If you're building an S Class, I'd have thought the book 'One of our submarines' would have been essential reading! It won't help you build the model but it'll give you much needed guidence on avoiding Japanese A/S vessels.
Seriously, it's not only interesting, I found it a really decent read. The photos of Storm will also help.
Secondly, on a more practical note: to find the volume of a irregular shaped object, you need to convert it to a regular shape, in this case a cylinder, and take it from there. So you'd divide the length up ... say every inch... and measure the perimiter at these marks with a piece of string.
You then average these measurements which gives you the average circumference. Divide it by Pi [3.142] and you have the radius? (is that right? Long time since I left organised education)
It's then a simple Pi times the radius squared which gives you the area of the object, you then times the area by the height to get the volume.
Have I got this right? It'll give you the ball park figure.