A way to have partials outside of the app/ directory?

I am building a site where only one section is dynamic the rest I want
to serve statically. Is there a way to use erb in the public/ folder
with static html pages?

You can just create the views and partials normally in the folder. If
you want to serve pages statically (or even just pieces of pages
statically) you should really just perform caching. You should perform
some level of caching on all your pages anyways. If you use page caching
then you will effectively be serving static pages anyways but this is
just the proper way to do it.