You must determine the targets actual position as accurately as you can for both plots, i.e. one at 0mins and 1 at 3:15mins.
Then you use the ruler to measure the distance between these two plots and what ever the ruler says the measurement is then thats the speed of the target in knots. (I would reccomend making a few more plots just to make sure though as accuracy increases when the target is nearer to you).
As for the plots, what I do is ask the Watch Officer for the range and bearing to the target. Then I immediately start the stop watch and mark my U-Boats position on the nav map. I then draw a circle with center point on my U-Boats position and radius equal to the range of the target. Next I calculate the bearing of the target relative to North and where that bearing line intersects the circle is where the target was when I started the stop watch. I repeat the whole procedure when the stop watch reaches 3:15. Et voila, the distance between the two plots is the targets speed.
Now, things get a little tricky when you're submerged and thus using the periscope as its harder to determine the range of the target, especially in a storm. Still, you should be able to make a decent enough estimate.
That said, if you managed to calculate the speed of the target before you submerge then have confidence in your plots. If he hasn't spotted you then the chances are he'll be where you expect him to be.
One last thing, once you're in firing position, immediately mark the position of your U-Boat. This way you'll be able to judge how far you're drifting.
Again, practice makes perfect etc.
Cheers,.....
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