The slope doesn't have to be very much because a boat is very heavy... Let's assume a 100mm/4" average slope (a guesstimate, based on what I've seen in locks -- could easily be bigger in a fast-filling/deep lock). For a 60' boat that's a slope of 1 in 180, so if the boat weighs 18t the "thrust" is 0.1tonnes or 100kg. That will accelerate the boat at g/180, about 1fps in 5 seconds, it'll hit the top gate at a couple of fps (BANG) in a bit over 10 seconds. There will perhaps be some drag from water flow, but which direction -- backwards (incoming water) or forwards (water bouncing off the bottom gates)? Yes you're right that the slope will be uneven, and I suspect that in many locks the water flows (and slope) are bigger than I've calculated above. I've actually watched this slope/tilting happening lots of times now, and you can use it to predictively keep the boat in place with the throttle rather than waiting for it to start moving (reactionary) -- watch for the bows to rise first (when the paddles open and the lock starts to fill from the top), then drop as the water "bulge" goes under the boat and put some reverse power on, then reduce this as the tilt decreases, then maybe a bit of forward to catch the "rebound". Sometimes you can keep the boat almost completely stationary near the bottom gates with no apparent effort, which is gratifying... 😊 P.S. Stourbridge duckweed in summer, uurgh... ☹️
