Fuel consumption difference between a small engine and big engine on the same boat

[shingo]

If we take for instance, a light aluminium 14' fishing boat of around 250#

Is there a big difference in fuel conssumption between let's say a 6hp, 9.9hp and 25 hp ( all in 2 stroke )

If we test the same boat at the SAME SPEED , will the fuel consumption be similar or will there be a big difference between the different size engine?

I know for exemple, my car, with the 150hp and 300hp version, if you go at 60mph, the difference in fuel conssumption difference is almost non existent.

Thank you

 

[blackhawk180]

Re: Fuel consumption difference between a small engine and big engine on the same boa

Obviously, the 6hp is not going to push your boat anywhere near as fast as the 25 so the top end speed is not possible to compare. None of those sizes is a real 'fuel gulper' and I doubt you'd notice much, if any difference between the 6 and the 9.9. I'd guess the 25 will burn more at minimum plane but has the capability of going so much faster should you want or need to. 9'9's are max hp in many lakes and will be OK for trolling and probably plane out with 2 in the boat.
In my experience, I burned more gas in the truck getting to the lake than I ever did trolling around all day with either a 6 or 9.9. No real experience with the 25... I'm sure someone else can help there.

 

[Frank Acampora]

Re: Fuel consumption difference between a small engine and big engine on the same boa

Your defining factor was "At the same speed." It takes a given amount of horsepower to move the hull at a given speed. It takes a given amount of fuel to generate a given amount of horsepower. So: at the same speed, fuel consumption would be close with all three engines, factoring in the added weight of the larger engine causing a little more consumption.

 

[LongLine]

Re: Fuel consumption difference between a small engine and big engine on the same boa

To really determine it you'd have to consider the rpm achieved at that given speed as well as the size and the number of cylinders. If the little engine achieves 15 mph at 1500 rpm yet the bigger achieves 15 mph at 900 rpm then yes they'll probably be very comparable. But if the big engine requires 1500 rpm then no way will the consumption be equal. On each intake of the rpm, the bigger cylinder sucks in more gas.

I fish a lot and have two engines on my boat. A 75 Hp 3 banger & an 9.8 2 banger (both 2 stroke). On a typical outing I can troll about 7 hrs with the 75 on approx 7 gal. Trolling at the same speed for the same time with the little guy takes about 1 1/2 gal max.

Tom B.
(LongLine)

 

[Silvertip]

Re: Fuel consumption difference between a small engine and big engine on the same boa

Look at it this way. A 9.9 HP engine at WOT will burn about 1 gallon of fuel per hour and on that boat let's say top speed is going to be 15 MPH. So in one hour you travel 15 miles and burn one gallon of fuel. On the other hand, the 25 HP engine burns 2.5 gallons of fuel per hour at WOT but it pushes the boat at 30 MPH so that same 15 miles is covered in half the time (1/2 hour). So to cover the same 15 mile distance as the 9.9, the 25 HP engine would burn 1.25 gallons of fuel and you would get to your destination in 1/2 the time. Now then -- is .25 gallons of fuel worth another 1/2 hour on the trip. The lesson here is that a big engine loafing can be just about as economical as a small engine working its guts out.