> 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.

Sorry John, but the % method doesn't do much for those who don't calculate. You have to know (calculate) the solid shaft weight before you can apply the percentage. One may as well calculate the void volume and weight directly.

For those who don't calculate, the following gives a rough idea of what a hollow shaft might save in weight, a 3/4" diameter hole, 18" long (roughly the length of a Greenland loom?) with 25 lb density wood saves only 2 ounces while a one inch hole saves 3 ounces. The savings might double for those paddles with a more conventional shape. Those with walnut or redwood shafts will save even more.

Bram