Polyurethane rigid foam stringer

[bradz]

Hi guys
I'm very slowly moving into the building stage (after completing the demolition stage) of my 19 feet fibreglass boat. The original sub-floor structure comprised of a fibreglass sub-frame stringer filled with expandable foam. Due to either poor construction, poor support on the trailer or a mixture of both, the sub-frame collapsed in a few locations. As such I have gutted the boat and am about to replace the stringers.

So, I have 3 options but am leaning towards 2 of them. The first is to use an Australian hardwood (Jarrah) for stringers and have 4 stringers in place of the 2 original wide foam stringers. The second is to use rigid polyurethane foam to form 2 stringers (6 inches wide) and covered in multiple layers of CSM and cloth and VE.

If I go to the foam option, do the stringers need to be trapezoidal (say 4 inches on top and 6 inches wide at the bottom) or can they be parallel (6 inches wide top and bottom)? I intend to install latitudinal bulkheads/supports roughly 3 inches wide glassed to these new stringers every 18 inches or so between the stringer and the side of the hull which will provide support to the side walls of the stringers.

Parallel will be much easier for me but I want to hear your opinions. Also, how many layers of CSM and cloth would you use to provide adequate strength given that the foam is basically a form only.

 

[tpenfield]

Perhaps you can post some pictures of your restoration, as that always helps with comments and advice. You can certainly use rigid foam for the stringer core and the rectangular profile (instead of trapezoidal) will be fine. I'd go 3-4 layers of a biaxial stitched cloth to build up to about 7mm thickness.

 

[gm280]

bradz, whatever you go with, the fiberglass will be the structural support and not the foam inside. The foam is installed merely to work as a place holder until you build the fiberglass structure over it to support the floor and cap. But like tpenfield above suggested, post a picture or three for us to see what you are dealing with. Then you will get better ideas and suggestions from a very well educated crowd on here. JMHO

 

[jbcurt00]

And if you build the stringers 1st out of foam, you'll need to protect the foam from the polyester resin before you try to encase and tab the foam stringer to the hull.

2in wide packing tape can be used, just be sure to overlap the layers slightly to ensure full coverage.

The stryene in poly resin dissolves/melts many (most? all?) foam. Like the faces in the 1st Indiana Jones movie......

Good luck w your project and yep, post some pix

 

[Red Herring]

There are polyurethanes that cope with polyester just fine, I built the whole aft deck structure out of rectangular profiles with a lot of glass on top. I did however buy some other urethane sheet that melts like butter in contact with resin. Not sure how to figure out which one is which apart from testing.

Some photos would help, and you can see a similar construction in my thread as well.

 

[bradz]

Thanks for all the feedback. I'll get some photos up in the next day or so.

Was outside grinding today. The temp was 38 degrees Celsius which is just over 100 degrees Fahrenheit so Im a little bit stuffed.
The foam Im considering is fine with VE as advised by the seller.

May still look at hardwood or marineply...just weighing up options.