William R. Watt wrote:
Note that the weigth of fibreglass and resin together is 96 lb per cubic
foot (Skeen's Elelments of Yacht Design, 30% fibregalss, 70% resin), and
a 1/4" thick layer is 2 lb per square foot. That makes 1/8" layer 1 lb
per square foot, and 1/16" layer half a pound per square foot.
snip

The above applies ONLY to polyester, mat and roving.

It definitely does not apply to knitted glass and epoxy where the
glass/resin ration is about 50/50.

Also, epoxy/knitted glass laminates are much thinner and lighter weight
than polyester/mat/roving for equal strength.

HTH

Lew

Lew Hodgett ) writes:

The above applies ONLY to polyester, mat and roving.

It definitely does not apply to knitted glass and epoxy where the
glass/resin ration is about 50/50.

The above applies ONLY to vacuum bagging. 70% resin is optimistic for a home
builder filling weave by hand.


Also, epoxy/knitted glass laminates are much thinner and lighter weight
than polyester/mat/roving for equal strength.

This is NOT a structural application. It's canoe hull sheathing, abraision
resistance and fir ply check checking.

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William R. Watt wrote:


The above applies ONLY to vacuum bagging. 70% resin is optimistic for a home
builder filling weave by hand.

Rubbish.

According to the total material invoices for the 55ft hull I built, was
able to achieve about a 50/50 wet out using hand layup techniques.

BTW, "hand layup techniques" is just another way of saying a lot of hard
workG.


This is NOT a structural application. It's canoe hull sheathing, abraision
resistance and fir ply check checking.

All the more reason to have a high strength, low weight, high strength
laminate that cloth and epoxy provide.

Lew

William R. Watt wrote:
Lew Hodgett ) writes:


The above applies ONLY to polyester, mat and roving.

It definitely does not apply to knitted glass and epoxy where the
glass/resin ration is about 50/50.


The above applies ONLY to vacuum bagging. 70% resin is optimistic for a home
builder filling weave by hand.

I don't agree either. 30% glass might be for a weak chopper gun
layup, but it's pretty easy to get up to 40-45% even with mat/roving
combinations. For knitted glass figure 50/50 without vacuum bagging,
maybe 54% if bagged.

Evan

On 18 Feb 2005 15:19:38 GMT, (William R.
Watt) wrote:
I don't know if you've started to build but meranti (luan) plywood is
cheap, rot resistant, and does not check. I assume because you are using
lumberyard plywood, as I do, that you are interested in a cheap boat. I've
built in lauan and virola (not rot resistant) and just painted. I've also
fished from cheap painted (fir and spruce?) plywood punts. You can't get
spruce plywood any more as far as I know.

I almost picked up the luan, mainly for the lighter weight, but the
interior glue threw me. My understanding was I needed to have
exterior glue or the resin could (and probably would) delaminate the
layers. Is that not true? my lumber yard does not carry exterior
glued luan in 1/4" (actually, 5mm)




email: dallyn_spam at yahoo dot com
please respond in this NG so others
can share your wisdom as well!

Dave Allyn ) writes:

I almost picked up the luan, mainly for the lighter weight, but the
interior glue threw me. My understanding was I needed to have
exterior glue or the resin could (and probably would) delaminate the
layers. Is that not true? my lumber yard does not carry exterior
glued luan in 1/4" (actually, 5mm)

You need exterior grade plywood. They all use moisture resistant glue
except for the fancy wall panneling. If it's kept inside the store it's
usually not exterior grade. If it's keep outside, even under cover, it
should be exterior grade.

I don't know what kind of meranti or lauan is available in the USA. Here
in Ottawa we just have the underlayment, used to put on floors under
carpteting and vinyl. It has one thick inner ply and two thin outer plys.
You have to look for sheets whith few or no edge voids and few face flaws.
I've seen some poor stuff and some excellent stuff. I have filled voids by
drilling small holes through one face ply and pumping in caulk or adhesive
with a caulking gun. That works well. I got one sheet with worm holes in
one face ply which I filled with little drops of polyester resin before
oiling (I use linseed oil on the interior). If I were covering the hull
with fibreglass and resin I would not have bothered.

