R/C Naval Combat

I would suggest the two 75 round cannons aft.
For several reasons.
First off. Two cannons will be easier to maintain, than three. And will probably be more reliable. And reliability is the most important factor in a ship.
And cannons in the stern so that you can shoot up any ships that give you chase. That way you are shooting up enemy ships while trying to not give
them a chance to shoot up yours.
I personally prefer bow cannons on my ships, but it will more often than not, get one into situations where you have to take damage in order to give
it.
You can always change the cannon positions at a later time.
But for a first ship, it is best to keep things simple, and not try to exchange damage with one's opponants.
The less damage one takes while on the water, the less time spent patching the damage in the pits. That leaves more time to keep the ship systems
in opperational order.
Mikey

How about having 2 75s forward with depression so you could shoot across the pond and when needed shoot close range.
Then you could stay far away from the enemy and shoot long shots and when they chase you they more than likely have now straight forward guns to shoot you with and Des Moines is one of the fastest ship on the water.

It's hard enough to hit things with a smoothbore cannon when you are looking down the barrel. Much less shooting across a pond from the shore line. While you can mount two bow guns for long range fire, you will probably not hit much. Also is there an issue with bb's skipping on the water with too flat a trajectory? That could be a safety concern.

I suspect your Remington has a bit higher pressure than the boats do. Isn't a standard test 2" of insulating styrofoam?