Doug fir pole mast

[benz]

While still very keen on using aluminum for my gaff rigger's mast, I'm exploring the possibilities of using wood in case I can't find a suitable piece of tubing (strange as it seems, just the right piece of aluminum is proving difficult to find). If I have to go with just a doug fir pole, how important is it to have a clear piece? Are some knots acceptable? If so, how many and how big?
Thanks in advance,
Ben

[Brian Duff]

while i am no spar builder, I can say that the couple gaffers i sail on here have big old knots in their masts. As big as my fist, but they are tight, for now anyway. Chappell says something about knots as big as a pencil are ok, clear otherwise....?

Whats the deal with your aluminum 'tube' , there is a huge assortment to choose from. WHat size do you want ?

[De_sv_Taz]

I am facing similar decisions with masts for a traditional unstayed junk... reluctantly considering wood due to difficulty finding industrial Al pipe in the requisite wall thickness and diameter.

The problem I have -- and it may be the same for the gaff rig owner -- is that industrial Al pipe in larger sizes comes only in even inch OD, i.e. 6 8 10 12 inch OD.

What I need is 9 inch OD or better yet 9.25. 9.5 would be a darn tight fit.

Then there is the problem of taper. To get some taper into the mast you want to sleeve sections of pipe with gradually diminishing OD. However, not many sizes of industrial pipe seem to sleeve tightly enough to make a really good mechanical fit. And if you start welding on T6 you lose the hardening and almost all the strength at the weld locations. Re-annealing is a major project for an entire mast.

Anyway, it's a tough problem if you need very stout, round x-section poles such as for a gaffer or junk. Steel sheet can be custom-made into faceted masts on a metal break, but the N-sided poles are (a) very heavy and (b) difficult to taper and (c) don't maximise material at radius.

My latest desperate notion is to skin a doug fir pole in thin aluminium, i.e. completely encapsulate the pole except for the butt end (I have a steel boat and don't want the aluminium in contact with the steel step). This would protect the DF from weather and eliminate the yearly rituals of oiling, and also provide abrasion resistance where the parrels and battens rub against the mast. I have no idea if this is realistic -- can Al sheet be bent closely enough to the mast to be a tight skin? can it be welded without too much charring of the wood beneath? can the seams be ground down smooth enough on thin plate w/o gouging through the metal? will it be too heavy? ... but I am thinking about it.

[De_sv_Taz]

Originally Posted by De_sv_Taz

My latest desperate notion is to skin a doug fir pole in thin aluminium...

I forgot to mention that part of the attraction of this desperate notion is an attempt to emulate a flitch beam, i.e. use the compression strength of the wood and the tension strength of the metal, for greater strength than wood alone but lighter weight than all metal.

I seem to remember that masts on old square riggers were often bound with iron compression bands (to prevent splitting?) but I can't recollect any instance of a metal skinned mast in my (not very extensive I admit) reading. This may mean that it's not a great idea, or just that they didn't have good light alloys back in the day.

Glassing a DF pole is also an option but given the amount of whip/flex in an unstayed rig I fear the glass layer would soon craze or fracture.

[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.
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

[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?