The game does not display exact damage numbers or unit hitpoints, which makes it impossible to accurately calculate them.
However, there are ways to estimate such quantities by comparing one unit's resilience to another. We can see how easier it is to sink a Snaikka compared to a Hulk, which allows us to come up with a relatively accurate ratio of hitpoints. Another way is to assign a damage value to one of the weapons, and use that as a reference for everything else. Another option is to calculate hitpoints based on the repair costs at a shipyard.
For the purposes of this wiki, the short-range damage of a Small Catapult is considered to be equal to 1. That makes it possible (though rather tedious) to measure the ship hitpoints in Small Catapult hits. Based on that we can see that two cannons will sink a ship five times faster than a single catapult, thus they can be assumed to do 5 points of damage per shot.
This system does not provide particuarly accurate values, but they should serve us well enough as a guideline.