Without too much work, a 4th level Explorator can get a tech use in excess of 110. This isn't even included rarer gear or assists from the rest of the crew (which can boost this to over 140).

This means that the Flank speed extended action can be safely performed every turn with no threat of hurting the engines, and allows what should be relatively slow vessels to move faster than even the fastest scout vessels (A grand cruiser effectively having a move of 1D10+6 movement).

The only situation I can do to balance this is to have the enemy ships have similarly stated crews, but this does lead to a slightly comical situation where the ships chase each other around the board at distances that are often longer than the range of their guns.

A couple of ideas would be to put a maximum cap on the number of successes (so for example, the vessel could only go up to double it's speed rating a turn) or to only allow the action every other turn. Does anyone have any other ideas on how to balance this action?

Yeah, you may have noticed that most of the ship combat is rather wonky when players get involved. I swear they were playtested with characters that couldn't actually do the job properly. Personally, I'd treat it like sprint. If you use it twice, your engines are overtaxed and are either crippled or incur a -10 penalty to all Ship Actions until you're able to repair them.