Each land unit in naval transport form has 5 hit points.
And they return fire when attacked by subs at .1 for level 1
A level 4 sub has 25 hp and does 4.5 attack damage on surface ships.
So without accounting for sbde or the x factor, a battle could go like this:
Sub attacks convoy. One attack by sub eliminates nearly one complete unit from convoy (4.5 of 5 hp) so lets round up to that:
sub attacks convoy of 36, and eliminates one.
Convoy returns fire, and does 3.6 damage (0.1*36) to the sub.
Sub can sustain the defense value of the convoy 6.9 times. (25 / 3.6) Of course the second hit will be 3.5, third will be 3.4, etc
So the sub will at most eliminate 7 units from the convoy before being destroyed.
Disclaimers:
In every battle there is an X-factor that means an attack might do from zero to 100 percent of the listed damage. This is in addition to the effects of terrain, forts, and home defense.
There is a limit to how many units of each type are most effective per group, called the State Based Damage Efficiency. Forum thread on SBDE https://goo.gl/HBY9fu The sbde of convoys is not listed, and may require testing as to the differences between a group of 36 infantry, and 36 units split evenly between several unit types.
If you parked the sub in the path of the convoy then the convoy would attack and the sub would defend, which would change the values in the equation.