Hi Guys,

You guys have got a great resource here. I haven't started on a boat yet, but have been reading this bulletin board for quite some time trying to learn all I can before start. The plan is to do it this spring and I'm currently leaning towards the Pygmy Opsrey HP or Coho. You'll probably be hearing from me more as I get a little further into the project. I'm also planning on making paddles and have a couple of designs on the drawing board (the engineer in me can't resist the design challenge) and have just worked through the math on the weight/strength tradeoff with hollowing the shaft.

Put simply, the center of a circular shaft adds very little to strength. Here's a couple of data points: Removing half of the diameter is from the center of a shaft (e.g, 1.5" OD and .75" ID) reduces the weight by 25% with a 0.39% reduction in strength. Removing 3/4 of the diameter (e.g., 1.5"OD and 1.125"ID) reduces the weight by 56% with a 10% reduction in strength. Is it worth it? That paddle moves up and down a lot, and a little of weight savings would quickly add up. I think my first paddle will have a hollow shaft, but I'll have to work the numbers to decide just how hollow. I'm thinking about doing some destructive testing on some samples first just to verify my design too.

-John D.

If anyone's interested, here's the logic behind my numbers. The weight of a circular shaft is proportional to its cross sectional area, A=Pi*r^2. The strength of a circular shaft is inversly proportional to its Moment of Inertia, I=pi*d^4/64. I'm assuming the paddle shaft to a simple supported beam with flexural loading. (and, while I realize that the actual loading of a paddle is a little more complex than this, I don't think this is a bad approximation). To arrive at the percentages referred to above, I calculated "A" and "I", reduced the diameter, calculated again, and compared the results.

> Hi Guys,

> While entertaining my own ideas of making a hollow tapered shaft by
> assembling 8 tapered strips of selected wood with carefully angled edges.
> It occurred to me that the actual volume of wood that might be saved is
> not really that great and the resulting weight savings (considering that
> cedar or an other a light weight wood could be used) may not amount to
> much. Which is not really relevant because it would not stop me from
> attempting such a project anyway.

> Has anyone calculated the weight savings? - just thought I would ask ;-)

> Hank