Options for removing old antifouling bottom paint layers are: sanding, stripping with chemical stripper, or blasting (usually with something relatively soft like walnut shells, to avoid damaging the fiberglass gelcoat beneath the antifouling). Since blasting was not allowed in the yard and I wasn't a sucker for punishment of sanding, I decided to try the paint stripper option. Let me emphasis just how horrible sanding a boat bottom is: the paint is a biocide so you have to wear protective clothing and masks, in 90+ hot muggy weather, as you exhaust yourself waving a heavy grinder upside-down, pressing it up against the bottom of the boat, as sweat mixed toxic dust rolls down into your eyes and you can't breathe easily through the mask. This is one of the worst boat-repair chores there is.
I bought pail of Peel Away paint stripper from the local hardware store (about $50/pail), and tried a test patch on my hull. This material is basically drain opener chemicals but in cream form, which you smooth over the hull with a 1'4" notch trowel. Cover it with the paper/plastic sheets provided, tape the sides of the paper down to make sure the stuff stays moist for as long as possible, and wait overnight.
The chemical stripper worked amazingly well when washed-off with a power washer. It came off easily, leaving only small stubborn spots of red bottom paint that sanded-off easily.
The area around my boat was an environmental atrocity, flooded with several inches of orange-colored slurry. (The main ingredient of bottom paint is copper, which is a biocide.) And once it all dried, the antifouling turned into dust and settled on everything including my car and my coffee cup. Yet this is normal in the yard, and worse has been happening here for several decades since the Navy owned the place. I suspect this is where I will catch cancer!
WARNING: Peel Away is highly corrosive so I suggest taking all the necessary precautions (especially eye protection and gloves.) A little bit of it was accidentally smeared on the inside of my wrist, under the glove, and now have a scab there. I'd hate to think what could happen if even the dust from the dried-up material got into anyone's eyes - so I suggest not only eye protection but also keeping a water hose nearby and handy as an emergency eyewash station.
The stuff I used was the regular hardware store version of Peel Away that comes with a plastic spreader, some pH testing strips, and the plastic/paper sheets used to cover the area being treated, and a packet of neutralizing powder (which I suspect is a mild citric acid. Vinegar also works.) Peel Away makes a "safe" version for use on boats that won't damage underlying gelcoat, which I also tried. (Since my boat had an epoxy barrier coat under the antifouling, I wasn't worried about gelcoat damage using the regular stuff.) The "safe" marine version is slightly runnier, so you need a better trowling technique especially on the upside-down surfaces of the keel, to avoid plopping the stuff all on the ground. They were both equally effective.
I had to buy 6 pails of Peel Away to complete the project but in the end I got a nice clean hull with no sanding. Of course, then I had to sand it all with 80 grit in preparation for the new coat of barrier paint. While sanding, I occasionally accidentally went through the old barrier coat and got to see the gelcoat underneath, which while crazed was still in decent condition. There's no sign of pox or blisters; boats as old as Whimsy generally didn't have problems with blisters. I figure, if there was going to be osmotic blistering, she would have shown it by now. However a couple of extra coats of epoxy barrier coating won't hurt either
Also, while the antifouling is gone, I want to check out the rudder shoe and investigate a worrying crack in the fiberglass around there.