There are two significantly different types of alternators, depending on where the how the return circuit to the battery negative is made.
In a "case grounded" alternator there is usually an attachment on the alternator for the battery Positive wire ONLY. The connection back to the battery Negative is through the alternator external case, and then through the engine block. This will not work on an Amel because the engine block is not continuously connected to the battery negative. These are the common alternators used on automobiles.
Occasionally, this type of "case grounded" alternator has a connection for a battery negative cable on it. The way to check if your alternator is "case grounded" is to look for continuity between the Negative terminal on the alternator and the the alternator's external case. If the alternator is "case grounded" it will show near zero ohms on this path. It is not suitable for an Amel UNLESS the case is electrically isolated from the engine block with the plastic inserts. Putting one of these on an Amel without the isolation parts will make a continuous direct connection between the bonding system (connected to the engine) and the battery negative (connected to the alternator case). This eliminates the "floating negative" that the Amel was designed with. It would put all the underwater parts of the boat at risk for stray current corrosion.
The other type of alternator with an "isolated ground" has connections for both battery Positive and battery Negative on the alternator. This is the best type of alternator, and hopefully the one you have. On this type of alternator, if you test the resistance between the Negative terminal and the case you will see an open circuit--no connection. If you have this type of alternator, it can be installed safely without the plastic isolation parts since it is already electrically isolated from its mount to the engine.
Great Harbor Cay, Bahamas