Thanks for the replies, Paul and Matt.

I understand that once you get to a certain thickness of layup, the
fibreglass begins to take the load while the wood inside acts simply
as a core. I also understand the concept of hard spots. I don't plan
on carefully designing this sheathing, though. This rig is designed
for a solid wood unstayed mast. What I hope to gain from sheating
isn't an increase in strength, it's protection from rot, reduction of
maintenance (no need to oil the mast) and elimination of mast chafe. I
was planning on one layer of cloth only. That's why I imagine that
I'll be in a situation where the mast is flexing as much as the wood
will allow it, and the fibreglass is simply following. That's why I
could be exceeding the epoxy's capability to flex. There's also the
shear force underneath the layup to consider.

Maybe there are different materials I could consider? I've heard of
plastic cloth and other resins (besides polyester, which is out for
obvious reasons), but I don't know much about them.

Paul: You mentioned that flexible epoxies are available, but not from
marine stores. Where could I find it?

Regards,
-Max Camirand

Paul
On Apr 18, 8:32 am, Matt Colie wrote:
Max,

Warning: The following answer may be too Engineerese for the general
public, my appollogies in in advance.

The real value of any modern woodworking epoxy is in it's capability to
saturate into the surface of wood and the fact that it remains flexible.

Yes, an unstayed mast will have higher flexural loads than a stayed
mast, but if you do the glass/epoxy correctly it will be the loaded
element and the wood will largely just hold it to form. As it is not a
case where the wood core could flex and the glass/epoxy would not, there
should be little shear load available to cause the separation of the two.

The matrix overlay will have to be carefully designed and assembled to
achieve stiffness as and where required and then taper as required to
not cause an abrupt change in stiffness at as undesired location.

Unidirection glass tows would be and interesting way to do this, but you
must be careful to cause hard spots as the section reduces (tapers).
Any place that parallel fibers overlap, this is a possible issue.

I have had several applications that were in sunlight for many years
without exhibiting any characteristics that caused me any concern at all.

Was this rig designed to use a solid wood unstayed mast (like many old
Catboats)?

If that is the case, what do you hope to gain?

Matt Colie