19xx Kobra 12ft. transom/stringer/floor restoration.

cp2009

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Hi guys,

I recently bought this small 12 feet boat, only to find out days later that the transom was completely rotten.

It had been repaired twice earlier I could see. First they put an additional plywood plate on the inside and glassed it in, later when that started to give in, they put a plate on the outside too.

So I removed the top and cut off a piece of the floor in the back end, so I would have sufficient working space to remove the old transom and insert a new one, aswell as inspect the stringers.
It turned out the stringers were pure fibreglass, however they were detached from the hull of the boat (see one of the pictures, I could lift it with my finger), again really bad glassing work, I could peel of the glassing they had layed onto the sides of the stringers with my hands, also it was a clear sign that the floor had been removed before and a repair of somekind had been made.

I suspect that there was originally foam in the boat, that had gotten wet and someone pulled the floor up and stringers aswell and removed all the foam.
Afterwards they simply glassed the stringers back on. Made some drains and closed the floor again. It's just a theory as I did not see any traces of foam anywhere, but the stringers are quite thin and the buttom of the boat isn't exactly very thick either.

For now I have epoxied a new transom in and just finished giving it 3 layers of glassfibres (after reading a ton of threads on here), so now it's time to move onto the stringers, however it seems most boats on here have wood stringers that are glassed in.
- I am not sure how to deal with them, so I could need some the expert advice.
My plan is to glue the stringers back on with epoxy (after grinding and cleaning), then cut the top off them and give them 2-3 layers of glassing on each of the sides attaching them to the hull, afterwards I would fill the stringers with 2 component foam and glass the top back on. I'd fill the rest of the buttom with foam aswell, then epoxy the floor back on (needing to replace the plywood plate on that first though).
Does this sound like the proper way to do it? -Also as I am not sure if there was originally foam in this boat, is it possible it was build like this without foam?

Here is a some pictures, so you can see what I am talking about.


As she looked before I started:
as_she_looked_rear.jpg


After the floor was cut out:
floor_removed.jpg


stringers_closer_look.jpg


Deck, stringers and floor:
top_stringers_floor.jpg


New transom glued in 2 layers of 3/4 inch plywood after adding 3 layers of glass first, as the end was very thin after grinding out the remains of the rotten wood:
new_transom.jpg


New transom glassed with 3 layers, keeping the temperature right:
new_transom_glassed.jpg



Any input or advice greatly appreciated.

Regards,

CP.
 
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Woodonglass

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Re: Glass stringers

Re: Glass stringers

Your plan is sound. I doubt there was any foam in there originally, just bad glassing. If she were mine I would use 1708 biax to glass the stingers back in with and use the Blue Sheet foam from Lowes and lay it between the stringers as best I could. You did an excellent job on your transom by the looks of it.

I'm just sayin...:D
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Thanks for you quick reply.

Is this the kind of foam you mean? http://www.biltema.dk/da/Bad/Kemikalier/Ovrigt/Divinycell/

I am from Denmark (far far away from the US), therefore I am not familar with Lowes :)

The foam linked above is called "Divinycell" here, it is very expensive, I think it would end up atleast $300 to fill the stringers alone, then it probably needs lots of resin too. It'd probably last forever though. Would you use it just on the stringers and make sure there is drain plugs for the remains?
 

Woodonglass

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Re: Glass stringers

Re: Glass stringers

Sorry about that. Here in the states the home builders use a closed cell foam that comes in 4 foot by 8 foot sheets by 1 " thick. It is used to insulate the walls of homes. It is usually Blue or Pink

http://building.dow.com/na/en/products/insulation/rigidfoam.htm

Not sure if they have it in Denmark but probably. It is NOT normally attached, just layed in loosely since it's only purpose is to aid in keeping the boat from sinking. It is closed cell and will NOT absorb water.
Here is a pic of an iBoater using it in his Aluminum boat between the ribs.
wDSC_2822.jpg


Lowe's is a building supply company where homeowners can purchase buildinging supplies for do it yourself projects. Do you have places like that in Denmark? I would suggest you ask someone in the home construction business if they could tell you where to purchase this product.

I'm just sayin...:D
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Ah great thanks. I will investigate it, if its Polystyrene (EPS), it's both easy and cheap to get overhere.

It sounds like a good solution, if the boat was not designed to have foam for structural reinforcement. This way as you say it'll keep the boat from sinking and at the same time I'll avoid condensation under the floor and needing drains.

Thanks again for your replies, much appreciated.
 

barbosam

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Re: Glass stringers

Re: Glass stringers

The foam he is describing is not polystyrene. EPS will absorb water and should not be used on boats. The foam he is describing is not much more expensive than EPS and is much cheaper than divinycell. Divinycell is meant to be used as a structural component sandwiched between two layers of fiberglass. It is an excellent product but its not really intended for what you are describing.

