Building a new polespear

I'm working on a new polespear, and using some different materials along the way. We'll see how it goes.

First, I ran some calculations, and decided to use a thin wall tube (5/8" OD x 0.049" wall) of normalized 4130 steel. That is a chrome-molybdenum steel, which is about twice as strong as plain vanilla carbon steel, but still relatively inexpensive. It's the stuff often used in race car roll cages and some home built aircraft.

As it turns out, this 5/8" tube is a fair bit stiffer than a piece of solid 5/8" aluminum, because steel is about three times as stiff as aluminum (other things being equal). At the same time, this hollow tube weighs about 1.5 lbs, vs. about 1.8 for a solid aluminum rod.

The only problem, of course, is rust. I turned a piece of G10 epoxy-glass rod that inserts about 1-1/2" into each end. I plan to put a carbon fiber sleeve over the whole thing, hopefully providing a tough outer skin to prevent rust. It should bond very well to the G10 at each end, and prevent any water from leaking in. At least, that's the plan . . .

If I did my math right, It should be stiffer than any aluminum tube, but the ultimate strength will be less than if a high strength aluminum like 7075 were used. But I couldn't find any 7075 tube to keep the weight down.

These are just calculated numbers for now, but comparing the 5/8" OD x 0.049" wall steel tube, to 5/8" OD solid 6061 aluminum rod, and assuming a 5' length, you should get:

Weight: 1.51 lbs (steel), 1.85 lbs (aluminum)
Predicted deflection, supported at the ends with a 20lb weight hung in the middle: 0.81" (steel), 1.15" (aluminum)

The 6061 aluminum would have a very slight advantage in the total load that it could support before taking a permanent bend. 7075 Aluminum would be considerably stronger.

I finally got around to finishing this polespear, and I'm pretty happy with how it turned out. It's right at 11/16" OD, with a steel / CF body about 56" long, and 12" of 5/16" shaft, tipped with a bowfishing tip. Final weight is 1 lb 15 oz.

It did turn out to be pretty stiff. Supported at both ends, it deflects only 1/2" with 20 lbs of weight hung in the middle.

All I need now is a band, and a warmer wetsuit, and I can try it out.