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Brian Gall
07-09-2001, 01:50 AM
I am just embarking ( for the first time ) to make a new oregon mast for my 60 year old 40 ft motor Yacht. I know the method of creating a round one from square stock but I want an eliptical mast tapering to the top. Can anyone help with any info on this. Also I shall create a laminated spreader in oregon. I am not quite sure on the dimension the spreader should be ie length/ width in relation to the height of the mast. The mast will be 3.2 mtrs. ( sorry were metric in NZ. ) Any help would be apprecated.

Mike Field
07-09-2001, 02:59 AM
Brian, it shouldn't be too difficult if you start by cutting the oregon with a rectangular, tapering cross-section, a whisker larger than the widest cross-section of the finished spar (to allow for final sanding.) Then taper both pairs of opposite sides with straight cuts to produce the appropriate taper on each axis. Finally, use a spar gauge to knock off all the arrises by the right amount, ready for sanding. (I seem to remember that Ian McColgin posted details of a spar-gauge here recently, but email me for the recipe if you need it.) You should find that the inside-out-belt-sander trick will do to finish the spar pretty well, ready for a final longitudinal sanding by hand.

Unless the mast is to be ridiculously narrow or to have a really dramatic rake, and assuming you only want to use it for signal halyards or a radio aerial, then the spreader width is really irrelevant -- the mast could tbe stayed quite adequately with shrouds direct to the chainplates. So just make the spreaders as long as looks right, which you would establish from a scale drawing. Make due allowance for the boat's beam, or if the mast is to be mounted on a cabintop, for the width of the cabin. To look right (to me,) the change in the shroud angle at the spreader should be at least 20 degrees or so -- anything less looks like the spreaders shouldn't be there at all. With the short mast that this is, this could mean fitting chainplates at the edges of the cabintop, or through the deck at half-beam, rather than at the gunwales.

[This message has been edited by Mike Field (edited 07-09-2001).]

Joel Herzel
07-09-2001, 09:21 PM
Brian -
My two cents is about grain orientation using Douglas Fir (oregon). Flat or plain sawn fir does very poorly when finished bright and exposed to sunlight and heat. Even painted fir will lift the grain rings and never hold a finish for long when cooked by the sun. Fir will make a good mast if the surfaces are all quarter sawn. The bird's eye method is the easiest way to accomplish this, in my opinion. You can also easily make the shaft oval, and taper the top portion, to boot. You are familiar with this method?

Joel

TomRobb
07-10-2001, 11:10 AM
Joel means bird's-mouth method. Do a search on birds mouth. The new issue of WB has a piece on oval and eliptical spars too. A 10.4 foot spar shouldn't be too hard to do single handed. Engineering it to deal with the stresses it will be living with may involve some figuring. You can't just make it the same diameter as the solid one. For the same strength it'd need to be of a larger diameter and probably have some artfully placed internal plugs in places where there are point loads.
A fun project, no doubt http://media5.hypernet.com/~dick/ubb/biggrin.gif

Brian Gall
07-10-2001, 04:16 PM
Originally posted by Joel Herzel:
Brian -
My two cents is about grain orientation using Douglas Fir (oregon). Flat or plain sawn fir does very poorly when finished bright and exposed to sunlight and heat. Even painted fir will lift the grain rings and never hold a finish for long when cooked by the sun. Fir will make a good mast if the surfaces are all quarter sawn. The bird's eye method is the easiest way to accomplish this, in my opinion. You can also easily make the shaft oval, and taper the top portion, to boot. You are familiar with this method?

Joel
Hello Joel, What does the birds eye method mean? and yes I am making it oval and tapered to the top. The fir is Quarter sawn and while we're on the subject of finish I want to finish it bright and am considering 2 pot. The rest of the boat gets the 10 coats of gloss treatment and I was thinking of going th 2 pot way for the mast. Has anyone got any thoughts on this?

Smacksman
07-10-2001, 06:36 PM
Does she have a steadying sail set from the mast? My smack Alberta's spreaders are ex 4" x 1.5" x 12' wide which is the same as her beam. But then she is a sailing boat with a 10" mast carrying 1200 sq.ft. of working canvas. Mike's idea is good - draw it out and see what looks right.

Joel Herzel
07-11-2001, 09:31 PM
Thanks Tom, and sorry Brian. Bird's mouth is the name of course, and I recommend WB #149 for a thorough explanation on how to do it. I hope this isn't an example of becoming a bird brain.
I think Tom's suggestions are all good and I know there were numerous schematics posted of bird's mouth cross sections which should turn up if you search under the name in this forum.
By the way, where are you in NZ? I lived in Nelson many years ago and worked for a boatbuilder named Jack Guard. I wonder if he is still with us......... Have you heard his name? At the time, he was one of the only builders left building carvel plank fish boats with Kauri.