This is a 3 piece Anode Kit. Please check that your propeller has a 3 seperate anodes around the propeller housing.
2-blade and 3-blade Propeller for (HR29), HR 31, HR 34 and HR 342
Older propellers do not have this anodes.
Change at least every 6-month. But please note that in some "hot" marinas it can be necessary to change the anode more frequently.