I want to open the glue topic up for discussion. I have never used anything but epoxy on any of my boat building projects (West Systems, MAS, and RAKA). That came to a frustrating head this weekend. I was laminating 2 pieces of 3/8″ ply for the centerboard on my Bobcat. I mixed a batch of epoxy and wetted the whole surface un-thickened. Then I thickened it a bit and spread it all out. I probably could have used a bit more, but I did not want to mix another batch just to cover a small section, so I spread it a little thin and figured it would be OK. I put the 2 pieces together and put weights all over it.
I came down the next day to find that I really didn’t have enough epoxy and I could see the thin gaps in various places around the edge. I spent the next hour taping, filling with thickened epoxy, and clamping those gaps. Again, I had a bunch of left over epoxy so I scrambled to get my new stem pieces lined up and used the remaining epoxy on two of the 8 pieces I need to laminate for that.
The centerboard will be fine structurally. I am not concerned about that. The problem I have is that the cost of epoxy seems to negatively effect the quality of my work and ending up costing me additional time. This will be my second stem because the first one failed due to my concern for my daughter being exposed (or stuck) to the epoxy (you can read about it here) Every time I mix a batch, I see the dollar signs flowing out through the pumps or sitting in an unused pot and hardening into a plastic ball of wasted money. Meanwhile, I have a tube of PL Premium sitting on my work bench staring at me saying, “Dude. Over here. I’m really strong, I cost 7 bucks, and my unused portion stays in the tube!”
I heard a quote from Russell Brown from Port Townsend Watercraft that I like to abide by. You should pick up his book on epoxy usage, “Epoxy Basics: Working with Epoxy Cleanly and Efficiently“. It is outstanding. I may be paraphrasing a bit, but he said, “You have to think that somewhere in the life span of every boat, someone’s life may depend on it holding together. I build my boats to those standards.” (The quote was from an interview on The Hooked on Wooden Boats Podcast)
That is the standard that I hold for my boats to as well, so I need a glue that will at least compare to the strength of epoxy for that given component. For example, I know that PL Premium will not compare to epoxy for something such as a fillet, but for laminating sheets of plywood together, it may. I will still use epoxy for fillets and fiberglassing, but if I can use something less expensive and less pain-in-the-ass-ish, for some of the components, I could be saving time and saving money.
So, my questions to the group. Please leave comments below. Regarding the stem and the centerboard of a 12′ catboat. Obviously, these are 2 parts that will be under a considerable load with some torque and twist over the years. The centerboard is done, but I have to still laminate 6 more layers of the stem. Do you think PL Premium is strong enough for components like this? Is there another single part glue that you would recommend? Could Tight Bond III be used? Which do you think is stronger? I have posed this question to the good folks at the Duckworks Magazine Forum as well and will update this post with their information.
Thank you for reading!
**UPDATE: John Welsford chimed in and answered my question in another forum. John is a boat builder and designer that has been building boats since he was 9 years old. He has built over 25 boats of his own design. http://www.jwboatdesigns.co.nz/
One of the things that I have done in order to earn a crust, is quality control management in factories that produce laminated wooden beams for structural use. This involves the use of some pretty interesting testing equipment including tensiometers, shear testers, torsion testers and scanning equipment to evaluate joints that have been pulled apart.
I dont have this equipment at home, but what I've done is make up my own testing gear, relatively simple stuff using load cells and a hydraulic press, running a bunch of test joints using air conditioning, uniform surface preparation and conditioning, uniform clamping pressures and curing time to ensure repeatability and consistency then doing comparative testing with both the company lab equipment and my own to benchmark mine.
Once that was done, I then ran tests on every type of adhesive I could find including some esoteric and specialised ones not generally available.
Using West system epoxy as the control, calling it 100%. I found that Resorcinol, specifically that produced as a commercial grade wood laminating adhesive by Orica was the best of the water resistant non gap filling adhesives at 112% ( mean of all tests with the best and worst dropped) and Urea Formaldehyde ( Aerolite 308) second best at 110%.
The polyurethanes did not fare as well. Gorilla Glue, which used to be advertised as "The strongest glue in the world" came in at 55% of the strength of West Epoxy, PL Premium at around 65%, Fullers and Selleys equivalents within a couple of percent of that figure, and no polyurethane glue higher than 70% of the strength of West system.
I noted that there was some variation among the epoxies, those which were 1 to 1 or 2 to 1 mix tested noticeably weaker than the 4 to 1 and 5 to 1 mix types, but the difference was only about 5% so for our purposes as home boatbuilders I consider that to be within tolerance for strength.
So what do I do with this information?
When designing for a skilled builder I can engineer joints much more closely, or if designing for such as the UN FAO where a production line will be using polyurethane applied by unskilled labour I design the boat with much larger faying (glue area5) surfaces in its joints.
So heres the warning, if the boat has been designed to use epoxy, use it. There may not be enough area in the joint to achieve sufficient strength to cope with the loading if you use an adhesive with lesser strength.