Brewing 10 gallons to be split in 5 gallon fermenters

[Clanchief]

OK, so I've taken on a gargantuan task, agreeing to provide a minimum of 500 bottles of homebrew for some friends of mine at their wedding. What I was planning on doing was brewing it up in 10 gallon batches, rather than my more typical 5, and then splitting it between two different 5 gallon fermenters. The reasons are that 1) I only have 5 gallon fermenters; 2) I doesn't take much more time to brew a 10 gallon over a 5 gallon and; 3) I don't have time to brew 10 separate time between now and the wedding JUST for the wedding. At least not if I want to make sure they have nice, fresh beer. I have one beer I'm going to do which I'll let age, but other than that, I want to be able to serve within 3 months of bottling. Here's my question(s):

For equipment, I use a 10 gallon rubbermaid homemade MLT. I have a 10 gallon kettle for brewing. For 5 gallon batches, this is great, and I have no problems. For 10, well, it concerns me a bit. First, I assume I'll have to basically have a boil of around 9.5 gallons, which will probably come down close to 8 when it's done. I would then top each off with another gallon of water. But if I'm trying to check the gravity during the boil, how can I calculate this when knowing that I'll have to add additional top off water? Also, how would I calculate the hop additions to get the bittering correct? Because it's such an important event, I want to make sure these are great beers, and I don't want the fact that I'm trying to save myself time to end up biting me in the ass when it comes to providing my friends a great product.

 

[BierMuncher]

Well, for reference. I have a 15.5 gallon keggle (a very handsome keggle thank you very much). And I have to work very carefully to prevent boilovers with a 13.5 gallon starting volume.

Trying to boil 9.5 gallons in a 10 gallon pot is going to cause real issues. The other thing is you didn't mention what kind of burner you have to fire up a 10 gallon batch. It may be worth at least investing in a 15 gallon boil pot and not having to worry about boilovers and accurately topping off a partial boil.

 

[Clanchief]

Arghhh...That's something I was worried about. Having just invested in a mill, I'm not sure I'll be allowed a 15 gallon kettle. But I can make the pitch. Just say it's part of our wedding gift. Haha.

 

[sweetcell]

the easy answer is to get a 15 gallon pot... no need for dilution. but if you're avoiding that expense:

first calculate what your post-boil/pre-dilution gravity needs to be. say you want to end up with 10 gallons of 1.050 (post-dilution), but you're planning on ending your boil with 8 gallong. use the second calculator on http://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/ to back into the required gravity: enter Wort Volume= 8 and Target Volume = 10. then play around with the values in "Current Gravity" until the "New Gravity" shows the number you want. in this case you need to brew 8 gallons of 1.062 to dilute down to 10 gallons of 1.050. beersmith also has a really handy-dandy tool to calculate this stuff.

as for IBUs - and this part i'm a little less confident about - i would look at the dilution %, in this case (10-8)/8 or 25%, and boost the IBUs by that amount. so if you want a 40 IBU beer, you need to brew a 50 IBU beer. the part that i'm not 100% confident about is whether IBUs dilute linearly. i'm assuming they do.

so there you have it: you need to brew 8 gallons of 1.062 beer that has 50 IBUs. plug away in your recipe software to figure out how to make such a beer.

but yeah, a new pot will make this a lot simpler

EDIT: fermcap can really help with controlling boil-overs.