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You can usually stack two hole saws on the same arbor, which you can check out at the hardware store before purchase, like the "oops" arbor Halekai shows.
The traditional way of doing the job was to put a plug in the hole and then saw it off flush, find the center, and drill the new larger hole.
You might also look in the electrical section at Home Depot and you'll see a tapered bit that electricians use to widen holes in electrical junction boxes. It's designed to go all the way from about 3/8" up to 2" I believe. They're a bit pricey but will work on steel and anything lesser. They look like a child's small spinning top only with cutting flutes on them. Fit right in your cordless drill. The only negative is the thickness of the material you may be going through. If it's under a half inch you'd probably be fine. You'd ream the existing hole from without and then from within to get to the size you need. any thicker than half inch though and I think you'd end up with too wide a hole at the surface to get your inside diameter correct, although you'd have a nice beveled cut. (g)
Another method is to afix a thin sheet of masonite or such as thin luan to either side of the hull so that it is in firm contact at the point of the thru-hull and is also secured well enough that it is not going to shirt about under drilling. Then find the center of your existing hole from the opposite side that the thin material is on and drill a small pilot hole. Then go to the other side and use that pilot hole to center your hole saw for drilling. You'll note that it is essential to keep the thin masonite, luan, or plastic from shifting until your hole saw has started cutting into fiberglas.
In any of these methods, the prep in making sure that you're accurately cutting and not just hacking about is the key. If the stacked hole saw method's inner saw fits too loosely inside your existing hole, for instance, you're going to have a heck of a time of it, hack it up, and find out really accurately how much torque your drill can really exert upon your wrist. In any of the methods, get everything lined up and fitted so you can make a nice cut at a controlled speed.
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“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.
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