Given that it is a slow firing gun, and it would take at least two shots to kill anything if you timed the hull break right, a Spire would be quite vulnerable. If everything went right, it would be a very deadly ship. If not, sand litter. I think the Spire would be the most risky ship to try this on.
On a side note, I am not sure how lochnagar would effect this gun, as it does not arm until after it strikes a ship. I guess it would explode in 1.6 seconds instead of 4 (if that was the 'push' time). The rockets would not push much before it exploded, and the timing would be pretty tight for breaking the hull. I can see where it could be useful, though. Or, it would arm right away and explode on contact. That would make it an extremely slow firing flak.
Let's one-up the fun by making it a ballista as well. Imagine this guy in brass and gears.