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I have heard a lot of home brewers talk about the vigorousness of the boil being quite important. Given that describing the vigor in words like "rolling" is ambiguous I was wondering if it makes more sense to try to tune the viroscity of the boil by tageting a particular boil off percentage?
For an hour long boil what percentage of the original volume should be lost?
It's far better to think of boil off in gal./hr. than in percentage. If you use percentage, that would mean that if you double the batch size you'd double the boil off. It doesn't work like that. I boil off about 1.5 gal./hr. whether I do a 5 gal. batch or a 10 gal. batch.
Regarding % loss, I can give you a recent example in litres (but the percentages are the same of course). I calculated that my boiler (electric) holds 0.962 litres for every cm depth. My last boil started at 23cm depth and dropped by 5cm over 1.5 hours, so I lost a total of 4.81 litres. Multiply by 0.6666 gives a loss of 3.21 litres per hour or approx 14% per hour.
Having said all this, it really is important to have the wort 'rolling', though. Personally, I think this is more important than a percentage (even though I do measure the loss). If the surface of the wort during boiling is bubbling and moving with plenty of activity - it seems to rise in a heap so that the surface is changing and uneven, then the loss will take care of itself. Ensure the roll and you will drive off the unwanted elements and achieve the 'hot break'.
Going for an exact percent can be tricky, since evaporation can be changed by factors out of your control (ambient humidity, etc.). In the summer I might get 10-15%, but in winter it will get up to 17%.
In my experience, rolling is about the only thing that matters, and that is to combat DMS. I haven't yet found reason to worry about a boil getting too vigorous, probably because I aim to make as much beer as my equipment will hold, so boil over is the only thing I'm trying to avoid.
This being said, I use electric heating, so I'm not worried about conserving fuel.
If you're just looking to conserve propane, then I'd use some of the other suggestions here. For my part, I see about 1.5 gallons per hour out of my 15 gallon keg/kettle.
I usually look for 12-15 percent per hour.