As I understand it, the only real need to sheath the hull is to keep the
fir plywood from checking and for that you need a pretty thick layer, from
what I've read.

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"William R. Watt" wrote in message
...

Dave Allyn ) writes:

I almost picked up the luan, mainly for the lighter weight, but the
interior glue threw me. My understanding was I needed to have
exterior glue or the resin could (and probably would) delaminate the
layers. Is that not true? my lumber yard does not carry exterior
glued luan in 1/4" (actually, 5mm)

You need exterior grade plywood. They all use moisture resistant glue
except for the fancy wall panneling. If it's kept inside the store it's
usually not exterior grade. If it's keep outside, even under cover, it
should be exterior grade.

If I'm paying good $ for ply I expect it to have been kept dry and at stable
moisture content. INDOORS. My supplier stores everything in the same
warehouse - from marine ply through exterior through formply, chipboard ....

Leave a sheet of ply flat on the concrete floor of the workshop overnight
and it's no longer flat - put it out in the sun for a few hours and it's
bent the other way. And if it's moisture induced curvature, it will try to
go back to being flat when you make it into a boat.
DF


I don't know what kind of meranti or lauan is available in the USA. Here
in Ottawa we just have the underlayment, used to put on floors under
carpteting and vinyl. It has one thick inner ply and two thin outer plys.
You have to look for sheets whith few or no edge voids and few face flaws.
I've seen some poor stuff and some excellent stuff. I have filled voids by
drilling small holes through one face ply and pumping in caulk or adhesive
with a caulking gun. That works well. I got one sheet with worm holes in
one face ply which I filled with little drops of polyester resin before
oiling (I use linseed oil on the interior). If I were covering the hull
with fibreglass and resin I would not have bothered.

As I understand it, the only real need to sheath the hull is to keep the
fir plywood from checking and for that you need a pretty thick layer, from
what I've read.

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Dave,

Neither fiberglass nor fabrics like Dynel® add significant strength and
the checking problem with BCX pine plywood is insignificant. Tape your
joints inside and out and paint with 100% acrylic latex exterior paint
of your preferred gloss. For more on this and other heresies see:
http://www.angelfire.com/nc3/davecarnell/

Dave Carnell

Dave Allyn wrote:

I am building a S&G canoe. I am going to use fiberglass cloth on the
entire outside for extra strength, and to prevent any checking in the
plywood. I'm useing 1/4" BCx plywood.

What weight of cloth would I want to use for this aplication? And the
next question, how would I decide that on future projects? Is there a
general rule of thumb for weight?

One last question, I know I need to tape the inside and outside with
fiberglass tape on the seams, but If I cover the whole outside with
cloth, do I still need the tape on the outside of the seams?

Thanks,

dave


email: dallyn_spam at yahoo dot com
please respond in this NG so others
can share your wisdom as well!

Dave Carnell ) writes:
Dave,


the checking problem with BCX pine plywood is insignificant. Tape your

Oh, I thought he meant BC fir plywood.

Nice to hear from you.



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Dave Carnell wrote:
Dave,

Neither fiberglass nor fabrics like Dynel® add significant strength and
the checking problem with BCX pine plywood is insignificant. Tape your
joints inside and out and paint with 100% acrylic latex exterior paint
of your preferred gloss. For more on this and other heresies see:
http://www.angelfire.com/nc3/davecarnell/

Who are you kidding? Perhaps that's true of larger boats with thick wood
skins, but fiberglass adds substantial strength when you're dealing with
light weight, thin skinned boats like canoes and kayaks. This is
especially true with sandwich construction where the core of plywood or
cedar strips is fully encapsulated in fiberglass.