Glue for birdsmouth mast?

[Welding Rod]

I am making a 15 foot long, 6 in dia mast for my flopperstoppers. It will be Douglas fir.

I mostly use epoxy but have read somewhere to not use epoxy for masts. I don't worry about cosmetics this is a workboat, so visible glue lines don't matter. It will get painted. Durable is more important. Daytime temps are 70-85 in shop, 10-25% humidity. Very dry wood. Been in shop 2 years in NM

Rod

[David G]

No reason that I can think of not to use the epoxy.

[Welding Rod]

That's what I thought. Just checking.

Thanks, Rod

[Nicholas Carey]

The reason for using epoxy with a birdsmouth spar (and the reason birdsmouth spar construction wasn't widely used, even though it's been around for nigh on a century at least) is that most adhesives -- e.g., casein, urea-formaldehyde, resorcinol, etc. -- require (1) well-fit joints, (2) and the correspondingly thin glue lines and (3) relatively high clamping pressures.

Epoxy, OTOH, is happier with relative thick glue lines and relatively low clamping pressures. It's also quite good at filling gaps and so isn't as dependent on well-fit joints. Epoxy also can be tweaked with additives -- microfibers, fumed silica, wood dust, etc. -- to get the desired consistency and strength needed for the job at hand. Further, unlike other adhesives (which see above list) epoxy has a relatively long open time, so you have some time to get your spar assembly together, adjusted and aligned before it needs to be clamped down.

The one argument for using an adhesive other than epoxy is that epoxy degrades under UV light. Presumably you will be varnishing or painting your spar, so that is in reality a non-issue.

Birdsmouth joints are not hugely precise. Consequently, it wasn't used prior to the advent of adhesives like epoxy, despite its advantages for assembly (self-registering), since the resulting spar would not be as strong as one built with traditional staved construction.

There is no reason to build a birdsmouth spar with an adhesive other than epoxy.

[David G]

It's not all THAT hard to get precision on a birdsmouth joint. As a result, I've successfully (AFIK... no complaints or failures yet) fabricated b'mouth spars using epoxy, resorcinol, plastic resin, and Titebond II & III. The resorcinol, in particular, requires not only very good clamping, but sufficiently high temps. I'm happiest using epoxy, though.

[Eric Hvalsoe]

2nd David's comment that a birdsmouth joint can be quite precise and can be significantly loaded with enough hose clamps - as nick explains epoxy is not a good candidate for furniture quality joints. I think epoxy is a bit of a nuisance to work with - and an essential tool in my, and most other boatshops.

Use epoxy if your not much concerned about the quality (tightness) of the joint. This is not necessarily a negative comment. Protect it with paint or spar varnish. Another very positive aspect of epoxy is the working time available, depending on the hardener.

I just did my first birdsmouth recently, using two different adhesives. The spar was glued up in halves (thanks Tim) with Tightbond III. Now somebody might pop in to say tightbond in manifestly the wrong glue for structural purposes. I am comfortable with it in this application, perhaps time will tell. Weldwood would also have been a good choice, although it requires mixing, and likes at least moderate temps. After glue up, I pulled the two Titebond halves apart, and fitted end plugs. The base plug was a couple of feet long. The plugs were not 'furniture fits', so epoxy was the way to set them. The plugs and the two spar halves were then set in one operation - all epoxy. The hose clamps were set just snug enough for squeeze out - less pressure than with the tightbond.

[keyhavenpotterer]

I'm no expert, but I have read up adhesives a fair bit.

Resorcinol is the king here.

1. It is not affected by UV light. A spar, sticking up in the sun is exposed to UV unless its painted and kept painted.
2. Resorcinol is not affected by heat. Again on a mast sticking up in the sun, I should think it could get hot.
3. Cyclic loading. After every movement and loading it springs back, fatiguing the joint. Resorcinol last longer than epoxy which lasts longer than polyurethanes. There's alot of cyclic loading go on up a boats mast.
4. Its not affected by seawater, as epoxy can be.
5. Resorcinol is easier to remove with a wet wipe and plane or sand off than epoxy when set.
6. Resorcinol maintains its strength at a high level. The tail off with epoxy and polyurethanes occurs more quickly as the degrade.
7. Its designed to optimally glue air dried timber at 12-16% moisture content. Wood to be stable outdoors is best glued up at this moisture content, not bone dry kilned stuff that has the capacity to absob moisture if its exposed oneday.
8. It costs less, and needs less than epoxy in a glue line.

You can get Resorcinol that goes off in summer, winter or 'normal' room temperature akin to epoxy with fast/ normal/ slow types.

Pretty much any adhesive starts of strong, but its over time that performance differences show up, and Resorcinol is the most durable best performing adhesive in long term durability studies with cyclic loading and weather exposure: Its important for a mast not to delaminate if its not made from one piece, or in glulam beams in the building industry where it's used alot.

For a dinghy, I'm sure epoxy is fine (UV, temp, cyclic loading etc are all limited) but well fitted staves glued with Resorcinol will be a superior product long term.

Resorcinol seems harder to find in the US which is a shame, a legacy of the second world war when we had to glue planes together like the mosquito bomber, here in the UK. Its also reasonable not to want to fork out for another pot of adhesive if your building an epoxy glu-lap boat, but a boat should be built to perform 20-50 years later to be truely well built (a typical small boats life before epoxy coating even)

I defer to more experienced builders though.

Ed

[Canoeyawl]

Having seen spectacular failures with resorcinol (some of them my own...) I would use epoxy and never look back.
There is the required glue line size and fit, the temperature issue (70° is a minimum, 24 hours is recommended) and it requires fairly high clamping pressures. Read the manufacturers technical data carefully, then do the math.
How many total square inches of glue surface will you have? It will be almost impossible to create the pressure required without inventing a clamping fixture for round work. I suspect you may have to deform the (delicate) hollow section to arrive at the recommended pressure.

[David G]

Having seen spectacular failures with resorcinol (some of them my own...) I would use epoxy and never look back.
There is the required glue line size and fit, the temperature issue (70° is a minimum, 24 hours is recommended) and it requires fairly high clamping pressures. Read the manufacturers technical data carefully, then do the math.
How many total square inches of glue surface will you have? It will be almost impossible to create the pressure required without inventing a clamping fixture for round work. I suspect you may have to deform the (delicate) hollow section to arrive at the recommended pressure.

The persnicketyness of resorcinol is real... but you overstate the difficulties. It can, and has been (many times), done successfully - including by myself.

[Eric Hvalsoe]

Clamping fixture for round work - otherwise known as a hose clamp. The crunched edges of the initial birdsmouth layup are faired off in the final rounding. When the veed edge is carefully milled the resulting hollow mast section is not delicate and is not deformed.

[Yeadon]

I recently built another birdsmouth spar.

[Yeadon]

Both sticks are birdsmouth spars. Main is spruce, around 16 feet. Mizzen is fir, around 11'6. I used Titebond III for both.

[Canoeyawl]

Not to exceed a .005" glue line, the moisture content of the wood is also critical.

Technical data here
http://www.dap.com/docs/tech/00030205.pdf
and more here
http://www.hallmarkfraulo.co.uk/asse...nformation.pdf