The two part foam would work great for what you are trying to do. Instead of glassing in the stringers, cutting the tops off filling with foam then reglassing, do you think you could flip that stringer grid upside down pour the foam in, once it dries trim it flush then flip it back over and install it in the boat? Might be a little tricky and require you to build some temporary structures and caps to contain the liquid foam. Just a thought.

I agree with woodonglass, I would use 1708 to glass it back in just make sure you do it mat-side down (one side of this material has chopped glass fibers which is called mat).

Great job on the transom looks like a high qulity job!
 

cp2009

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Re: Glass stringers

Re: Glass stringers

barbosam,

Thanks for your reply. Your suggestion is appealing, I think it should be possible to flip the stringers and fill them with foam, would save me a bit of glassing work.

I'll look for the 1708, otherwise I have my eyes on some 16 oz biaxial, which can be used with epoxy aswell.

Thanks to both of you for the props on the transom replacement, it went quite well thanks to the excellent threads on the topic in here. Today when it was all cured, I found a few airbobbles on the top of transom, so I'll need to grind that down a bit and re-do the top/edge to the outside.
 

Yacht Dr.

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Re: Glass stringers

Re: Glass stringers

barbosam,

Thanks for your reply. Your suggestion is appealing, I think it should be possible to flip the stringers and fill them with foam, would save me a bit of glassing work.

I'll look for the 1708, otherwise I have my eyes on some 16 oz biaxial, which can be used with epoxy aswell.

Thanks to both of you for the props on the transom replacement, it went quite well thanks to the excellent threads on the topic in here. Today when it was all cured, I found a few airbobbles on the top of transom, so I'll need to grind that down a bit and re-do the top/edge to the outside.

I would not foam your stringers... It only 13'.

There is from what I can see no structural value filling with foam.

Glass tab with mat and glass ( biax is basically the same as 1708 less the mat ). mat first anyways with biax or 1708..

YD.
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Yacht Dr.

Thanks for you reply.

I am still concerned if this boat was originally constructed with foam under the floor and inside the glass stringers for structural reinforcement. I found a website with the same brand boat, not the exact same model but close and the guy had wet foam in his, no pictures of the stringers etc. though.

Overall the boat is very light and thin and stringers are like two layers of glass only and do not feel not very strong.

Obviously they will be a fair bit stronger after I give them some additional layers of glass, but will it be enough if it was originally re-inforced with foam.
I am still thinking of the best solution, it would be nice with no foam as it saves work, money, time and I am absolutely sure nothing will absorb water under deck.
 

twkjr 1250

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Re: Glass stringers

Re: Glass stringers

i have a 23 ft cuddy cabin boat that has hollow fiberglass stringers and foam outside each stringer to the hull up the lenght of boat. pics should show you how it was done . if you need better pics let me know. if you do foam use the pink or blue foam board or even noodles . still wondering why most boat manufacters did not use this glass stringer system. good luck with it.
 

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Yacht Dr.

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Re: Glass stringers

Re: Glass stringers

Cp2009...

Shure looks like a Molded stringer system to me..thats ONE part stringer Installed..

If it was foamed with say a 2 part foam you should have hole ports on top where they filled them.

Ok..I understand the aprehention about foam being a structural part of the stringers.

IF so then your HD foam...or Noodles WILL not give your structural..only floatation.

You would have to 2 part pour foam that then glass it in ( or glass it cut holes and pour .. RisKy.. ).

I would Tab those back in..but not overlap the tops ( or it will mess up your floor base ).

More pics :) .

YD.
 

cp2009

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Re: Glass stringers

Re: Glass stringers

twkjr 1250

Thanks for your reply and photo's, also looks like a rare construction. Did it have drains for just the stringers?

Yacht Dr.

Thanks again for your reply. I see what you are saying, if the stringers are molded there would need to be traces of holes for pouring foam in them, if there was ever foam in them. There definately is no traces of that and I am pretty sure they are molded.

I have attached a few more photos of the stringers. Going by your info I think I will do them without foam, they get a bit stronger anyway once I put some more layers on them.

stringers1.jpg


stringers2.jpg


stringers3.jpg
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Forgot to mention if anyone wonders, yeah there is a plywood plate in top of the middle stringer, it was used to screw the floor to the stringer. It nicely rotten now and will go out aswell. I plan on glueing the floor to the stringers.
 

twkjr 1250

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Re: Glass stringers

Re: Glass stringers

no there were no drain holes for stringers . I guess they figured if glassed in properly there would be no water intrusion, but you know water seems to find a flaw in workmanship always. so be extra careful when glassing them back in . seen one other boat on here with similar stringer design as mine.
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Small update.
After a long and cold winter, temperature today was good enough to do some more glassing (the project had been on hold since my last update, due to temparatues making it impossible to do any further glassing).

