Hollow mast

[signs]

I'am making a wood mast for my 18' sailboat and and plan on making it rectangular, & hollow.
at the base it's 2.50" x 4" and tapers to the top. I plan on using sitka spruce 3/4" thick is this heavy enough thanks.

[George Roberts]

What do the plans call for for a solid mast?

What rigging is there?

[signs]

i bought the boat used and it didn't come with a mast. as far as rigging there are 2 shrouds on each side along with a fore & back stay.

[Nicholas Carey]

Do you have the sailplan for the boat?

You should probably get a copy of Skene's Elements of Yacht Design (available used...googling should do it for you, or any of the usual places).

And you should maybe get a copy of Brion Toss's The Complete Rigger's Apprentice, as well (available from http://www.briontoss.com/ and the usual places.)

That being said, Skene's rule for wall thickness of a hollow spruce spar is that it should be 20% of the spar diameter. Since your spar is rectangular, the rule changes a bit: the port and starboard walls should be 20% of the athwartships dimension; the forward and after walls should be 20% of the fore-and-aft dimension. I believe this 20% figure is used as well by both Nevins' and Herreshoff's scantling rules.

If you use douglas fir, this thickness can be reduced by 5 percent or so to 15% as douglas fir is somewhat stronger and stiffer than spruce, so the reduction in scantlings more-or-less makes up for the difference in weight between fir and spruce.

If your spar is deck-stepped instead of keel-stepped, you'll need to increase the spar's section (diameter). A keel-stepped spar of a given diameter is stiffer than a deck-stepped spar of the same diameter and deck-to-masthead distance, as counter-intuitive as it may seem.

Hope this helps.