I had the same issue. I just want a helicopter to sit 500 feet of the starboard beam, and track with the carrier for the entire mission. Matching speeds didn’t work, and matching arrival times didn’t work. The helicopter is always a slightly different speed vs the boat.
I did find a solution though, but it take a bit of trial and error.
1) Set up your carrier, and put a helicopter abeam at whatever distance and altitude you want.
2) Set each one with a waypoint down range, spaced the same distance apart as they were in the start. I used 500’. This should be “Waypoint 1” for each vehicle.
3) Change the waypoint setting so they both are arrival time, not speeds. Set the same arrival time for both helicopter and carrier. In theory, this should be all you need, but it’s not.
4) Load the mission, and go to BOAT VIEW (F9 I think). Move around the viewpoint so the helicopter is positioned in front of the carrier. Time warp to max, and watch and and see if the helicopter moves forward or backwards in comprising to the carrier. Likely the helicopter is moving faster. If so;
5) Go back to the editor and move waypoint 1 for the HELICOPTER closer to its starting position. For my 90 NM leg, I needed to move it back by about 6.5 miles. That’s around 7% of the track, so start with 7% of your total track to get you in the ballpark.
6) After you moved the waypoint, load the mission and repeat step 4. Is the helicopter still faster? If yes, keep moving the helicopter waypoint closer to the start. If the helicopter is slower, move the helicopter waypoint farther away.
7) Keep doing this until the helicopter has no movement in relation to the boat.
It took me about 6 or so adjustments until the helicopter was completely stationary. As a final test I loaded the mission and time accelerated, then came back a few minutes later. The helicopter was still dead abeam the ship. Yes this is annoying to do, but so is having the helicopter be in wrong place when you come back from an hour long sortie.