What do you mean by "pretty good size"? If it is 12" or less, you could do it with a hand rip saw. If you're talking about 24" wide cuts, you could use a two-man crosscut saw and cut vertically like an old fashioned pit saw. I agree with Fish Factory, though. Between the expense of the saw and the time it would take, it would be far more practical to find someone with a portable sawmill or a large chain saw with a ripping chain. In some cases, you could bring the log to the mill, have it cut into whatever you want, and take it home with you in one trip. I do that sort of thing all the time. Here are a couple of photos of a walnut crotch on my Norwoodband sawmill.
However you get it cut, do it soon.
Best to get somebody to cut it who operates adequate machinery.
Some days, DIY isn't safe enough.
From a wood carver's pespective, many woods that carve & cut easily when green/wet
really tighten up as they dry. PacNW Alder for example is a very popular wood for dishes and masks. It goes from easy to ebony when it dries.
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