Keel stringer and plywood strips

zorak

Petty Officer 2nd Class
Joined
May 19, 2009
Messages
164
Hi all.I have been searching for a previously posted answer to my question but came up empty.In the process of removing the keel stringer i have noticed that the stringer was merely placed in the center of the keel along with 5 pcs of 3/4" thick plywood strips butted up to each side of the stringer .I assume this was done to allow the wood to conform to the shape of the keel.It appears that the stringer and strips were bedded with a chopper gun then covered with what appears to be roving, possibly with only 1 layer.This of course has delaminated to the point that i could remove the strips and keel stringer by hand just by cutting the glass next to where it is tabbed to the inside of the hull.I had planned on using resin and csm to attach the strips to the hull then 2 layers of 1708 over the top of them to attach to the hull.I have begun to think this design is inherrently weak.I think it would be better to bed the stringer in PL then tab it in place , put the plywood strips down over the tabbing and then cover with glass ,possibly 2 layers of 1708.All of the other stringers were tabbed in with a chopper gun and there are many more strips of 1/2" plywood that were stuck in place and covered with one layer of csm.I am looking for ANY suggestions here as i have not ordered any material yet but i am dead set on using poly resin as the PO had used poly to replace part of a stringer and even with his poor surface prep it is very sturdy.Thanks in advance
Dean
 

zorak

Petty Officer 2nd Class
Joined
May 19, 2009
Messages
164
Re: Keel stringer and plywood strips

Hi. I must have either stumped everyone ( Not very likely ) or no one is sure what i am talking about.I have attached a crude hand sketch of what i intend to do about my keel as far as keeping with what the manufacturer originally did except i don't own a chopper gun.The only thing i want to do different from my original post is instead of using CSM to bond the plywood strips to the hull i was thinking of using peanut butter instead,then tabbing the stringer to the hull with 2 layers of 1708 and cap the stringer with 1 piece of csm.I would really like some input or ideas on this as i am almost done grinding and ready to start building.Thanks in advance.Dean
 

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Joined
May 28, 2009
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Re: Keel stringer and plywood strips

From what I've read here the pl is the way to go on bedding the stringers. My boat only has one stringer which is the keel stringer and I had thought of doing it the same as yours with side boards running the length of it. There is a problem of hard spots on the hull that seems a little confusing to me. They say you don't want any plywood to hull contact which will cause a hard spot, but the old stringer I took out was direct plywood to hull (resin bed). For what it is worth I think your idea is sound and would be stronger than the one I have in my 15' trihull. You might use the pl to bed the side boards to the hull bottom and to the center stringer before glassing, that should be bulletproof..
 

zorak

Petty Officer 2nd Class
Joined
May 19, 2009
Messages
164
Re: Keel stringer and plywood strips

I originally thought of using PL on the side keel boards as well but that is an awful lot of Pl.I will have to see which is cheaper if either way is ok to go.
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
Re: Keel stringer and plywood strips

The sketch helps a great deal, not that many boats used plywood strips like that.

The easiest method would be to forget the plywood strips and just add an extra layer of 1708.

A great deal of stiffness is gained by continuing the glass over the top of the stringer. Think of an I beam.

Bedding the plywood stringer is to keep the plywood from only contacting the hull in a few random locations, this is where the hard spots form. If you could cut it and where able to achieve full contact for the entire length of the plywood, then the load would be spread out. What you use to bed the ply doesn?t really matter that much, its not really being used to help bond the plywood to the hull, with something soft like PL you're using it to space the ply off the hull slightly, with putty you're filling the void completely (full contact).
 

zorak

Petty Officer 2nd Class
Joined
May 19, 2009
Messages
164
Re: Keel stringer and plywood strips

Thanks guys for the responses.I Plan on capping the stringer where possible ,however since the decking screws to the top of the stringers i don't think i have the space to cap it unless i subtracted the glass thickness from the total hieght of the stringer.I am going to use pl really just to attach the stringers as they were originally bedded with a chopper gun and i have a pretty flat space where the original stringers were.I think i will use 2 layers of 1708 to tab the stringer to the hull,then use peanut butter to attach the strips then cover the whole thing with 1 layer of CSM or 1708 which is 10 times better than original.I don't think the keel on this boat will be strong enough without the strips for support plus there are other strips outwards from the keel that add extra support mainly for the trailer bunks i think.The hull is no more than 1/8" thick in places.Opinions anyone?
 
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