On Mon, 26 Apr 2021 16:18:46 +0100, newshound
wrote:

snip

I was thinking we end up with a fairly substantial (where it matters)
fibreglass tube frame, even if the steel was to rot away inside. ;-)


The problem is that the elastic modulus of fibreglass is very much lower
than that of steel. So that once you are carrying a significant part of
the load in the fibreglass, it wants to stretch much more than the steel
does. This mis-match fractures the bond at the interface.

Understood ... *if* it does (at any level that matters etc).

From my personal experience and certainly when laminating clean
shot-blasted steel with fibreglass and where the glass is
proportionally substantial compared to the base steel, there hasn't
been any separation.

And with concentric tubes, there isn't anywhere the inner can really
go if tightly bonded inside a fibreglass skin is there?

I have actually been quite surprised how well fibreglass, laid up on
clean and 'rough' finished (eg, shot blasted or very course wire
wheel) steel really bonds.

The problem is that most people (not you of course g) don't 'get'
how clean or how good the prep and process needs to be (inc humidity /
temperature / pressure / viscosity) needs to be to give any form of
adhesive bond a fair chance.

Most things I glue together tend to stay together because I have a
reasonable ides of what's likely to work before I even give it a try.
[1] ;-)

Cheers, T i m

I have the daggerboard casing out of my smaller folding dinghy because
it leaked when I first tested it in my mates pool.

When I took it out (6 screws and some flexible compound) the bottom
surface on the casing was far from flat / uniform so I have sanded it
down till it was (on my linisher). It's not flat as it has to follow
the curved contour of the hogpiece so I have bought some hardwood
sheet about the same thickness as the material I had to remove that I
now need to bond to the bottom of the existing (marine ply) casing.

To give that the best chance, I need to first pre-bend (steamer) the
plank to the same curvature (2D luckily) as the hogpiece then bond
with a strong and fully waterproof adhesive (probably a two part
resin) and PB / stainless screws to the casing (that will be left in
there) .

Sand, varnish and then re-fit into the boat by making up a 'wet'
gasket of CT1 that will be slightly tightened once cured to give it
some compression. It can't be bonded ''hard' as the hogpiece flexes
quite a bit between the folded and unfolded states. Also, being a
folding boat it's all a bit 'live' so anything acting as a seal needs
to have some give in it.