@Clare I saved right when the rain started, tried and it didn’t work, then I quit, reloaded and repeated the same steps and then it worked. So all conditions were the same, including the storm. The only difference was my position on the raft. (and the fact that I reloaded, weird things may happen when you save, quit and come back as opposed to saving and moving on without quitting)
As for the raft… 7 wide, 4 long, mixed materials. Rudder is at central “column”, between 1st and 2nd row (so you stand on 1st row while operating it), animals were in 3rd row (at left and right of the central column, both tire base without a floor), motor is also at center, attached to the back of the 3rd row (4th row has a gap in center). So yes, I was closer to the animals when using the motor (same row).
Animals were killed (but not skinned) in previous visits to the island, they were left there for weeks waiting for the right opportunity. My Achievement Screenshot is not very helpful since I used Large and Small Crabs, so the raft was partially submerged with so many animals