Have you seen one in person and put a tape on it? I just measured mine and it is over 17" at the bracket end, more like 17.5".
The tripod chair sits in the sled with your feet our front, and in this configuration I have enough room out front for 2 holes (one for sonar, one for rod), sonar, and buddy heater. I don't know why you would want to sit in front of the sled, you would for sure not have enough room.
Construction is decent, you need to keep an eye on stitching as I had one start to come loose. I just sealed them all over with some Aquaseal. The brackets for the poles is double with the poles bolting in between it. This looks more sturdy than the Clam models I looked at.
Got it at the beginning of last season and it worked well for me, I would buy one again.
Burt