I have a confession to make, I don’t like fiberglass. Well, to be more precise, I don’t like epoxy, I have nothing against the fiberglass cloth itself. I don’t like buying epoxy, it is expensive stuff and I’m ‘cheap’. I don’t like measuring epoxy. I don’t like mixing epoxy. I don’t like the smell of epoxy. I definitely don’t like cleaning up epoxy spills and drips, and I don’t like all of the ‘disposables’ (the cups, the stirrers, the brushes…) required for epoxy work.
Given my dislike of epoxy, I was intrigued when I found "Ken Simpson's “Tape & Glue Process" on the Duckworks website. His method uses common waterproof wood glue in place of epoxy. Waterproof wood glue is much cheaper, roughly one quarter the cost of epoxy when bought by the gallon. It doesn’t need to be measured or mixed, has a very low odor, and cleans up with water (brushes can be washed and re-used). The “Tape & Glue” process minimizes or eliminates all of my dislikes of using epoxy! I was intrigued indeed, but how much strength does it add over a plain glue joint with no reinforcement? Is it worth the effort? I decided to perform a test.
To start with, I ran out to my garage and hastily created a fairly typical chine joint. As with most things I do hastily, I didn’t do the best job. It consisted of a pine chine log, approximately 1-1/8-inches by 3/4-inch and two scraps of plywood. One scrap was from a “handy panel” which was 1/4-inch actual thickness consisting of five relatively even thickness plies. The other scrap was SurePly underlayment which is also five plies, but the outside plies are very thin compared to the other three plies. The SurePly measures 5.25mm actual thickness. After gluing with Titebond III wood glue, I rounded/beveled the outside corner by approximately 1/4-inch. I then covered half of the length of the joint with bias cut fiberglass cloth set in Titebond III wood glue per Ken Simpson’s method.
After allowing the joint to dry for 24 hours, I cut it into four 1-inch wide samples, discarding the ends. I cut two of the samples from the un-reinforced area; two from the “Tape & Glue” reinforced area. I cut the samples using a power miter saw with a length stop to ensure they would all be very close to the same width. The ‘legs’ of each sample were also cut to a uniform length, approximately 3-inches.
I took the samples to work and used a 500-pound capacity force tester to apply a ‘closing’ force to the joints. This is the type of failure which the “Tape & Glue” reinforcement should help to prevent. I recorded the maximum force required to break the joint. I did the testing during my lunch and with my boss’ approval of course.
I found that the un-reinforced joints failed at 25.2-pounds force (lbf) and 32.0-lbf for an average of 28.6-lbf. In both samples the layers of the SurePly separated starting at the outside corner. Separation progressed to near the corner of the chine log, at which point the remaining layers fractured.
|Failed Un-reinforced Sample
The joints assembled with the “Tape & Glue” reinforcement failed at 41.4-lbf and 45.2-lbf for an average of 43.3-lbf. In both samples the “Handy Panel” fractured at or near the corner of the chine log.
This preliminary testing taught me a few things. First, the corner on the chine log creates a spot in the plywood where stress is concentrated. Evidence of this is provided by the fact that all samples fractured at this point. On the reinforced samples the fiberglass reinforcement ended before or very close to the corner of the chine log on the “Handy Panel” leg. Extending the external fiberglass reinforcement farther along the plywood might prove to distribute the stress over a larger area. Also learned was that separation of the layers of the plywood might be thwarted by use of the “Tape & Glue” method.
After thinking on what I had learned, I decided more testing was in order. I created a second chine joint much like the first. This time, I used a 3/4-inch square pine chine log so that each piece of plywood would be supported by it evenly. I also used only SurePly plywood. After gluing, I covered approximately 1/3 of the joint with fiberglass which was cut such that it would just extend to the corners of the chine log, approximately 3/4-inch down each leg. I also covered another 1/3 of the joint with fiberglass which was cut such that it would extend approximately 2-inches down each leg, well past the corner of the chine log. After reinforcement had been applied, I realized I had forgotten to round/bevel the corner on this second chine joint. I cut test samples as I had done before, except this time I tried to cut three samples from each section.
|Prepared Second Batch Samples
Testing these samples revealed that the un-reinforced samples failed at 24.6-lbf, 25.0-lbf and 27.4-lbf for an average of 25.7-lbf. This time, the SurePly layers did not separate. Perhaps rounding the corner promoted that previously? All three samples failed in the same area and in the same way. The plywood fractured at or near the corner of the chine log, supporting my previous testing and conclusions.
|Failed Un-reinforced Samples
The samples with the 3/4-inch reinforcement fared slightly better than the un-reinforced samples. You may remember that earlier I wrote that I tried to cut three samples from each section. The 3/4-inch reinforced section was the last section to be cut into samples. Due to the small amount (1/8-inch) which needed to be trimmed off of the final sample, I had some difficulty. Not wanting my fingers less than an inch from the saw blade, I tried to hold it with a scrap of wood. That resulted in my miter saw ‘eating’ the final sample. Better the sample than my fingers though.
The 3/4-inch reinforced samples failed at 31.4-lbf and 38.4-lbf for an average of 34.9-lbf. These samples failed in much the same manner as the un-reinforced samples. The plywood failed at or near the corner of chine log. This was to be expected since the reinforcement didn’t extend past that point.
|Failed 3/4-inch Reinforced Samples
Now, the main event, the 2-inch reinforced samples, these proved to be the strongest of all. These samples failed at 43.8-lbf, 44.0-lbf and a whopping 49.0-lbf (pictured just prior to failure below) for an average of 45.6-lbf. These three samples also all failed in the same area and manner. The fiberglass eventually tore and the plywood fractured at or near the corner of the chine log. Though it wasn’t measured during testing, I did notice that the 2-inch reinforced samples flexed much further prior to breaking than did the un-reinforced and 3/4-inch reinforced samples.
|Testing 2-inch Reinforced Sample
|Failed 2-inch Reinforced Samples
To sum it all up, “Tape & Glue” reinforcement of a chine log joint appears to increase the resistance to ‘closing’ forces. With the 2-inch reinforcement, the load required to fracture the joint is nearly double what is required to fracture an un-reinforced joint. The 2-inch reinforced samples had an average breaking force of 45.6-lbf versus 25.7-lbf for the un-reinforced samples. As the differences in the 3/4-inch reinforced samples and the 2-inch reinforced samples show, reinforcement should extend past the corner of the chine log. This test didn’t make clear how far reinforcement should extend, but I suspect after a minimum distance, increases in strength would be minimal. As I suspected after the initial testing, the chine log corner creates a stress concentration point. If this stress could be somehow otherwise distributed, I do not doubt even more force would be required to fail the joint. After performing this testing, I think the “Tape & Glue” method of fiberglass reinforcement is preferable over un-reinforced joints. It may or may not be stronger than fiberglass and epoxy joints, but as noted in the introduction, it does eliminate most of my perceived drawbacks of epoxy use. Given that, I plan on employing the “Tape & Glue” method on my current project, a Puddle Duck Racer made of SurePly with internal chine log construction. It should provide additional strength over un-reinforced joints. It should also seal the edges of the plywood against delamination and water infiltration as well as provide some measure of abrasion resistance in the chine area.
Happy building and happy boating,