“Magical” Catamaran Beams

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.”

"why can't you do a beam like this?"

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.

they think the 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.

what the beam really looks like

That is compared to a standard beam. Like this…

the standard beam

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.

4 thoughts on ““Magical” Catamaran Beams”

  1. So why not a triangle with a blunt edge. It would hide under the seat in the back. I didn’t do the math here as it maybe needing to be 24″ think for the numbers to work. Forget that my mast section isn’t 4″ thick.

    For the home builder think pulltruded tubing aka 1/2 over 3/8″ work real nice in 2 corners. The long side of the triangle shouldn’t need one. Last time I priced this stuff it was about $10 a foot. But the it made a wing shaped mast for my F27 that survived hitting the mast roller from near vertical. Well if you call 2 years of hard use after without any need for a repair. The basic mast is a trangle inside where the load of the bolt rope case and the shear web in the middle do the high loads the 4 layers (more layers on the bottom as most skippers like to punch holes in for thing like halyards) of carbon. So a triangle on a 10 cord mast about 3.5 inches think is pretty strong.

    In Southern California the carbon trees are a bit easier to get than clear grain wood.

    1. there is a reason an I beam has a big flange top and bottom. strength comes from having a lot of beef as far as possible from the centroid. and a box beam is like an I beam, but with the web split and put on both sides. so if you cut away a lot of the top flange, you lose a lot of strength. you can calculate how much section property any shape gives you. it won’t be as much as a box beam.

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