New stringers

S

sailingstu

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Seattle
Builing new engine stringers in my Mariah 31 for a yanmar 3gm30f. I'd like to avoid using wood. I was doing the calculations on density, and noticed that the pink home depot insulation foam has a density 5.33. High Density Marine foam from generalplastics.com recommends between 6-20 pcf for stringers. Given I was planning to use plenty of glass; is this going to be close enough? Or should I just go with my original idea of building them out of G10 and not worying about anything.
 
El Borracho

El Borracho

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My engine beds are hollow. About 6-8mm of vinyl ester fiberglass layup not unlike a tub made from 2-by-4 lumber. For a 50 hp engine. So any foam would be okay as a molding form. I suspect they were built in a mold, inverted, and then bonded down with a layer of heavy cloth. Don‘t use any wood, yikes! Hollow with access for nuts on the fasteners seems easier and more reliable than the alternatives. G10 seems complicated, expensive, and would still involve much glass work, right?
 
SloopJonB

SloopJonB

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Great Wet North
Pink foam is basically Styrofoam and will get dissolved by poly resin.

I've used it with epoxy for cored panels in non-stressed applications and it worked great.

If you just want it as a former for epoxy laminations it will do well.
 
M

mckenzie.keith

Aspiring Anarchist
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Santa Cruz
sailingstu said:
Builing new engine stringers in my Mariah 31 for a yanmar 3gm30f. I'd like to avoid using wood. I was doing the calculations on density, and noticed that the pink home depot insulation foam has a density 5.33. High Density Marine foam from generalplastics.com recommends between 6-20 pcf for stringers. Given I was planning to use plenty of glass; is this going to be close enough? Or should I just go with my original idea of building them out of G10 and not worying about anything.
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G10 doesn't seem like a good idea to me unless you maybe cut a tube in half lengthwise or something. Seems like a lot of work if you build it up from plates. And most of the benefit of G10 is that is has known strength specfications. But if you join plates you kind of lose that advantage and might as well lay up your own stringer so there won't be any glue joints.
 
S

sailingstu

New member
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Seattle
mckenzie.keith said:
G10 doesn't seem like a good idea to me unless you maybe cut a tube in half lengthwise or something. Seems like a lot of work if you build it up from plates. And most of the benefit of G10 is that is has known strength specfications. But if you join plates you kind of lose that advantage and might as well lay up your own stringer so there won't be any glue joints.
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Sorry. My thought was to use the G10 to build the stringer tops and sides and just epoxy glass it all. Was trying to keep it all hollow like the current designed engine mounts, only in epoxy and not polyester. I know it's more expensive, but the idea was that I wouldn't have to worry about strength except in the places I'm laying up glass.
 
Zonker

Zonker

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Canada
The cost difference is probably < $100. The better bonding properties of epoxy sticking to old cured polyester is important in this area.

The pink/blue extruded styrofoam isn't structural. You can use it as a form, but don't think it adds any strength. The glass alone has to be strong enough.

I think G10 for the top (big rectangle) but kind of hard to shape and join the sides of solid precut glass.

Build more like this. Green = G10/pink=foam. Lap the Blue fiberglass layers min 2" onto hull bottom, adding 1" for each layer underneath (3/4" if really tight). Use +/- 45 ~12 oz biaxial because it makes a great side shear web and drapes nicely. I'd use 5 layers if you're decent at glassing, maybe 6-7 if you're a beginner just to be sure.

You can use 1708 which is a mat/biaxial stitched together because it builds up thickness fast but it also soaks up resin like a sponge. 3 layers if good, 4 if beginner.

Round the edges of the G10 so the glass will go around the corners.

Use peel ply to finish

1686157924789.png
 
Zonker

Zonker

Super Anarchist
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Canada
Most have styrene melting binders, some have epoxy compatible binder.

But it's the binder that dissolves in styrene, the mat is still there and is glass.

Ultimately it doesn't really matter. It still wets out, just slowly! Lots of pushing and poking with a brush or roller to get the fibers saturated. I agree it's not ideal but if you have some on hand you can use it.
 
M

mckenzie.keith

Aspiring Anarchist
2,360
1,078
Santa Cruz
Zonker said:
The cost difference is probably < $100. The better bonding properties of epoxy sticking to old cured polyester is important in this area.

The pink/blue extruded styrofoam isn't structural. You can use it as a form, but don't think it adds any strength. The glass alone has to be strong enough.

I think G10 for the top (big rectangle) but kind of hard to shape and join the sides of solid precut glass.

Build more like this. Green = G10/pink=foam. Lap the Blue fiberglass layers min 2" onto hull bottom, adding 1" for each layer underneath (3/4" if really tight). Use +/- 45 ~12 oz biaxial because it makes a great side shear web and drapes nicely. I'd use 5 layers if you're decent at glassing, maybe 6-7 if you're a beginner just to be sure.

You can use 1708 which is a mat/biaxial stitched together because it builds up thickness fast but it also soaks up resin like a sponge. 3 layers if good, 4 if beginner.

Round the edges of the G10 so the glass will go around the corners.

Use peel ply to finish

View attachment 596033
Click to expand...
Would you also fillet at base of foam so the glass lapping onto the hull doesn't need to make a sharp turn?
 
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