to answer your question on speed up of the mill at the end of the cut, well here goes----
depending on the size of the cut ie: (face cut) by this I mean cutting a 6 inch cut versus a maximum cut of 28" would of course produce different cut termination results.
As the saw approaches its final vertical cut the sawyer is fully aware if the water gate is fully open or only partially open--if fully open then there is a tremendous increase in speed and he needs to close the gate immediately to slow down rotational speed, whereas if the equipment is sort of coasting along with a minimal amount of water to affect the cutting action then at times no adjustment is necessary during the final cut.
My method of calculating horsepower is maybe not the most scientific in the world but I believe it gives some approximate results, or results that are in the ball park area.
I was fortunate to have been asked to power a belt drive 1860's shingle saw, by extracting power from the barrel wheel. Now I knew from my experience that to drive this circular bladesaw up to speed and do work with it you would need approx 800 rpm or it would wander in the cut, and the speed needed to be held constant.
I knew before hand that because the rpm's of the barrel wheel being a max of 125 meant that a series of pulleys of proper diameters would be needed to come up with the final 800 rpm's, meaning to me that to just get the shingle saw up to running speed would be a great effort on the part of the barrel wheel.
Now one thing that was sort of questionable in my mind was that the shingle saw also had a cast flywheel which would create power once up to speed, would it be enough to do some work, alittle work, or no work--that was the 64 dollar question.
Anyway we forged ahead with directions from my superiors at that time. I had many misgivings about safety especially of the cast flywheel, Iknew that excessive speeds was quite hazardous and a calmity could result if the operator could not keep the saw from running wild after a cut was finished. to this end a safety brake on the flywheel was installed.
To make a long story short, we were able to cut shingles by allowing the machine to pick up speed do a cut and repeat again.
after this trial period I was asked to cut a large quantity of pine shingles for one of our restoration projects, and to this end we removed the shingle saw and in another location powered it using a massey 35 hp modern engine. Now it made it work and we could cut fairly steadily.
As a final notation to this discussion this same shingle saw was used for many years powered by a 2 horse tread mill, which I operated daily, it actually could saw shingles steadier than the 6 horse power of the Mulley saw mill, but not quite as steady as the massey tractor.
I always wondered why the 2 horse tread mill seemed to have more power than the Barrel wheel.