I have an older (late 80's) 25 HP 2 stroke Mercury on a 14'10" Johnsen. I am planning on redoing the decking and am doing my research from those that know more than I do. I currently have a 6 gallon tank, but am curious if a 3 gallon would be sufficient. Is the weight differential significant enough to make the change? Or should I just draw my decking plan around the 6 gallon?
Duck is absolutely correct! If 3 gallons is sufficient for your type of fishing (proximity of ramp to fishing spots), use the 3gal. Less weight, plus you won't have gas sitting around in your tank if you have long periods without trips.
On the other hand, I have a 6gallon on my Johnsen 14' with 25hp 2stroke Yamaha because some days I like to run around to many different flats
I'd design the deck to accommodate two 3 gallon tanks. I like to have 2 tanks that carry enough fuel for a long day. You run tanks empty that way instead of topping off a single tank forever. Most of the time, the fuel helps balance the boat, so I don't sweat carrying a bit extra unless I have to physically carry it to and from the boat because I am at a primitive launch.
Another plus is the very timely reminder running a tank empty gives you to start heading back toward the launch.
The amount of fuel you carry will be a factor for more than just weight and balance... Many years ago I had a micro (old StarCraft 16 with a bonefish interior) that had two six gallon tanks - and like most I figured - run until you empty tank #1 then come home on tank #2.. That worked most of the time (and I wasn't using the skiff with passengers so the only one who ended up poling home was me....). Once or twice, though, I did have to pole the last mile or two... I just didn't know any better (I was younger then - my only excuse).
Years later, once I began guiding full time I learned that the right way to do things as far as fuel was concerned is the "rule of thirds" -just like you're forced to do with an airplane... One third of your fuel outbound, one third of your fuel inbound, and one third in reserve...
With a tiny micro using a small motor and fishing just a few miles from your starting point you might try a 3 gallon tank and see just how much is left at the end of the day - but a six might give you enough cushion that you'd never have fuel worry. If a six leaves you little at the end of the day then I'd want two tanks (two sixes or a six and a three...). Remember most guys on the water couldn't tell you exactly what their motor burned in a day's fishing -so they have to find out the hard way...
I have same boat with 25 Yamaha 2 stroke. Battery up front inside what was the front bench seat. Six gallon tank riding back in Port corner. Keep the 6 and don't top it off if running solo. floats level with person on front and back decks. 4-5 MPG but never really averaged it.