Preboil For 70 Liter Kettle

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guys i have just bought 3GGG ,s beerbelly 70 liter kettle for doing double batches, usually around 1040/1050 beer,

in my single 40 liter kettle i used to preboil amount of 28 liters to get 23 liters after 60 minutes boil.

when i do double batches .what preboil amount should i have into the kettle to finish with 46 liters, anyone have the 70 liter beerbelly that i can compare to, i will adjust my volumes after i have run my first batch through.

My pre-boil in my 70L SS pot from Craftbrewer is typically about 52L for a 43L knockout... completely stress free, not having to watch for boil-overs.

edit: So i guess 55L should do it for you
For 46L into the fermenter i start with 64L. This is based on a 58cm wide pot with an evaporation rate of almost 11L / hour and doing BIAB

Heres all the data

I would have thought somewhere between 51 - 55L if looking for 46L post boil in pot. If it has the same diameter as the 40L then the boil off will be about the same - given the same rolling boil strength.

In my 50L keggle I normally have a pre-boil of 46L and watch it like a hawk until it's in a good boil, then top up 5L post boil. Gives 47L post boil (3L lost to break, 2L loss to cooling). End up with 42L into fermenter.
Sorry to jack the thread but I have the same question but with a 31ltr pot and wanting 20ltrs to go into the Fermenter... any help appreciated B)

@Stillinrehab. Depends on a few factors - surface area, intensity of boil, loss to trub/hops etc. but couple of options:

1. Plug it into beersmith and get an estimate.

2. I reckon given a tightass boil, 26L in.. 60min boil 20L out. 90mins to drive off DMS, 30L in, 20L out.

Time, Trial and error will get you to know your system best
thanks for the info guys appreciated.

argon what size burner are you using for your 50plus liters, i have a 3 ring burner but i think i will need to get a bigger adjustable regulator to do the job.
might have to drill out the holes a bit too.