Welcome to the forum John.
Diesels use a lot of air.
The engine compartment can get hot. Rather than overheating
the engine (the water cooling
usually has plenty of reserve capacity) the hot intake air makes the engine less efficient and it produces less power particularly if it struggling for air. The ancillary components like the alternator
will run hotter which is not great for their life expectancy.
Long term you should replace the fan, short term its fine to run without it.
An old crude trick to see if the engine is struggling for air is to crack open the access hatch
. (When the engine is working hard) If the engine speeds up it was not getting sufficient air. You can also measure the temperature in the engine room. These tests will give you some idea of how essential the fan is in your particular installation
Unfortunately boat builders have a habit of placing equipment
where it is very difficult to service
. With a bit of patience and a lot of swearing there is usually a way. Can the fan be mounted somewhere else?
Check the fan is getting power with a multimeter before you replace it. It is not unusual for the problem to be a bad electrical
connection somewhere in the circuit rather than the fan itself.
There are some tricks such as bluetack on the heads of the screws so they stick to the screwdriver and taking pictures with a small digital camera
to see what is going on in areas that you cannot get too, or to find a dropped screw. Good luck !