If there are no hop additions planned before :20 remaining, is it still necessary to boil for 90 minutes?

I understand why hop additions happen after the first 30 minutes - wouldn't it be a time and propane saver to cut the boil to 60 minutes when only hopbursting?

You're not wrong with that idea.

My last brew I targeted for 70 mins. and an 8% evaporation rate. I noticed in the BIABacus that the efficiency drops when boil time drops.

I've read that the minimum is 60 mins. at 6% evaporation rate, but that's at the threshold of DMS risk. DMS for the most part goes away during fermentation.

My normal evaporation rate has been 11%. I don't see anything wrong with 60 mins., except for the fact that Pat advocates 90 mins.

Rogue brewery boils for 45 mins. for their Dead Guy ale now, with​ a fair amount of whirlpool hops.
MS

There's a few things (Edit: actually, a lot of things) worth understanding here.

- Hop addition times are really not that relevant to boil time. For example, if you add x grams of hop A at 90 mins versus adding it at 60 mins, there is very little difference in the end result. So, boiling is not really about hops.
- A home-brew 90 minute boil is not the same as a craft brewery 90 minute boil. Certain techniques might be employed in a craft brewery that are unavailable to us.
- You can't generalise on boil times. Some styles you can get away with y minutes while other styles boiled for y minutes might fail.
- The lower the boil time (or, more correctly, the evaporation rate) the lower the kettle efficiency. This is due to your grains being washed with less water.
- Increased boil time allows some chemical processes to complete.

Why the 90 Minute Boil is Recommended

It's a safe time that works across all styles.

What if I Drop to 60 Minutes?

You will experience a decrease in kettle efficiency.
You may or may not, depending on beer style and your water conditions, experience clarity, attenuation or flavour problems.

What Should I Do/Consider?

Experiment but don't generalise or assume. There are many scenarios to consider. Here's two examples....

- Just because a certain minute boil worked on this style doesn't mean it will work on another.
- Just because a certain minute boil worked on this recipe doesn't mean it will work again (unless you can control your water & weather)

Oh, and here is a link we can never post enough!

Hope that helps ,
PP