replaceing mat

My scantlings call for 17oz bias, mat,and 6oz finish, my designer said I could replace the mat with glass I want. He did'nt tell me if I needed ounce for ounce, only the mat is there for thickness. I have bought 18oz 0/90 and 12oz 0/90 to replace the mat, would I be better off replaceing the 12 oz bias with a 5oz kevlar for puncture resistence, and a 5.7oz s-2 glass for my finish glass , for a weight saveings of 6 oz? Thanks for any advice rick

 

What's wrong with the original specifications?

 

would use twice the amount of epoxy to wet out the mat, the extra bias or s-2 glass will make a stronger stiffer , and lighter hull, kevlar might add some puncture resistance.

 

Mat is usually only made to be used with polyester or vinlyester. The adhesive will not be dissolved by epoxy and have a poor bond. If you make a thinner laminate than designed, it will be too soft. A panel stiffnes is a functioin of the cube of the thickness. That means that a panel that is half the thickness will be four times softer.

 

I will not be using any mat, it will be replaced either ounce for ounce with 0/90 bias or with some S-2glass and kevlar.

 

If the mat is there for thickness, then make sure whatever you put back, is the same thichness. Which probably means you will need heavier glass.

 

Thanks Herman, it,s there for thickness. I think my question is would kevlar make up for a little thiner layup. My scantlings are 1 1/2 oz mat, 17 oz 45/45, 1 1/2 oz mat, 6 oz finish.

 

Kevlar will hardly make things stiffer, thickness is the key.

 

Thanks again, I'll keep it thick, I think I am about right. My build is a Horstman tristar 31. In one of his books he writes that "general" fiberglass thickness for boat length 20'-30' at .110 , 31' - 35' at .150 , I should be at .136 with 17oz,18oz,12oz,bias and 6oz cloth. Adding up his layup scantlings it hits around .19 . He also writes that in multihull construction you might want to use less mat because of its weight building properties and use 1/2 oz or 3/4ozmat between woven roving. A builders choice. He was wrighting about fabmat ( 2415, 1610 ).

 

I started a reply but it got too involved. You're building a pretty big and expensive boat at 31', but you need to get some basics down first. Scantlings are frames and reinforcement for the hull skin. What you are talking about is a composite layup schedule for a hull skin. It is engineered for the way the different materials work and the direction of stresses applied to it. There are different types of strength. Basically there is tension, compression and shear. Changing the layup schedule can drastically effect them, especially in lightweight layups. What sort of resin is used is also a big factor.

You have to tell what the (6 oz) 'finish' is. Is it the inside or the outside of the hull? Male mold, female mold, a covering for plywood?

You have to tell what the resin is, polyester or epoxy. They have different properties. As does fiberglass, cloth, and mat woven roving from each other, as does kevlar from them, as it is a different material with different properties.

A 12 oz 0-90 is not the same as a 12 oz bias and not all 12 oz bias's are the same.

 

Ok, so if you are building a Horstman tri, is this the outside skin of a foam core sandwich that you are describing? because if it is and you are using epoxy as you indicated in post #3 you wont be wanting to use the mat (for the reason gonzo indicated) but the stiffness of the laminate is not an issue at all (the core will provide the panel stiffness)but and what you are planning is fine although im with SamSam on this, are you talking 18oz woven roving or stitched fabric, its a little confusing because you are not giving all the info. So,what is the actual laminate schedule,including the inner and outer skins and core. Neat project bty.A friend of mine built a cold molded 35 years ago.
Steve.

 

Sure, there it is in post #3 - epoxy. I also forgot to add that to get an opinion about replacing the mat in the layup, you have to tell what weight the mat is.

The chart here in post #3 might help you, "Weight, thickness and resin requirements for common fiberglass materials".............

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/fiberglass-thickness-8686.html

 

Thanks for all the replys, I may be causing some confusion with the way I,m asking my queations and not giving enough info to get an answer. So first I'll state what I,m trying to do. I want to to get the lightest lay up I can without using mat or losing stiffness and puncture resistance. The designer is ok with replaceing the mat , and in the upper post I,m trying to show that the designer is ok cutting 1/2 the mat out of the layup as stated in one of his books. So here is his layup.

Layer material thick in

mat 1.5 0.051
wr 24 0.048
mat 1.5 0.051
core 0.625
mat 1.5 0.051
wr 24 0.051
total 0.874

layer material using DBM
mat 1.5 0.051
DMB 1715 0.077
core 0.625
DMB 1715 0.077
total 0.831
My material on hand is 5/8 H80. 5/8 renecell 240E, Bias 17oz 45/45, bias 18oz 0/90, bias 12oz 0/90, 3 and 4oz s-2 glass. using this material my thickness would be 0.753, takeing out half the mat on Eds layup would come up to 0.7545. For a finish Ed says use gelcote or a 6oz cloth, so can I do away with the 12oz bias and replace it with my finish cloth and a 5.4oz kevlar. Would the 6oz and kevlar maintain my stiffness and add punture restance, would it be worth the extra money? thanks rick.