Doug fir pole mast

[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?

[benz]

Hi Taz,

I'm familiar with plug welding, but would ideally like a solid piece. My mast must not taper, at least for most of its length, so the gaff jaws or saddle don't get all baggy at full hoist. If I have to, I can have a mast that ends at the hounds (where the lower shrouds come in), and then use a skinnier pole above that--but that's a complication I was hoping to avoid. I like simple.
How 'bout (for your project) doug fir soaked with CPES epoxy, (available from Jamestown Distributors), then varnished a lot before the mast is stepped? once the mast is up, if you keep a good several coats of Mother's carnauba wax (Mother's smells soo good!) on the varnish, the sun won't eat it up, and a coat of wax every so often is almost a joy to apply.
If I have to go wood, I'll ceratinly do that. By the way, Bird'smouthing is exhaustively discussed on the wooden boat forum run by Wooden boat magazine, with useful links to pictures. Google it and see--pretty cool stuff.
Best,
Ben

[cwa]

Ben,

You could use 6" aluminum pipe as a mast.
That is what I am using for my boat.
Doing a splice is rather easy.
The wall thickness is a little more than the 3/16" you were looking for as it is over 1/4".
You will have the same diameter for the length you want.
I'm doing 44 feet so there will be two splices for me to get that length.

De,

Nice to see your website about TAZ.
Evan had shown us your boat as an example of his work before starting on ours.
You could consider an aluminum pipe mast as well and just shim the butt end to go into the tabernacle with a tight fit.
Aluminum pipe is very inexpensive in Canada and that helps with the budget.
Hope to meet you this coming summer when we have Evan do a little more to our boat.
Are you still at Newcastle?

Carl

[benz]

Hi Carl,

How are you doing your splices? are you sleeving and plug welding? I'm under the impression that a gaffer needs a mast section that is initially stiffer than a marconi rig would need, since the lowers land so much higher up. If this system has proved sufficient for someone in the past, I'm open to it--but I'd sure like to hear from people who've used it before, or one of the master riggers who weigh in from time to time. Nothing like experience to take out the guesswork.
Ben

[cwa]

Ben,

The splices are sleeved and plug welded.
Easy to make the sleeve by reducing a piece of pipe to the required dia. and inserting inside the joining pieces.
As for getting stiffness, I believe that the .275" wall thickness would really help there.
Actually I'm having a fellow at Beecher Bay on Vancouver Island make my mast.
He has done a number of masts (I've seen two examples so far) using this technique but haven't asked about ones for a gaffer.

I just talked to Ken (the fellow making my mast) and he has made 2 masts for gaffers using this technique.

Carl