While the ABYC allows for the use of annealed copper fuel lines they are, and can be, a poor choice on a small boat due to the sever vibrations caused by the small aux diesels.
We don't have a small aux diesel, we have a small aux Atomic 4
. Besides: The line isn't copper all the way to the engine. The line running aft from the fuel/water separator/filter changes to a flexible line a little way back, before it leaves the bulkhead, and that short length of flexible line then loops over to the engine.