1515 1y is prefect for the jae .21 FE. as is the UL-1. I have modded the .12 plans enlarged to 29" tub, a little more than 1.5 inch longer than the "typical" .21 boat which is usually 27" or so. 4s1p 5000mah, and 1515 is fine. If you want to make it all fit nicely...make a bubble hatch and you...