Just a thought that hit me, but what about using six or eight tapered strips of sheet aluminum and welding them into a hex or octagonal tube? A lot of welding, but little or no forming. I think they make tapered lamp poles this way.
Thank you for the extensive replies, guys. I like where this thread is going, there are some really great ideas here.
Originally Posted by petersen
If you have the expertise to weld a "longseam" or two as you suggest in your original post,..why not four " longseams"??
You could consider simply cutting tapered strips of sheet or light plate, and bending (brakeing) a 45* crease down the center of each strip and assembling four strips to create an eight sided
tube with four longseams.
Make the strips 4' or 5' or 2 metres long; or whatever is practical to brake. And start with two long strips and two short strips so you can join a next section with transverse welds staggered to avoid a full circumference heat affected zone.
You could use heavier material at the bottom. and lighter as you build toward the top, and taper continuously from bottom to top.
Much less complicated metal forming task, But the welding supplies would probably exceed your unrealistic budget.
Very interesting idea. This sounds a lot like making a wooden mast from wood strips (birdsmouth method). I little idea of how much welding would cost at this scale, I never welded so much in one go. I will have to run some maths.
As I understand, this would end up in a 8-sided mast that I would have to round using the water pressure method?
The shape of the mast is rather important. You'll note that many of the OP's intentions centered around making a mast that would end up teardrop shaped to some extent. Yes, a round mast can work, but it is the first point of contact to the air which is generating your power. Recall that sails behave more like a wing than a parachute on most points of sail. So having a fractal shape would do you an aerodynamic disservice by "dirtying" the air over your main sail from flow separations and other discontinuities. OP could resolve this to some extent by using a sail which slips over the mast and under tension has the proper shape, but with an octagonal mast it would put stress concentrations into the sail sleeve, and still not be an ideal aero profile.
I worked in the industry of making round, tapered aluminum tubes for lighting, flag poles, etc. The size of tube you desire is considerably smaller than this industry makes (the smallest diameter I've seen is 60mm and in 18' that would start with a tube between 100 and 130mm diameter that is cold-spun down to 60 at the small end). Your wall thickness of 3mm (.125" actually) is common. These tapered tubes are made from seamless 6061 or 6063 alloy, purchased at T4 temper that is increased to T6 strength after fabrication is complete. You are not going to get this industry to make you the tube you say you need, however I suggest you explore the FLAGPOLE industry for something similar. On the other hand, why not just BUY a tapered aluminum boat mast? Alternately, there are suppliers in the U.S. of straight aluminum tubing and pipe in the sizes you need that can be telescoped together and secured, perhaps by plug welding. I think the ideas of cutting, bending, and welding from aluminum sheet will entail a lot of labor and you will end up frustrated and with a pile of worthless scrap. (By the way, if you do any welding keep in mind there is such a thing as the HEAT AFFECTED ZONE where material strength will be lost.)
I believe you are planning to use this mast SELF SUPPORTING, with no guy wires (I don't know the nautical term) to support it. In that case your tube diameters are too small to support the loads. Keep in mind also that aluminum is 3x more flexible than steel. If you are determined to use aluminum tubing, go to EBAY and look for ALUMINUM FLAG POLES. They are very reasonably priced, and made in telescoping sections (cheaper to ship). There's your answer.
I'm not sure exactly how I came across this thread, but if the OP is still lurking around, despite asking for something roughly unattainable, I think I have this one figured out:
Don't taper the pole. The goal here is to match the strength of the section to the stress applied, and keep the center of gravity as low as possible. You can do that by tapering it, like the big boys do, but not cheaply, as is obvious to everyone here.
So instead, work on tapering the thickness. If the pole was 70mm the whole way up, but the wall reduced from 3mm at the base to 1mm or whatever at the tip, you end up in the same position. Better, actually, as the bending strength of a big thin hoop is better than a small fat hoop.
So chemically mill it. Stand the pole up on end, put a rubber stopper with a small hole in the middle on the base. Run some tubing from that hole to a catch drum.
Mix up some pretty dilute muriatic acid and fill the tube up from the top. It'll slowly drain out the hole at the bottom. So the top will be in contact with the acid for maybe ten seconds, and the bottom for maybe two minutes, or whatever. Everywhere in between will be a smooth transition from one time to the other (assuming you can pour fairly fast)
You'd want to size it so you etch off a couple or ten thousandths per fill, and just keep collecting and pouring the acid (you'd have to make up for what's consumed) until the top side is as thin as you want.
You'd probably want to do the process development on smaller test poles to be cheaper, but this will 100% work, is reasonably safe and is very garage friendly. All you'd really need is a tall ladder, or a roof, or a pit.