Need help in stringer design (All fiberglass and NO wood)

[Saskatoon2005]

Here is my drawings for my stringer designs: I don't know if you can read my writing so I will spell it here....

The first picture (upper left corner) says, here is one idea I have for a stringer. This is the cross section of it. I know this design is used in aluminum boats. Do you think this made in fiberglass would be strong enough?

The second picture (lower left corner) says, here is another cross section of an idea I have for a stringer. This one has wood on the upper part but but right next to the floor (of boat) so water would not reach it, even if the hull was compromised. Does this seem stronger than the one above?

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

maybe this will work....

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

Any ideas? C'mon guys, you gotta have something...LOL!

 

[deejaycee_2000]

Re: Need help in stringer design (All fiberglass and NO wood)

why not try aluminium? .....

 

[Mark42]

Re: Need help in stringer design (All fiberglass and NO wood)

The second design matches other fiberglass stringers I have seen in factory built boats. You might want to consider carving up foam panels into the shape you want, then apply glass.

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

I would try aluminum, but the cost I fear is way to much, but thanks for the idea deejaycee 2000.

I think I like the second design better as well. Yeah I agree with you that I will need to make a mold, probably out of wood...I will take an old stringer out of the boat, make a mold by building up wood around it , apply mold release, and layer up some nice fiberglass stringers...Now I just need some sort of closed cell foam for the inside, so I don't have to ever worry if water does get into the hull....any ideas that way?

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

One problem with the second design is that you're relying on more torsional strength of the fiberglass wall than the in the first design. The first design is almost purely sheer strength. You'll need to build thicker glass walls to handle the additional torsional forces. The other thing is, just because you've got the wood at the top don't think that rot isn't an issue. How many stringers rot top down from the fastener holes?

If you're worried about cost for the aluminum stringers, get a fab shop to price it out. That's pretty simple job on a metal brake. Nice thing about aluminum is you could make it in small sections and join the pieces together with plates and rivets. That would cut the cost down significantly over one big piece that would have to be bent on a mandrel rather than a brake (or one freakin' huge brake).

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

That said, i'd probably do one thing different in the first design. Extend the top and bottom horizontal pieces to overlap the vertical a bit. Sort of an offset I-beam. Fiberglass doesn't flex like metal, so the likelihood of cracks along the right angle joint is higher. Reinforcing both sides of that right-angle joint would seem like a good idea.

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

(Okay you got me rolling - I'm now in engineer mode)

All that being said, I think the second design is going to have less problems over time if you can keep the water away from the wood. The first design places all the torsional forces on those two right angles. Whereas the second design more evenly distributes them along the structure. This will allow you more margin of error for any construction quality issues as you're fabing this up at home - something important to consider.

In a nutshell:

With the first design you better get those right angle joints done right - they're your points of failure.

In the second design your 30-degree (or so) angles aren't so critical since the rest of the structure is supporting any torsional forces. Make sure that wood is well-encased in epoxy and your fastener holes are pre-drilled and sealed.

 

[kalla man]

Re: Need help in stringer design (All fiberglass and NO wood)

....i believe that fiberglass does flex, i mean...
thats the whole reasons stringers are in boats.....to keep the fiberglass hulls from flexing (as much)...

The second design is the best.
ive seen C-shape fiberglass cast stringers,
you dont even need the flares on the bottom
if you are just glassing it into the boat
just make sure your mold is right on,
popping out your first casting and finding out it
wont fit could be very discouraging and COSTLY.

IF you make the top thick enough i dont think you
will even need the wood... if its thick enough to
provide sufficient structure for the deck and hull
its prob. about thick enough to hold a deck screw.

good luck, put up pics when you get them done
Id like to see how they come out.

