As a first attempt they look good. And I see no problems with them at all.
Now try a cargo ship of various eras. Generally I see no problem with you drawing anything you want. so just go for it, build a little experience and when you do that, then you can easily modifie other drawings. (But remember, it is nice to ask the original artist of the drawing you want to modify. I always good to be nice!)
I haven't studied the picture of these submarines, but I'll give you a few hints:
Panelings: think how they would look at a photo, or when you see them at long distance, equal to the size you draw them, what can you see and what can you not see with you'r eyes. will panels be visible at long distance and if they was close to you?
Shading: for round forms, we shade things that are generally at a 45° angle.
Note how the light hit on this object, and how the shades look. In shipbucket we do a simplified version of this, we do not do a gradient shading, but 3-5 different shade colors, depending on surface.
- light color toward the light source.
- a darker color for shadow.
if there are two surfaces connecting each other at an angle:
- 90° we mark with a black line, since it is a break in the shape.
- anything under 75-80° we draw with a darker color to mark it is a break that is not 90°.
Here a link where you can see how various of long term artist on this forum started, drawing:
http://shipbucket.com/wiki/index.php/Fi ... tarted_out