Hello Amerigo: First of all, welcome to this forum.
About modifing the speeds for the uboats (assuming that you were reffering to uboats when you say vessels), there are several files that manage this info. I did it using a program called Mini Tweaker (made by Timetraveller)
http://www.delraydepot.com/tt/sh3sdk.htm. It makes things very simple and it's worth using it. It's very simple, all you have to do is open the file NSS_Uboat2A_Sim.txt for the IIA sub (Mini Tweaker finds the right file by using .txt files that you must install in the same directory as Mini Tweaker), just as an example . The file in question shows you 5 menus: General, Diesel Propulstion, Electric Propulsion, Flooding and Torpedoes.
In the Diesel Propulsion menu you will see an item called: max_speed
There is wher you must insert the speed you want. But notice that you must tweak the knotsSurfaced item as well. You will have to try several combinations, until you find the right balance between speed and the distace you can travel at a given speed (knotsSurfaced), otherwise you will notice a serious drop in the distance you can travel at that speed. You can also change the value in milesSurfaced to increase the distace your boat can travel. You must follow the same procedurewith the Electric Propulsion.
About the hitpoints, you must change the values in the zones.cgf filesthat is in the following directory: SilentHunterIII\data
There are different blocks of values for different kind of vessels.
I hope this information helps you.
If you need more informaton or other details, just let me know.
CapZap