So I have started to look into the FORTRAN a little.
At this point I must say that I have been sent a batch of .for and .f90 files to try and figure out how the code operates. The software team that built the code have all left the organisation along with a batch of files and instructions on how to compile the lot into an executable that represents a marine navigation simulator - a bit like a flight simulator but for navigating and mooring ships. The code has a Visual Basic (VB) user interface which has levers and switches to represent the various ship controls. This VB interface sits on top of the FORTRAN core that calculates the physics of the various forces the moving ship experiences. That's everything from propeller thrust forces to wind, waves, drag and any tug related forces. This, along with a representation of the surrounding geography and any water flow details allows simulation of a port environment within which the user can manoeuvre the ship in to the dock.
So - the first issue relates to the use of a FORTRAN differential equation solver in a real time simulation environment. Certainly not seen anything like this set up before personally! Now need to have bit more of a delve into the code to see exactly how this is set up.