I have a flush system that allows me to turn off the raw water and bypass it with hose water. My question is does the B3 drive need to be in the water or can I run the motor via the flush system while its on the trailer?
You probably have the hose connection somewhere between the transom raw water supply hose and the raw water pump on the engine, and a valve to prevent water from running back out the drive?
The Bravo III doesn't need water in it at all to run since you're not going to run it "loaded". The entire drive is actually cooled by being in the water, but running it unloaded, it produces little heat.
If you connected the hose to the raw water pump discharge, you would want to remove the drive belt to prevent damaging the impeller.
Regards,
Rick