BoatBanter.com - How thick to make the fairing? & vaccum bagging a hull

I'll have to look into the mat issue some more I guess. But one thing I'll
say about your statements about mat versus yarn versus strength is that most
people love to say that mat provides little strength compared to yarns.
That statement is true *IF* you say *tensile* strength, which means 'in
tension' or 'in a pulling direction'. Since that happens to be what most
seem concerned with, I suppose the statement is mostly true. However, if
you study composites, you'll find that mat is known as an omnidirectional
strengthener and it improves *sheer* strength in all directions. If you
know the direction of sheer ahead of time, then you can orient yarn
materials appropriately and do better. But if you don't, then mat is a good
solution for improving sheer strength in all directions. It adds some
tensile strength as well, but somewhere better than straight resin and
somewhere short of oriented strand or yarn. Mat is not without it's
benefits, other than 'just being a filler' for example.

Brian

"Glenn Ashmore" wrote in message
news:_vnOb.15922$Mb7.5785@lakeread04...
Good question but the answer is a bit complicated. Epoxy will bond to
mat but the bond is a bit weaker because the bond is to the sizing more
so than the glass. I use DB1808 for tabbing non-structural parts
primarily because it maintains its shape when it is wet out better than
straignt DB180 but it still makes me a little nervous. It is very hard
to get the mat to wet out. Even when you get it soaked in epoxy and
work it a lot you can still see the mat fibers which means the binder
has not desolved.

My main objection to straight mat is that it adds weight and consumes
resin at a much higher rate than it does strength. It is really more how
epoxy is used to take advantage of its properties. That is why
uni-directional fabrics are favored over woven cloth and cored
construction over solid layups. The goal is the maximum strength for
the elast weight. Besides having higher bond strength epoxy has a much
higer modulus than polyester. That is, it stretches less for a given
amount of stress. To get all the value from of epoxy it should be
matched to the fabric. Glass is the primary load carrying component
in a composite but it can only take the load in the direction of the
fibers. In tension the crimps in woven cloth have to straighten out
before they can take the load. The resin has to yield until the fibers
straighten out. With a lower modulus, polyester will stretch while
epoxy must take the load it was not intended to do.

With mat the situation is even worse. Only about 1/12 of the random
fibers in mat are oriented to take a load at any particular time and
then over only a very short distance. The epoxy must then carry it to
the next fiber. Mat is intended to fill the weave between load carrying
plys and add bulk. It soaks up a lot of resin which is not acceptable
with lower cost polyester but it is a waste of epoxy.

Brian D wrote:
Yes, very good answer. But one question looms still...

Someone in the post below said that mat it held together with a
styrene
based binder that dissolves in polyester resin but not epoxy resin.
Then
how come System Three, RAKA, and other companies sell 1708 (17 oz cloth
plus
8 oz mat) that is compatible with epoxy? Surely these outfits must be
selling material with something other than a styrene based binder in
it???
Can anybody offer a good answer on this?

Thanks,
Brian

"Pete" wrote in message
...

A lot clearer now, thanks.

Pete

On Fri, 16 Jan 2004 17:26:07 -0500, Glenn Ashmore
wrote:

Lets get the terminology straight first.

Roving is a thick yarn loosely woven fabric used as the structural
component in a polyester lay-up.

Cloth is a thin yarn relatively tightly woven fabric used as the top
ply
in polyester and epoxy lay-ups.

Mat is a loose felt like glass fiber with no weave. it is just random
fibers mixed with a styrene based binder and lightly pressed into a
sheet. It is used between layers of roving to fill the voids between
yarns and under the outside cloth to prevent the weave of the roving

from printing through as the polyester cures.

Knitted fabric is straight, unwoven glass yarns that are held together
in a fabric by very light weight threads knitted across the yarns. The
yarns can run the long or warp direction, across the short or weft
direction. Two or three plys of yarn can be knitted together on the
bias (45/45) or warp and weft (0/90) or some combination. It is called
Bi-axial or tri-axial.

The numbers used on US fabrics are the ounces per square yard.

