Tabbing Coosa stringers with CSM & VE?

[BrettinVA]

Hello everyone. The other day I saw a boat restoration where the builder used Coosa for the stringers and tabbed them to the hull with a few layers of CSM and VE resin - no cloth or resin over the top of the stringer. The deck was also done with a couple layers of CSM and VE then webbed with gelcoat. It was a really good looking job but what are you opinions of using CSM/VE v. 1708 or roving?

 

[missinginaction]

I can't speak to the deck aside from saying that using mat up there would make a heavier than necessary structure. As for the stringers, I don't think it was a good idea to leave them bare and just tab them in. Coosa is pretty tough stuff but still....... The glass that is installed over the stringer and tabbed onto the inside of the hull is what gives the stringer it's strength. I replaced engine stringers on my boat when I did a restoration a few years ago. I used Owens Corning Formular 250 (this is just extruded polystyrene closed cell rigid foam insulation material. You can get it at Home Depot) You can easily snap a 2" thick piece in half over your knee. Where the engine beds were located, I built sections of solid wood into the foam stringer for compression strength (through bolting the engine beds). Why am I telling you this? To make the point that whatever you make the stringer out of is not very important. What is important is the the glass/resin layup that goes over the stringer form. I measured the thickness of the original layup and also used information in one of David Gerr's books, The Elements of Boat Strength, to determine the thickness of my layups and tabbing. My boat's been on the water for some 5 years now. Nothing has moved, no issues with engine alignment. No problems with the stringers at all. One thing I have learned from more experienced people on this site is that roving or cloth is preferred over CSM or bi-axle stitch mat for structural work, such as stringer fabrication. It's stronger and you don't use as much resin so it's lighter.

 

[BrettinVA]

Woven roving that I've seen uses way more resin that 1708 or CSM - so I'm not sure what type you are talking about.

 

[PAR]

In this application CSM is just a bond promoter and not really part of the structural elements in the laminate stack. Alternating CSM and roving, using a combo fabric (1708, etc) or alternating CSM with and knitted fabric makes a lot more sense. I suspect the laminator you saw didn't really understand what he was doing and knowing what he was doing, would soon be buried under the deck, out of sight and mind, he saved some money and labor and made a really weak laminate, that will likely last longer than any warranty period the shop offers. In fact, it seems pretty stupid to use Cossa and VE, then to just CSM it in.

 

[BrettinVA]

It's his personal boat and he did the work himself - not trying to get anything past anyone. This is a small classic speedboat and
he uses it frequently. It has a number of blukheads or ribs building a framework with the stringers. He was showing me because he's a huge fan of the coosa and one called thermolite or something and I'm getting ready to do a deck in a project boat. His is holding up nicely and stays outside, wide open on a lift over salt water. I used epoxy and 1708 on my last project - not sure if I'm gonna spend like that again.

 

[Steve W]

You don't usually use epoxy with 1708 because you don't need the mat that is stitched to the double bias. A better choice is 1700 double bias with epoxy, 1708 with VE. It does work because the mat is thin but it just adds unnecessary weight in both the mat and the extra resin to wet it out. As to using just csm with coosa, yeah, not a smart move but I'm sure it will still work out fine as long as he bonded the coosa well. It does have a lot of stiffness on its own and csm is not exactly without strength. As Par said CSM is really just a bond promotor and was necessary in the old days when using woven roving, not really needed with the low profile stitched fabrics.

 

[fallguy]

csm is still handy to prevent biax deformation as well; not this application though