I'm guessing the smaller/inner collar zinc that you're referring to are not for the
saildrive, but for your prop. The SD20 sail drive isolates the prop through the use of rubber dowels. This protects the
saildrive & gearing from catastrophic failure when the prop hits an obstruction. However, it also isolates the prop from protection, and the props are expensive.
What model prop do you have?
if you have a Varifold 2-Blade folding
Propeller you can find the prop zinc by googling VF2SDA.