water volume

I've done 5 all-grain batches and everything is going perfectly well (especially the beer!) except for one thing - VOLUME. I seem to be having a really hard time calculating anything. My first brew was fine, and I calculated about a 25% boiloff rate. Then, my second brew I used the same info for mash tun loss, grain soakage, boil-off rate, etc and ended up with about 6.25 gallons of wort in the fermenter (was going for 5.5). Then, on my next beer i ended up with 4 gallons of wort in the fermenter. I'm so frustrated. But the worst is the most recent - I measured 7.75 gallons of wort pre-boil. My boil time was 60 minutes. I ended up with 4.25 gallons in the primary, MAYBE 4.5. What could it possibly be? That's 2.5 (or more) gallons lost in 60 minutes! i guess i'm leaving about .25-.5 galons of trub in the kettle... but that's still a 2-gallon per hour + boil-off rate. And remember, i had one beer that ended up 3 quarts too much (which is much less of a problem . My kettle is pretty wide i guess, and my boils are not insanely vigorous. It just seems like I have a huge variation in boil-off rate. HELP i'm so pissed off.

What you're looking for is consistency in this process. If you're brewing outside with a propane brewer several things can impact your results, like wind and ambient temperature. What you want to do is make sure you get it to just a rolling boil, not where the wort is jumping out of the pot with giant bubbles rising to the surface. If your boil is rolling and looks similar each time, you'll have better consistency.

How are you measuring your water volume? I would suggest using gallon jugs to measure it out, and make sure you know how much your kettle holds. My calculations always use more sparge water than necessary, even when batch sparging, and then I cut off the sparge when my kettle is at the right volume to account for boil off rates. Of course, this may not help if your boil off rate isn't consistent. Maybe try calculating with a couple dry runs with water, or track each brew. The key though is that your boil is consistently at the same level of vigorousness and your inputs, like volume of water used for mashing and sparging is measured properly.

It sounds to me like one of your main issues is that you're not accurately measuring your volumes. Guestimating your losses to kettle isn't good enough if you want to get the same volume to the fermenter every time. I also second the suggestion to prepare more sparge water than needed and just use enough to get to your desired pre-boil volume. Each batch will have slightly different absorption etc, and this will prevent those inconsistancies from effecting your volumes. Besides that, it's always better to have it and not need it than the other way around.

A wider kettle is definately going to have more variation when it comes to boil off, but you should be able to get it at least close each time. A rolling boil and a very vigorous boil often look similar, and remembering exactly how hard it was boiling last batch isn't easy. Maybe try turning the flame down until the boil stops, and then turn it up just enough to maintain a rolling boil. This should help ensure a more uniform boil off between batches.