Had a lot of experience with this with a P3P this past summer, many launches from a relatively large (105') aluminum boat while moving at 7-8 knots. I had no issues. I did a lot of research before I ever tried it so it is good to do your homework first, like you're doing. My tips:
1. Do NOT calibrate compass on the boat. Do a good cal on land before you head out to the boat. First time I calibrated on the boat, while at the dock, it flipped out and tried to fly away on me right after launch and it was a fight to get it back. Next I calibrated on land, it flew normally, then I took it on the boat and did not fly it again until we were 150 miles from where it was calibrated... decided to chance that a bad local cal was worse then a good cal done far away, and believe my reasoning was sound because I never had issues with many flights despite the cal being done 150 miles away.
2. Very critical, in my view the most critical when launching from a boat... Launch in ATTI mode, never launch in GPS mode! My strategy was to launch in ATTI, gun it hard and get it up and away from the boat quickly, then switch it back into GPS mode for the flight. Of course it will try to maintain its position if you launch in GPS mode, but the position it wants to hold quickly moves away from the boat even if the boat is just drifting, which can cause the drone to suddenly accelerate off in an unintended direction as it attempts to keep that position, leading to a potential crash into the vessel superstructure before you can react. This happened to somebody I know, and there are a lot of reports and videos of this as well. So launch in ATTI, fly in GPS mode... landing back on the moving boat I also did in ATTI just to prevent any potential erratic behavior during the landing process.
3. Homepoint... Reset this now and again to your current position if your moving. Of course by default this is where the drone fired up its motors which could be some distance away from you. When I was doing these flights (mostly July) a lot of the new modes like Follow were not available so I would reset the homepoint once we (boat and drone) had moved about 1,500 feet from the last reset, just to be sure. Maybe not as critical now but keep in mind you should reset the homepoint now and again. I've heard others mention setting the Homepoint to a point on land somewhere (in my case land was 30 miles away at the closest) -- might not be a bad idea, just depending on your situation.
4. Floats... Add something to help it float in case it ends up in the water. In the case of the person I mentioned having an oopsie under #2, he got the drone back even though it was in salt water thanks to floats... rinsed in fresh water, battery was toast, but the drone dried out and has been totally fine (he says). I would fly with 4 keychain floats zip-tied to the drone legs. Here is a picture... The floats had no discernible effect on performance and could not be seen in the images/video it took
5. RF interference... I had the captain turn off the ship radar during my flights initially. BUT later he switched the radar on during one of my flights close to the ship and it had no effect, so we left it on after that. So it is not something I would worry about anymore.
6. Bring it back earlier then you might over land. I would start bringing it back at 50% battery, give myself plenty of time for the sometimes tricky landing.
7. Launching/Landing... Never tried hand launching. I was launching and landing directly on the back aluminum deck, no issues. On a small boat you could hand launch I suppose, or just launch on a box proped up near the boat gunwale... just get it out and away quickly, as discussed above. Landing by hand-grabbing is not something I did on the boat but have done on land; I could see this being fine on a small boat with limited deck space for landing.
8. That's about it... I was a bit of a nervous wreck my first few flights from a moving boat but it wasn't as bad as I thought it might be. Good luck.
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