1. Yes, "small" is just a relative term. That just means your author was used to sailing on bigger boats/ships and/or sailing alongside bigger yachts.
2. I've worked through archival material in the past, so I'm more relaxed about spelling in manuscript (in contrast to typescript or print). Oxford
English Dictionary lists the orthography "lea" in its list of variants of lee, but my copy does not have a dated citation of the usage of lea in that context. I think you already have the idea that the rail of a
boat is likely not a hand rail, it's just the gunnel/gun wale that is the upper edge of the side of the vessel's
hull.
3. 'trolling' has long been a synonym for sauntering, marking a speed that is not a fast
race speed, but a comfortable speed to let one look around (and in the case of people walking in urban areas, the right speed to show off, to be seen, to attract the attention of people whose gaze you might want - and hence its use on
internet forums!). On a
boat that has a top speed of 9 knots, 5 knots would be sauntering speed.
4. I took a quick look at my electronic chart of
Lake Ontario, seeing (for the first time for me; I've only sailed the coasts of CA and HI) Oswega, Sodus, etc. And sure, if a person had a sleek
racing yacht around 40 - 45 ft, were familiar with the Lake (and particularly on a moonlit night), then if they were on a vaguely easterly course with a roughly southerly
wind of 18-20 knots and so were on a beam reach (or even a broad reach) with full sail, I see no reason why they couldn't do 9 knots. And the boat would likely be heeled over, with its lee rail awash. I think it's much more likely that 9 knots is boatspeed than
wind speed. Any
English speaker who estimated wind speed at 9 knots (and not the round figure of 10 knots, about the wind speed that produces white horse or white caps) would prompt me to ask how they knew.
5. The lead of a lead line, used to sound the
depth, has a depression in its lower surface. That depression is "armed" with fat, lard, or grease before it is cast
overboard. And the arming captures grains of the bottom, so
inspection tells one whether one is over rock (nothing adheres); mud; or sand. The chart of
Lake Ontario shows me patches of rock, clay, mud, stone, shells, and sand. But (at least on my chart) those annotations for the bottom are in the shallower
water (what is called "soundings" because one can 'sound' with the lead line). Lake O, according to my cursory
inspection of my electronic chart, has patches at 244 metres
depth. That's beyond most lead lines. But I don't see your author saying he employed his lead line on that occasion, rather he was noting how, when becalmed, a
captain can keep his crew busy and gain additional position information by doing so.