I also found out that it is a mistake shapers often do with wide tailed boards, to put the rear fins so much on the rail like:
The logic seems to say "Board is so wide, I must put the fins on the rail for them to have some bite" but I guess it may end up either with no fin acting as a stabilizer, or the issue well known on planes when 2 foils too far apart will undergo wildly different relative speeds in turns and become unstable?
Boards set up this way tend to behave like shopping trolleys in the rail-to-rail transitions: you feel the board "floating" in the yaw axis...
Things I have found that work well to counter this:
- move the front fins backwards, for a more "Simmons"-like ride, but you lose looseness.
- use the McKee setup (pull the back fins towards center). The rear fins should act as a splitted central fin rather than splitted side fins. On the Gulliver it could mean having the boxes inside the channel, with reduced toe-in
- use a trailer fin, like you did. Note that the knubster can not work: I had some boards where a nubster worked well, and others where it was always in turbulent flow (I could feel the wobbles in the rear foot, adding drag but no hold), but a trailer fin was more efficient and didn't stall. You can omit the trailer for small waves to add looseness and speed, and add it for bigger waves. So it is great that the gulliver have a central box, but anyways it is quite cheap to have a central box added in any case.
Note that nubster are very easy to make. Just get hold of some ultra-cheap plastic fins and sand them.