Swapping masts and re-rigging

[mariner2k]

I am going to be replacing a rotted main mast on my 45' ketch. The current mast is a box mast with a single spreader rig. The replacement is the same height dimension. (45') The cap/pully and gooseneck measurements are perfect. However the newer mast is a round mast and a bit less beefy. (about 8-9" diameter at it's thickest point) It was originally rigged with two spreaders.
The question is: Do I need to keep the double spreader arrangement or can I rig it identical to the original box mast?
Once again the major difference between the two is one being a box/oval (rotted mast) and the other being round. Also between the two boats, the round mast had a 20' boom and a much larger mainsail, with about the same headsail area. The existing box mast has a 16' boom leaving a much smaller mainsail area. In other words the newer mast will be carrying much less sail area than it used to carry. Is there a trade off?
Thanks, Kevin

In addition, does the dimension of the mast matter as much as the geometry of the rigging?

[Brion Toss]

Hello,
Unfortunately, none of the variables you mention, by themselves, lead to a meaningful answer. What are the loads? Do the moments of the old mast suit those loads? Are the moments of the new mast at least as large? The fact that the new mast was double-spreader might indicate that it is lighter, relative to the load; that's one big reason to go with double spreaders. But we just don't know, so any selection would be arbitrary at this point. I think you need a consultation before deciding.
Fair leads,
Brion Toss

[mariner2k]

Fair enough, makes sense. I'll work on that as best as I can and consult with you, with hopefully the pertinent info. I'll probably have some questions in the mean time. Most of my info so far would come from your book. Do you have any other publications to help figure loads, or can I find most of what I need from the RA book?
On a very basic note: Is it possible to use a narrower mast with the correct rigging?
The rotted mast is a 9.5" x 6.5" box, (tapering toward the top) and the newer mast is a 7.25 diameter hollow spruce mast, tapering down to about 5" near the top. The shroud lengths would need adjustment as a result of double spreaders. The spreaders should be able to go in their original positions. The stays will be identical. I'd probably go back to running backstays which the old boat/mast carried. If nothing more than for peace of mind. The mast heights are identical. Probably go with soft eyes over hounds.
This is all I have for now. I just wish to know if this endeavor is worth persuing? I hate to waste a good mast.
Thanks, Kevin

[Brion Toss]

Hi again,
I think you can get at least in the neighborhood with my book. Supplement with Skene's or Larsson & Eliasson. By definition, a double-spreader mast can be lighter than one with single spreaders. Run those numbers. Note that your round mast, if it is big enough fore-and-aft, will be bigger than needed laterally.
Fair leads,
Brion Toss

[mariner2k]

Will do. I have to remove the cobwebs from my geometry/algebra first.

The existing mast is rigged with a checkstay as opposed to a running backstay, going aft at a very shallow angle, propably not more than 10 degrees, about 2/3 up the mast. The old boat/ mast was rigged with a running backstay opposing a releasable inner forestay.
Given the shallow angle of the checkstay, how functional is it?

[Brion Toss]

Hello again,
Plan on replacing the wires you are talking about. We call them aft-led intermediates, and they are right up there with twin backstays in my least-favorite-wire department. Turn them into runners.
Fair leads,
Brion Toss