i second everything that yankee stated.absoluetly no need for 1000w ducer,actually dont even need 600w for what we do,but a traditional sonar unit with an airmar 50/200 600w is all that is needed for trolling.i run a transom mount ducer and on high frequency never loose bottom and can clearly see bait running 30 mph.put it on low frequency for trolling and you will see everything you need to see under your boat,with proper tuning and correct ducer angle.i know you are looking to go with a single unit,but like rick said,i put a variable chirp 100-140khz with only 300w ducer unit on my boat last year and i am extremely happy with the detail it shows between fish and bait.so between the traditional 50/200 and the chirp 100-140,i feel like its the best setup for trolling.