Thread: Birdsmouth - epoxy vs polyurethane

Building adhesives (PL Premium, etc.) are designed for static loads, not the dynamic ones, seen on boats and especially a mast.

PU adhesives (Gorilla glue, etc.) aren't gap filling (they foam, but this isn't a structural gap filling thing) and they cost more than epoxy, ounce to ounce.

Lastly, neither of the above adhesives will seal out moisture as epoxy encapsulation will. If you want to use something other than epoxy, consider resorcinol instead.

Thanks Paul.
Alas my timber skills are not fine enough to get effective bonding in resorcinol.

These are only the non load bearing top decoration and a stiffening plug for the bottom metre, not the actual mast.
That's an aluminium tube. I always intended to encapsulate them inside & out with pox.

I have now done the tip in PU. I'll wind a bit of 2 oz glass tape into the pox coating once I finish shaping it. I might screw my halyard block to the timber rather than puncture the ally mast too.

I will use pox in the bottom plug. Found I can do most of the 16-siding with a sander, just using the plane to 32- & 64-side it. For one thing, it is more likely to get wet than the mast tip. It will also be taking the foot & partner bending stress from the ally tube mast.

cheers
Alan J

Hi Alan-After just completing a flagpole refurbishment(Solid timber, see attach), I am looking at doing the same as yourself and just upgraded the spindle moulder with new cutterhead to do a birdsmouth type.

I researched extensively types of glue. All the purists reckon Resorcinol is still the go,but leaves the dark glueline.

I would enquire the gurus about using epoxy but with sanding filler instead of silica.There will be some loss of strength though.Will save you a lot of blunt tools and sanding gear.PBO Jan '14 and Wooden Boat No 238 have good articles on this topic.

Regards Scott.

Bond strength in the bottom plug is probably not a big issue.
So long as the strakes don't slide on one another, they will be held in place by the ally outside & the birdsmouths inside.
The shortest birdsmouth would be about 500mm, the longest 1200mm.
They're simply being asked to soften the hard-points at foot & partner.
So sanding filler in the pox is probably not a problem.

With masts all the usual choices will have sufficient tensile strength, but this isn't what's important, it's modulus of elongation. If an adheasive is designed to have a dstatic loading as it's primary holding goal, (such as construction adhesives), then you can bet it's elongation properties have suffered to offer the great static loading quality. Think of this in terms of "toughness". One glue may be intially tougher, when looking at the spec's, but can it do it in a dynamic situation, with things bending and moving continuously? As I mentioned previously, hanging new paneling on your family room wall is a classic example of a static load. Once it's up, gravity pulls in the same direction, until it's removed and no other loading is testing the bond. A glue designed to tolerate dynamic loads will have a high elongation modulus, so it can eat up cyclic and repeated loading without breaking, creeping deforming, etc. When a mast bends from a gust, the leeward side of the mast adopts a smaller radius curve, than the windward side of the stick. This places the glue and the attached fibers in a rolling sheer, with greatly differing amounts of loading (the windward side is stretched, while to leeward it's compressed). This is where the toughness aspect of the adhesive formulation comes to play and why epoxy and resorcinol are commonly recommended. I've seen Urea Formaldehyde glues do well too, which has many of the same issues of resorcinol.

So, short version PAR is that sanding filler in the pox is likely to be a problem?

WP_20150129_001[1].jpg
That's the mast tip well on its way. Haven't weighed it. No tip would be lighter, but This will look nicer.
The gaps between the staves where the PU was visible absorbed a lot of pox before they sealed.
Almost as if there were no glue there. Clearly PU not a good choice anywhere water needs to be kept out...

The bottom plug will definitely be glued with pox.
I'll do most of the rounding with the sander to preserve my metal edges.

Sanding filler is dependent on what fillers are employed, but structural bonds will typically be a tough sand if you have some ooze out. I use a wood flour, talc and silica mix for masts and spars. The wood flour is the fibrous bulking agent, the silica the thixotropic (viscosity control) agent, with some talc tossed in to make it creamy and a touch easier to sand. I use carbide or diamond impregnated blades on goo. Save you good carbon steel for things that are a bit softer.

I've used PU's and have never liked them. They're technically water proof (okay some of them), they setup quickly and are stronger than the wood fibers under load. I've found they tend to go brittle with age and the foaming gap thing has me off enough, to not want to use it much any more.

A lot of defects in that short hunk of mast tip, though the work looks clean, milled straight and good, tight workmanship.

It was a bit of a brute to shape, being first ever attempt at birdsmouth & all.
The plank I cut the staves from was warped & twisted, which got me exactly where I wanted...
The staves are actually under no stress at all - curling inwards like that is their natural lie.
A coupla turns of masking tape around it to hold it together.
Once white paint goes on it to match the power coat on the ally mast...

Gunna make new staves for the bottom plug - 10mm or 12mm. 19mm is waaaaay too stiff.
I'll use the 19mm staves as framing elsewhere.