I wont be holding my breath anytime soon..
This one could take some time!
~~~~~~~~~~~~~~~~~
THE PROJECT.......
s99.photobucket.com/albums/l309/kallaman/

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

Yeah, it'll flex. But, not like aluminum will.

A more appropriate term is Al has good ductility, while fiberglass has significantly less.

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

Excellent replies!!! Now, what if I throw this out there...

What if I use the second (lower pic) idea and remove the wood from it, and pinch the top together so you end up with an upside down "V". Will I have enough space to attach the floor to? Do I have to screw the floor down? Can't I just wet the tops of the upside down V's with resin and "glue" the top down? I guess I would have to weight it....hmmmmm (don't mind me, I always think outloud). Or what if I make the upside down V's and fill them with foam, or something to make them more rigid...how to get the floor attached to it after I make them....how about glassing the V stringers to the bottom of the floor, and then installing the floor as one unit? Seems very difficult, even for my 14' runabout.

Also, I am looking for another way to foam it with some type of closed cell design. I mean I don't want to do all this work just to have the foam get soaked and later have to be removed...I want this to stand up to years more without any problems. What else can be used for bouyancy that will not soak up water. I have heard of plastic bottles, but that seems silly to me. Any ideas for this as well?

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

I think you're better off putting a curve at the top than a "v". A peak leaves very little room for error for fasteners, and a curve would make glassing a lot easier. You could just leave a plastic pvc pipe in the top behind the glass for fastener backing - it just needs to stick to the glass long enough to take some drilling and screwing. It'd act like a giant washer against the fiberglass. As an added benefit it'd make for some nice wire ducts if it were wide enough not to interfere with the screw ends.

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

So, if I used a pvc pipe to make the form/mold, then it would be easier to lay the mat up and would resin hold the pipe in place? I have heard lots of negative things about using pvc in boat building...Hmmmmmm

I could cut the pvc in half for the form/mold...I like the idea of having a non-rotting backing for something the screws can bite into. It would be more difficult to get the right height as the curve adds a little more grey area...LOL! It could definately be done, and not having the flairs at the bottom would help in fitting the "stringer" into the hull, allowing for trimming. Then glassing the bottoms to the hull...

I just wonder if the curve on the top would make it less stress resistant. I wonder if stress would crack or break it over time...but I love the idea on no-rot...

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

What size pvc pipe should one use (if they decide to try this) in this project? Any ideas? I am surely going to make a mold up of it and then lay it up. Pop off the new stringers and install.....

It is still winter here and I am waiting patiently (well not really...LOL!) for spring to come here in Canada. So it will be a while before I get this project up and running, just doing the research part of it so I can have everything planned out when the time comes....LOL!

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

PVC will make a very weak mechanical bond with the resin. It should be enough to hold it in place for the screw backing. You won't get any real structural benefit from it, but that's not really what it's for. I guess I'd go 1" or better pvc if I were doing it.

As for structure - arches are very strong.

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

1 inch doesn't give me much of a curve at the top, but it will make it easier to reach the proper height and not much trimming to get it to fit...I wonder if I should lay up fiberglass mat along the bottom of the pipe when I am making the stringer from the mold. What I mean is...

make a mold with the 1inch pipe as a mold (using mold release to remove the fiberglass from it when cured) and then glass in the pipe to the underside of the upside down V. This will make sure is does not fall off the upside down V when trying to screw into it.

...also when you say pvc, do you mean the black pipeing for plumbing or the white pipe!

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

pvc is the white pipe, abs is the black pipe, cpvc is the beige stuff.

abs is a little more brittle, but I suppose either one would be fine. I'd go with whatever has the thickest walls to hold the screws in.

I say 1" or better, meaning eye up some different sizes and see which one you want to work with. It may be 1.5" is better.

I'm just shooting from the hip here.

 

[iwombat]

Re: Need help in stringer design (All fiberglass and NO wood)

The idea is, you want to give yourself at least 1/4" of leeway side-to-side in where you place your fasteners through your decking material. So, within 1/2" of the peak you want the top of your stringer to be pretty close to flush with the backside. A larger radius is better for that purpose.

 

[Saskatoon2005]

Re: Need help in stringer design (All fiberglass and NO wood)

Hey iwombat,

I totally agree about having a larger peak in order to have a good flush seal with the flooring. Yeah I also agree probably 1.5 in I was thinking, no smaller....

and what do you think about glassing the pipe all the way around so it won't fall out from the upside down V. Do you understand what I am saying?

...and finally what can you advise as to a non rotting floatation system to install under the flooring in the hull? I don't want that foam that everyone has soaking with water when they go to replace their floors. I want something that won't suck up the water, repel it even.....LOL! I have only heard of that pour foam, 2 litre pop bottles (not keen), and styrofoam sheeting (the kind used for insulation purposes). As well as foam in a can under pressure that you spray into the hull area before you lay the floor. Any other ideas? Which is the best?