I just bought a 16ft aluminum v-hull boat with a 35 hp evinrude outboard that comes with a 6.4 gallon tank.  I was just wondering how much water or how long will that 6 gallon last.  

I understand that the answer may vary depending on the body size of the water and how often the motor maybe used.  

I just want to read some of your estimated answers or personal experiencesjust to get an idea, thanks.  

A good rule of thumb is that an outboard will burn 10% of it's HP rating in fuel per hour at Wide Open Throttle. So that means your 35 HP will burn 3.5 GPH at WOT so your 6.4 gallon tank will last 1.8 hours at WOT. Your run time will increase, dramatically,  at lower throttle settings. You will have to figure out how fast your boat goes at WOT to figure out a range.

Okis it ok to be able to have more than one gas tank in the boat.  Let's say that if I added another 6 gallon tank on board to switch to when the other one goes dry.  Is that legal?  

Oh yea. you can do that.  I have two six gallon tanks in my boat for when I go to bigger lakes.

Thanks for your responses, I appreciate your help.

By all means, if you have room for two tanks, do so.  Another thing to remember is the one third, one third, one third way of thinking.  One third of your fuel out, one third of your fuel back, and one third in reserve.  

By all means, if you have room for two tanks, do so.  Another thing to remember is the one third, one third, one third way of thinking.  One third of your fuel out, one third of your fuel back, and one third in reserve.  

Good thinking.

I've been known to follow the one half, one half method. Which means I'll eventually get a workout with our emergency oar  ;D

1 hour of running is more than most people do in one weekend  

Another thing to remember is the one third, one third, one third way of thinking. One third of your fuel out, one third of your fuel back, and one third in reserve.

Good rule of thumb!  Always remember to factor Murphy's Law into the equation.

Also, and I'm sure you are aware of this already, your boat will run more efficiently when it's "on plane" and pushing less water.

I think you will be shocked at how long you can go on 6 gallons.  The boats I have used with portable 6 gal tanks (and there have been many) will go a LONG way.  

I would fish a bit with it before buying a second tank.  You may fish all day, but the motor may not run more than 45 minutes.

I have an 1983 50hp Merc with a 6 gallon gas tank.  From my experiences, I have never used more than 4-5 gallons in one day.  Like flechero said, you will be shocked how long 6 gallons lasts, especially with your 35hp motor.

My Tracker with the 50 HP Merc does not burn that much gas.

However, please follow the rule James' said as it is the one the pros at the old Bassmaster University told us to use during one of their seminars.

I have a 25 HP merc with a 6 gallon tank. I burned 3 gallons in 1 day (Lots of WOT, pretty long lake). I just went out again today for 1.5 hours doing waypointing and did alot of WOT and still have 1 gallon left)  again, you'll be suprised how long a 6 gallon will last.