In my world, there are two countries constantly in cold or warm war with each other, and one of them has quite a lot of magi in their army. In my RPG campaign, the party is sailing one of the best-equipped ships of the other side. So I made some countermeasures against enemy magic. Would any of them be dangerous or make the ship slower or worse in some other way?
The easiest way to get rid of an enemy ship is to summon a strong water elemental and let it damage its rudder. So the ship has a system of pipes with some special oil inside, protecting the rudder. The oil itself repulses the elementals, and on the inner side there is the ship's kitchen, where the oil can be warmed up, so that it's even deadlier for the elementals. EDIT: the pipes make a cage around the rudder. They will be at least a meter from each other, so they wouldn't completely disturb the flows around the rudder. But especially when warmed up, they will affect them somehow; I don't know whether this will be important or not. They are made of bronze, with the parts regularly in contact with water silver-coated (yes, it was expensive).