Every couple of years I have someone ask me why my cat designs need to have aft beams. This occured again last month. They seem to have the tone that there must be a structural secret I just have not figured out yet. I did just get this picture. “Why can’t you do a beam like this? Ask them how they did it.”
People. Remember where I always say “with enough carbon and enough money, you can probably do anything you want to.”? There are limits. They probably think a sleek cat beam looks like this.
It actually looks more like this. The boat salesmen hide a beam like this so it will look thin or even gone.
That is compared to a standard beam. Like this…
So why don’t I catch up and do the flat beam? Remember how I always talk about efficient cats? Look at this.
The tools we designers use to compare beams are the moment of inertia “I” and section modulus “SM”. (assuming shear is satisfied) Suppose the regular beam is 12″ x 36″ by 0.1″ thick laminate. That regular beam has a SM of 99 in^3 (inches to third power) and a moment of inertia I of 2104 in^4.
In the horizontal mode beam, it only has a SM of 44 and the I is only 272 in^4. To get more strength and stiffness it must be made thicker and heavier. Even if the horizontal beam laminate is made 7 times thicker and 7 times heavier, it only has the same strength as the regular beam and still only a third of the regular beam’s stiffness. The flat beam boat vendors hope you don’t know or care enough to appreciate the difference. They can’t claim efficiency and still have horizontal beams.