which allows the f/g to weaken through use and loading. I have not found rot yet but there is moisture.
I have not observed deflection while sailing but I have not looked. I should. Thank-you
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Old 29-06-2013, 11:10   #10
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Re: Soggy Knees, New Idea?

The plywood is more or less just a form to mold the fiberglass knees around. All the strength is in the glass, not the wood. The plywood only adds strength to the knees by keeping them from flexing inward. Injecting thickened epoxy/polyester resin into holes drilled from above should do fine. Drill the holes on 1" or so spacing with a drill bit large enough to scour out the wood from side to side, fill with resin and Bob's your uncle. You don't need to scour out all the wood. If chainplates are bolted to the knees, clean out the wood where they are through bolted. None of the above is needed if the wood is not rotten and you can can seal out future leaks.
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Old 29-06-2013, 11:11   #11
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Re: Soggy Knees, New Idea?

sorry to drift... but I think Soggy Knees would be a great boat name!
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Old 29-06-2013, 11:17   #12
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Re: Soggy Knees, New Idea?

Have you fixed the reason for the water ingress?
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Old 29-06-2013, 11:34   #13
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Re: Soggy Knees, New Idea?

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Have you fixed the reason for the water ingress?
Yes that was the easy part.
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Old 29-06-2013, 14:27   #14
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How did the water get into the knees? I'm thinking your right that you could go for epoxy injection. Maybe flush it with denatured alcohol. And dry it as well as possible then add a few layers of glass
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Old 29-06-2013, 15:42   #15
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Re: Soggy Knees, New Idea?

Yes, you guys have me thinking now more about the CPES. I spoke with a salesman yesterday about it but I always have to question their motives. He seemed more interested in the potential for a sale than to be of genuine help solving the problem. Maybe I'm just too cynical.
Anyway, I think we have it. I do not actually have a problem... yet. But since I have drilled and identified where the moisture is, I might as well go ahead with the original idea. That was to fill the holes that I've drilled into the knees with penetrating epoxy and fill with thickened epoxy sealer when done. I will try the denatured alcohol too.
It is not all the knees that need to be done and I can still monitor the situation.
I do plan to leave a weep hole at the bottom of every one and think that there should be such a drain in all knees to begin with.
One of my knees had so much water collected in it that when I drill my first hole at the bottom, it came out in a gush. That was not good. Thankfully, it was the worst, it was accessible and it was only a running backstay knee. The second to the worst though is the backstay knee but it not rotting.
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