What size gas tank?

The question is 3 or 6 gallon gas tank. I am planning to put a 9.9 or 15 HP motor on my boat. How much gas does one of these motors use? To save space I'm thinking 3 gallon, but I don't want to ever run out. My last boat had a 50 HP Mercury and I typically used around 4 to 5 gallons per trip. I would assume a 9.9 or 15 gets much better mileage than a 50 HP. Thoughts or recommendations?

I run a 3 gallon tank with my 9.9 Honda 4-stoke on our 1648 and have never come close to running out.

I started with a 6 gallon tank on the rig below. Switched to 3 since I never came close to running out, even with long runs.

rickybobbybend said:

I started with a 6 gallon tank on the rig below. Switched to 3 since I never came close to running out, even with long runs.

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You got the right idea...cva34

I had the same question last year when I upgraded from a 9.9 2 stroke with 6 gallon tank to a 20 HP 4 stroke. Everyone told me the 4 stroke would use less gas than the 9.9. I went with the 3 gallon and can run several days without needing to refill. I don't run all day long, but go to a spot, and usually drift or anchor and fish. There is probably gas consumption numbers somewhere online. One member told me they run a 3 gallon tank and keep a half or one gallon can in the front of the boat just in case. He said he has never needed to use the backup can, but it is there for piece of mind. So, that is what I do when I go to a bigger lake and plan to be using the motor alot. I have a half gallon can I bought at some point for my chainsan. I just dump whatever is in it (reg or mixed) into the truck and put fresh in each time I need to use it. I am very glad I went with the 3 gallon, so much easier to find a spot to put it in the boat.

The old rule of thumb for outboard motors was one gallon of gas per hour running time for every ten hp. So your 9.9 will have three hours run time with a 3 gal tank. That formule is for wide open throttle, if just putting around you can double that time frame. However there are many factors that increase fuel usage such as heading into a strong wind, choppy water, heavy load in the boat. I always figured a third to get out and a third to get in and a third for safety when I calculate how much fuel I will need. Hope this helps.

Airshot

I had a 6 gallon. Never ever came close to running out. I switched to a 3 gallon. Still haven't come close to running out. I am running a merc 20 four stroke

I use a 12 gallon tank. 1648 with a 25hp motor. I sometimes go quite a ways down river so the extra fuel is nice.

I like having lots of gas. Being stuck out of gas on the water is different from sitting on the side of the road. If I decide to make a long run I like knowing I have plenty to get there and get back home.

Can carry a backup can for long runs. If I know I will be running a king ways I will swap the 3 for the 6

I go on a lot of trips where I'll be using the boat for multiple days in a row ..... and re-fueling is sometimes a hassle ..... so I always get the largest tank that will fit nicely in the boat.

If I only ever fished one day at at time, a 3 gallon would be fine for just about any motor smaller than 25HP.

Thanks to all of you for your advice. I think I'm going with the 3 gallon. I can take a back up when needed.

I make a 90 mile run every now and then on twin 6-gallon tanks . . . . 15HP 2-stroke on 14' Lowe semi-V.

airshot said:

The old rule of thumb for outboard motors was one gallon of gas per hour running time for every ten hp. So your 9.9 will have three hours run time with a 3 gal tank. That formule is for wide open throttle, if just putting around you can double that time frame. However there are many factors that increase fuel usage such as heading into a strong wind, choppy water, heavy load in the boat. I always figured a third to get out and a third to get in and a third for safety when I calculate how much fuel I will need. Hope this helps.

Airshot

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Thats the way I do it Kinda With whole numbers ; put a (.) before last diget..And your in the ball Park 10hp=1.0 gph
100hp= 10.0 gph
300hp=30.0 gph
just another way to skin a cat..all above info is correct and is quite close for normal boat;;; X-2 for airshot