[De_sv_Taz]

Originally Posted by benz

Thanks Brian--after posting that question I saw several pictures of pole spars that seem simply riddled with knots, and some even with longitudinal cracks (!). Trouble is, I'm an inveterate worrier, and I know I'll fixate obsessively on even a modest knot, as long as the night is dark and the wind strong.

longitudinal checks are -- so I am told -- not to worry about, it's the horizontal or spiralling ones that should scare us (I currently have solid DF masts, and a couple of years ago I broke one, so I'm a bit sensitive on the subject :-))

As for aluminum tubing, all over the interenet I can find 20-foot lengths of suitable dimensions ( I need 6' OD, with a 3/16" wall thickness), but I need a forty-foot piece. The flagpole companies I've asked only go to 35 feet (anything longer they join two pieces with a sleeve), and their poles are generally tapered, which I don't want.
If anyone knows of an aluminum tubing source, I'd sure be keen to know it.
By the way, d-sv-taz, if you skin a pole with fiberglass, it will have to be very thick in order for the wood beneath not to compress and then pull away where things press on it--I tried that once on some oars when I couldn't get leathers, and it was a terrible idea.
ben

thanks for that data point.

interesting that you don't want a tapered pole...

am told it is possible to join two sections with a sleeve and a technique called plug welding, where only little spots of welding are done, widely spaced, through holes in the outermost pipe or tubing (btw, pipe and tube use quite different sizing conventions -- you knew that, right? so 6 inch pipe is not the same as 6 inch tube. onlinemetals.com has good human-readable writeups on pipe vs tube and various grades of Al). this reduces the area that's weakend by the heat of welding, and with the additional wall thickness of the sleeving material (inner ferrule or outer sleeve) it should be plenty strong. this may address my concerns about spar construction using T6 and the strength lost at weld points...

also note that the center of a solid wooden mast doesn't do much for strength. there's a lot to be said -- in theory anyway, based on the math -- for hollowing out the centre or building up the mast by the birdsmouth technique... I found that the difference in strength between a solid 9 inch DF pole and the same pole hollowed out to a 2.5 inch wall (removing a 4 inch diam section from the centre) was not a whole lot, and it's a pretty significant weight reduction... but maybe the wooden boat folks here have countervailing wisdom to offer about keeping the growth pattern of the wood intact?