I would like to publicly thank the engineers at Johnson Pumps for helping me come up with a solution to the trim-tank pump. The trim-tanks are very big and they should not take too long to fill. This can easily be done if you use a big, heavy expensive impeller pump. But besides being big, heavy and expensive these pumps are also usually fitted with non-watertight electric motors, which makes placing the pump in the bildge problematic.
When I told the engineers at Johnson about these problems, they came up with the idea of connecting two of their biggest submersible pumps in series. This was a little tricky because these pumps are not made to be connected to a piping system. But after plenty of searching I was able to find a good solution to connect the pumps to the piping:
This gives me a pump that is completely waterproof, and weights and costs about 20% of a large impeller pump, but still gives about 60% of the capacity. With this pump it will take roughly 10 minutes to fill the trim-tank.