Anodes should be replaced as necessary.
Aluminum or Zinc for the anodes?
Zinc, when exposed to air or freshwater will create a protective coating over itself and then stop working until you scrape the surface and remove it. This is apparent in the photo gallery when you look at my zinc vs aluminum anodes.
The Yamaha F300 is raw water cooled, thus when the motor is tilted up, out of the water, the internal engine anodes get exposed to air. After every trip we also freshwater flush the engine to get the salt out, which exposes the internal engine anodes to freshwater. For these reasons, the 8 engine crankcase anodes, the two thermostat anodes, and the rudder anode on the F300 are all aluminum, as OEM from Yamaha.
The Ranger Tug R27-OB also has an engine transom anode and trim tab anodes. I've ran zinc (the boat came with) and have since switched to aluminum. Aluminum works so much better. Costwise, aluminum anodes are slightly less expensive and last a little longer than zinc.
Here's some photos of aluminum and of zinc after 6 months in the water. Also note, Channel Surfing is moored at the Port of Everett which is brackish water (at the mouth of the Snohomish river). Being an outboard, she's also hauled out of the water several times a year for engine maintenance.
It's not wise to mix anode metals as they end up protecting each other instead of protecting the underwater boat metal.
On the R27-OB, I use the following anodes and replace them often.
Trim Tab Anodes, Aluminum
R-3A Rudder/Trim Tab Aluminum Anode - 3-3/4" Dia.
Item Code: (R-3A), Quantity 2.
Engine Transom Bar Anode, Aluminum
6G5-45251-01A Yamaha Outboard Handle Bar Aluminum Anode
Item Code: 6G5-45251-01A, Quantity 1
Engine Trim Tab Anode, Aluminum
6CE-45373-00 Flat Trim Tab Aluminum Anode
Item Code: 6CE-45373-00, Quantity 1
Trim Tab Anode - R-3A Rudder, 3-3/4" Dia
Engine Transom Bar Anode - 6G5-45251-01A
Engine Trim Anode - 6CE-45373-00
Last Updated: 9/26/2023