If nothing else helps you could program the controller to use only two or three engine (speed) settings. 1/3 ahead and Full ahead would be something useful, maybe the minimum speed ahead too. At times in stock SH3 one can't use keyboard commands anyways, because there is no preset key for 1 knot forward (silent enough) or 8 knots forward (most economical speed for a type VII). There might be mods that change the speed settings, though.
Another solution would be downloading a keymapper program from the Downloads section and look though all the commands if there are commands for slower and faster.
http://www.subsim.com/radioroom/down...o=file&id=1016 might be useful.