A-130 is 13 oz/sq.yd. unidirectional knitted E-glass. DB180 is 18 oz
45/45 bidirectional. CD120 is 12 oz 0/90 bidirectional. When there are
4 digits it is usually some kind of combination of fabric and mat.
DB1808 is 18 oz biaxial with a layer of 8 oz mat laminated to it.

Here is a page with all the Knytex fabric specs.
http://www.fiberglasssupply.com/pdf/...tedFabrics.pdf

Another word of advice, never buy fabric that has an epoxy maker's name
on it. Check around for a fiberglass supplier that handles the major
maker's lines. You are going to be buying at least a couple of rolls
and the prices get a LOT lower in full rolls.

Pete wrote:

On Thu, 15 Jan 2004 18:19:50 -0500, Glenn Ashmore
wrote:

First of all don't use mat with epoxy. The binder that holds mat
together is designed to desolve in the styrene in polyester. It will
not bond to epoxy. Even if it did, mat is primarily for adding bulk

and

is terribly inefficient for carrying the loads in a strip composite
hull. Loose thick "Roving" also is less than desirable with epoxy.
It
takes to much resin to fill adding little to the strength and a lot
to
the weight. Uni-directional and bi-directional knitted fabrics are
used to match the strength of the epoxy. Two layers of DB180
bi-directional is about 3mm. Two layers of A-130 unidirectional is
about 1.5mm.

Ah, now this is something I have not heard of, now I am really
confused. Maybe it is just a question of terminology because the stuff
I'm talking about is specified by the designers Van de Stadt. I say
mat probably in ignorance but the exact specification is for 2 layers
of uni-directional roving laid diagonally, 90° from each other. I
suspect from your comments that I should be sayiing fabrics, I really
had no idea there was a difference between glassfibre for epoxy and
polyester.
Unfortunately for me, the reference A 130 doesn't mean anything to
me, is it an international or national reference? West Systems or
their distributors here do not supply anything like my specifications,
but SP Systems do and it's called unitex woven low crimp
unidirectional fabric, fibre type E-glass. Looks like I've got some
checking to do.

With that out of the way, vacuum bagging is definitely worth if if
you
want the most strength with the least weight. It is not all that
hard
to do. Check out my web site to see how a team of 6 volunteers with
no
experience glassed and bagged my 45' hull.

I've been checking out your site for a while, and after years of
procrastination I have got from it most of my inspiration to finally
go ahead and build myself. Thanks from me and quite a few others I
should think.

Fairing putty on a well laid hull ranges from none to what ever is
necessary to get a fair hull. It is not for filling the weave but
for
evening out the overall lines. Normally when lofting station molds
you
don't consider the fairing putty thickness. If you do everything
exactly right you won't need much putty. If you deliberately allow
for
it you will be forced to use a lot more than you would otherwise.

Thanks for your help

Pete wrote:

On average, how thick is the putty fairing layer over the mat on a
strip planked hull? I guess it may depend on whether vacuum bags
were
used (that's a question too - what's the general consensus on vacuum
bagging a hull?; what are the gains 'cos it looks like loads of
work?).

I'm just trying to cut the molds from the designers lofting
templates
and I want to know how much to reduce the size of them by, to allow
for the overall hull thickness. But I also need to order the right
amount of filler too.

The glass suppliers have told me that two layers of 500g m² mat of
uni
directional rovings laid at 90° add up to about 0.75mm after layup -
sorry about the metric measurements for the Americans, I haven't a
clue how many ounces/yd² that is - which sounds negiligble to me,
but
I have read somewhere of around 3mm or just over 1/8 inch may be
fairing filler.

This sounds a lot to me (after my many attempts at plastering over
defects, I feel that the more you put on, the worse things get). It
seems to me that if easily sanded fairing is used it must be a bit
soft, so would the minimum necessary be used?

I'm talking epoxy here BTW, and I'm assuming that I will make a
reasonable job of laying up the mat .............. hmmm, I can hear
the gods of Famous and Doomed Last Words stirring from their slumber
already.......

TTFN
Pete

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com