Your best bet is to go to Subsim Review. They're a website devoted to all things in naval gaming, in particular the Silent Hunter games.
I still have SH3. SH5 adds the ability to roam the inside of the U-boat at will, but if that was the only improvement in gameplay, then I stick with SH3.
The only other thing I would do is add this nifty mod called Jace11 Bearing Details, and another called Nav cursor Protractor Tools.
They allow you to use your map sheet to do your own plotting easier. The bearing details changes the look of your u-boat on the map so you have a 360 degree compass rose around the boat, useful sometimes to manually draw out lines. The Protrator tool changes the look of your mouse pointer to instead be another 360 degree compass ring (a rose that is upside down and counter clockwise for the numbers). This lets you drag out a line and use the rose to get it pretty accurate on how you draw it.
This lets you do this.
If you take a bearing and range measurement with your stadimeter periscope, you can go to your map screen and draw out the bearing and range, then make a point. If you do that every 3 minutes and 15 seconds, you will see a line developing.
Why 3 minutes and 15 seconds between readings? Well, it is math. Distance = rate x time. I'll spare you the math, but a ship going 1 nautical mile an hour will travel 100 yards in 3 minutes and 15 seconds.
If you make a running plot like I do, you will see the enemy ship has moved 700 meters between making a plot of him. You now know his speed, which is 7 knots. You also can see his course and can extend that out on the map, run ahead and submerge about 1000 meters off of the line and wait. You can also set the speed of the target to be 7 knots in the torpedo data computer.
Eventually he will be along and run right through your crosshairs.
