The other day, I scrolled down the KBBS far enough to come across a string with an interesting end. A nicely written post from Lee had suggested that George Roberts defend some of his positions with substance (I share Lee’s opinion). George responded with a few links to his web site that were intended to do that.

I have not had much time to go over what George presented, but it may be helpful to others for me to make a few initial comments. Before I do that, I want to assure George that I intend to be constructive, not only for the benefit of the readers here, but also for his benefit as well as to help maintain the mostly good tone of discourse on this board.

In general, I think that the material shows some good thinking and analysis. The rational behind his approach is stated well enough, but it does require some basic understanding of engineering mechanics to follow, something that many of the contributors to this board are equipped to do. There are some areas where the presentation could be improved for clarity. I hope that most of these places are just proof reading type omissions.

There is one data set, comprised of two tables, that consist of calculated energy at failure for different lay ups. George states that “if one wants to argue about the suitability of lay ups, they need to refute these predictions”. First, in order to evaluate the predictions, one needs more specific information. A good place to start would be to include the units associated with the data – that is simply good engineering practice. Second, he needs to be specific about the CLPT calculations, including elastic properties, failure properties, failure criterion, specimen dimensions, and loading conditions. Without these specifics, no direct comparison between his calculations and experiments could be made. Second, his predictions are not the center of thought on the subject. For those who are open to argument in the best sense of the word, any other well reasoned, well substantiated approach to lay up design would be a valid position from which to argue about the suitability of lay ups

In the following sections, George explained his approach to determining his ideal wood to epoxy/glass ratio. I genuinely enjoyed this section because it showed the power of what good generalizations of mechanics equations can do for analysis. I did notice that his expression for moment of inertia (better described as area moment) seemed to have the wrong units, but perhaps he is putting the beam width someplace else (it should be a constant). One detail had me smiling a bit. The geometry that George used in this analysis was a plate, supported at the ends, with a single line load across the width. This sounds suspiciously like three-point bending to me, but in the past he has strongly objected to the use of three-point bending to evaluate kayak lay ups.

In the Summary, with respect to suitable values of stiffness, static, and dynamic strength George wrote; “The problem is that there is no agreement as to what values are necessary.” This is absolutely correct. However, George has strongly criticized others for not explicitly stating what lay ups are suitable for what conditions.

Although I have some issues with Georges work, and I need to look at what he has posted more carefully, I think that it is a nice contribution to our collective efforts to build good boats. Having said that, I would like to thank George for his contribution. I would also like to thank Lee and others for keeping up the pressure on George (and the rest of us) to back up our opinions and maintain decency in our attitudes. Most of all, we can thank Nick for putting up with the occasional sniping and throwing of hand grenades that occurs from some of his guests.

Sam