Last trip I took I went through 11 gallons (half of my fuel) and wasn't at the half way mark of my planned trip so I had to turn around. I had a 2.5 gallon spare can, but I didn't calculate that in to my fuel consumption because it was intended to get me home if I miscalculated. That day was an 8 hour day, but it included 2 trips through a portage crossing (around 45 minutes to get through each way) and stops for swapping/filling tanks. Yes, lots of tiller holding, but its the only boat I have I can "explore" in. I bought a regular boat seat a few weeks ago after some exploring treks I made earlier this season. I needed a padded seat and back support. So I got that covered. I thought about doing a console conversion, but I think that is a bit far out of the question. Until I can afford another boat I'm making do with what I have.
Based on that last trip I need another 10 gallons to make the trek comfortably on the main motor. My running fuel was 22 gallons, that I planned to burn, then 2.5 gallons on top of that. So add in another 10 gallons and that is 32, but at that rate I'd like to throw in a 5 gallon can as a spare in place of the 2.5 for a total of 37 gallons. With 3x 12 gallon tanks that pretty much hits the mark at 36. I still need to check the burn rate of the spare motor, that could be another big factor.