I decided to glass 2 layers to the inside of the hull bottom, because I felt it's very thin by the original design and also there were small stress cracks here and there, probably due to the "loose" stringers.
insidehullgrindeddown.jpg


Lots of grinding :)

2layers2hull.jpg


2 layers of rowing/fabric epoxied.
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Got a bit more done this weekend, after grinding the bottom of the hull again it was time to glue in the molded glass stringers. I attempted to mix some more solid epoxy with microfibers and balloons, the first couple of attempts were still too thin, so it made a nice epoxy mess -more grinding later :/

Anyhow stringers got glued in well, the "flaps" on the sides of the stringers have bend a bit out of shape, so I couldn't get full contact on all outer edges. Next weekend I plan to grind down the edges of the "flaps" so it makes a smooth transistion to the hull and then I will apply 2-3 layers attaching the hull to each flap&side of the stringers.

Also planned is the actual floor that has to go ontop the stringers. Originally it had a thin plywood plate between the floor and stringers, most of it was rotted away, so I could not measure exacly how thick it was, but I estimate it was about 0.1968 " (5mm).
I was wondering if I could simply make that thickness with glassing on to the backside of the floor and get similar strength to the thin plywood plate? I also noticed the floor is not even, it bends slightly in the middle (mid stringer is a bit lower than the side stringers) probably to get water running to the bilge area.

Here's a few pictures of this weekend.

stringers_glued1.jpg

Stringer flaps were pasted with epoxy and the hull got expoxied, then put together with various weights on to keep it pressed together while curing.

stringers_glued2.jpg


stringers_glued3.jpg


stringers_attached2transom1.jpg

Stringers attached to the transom, a few layers more will be added when I glass the stringers, now it has the connecting-shape, as there was a little gap from the cutout.
Loads of epoxy from the gluing ran to the bilge area.
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Today I got a lot done, edges of the stringer flaps were grinded down, so they have a smooth transistion to the hull, I was happy with the result every stringer is really well in contact with the hull.
So it was time to glass them in, to further strengthen the bond with the hull aswell as stiffen up the stringers themselves.
I got the center stringer and the inside of ther outer stringers glassed with 3 layers of 10 oz (300 grams) woven fabric using epoxy, using overlapping sizes (the final layer actually went from top of one stringer to curve up slighty on the other stringer (from each side) so the hull is now enforced with 4 layers between the stringers(5 and 6 for a smaller part). I got a good feeling about it, it might be a bit overkill though :)
I also got the floor done, for this I used polyester for a change as I was too cheap to use epoxy for this task, it got 2 layers of heavy matt, I think 20 oz (I had it in the garage from a few years back) and finished it with a layer of fabric, it really sucked up a ton of resin, been a long time since I used polyester and I wanted to do it all wet in wet to avoid regrindning&cleaning, so I was very busy as it sucked away the resin as quick as I could make it almost. The floor feels very solid (and heavy :/) now.

Hopefully tomorrow I can glass in the outside of the outer stringers (should be even easier) and maybe glue in the floor. Finally I can see an end to the grinding, although not quite yet but soon!

Btw. is there any issues using topcoat/gelcoat ontop of epoxy glassing? - as I guess topcoat is based on polyester, it might not work, its mostly an issue on the outside of the transom, inside it I can use epoxy primer & then 2k polyurethan, perhaps I can do the same on the outside if its not going to work with topcoat?

Anyhow here is some pictures of todays progress:

stringers_edges_smoothed.jpg

Edges grinded.

stringers_glassed.jpg

Stringers glassed with 3 layers of woven rowing epoxied.

stringers_glassed_closeup.jpg

Closeup of the glassed stringers, the hull now has 4 layers between each stringer and 5-6 layers on the edges af the stringers.

floorpoly.jpg

Floor reinforced with polyester 20 oz matts and a finishing layer of fabric, to replace the original thin plywood plate that had rotten away.

floorpolygrinded.jpg

Fully cured and edges grinded, its ready to be epoxied to the stringers almost (need to grind where it has contact with the stringers).
 

Woodonglass

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Re: Glass stringers

Re: Glass stringers

Poly will NOT stick to Epoxy!!! Epoxy WILL stick to almost anything!!!!
 

cp2009

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Re: Glass stringers

Re: Glass stringers

Thanks for the replies.

@Woodonglass
Yeah I am aware of Polyester not sticking to Epoxy, just that I kinda heard that gelcoat/topcoat isn't all polyester based these days as it used to be, but I take it gelcoat/topcoat onto epoxy is still a no no then? and will 2 component polyurethan paint be a good alternative to match up with the existing gelcoat, or is there something else used on epoxy for the outside skin that ressemble gelcoat?
 
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