Small Pot, Large Batch?

Since I'm keeping things small on the boat, and have been watching instruction videos on youtube of kit brewing, this question came up:

Can I make up a nice boil of LME, DME, partial etc with hops in 1.5 gals of water and then add 1 gal of cold water after it's all done to make a 2.5 gal batch? Seems like it would help chill the wort, would reduce the size of the pot needed and reduce heating time/fuel usage.

The kits (like coopers) add a mix of warm water to dissolve the LME and then add a bunch of cold so it seems like it should work for the kits. I'm not as sure about partial mash, steeping grains, adding hops and LME to a regular boil etc.

The chemist in me says this is a fine idea, while the Welsh tea-brewer in me thinks that I may be missing some essential voodoo here....

BrewOnBoard

Most extract brewers do partial boils (2.5 gallons, then add 3 gallons to their wort in the fermenter). If you are doing 2.5 gallon batches you should be fine, just be sure to compensate for boil off. If you are only boiling 1.5 gallons, you might only yield 1 gallon wort and have to add 1.5 gallons water. Happy brewing!

Other than adjusting the bittering hops, there isn't any problem. Higher wort concentrations reduce the rate at which the bittering chemicals dissolve, so you need to use more hops. If you are doing partial mashes, you can also add the extract at the end of the boil to raise utilization.

There are various software packages that aid in making bittering adjustments.

There is a limit to how bitter you can make a wort (100 IBU), so if you boiled 1.5 gallons and added 1 gallon, the maximum bitterness would be 60 IBU. Not a problem in 98% of beers.