[sbw1]

One cause of low coolant is air in the system after it has been drained and refilled. We have 8.1 Crusaders and the set up on our boat sounds very similar to yours. The owners manual talks about the importance of changing the antifreeze when the pH reaches a certain specified level that indicates it is time to change the coolant. On our boat it lasts about 4 years. I've noticed that the fluid levels on the overflow tanks will be about midway between the add level and the full level after the tech does the work. The first time we run the boat after the change, the levels will drop, some times below the add level. Both sides will be different. When the boat cools down, I add antifreeze to each side as needed to bring the levels up to the mid points again. I may have to do this 2 times and sometimes three before levels stay constant. Once equilibrium is reached levels stay the same for the four or more years between changing the fluids again. Levels should stay the same or your have a leak somewhere. Also, it is critical that you add antifreeze that is compatible with the engine spec and do not mix types. Mixing will ruin your engines.