So stands the mighty oak

Posted by Bruce Niederer on 19 February 2010 | 13 Comments

The Druids waved their golden knives and danced around the Oak when they had sacrificed a man; but though the learned search and scan no single modern person can entirely see the joke. But though they cut the throats of men they cut not down the tree, and from the blood the saplings spring of oak-woods yet to be ... from ‘The Song of the Oak’ G.K. Chesterton

People have been building boats using White Oak for centuries, sacrificing blood, sweat and tears to engineer wonderful and enduring vessels of all shapes and sizes.

Oak was often used because of its desirable properties and behavior. It is dense, strong, and rot resistant. In the days before glues and adhesives, oak planking was used because it would swell considerably which resulted in tight and sound hulls, meaning little leaking and dry interiors. Of course, time marches inexorably forward and eventually builders began using adhesives to augment or, in some cases, replace mechanical fasteners.

We’ve been debating the issue of gluing oak ever since.

There are many who argue that adhesives have little use and/or no place in wooden boats. Sometimes that statement is qualified to apply only to “traditional” wooden boat construction. We have over 40 years of experience, data, and history to support our argument to the contrary and I will share some recent test data here. So get ready people—it’s GBI as Mythbusters.

These days White Oak is used more for keel timbers and frames, less so as planking. Regardless of the application, adhesion, while certainly important, is only part of the equation necessary to success. Still, it’s a good place to start.

When we say adhesion what we are referring to is tensile adhesion measured with a PATTI instrument according to ASTM D-4541. There are no peel or shear forces involved.

Adhesive	Surface prep	Avg. adhesion (psi)
105/206	none	1625
105/206	80-grit hand sanding	1291
105/206	60-grit orbital + 60-grit hand sanding	1797
105/206	50/50 blend PEG*/water - no sanding	1774
*polyethylene glycol

Adhesive	Surface prep	Avg. adhesion (psi)
G/flex 650	80-grit hand sanding	1935
G/flex 655	80-grit hand sanding	1780
G/flex 655	wiped twice with alcohol pad*	2212
*70% isopropyl alcohol in water

What we learn from this data is surface prep can make a difference in adhesion as well as choice of epoxy. While G/flex yields significantly better values, standard WEST SYSTEM® 105 Resin/206 Slow Hardener did pretty well without any surface prep at all, which does lend some support to our stated position that epoxy can bond White Oak. But as I said, this is only one part of the whole story.

ASTM D905 Short Block Shear Test

To get a more complete understanding of adhesion to White Oak we recently completed short block shear testing according to ASTM D-905. We had two goals: first, to test adhesion to White Oak under a shear load and second, to collect some more data on our new Six-10® Thickened Epoxy Adhesive.

All the specimens in this sample population were hand sanded with 80-grit followed by a thorough wipe with alcohol pads. The load is applied to the sample in the direction of the grain. The results are as follows:

Adhesive	Avg. shear strength (psi)
105/206	2866
G/flex 650	2968
Six-10	2834

White Oak test specimens ready to take their turn on the test machine

Every broken sample in the test population, regardless of which of our epoxy systems was used, resulted in 100% wood failure. But that doesn’t mean all things are equal.

What are the practical implications we can assign to the observed results? Given that all the samples received the same prep, all of them resulted in 100% wood failure and all the values are very close, it is fair to say the glue is stronger than the shear strength of the oak. The slightly higher values achieved with G/flex can be explained by noting the extent of the wood failure. Because of the tough flexibility of G/flex, more wood got involved in resisting the applied shear force and so the value was somewhat higher.

The results of testing we did on another dense wood from South America, Jotoba, support this conclusion. The White Oak we tested had a density of 10.461 g/in3 = 39.9 lbs/ft3 while the Jotoba had a density of 15.80 g/in3 = 60.2 lbs/ft3 which is 33.7% more dense than the oak. If density plays a role in adhesion we should be able to see that here. First let’s looks at the results:

Adhesive	Avg. shear strength (psi)
105/206	3,319
G/flex 650	4,031
Six-10	4,088

Looking at the photos of the broken samples we see very similar results. Every sample has 100% wood failure and the average shear strength values are consistently similar. Again, it’s fair to say that the glue is stronger than the shear strength of the Jotoba and that the higher values with the Jotoba reflect the increased density and thus the increased shear strength over the oak samples.What we learn from this test data is that, contrary to the opinions of the naysayers, with the proper surface prep White Oak is quite bondable. It’s instructive to understand what these numbers really mean and how they might apply to something real, like a boat. For tensile adhesion picture this: you could lift a block of aluminum weighing just under a ton by gluing a piece of White Oak to it.

An interesting point to make in that regard is the use of a wipe with an alcohol pad, a practice I am following more and more. Our current theory as to why this works is that the alcohol has some water in it and the wipe raises and opens the grain which allows for better penetration and therefore involves an increased amount of wood surface to better share the load. Regardless of the mechanism the data doesn’t lie: that surface prep works well.

Still not fully convinced? Good. Neither am I. As encouraging as this data is, I can’t help wondering if it accurately describes what we can expect in the real world. These samples were made in the lab under ideal conditions of moisture content, humidity, temperature, etc. How often are boats built that way? Good question. To try to answer it we are conducting another series of tests where the samples will be tortured in our environmental huts to really get that oak moving.

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