[tygo]

Just how vigorous of a boil is necessary?  My kettle is a pretty wide 15 gallon cooking pot and my boil off rate is pretty high.  In an effort to get that down to something a little bit more reasonable I'm attempting with the brew I'm doing right now to keep the burner as low as I can without it going out and just have the wort very gently boiling.  It's boiling, and there's surface action, but it's not what I would describe as a vigorous boil.

Is that good enough?  Is the boiling action going to be sufficient to get done what needs to be done (hop isomerization, driving off DMS, etc)?

[denny]

I'd say that as long as it's "rolling" you're good.

[dhacker]

Just curious . . what your boil off rate was per hour??

 . . that is, what it was before you adjusted.

[hopfenundmalz]

I always do a vigorous boil.  Or so I thought.  When I was at Sierra Nevada, they were boiling on the 100 barrel system, and when they opened the doors and the steam cleared, you could see the wort jumping up 1.5 to 2 feet or so.  That is what I remember, but I was involuntarily taking a step or two back, so I could be wrong.   

Breweries are said to have higher utilization, and this would be one reason why.

[tygo]

Just curious . . what your boil off rate was per hour??

 . . that is, what it was before you adjusted.

23% or so.  Even with the less vigorous boil today it was still around 20%.

[hopfenundmalz]

I am usually aroud 12-13% for the boil off.  In Brewing Classic Styles, Jamil Z. has 15% for his recipes if I remember correctly.

[dhacker]

Yeah, I'm also around 12%. All I can say is the wide mouth of the 15 gallon pot must really make a difference! 

[Mikey]

Just look for some nice activity and you'll be okay.

[tygo]

Changing the topic slightly but what are the implications of having too high of a boil off rate? 

I remember an episode of Brew Strong that discussed it and I believe there was some discussion that it had negative effects on flavor stability, but I can't remember exactly.  I need to go dig that podcast out again.

[mainebrewer]

I also have a 15 gallon kettle and, with a rolling boil, I average close to 25% boil-off.

[denny]

How can you guys measure boiloff in %?  When you double your batch size, do you boil off twice as much?  I know that I don't!

[bluesman]

I look for a steady rolling action across the top surface of the wort.  The wort doesn't have to be dancing around or rising up, just enough rolling action to keep the liquid turning itself and constantly exposing more liquid to the top surface of the boil.

[richardt]

I brew 10 gallon batches in a 20 gallon SS kettle outdoors on the patio under an overhead fan. 
It's usually hot enough in Florida (ambient temps during the day + burner going).
I've wondered if the overhead fan helps speed up the evaporative process during the boil?

I've been thinking about turning the burner down a bit--enough for a boil, but not full blast/rapid boil.
I guess I'm trying to avoid melanoidin reactions with my paler beers.
I'm doing all-grain (mash and lauter in igloo coolers) and only use the SS boil kettle for heating the water (liquor) and boiling the wort.

Anybody have advice on these two ideas? Does the fan make a difference?  Should I turn the burner down or let it rip?

[hopfenundmalz]

How can you guys measure boiloff in %?  When you double your batch size, do you boil off twice as much?  I know that I don't!

Amount of volume, and it agrees with promash in %/hour.
It would be the same for a big batch vs small batch if I calculated in gallons/hour, as the BTU input is the same (assuming constant atm. pressure and relative humidity).

Or I could be missing something.

[tschmidlin]

How can you guys measure boiloff in %?  When you double your batch size, do you boil off twice as much?  I know that I don't!

Amount of volume, and it agrees with promash in %/hour.
It would be the same for a big batch vs small batch if I calculated in gallons/hour, as the BTU input is the same (assuming constant atm. pressure and relative humidity).

Or I could be missing something.

I think Denny's point was that you don't measure the percent, you calculate percent and it will vary by batch size.  Gallons/hour is a better way of reporting your boil-off rate.