Excessive Evaporation??

I used a 15 gal stainless kettle for the first time Saturday. I scrubbed it and boiled 10 gals of plain water for an hour to remove any finishing compound left behind during manufacture. I surprised at the amount of evaporation that took place but I didn't measure it.

I did my 1st all grain batch and collected 7 gal of wort from the mash and batch sparge per Beersmith and my equipment entries. After the boil I was left 4.75 gal of wort. Based on my test boil results I had 2 gal of plain water boiled, cooled and waiting. Am I glad I did that! I was shooting for 5.5 gal or wort.

I calculated 32% loss. I did it outside in low 50F weather. Am I using to vigorous a boil? Would covering the pot say 75% help? Do I need to start with 8 gal wort?

thats normal. Vigorous is good. Don't cover the pot. I boil off 2 gallons in 60 minutes.

Just by reading your post your kettle must be rather large in diameter like 20" (guessing and a number i'll work with) which is 314 sq/in of open surface area.
A 15.5 gallon keg converted to a keggle with a 12" open top has 113 sq/in of open surface area. As you can see a lot more boiling surface area exposed hence a higher evaporation rate under the same burner boil conditions.
I plasma cut a friends boil keg to a 10" diameter open top for 78.54 sq/in of surface area, this reduced the evaporation losses more vs a 12" top more but a smaller diameter coiled chiller must be made. The 10" opening is .250012 the area of a 20" diameter pot or your pot has 3.9979 times more area than this 10" opening keggle. So far no bier quality effects with 10" vs a 12" opening, I would not go below this 10" opening size myself as you need to have some boil off. I could open it up to 12" if needed but so far no problem at 10".
Other factors like wind, air temperature, humidity, altitude, how vigorous of a boil and how long all effect your boil off.

just adjust your boiloff rate in beersmith and it'll compensate your recipes for this rate.

Thanks for the replies. I changed the evaporation rate parameters in Beersmith and I will be adjusting my water volumes accordingly.

Beersmith listed a suggested range of evaporation rates between 5% - 15%. That prompted this thread as my rate is 32% and I thought that was way out of line.