Fiberglass questions

[Jason Klett]

Good morning. I just got the plans for a Marissa 18 and am a little confused on the fiberglass. The materials list says 300' 12oz bi-axial tape. Is this the same as 1208 biaxial ? What width would I need? I saw the fiberglass kit on the website shows 70 feet of 6" 1700 Bias tape which is throwing me off even more. I have built two small stitch and glue boats and a plywood on frame jonboat so I'm not totally new, just that this is my biggest(most expensive) one yet so I don't want to screw it up. Thanks in advance for any help.

[Alan Stewart]

Good question. The glass in question is specifically just for the inside chine reinforcement. The glass pack we send with the kit includes the 6" bias tape for this which isn't exactly the same as any bi axial tape or cloth. Bi-axial anything just means that the cloth contains fibers that run in 2 directions which can be straight weave with fibers running long ways and cross ways(0/90) or Bias weave with fibers running "45/45". It's specified "bi-axial" because they also make "uni" cloth, "tri-axial" and even "quadraxial" cloth with more fiber layers and more directions. 

What is bias? 

Bias tape or cloth is glass that is woven "on the bias" meaning the fibers run 45 degrees to the long axis of the cloth. Below is a picture of what that looks like. These conform more easily to complex shapes. The one below has no "mat" on it. 

 

Here is a picture of straight weave tape. with fibers at "0/90" what most people use for stitch and glue boats. It has a selvage edge that is slightly raised so one edge takes a bit more work to sand smooth unless you cut it off first. 

 

The picture below shows the chine joint on Marissa glassed with regular biaxial cloth cut into a strip which is totally fine. You don't HAVE to use bias cut tape for this section but it's better/easier for 2 reasons. Firstly, the fibers lay over the chine batten at 45 deg so they don't have to bend as hard around the corners so the glass lays in a lot easier than if the fibers have to make 90 deg bends to conform. And secondly all of the fibers cross the joint as opposed to half running along the length of the joint making it stronger. Anytime you use straight weave tape over a joint only half the fibers actually cross the joint, still this is fine for almost all applications unless trying to highly optimize for strength to weight ratio such as in aerospace applications. 

 

So what is the bottom line... 

Marissa can be built with nothing but 10oz woven cloth on a roll if you want to keep it simple. You can glass the exterior, cut it into strips and glass the chines inside, the bulkheads and stringers and everything. The chine specifically calls for 20oz of glass so 2 layers of 10oz. on the exterior you just overlap the bottom and side glass and you get 2 layers on the chine. On the inside you'd use 2 layers of 10oz cut into strips between each frame. If you use 17oz bias tape on the inside of the chine then just 1 layer is enough. The rest can be 10oz cloth cut to fit each area (bottom stringers, bulkheads to sides, etc...) Anywhere that 2 pieces of plywood come together in a "T" joint we put a fillet and reinforce with fiberglass OR we have a solid wood "cleat" where we can't access the joint for example when gluing down the cockpit sole. 

 

The material list calls for 1200 bias tape because it's more versatile and has the benefits of bias tape. Use 2 layers on the inside chine to get it over 20oz and then continue using it inside to tape the bulkheads to the hull sides, on the console, anywhere you'd use glass tape. 

 

And then... 

It gets more complicated with different combos of glass like when you add "mat" which is random glass that helps stabilize the weave and act as a binder for polyester resin (still the majority of use cases). And one further distinction is between "woven" glass and "stitched" glass. The difference between the glass fibers being woven "over under" like you would imagine vs the layers being held together with a "stitching". Stitched glass is stronger and woven glass is typically only seen up to 10oz weight also called "plain weave". above 10oz most glass is stitched with the exception of "woven roving" which we don't use for our applications. 

If you browse through some of the glass on US Composites site for example they have good pictures of the different stitched glass.  

https://www.uscomposites.com/specialty.html

 

A stronger lighter Marissa could be built for example with vacuum bagged 1208 glass (vacuum bagged to reduce excess resin) or carbon fiber. But the savings compared to the extra effort is in slim margins territory.

[Jason Klett]

Thank you so much for the detailed explanation of it, I really appreciate that! That answers everything I was getting confused about. 

[AmosSwogger]

Thanks for that explanation.  Very helpful.