If I remember mine correctly, I just laid out a strip of the copper tape about six inches long on a board, copper-side up, and taped down both ends with regular Scotch tape. Then I laid a steel rule along the centerline of the copper tape and used it as the guide for the Xacto knife. If you don't try to cut more than a six-inch length of the stuff at a time, you'll probably find that you can eyeball the centerline of it well enough.
Remember that the cut doesn't really need to run precisely down the center of the tape. Each row of plates is going to overlap the one below it by a couple of scale inches (say, 1/64" or even a little more). If one edge of a plate is a little uneven, let that be the edge that gets overlapped by the plates above it.
The Phantom is a near-ideal subject for a first effort at "copper sheathing." If I remember correctly, I did mine in one long evening - with another to add the shallow dents in the plates to represent the nail heads. (That's why a I have small, cheap stereo system in the workshop. That sort of job is great accompaniment to an audiobook.)
I think that copper tape that comes with the kit is the stuff that's used in making stained glass windows. Several companies that sell supplies to stained glass enthusiasts have sent me catalogs. All of them sell pressure-sensitive copper tape like that. Unfortunately the 1/4" width seems to be the narrowest that's available. But they do sell it in fairly large sheets - with the adhesive on one side. That might be worth investigating.
For the record, it's worth noting that copper, strictly speaking, may well be technically wrong for a ship of the Phantom's period. By that time (the 1860s), copper was being replaced by the much more durable "Muntz metal," which was an alloy of copper and zinc. It apparently was about the same color as brass; by the mid-nineteenth century Lloyd's Register was listing ships as being "yellow metalled," "coppered," or "brass bottomed." (My source here is Mr. Campbell's fine book, China Tea Clippers, which has an excellent chapter on hull sheathing.) My big ongoing project is a 1/96-scale model of the clipper Young America, which is known to have been "yellow metalled." I've located some .001 brass sheet, which probably would look about right for the purpose. But I have to confess I have reservations about it. A copper-bottomed model looks so, so nice - and I can't recall having bumped into one that was actually plated with brass.
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