Fuel efficiency of outboard motors for fish/ski boats?

Question

I got a 16' bass boat this summer and I am astounded at how much gas it uses. I use 4-5 gallons everytime I go out fishing for a couple of hours and my motor is a 1990 Mercury 115 (2 stroke).

I am considering getting a new boat and am wondering about fuel economy. My guess would be that smaller engines will burn less fuel and that newer engines (2000 or newer) will be more fuel efficient than older engines. However, I want to put my assumptions to you experts.

1) Will lower horsepower outboards get better "mileage" than bigger ones?

2) Will newer engines get better mileage than older ones?

3) Do 2 stroke engines getter better mileage than 4 stroke engines?

Any other insights that you have would be appreciated?

Answer 1

For many years the standard for determining approx fuel consumption for 2 stroke outboards was 1gal/hr for every 10 hp. Basically a 200 HP would burn 20gal/hr AT FULL THROTTLE.

With today’s newer engines fuel consumption is notably lower. 2 stroke direct injected engines tend to burn far less fuel at idle than there older counterparts. They also save notably on fuel at higher RPMs but keep one thing in mind. It takes FUEL to make horsepower! A new 200 hp engine will burn less at full throttle than an older version of the same size but the difference is not as substantial at the top end of the RPM spectrum as at idle speeds.

Four stroke outboards tend to burn cleaner than older two strokes as well. Once again the savings are much more notable at idle than at high speed.

One very important factor in efficiency is power to weight ratio. At low speeds a heavy boat and a light boat will burn very similar amounts of fuel. As you accelerate the vessel; however, the amount of thrust required to get a heavy boat "on plane" increases greatly over that of a lighter boat. As a result the heavier boat will burn more gas.

Both engine types 2+4 stroke offer advantages and disadvantages.

At extremely low speeds (in my experience) most DI two stroke motors will get better mileage than their 4 stroke counterparts. As RPMs increase into the midrange DI two strokes start to loose mileage to the 4 strokes. Once you have the boat up and running I think the two strokes will eventually even out the mpgs as you approach full throttle.

I think 4 strokes match up very well with smaller aluminum boats and pontoons. They don’t really have the torque to hold a large plaining boat "up" without requiring more fuel to offset their increased weight and lack of torque.

2 strokes are matched with faster + heavier vessels where the thrust to weight ratio is most advantageous.

If you are going to look at new boats check out some of the manufacturers info on mileage vs. their competitors. They all "fudge" their answers a little but you should be able to estimate the trends.

The 90 Merc 115 wasn't a very fuel-efficient unit so I think you'll find quite an improvement with either of the newer tech engines. Just don’t ask for too much! It takes fuel to make power! Also remember you can plane a boat easier with a large motor and then pull the throttle back into the midrange to decrease fuel use while still keeping it on plane where a smaller engine would have to be held at full throttle to hold the same speed.

Whatever you do talk to your boat dealer about other options for fuel economy. Props can make a big difference as can loading and weight distribution.

Hope I didn't muddy the water too much

Answer 2

Generally when talking about fuel usage on a boat you talk about gallons per hour at specific RPMs.  You will find the newer outboards, especially the four strokes, are much better on fuel usage then the older two strokes.  Lower horsepower engines will not necessarily get better fuel economy, if the engine is too small for the boat then it will have to work harder to go the same distance and therefore use more gas.  Four stroke engines get better gas millage, but they also weigh about 20% t0 30% more, so you can loose some performance.  Some of the newer two stroke technology (such as Evanrude’s E-Tech) are coming close to matching the four strike’s fuel performance without the weight or maintenance issues of a four stroke.  Also you didn’t say, but I suspect that a 1990 115 may not have automatic oil injection, which means you premix the oil and gas.  This can reduce fuel economy; the mixture is made for the worst conditions of a motor running (high speed or RPMs) so at slower speeds your mixture will be heavy on oil.

Another factor in outboard performance is the installation of the motor, too high and you can cavatate, too low and increased drag which reduces fuel performance.  An outboard mechanic can check to see your motor is mounted correctly.  And although you don’t want too small a motor for your boat oversizing will also cost you more fuel, and most of the time boats don’t have to go fast to get to the fish.  So unless you want to fish big money tournaments I would recommend you follow the boat manufacturer’s recommendation for the smallest horsepower for whatever boat you get.  Good Luck!

Answer 3

Buy the most HP you can afford for the boat. Smaller motor will work harder therefore less efficient. I would look at a 115 Merc optimax. Evinrude ETec or a Yamaha 2 stroke. These are the best in 2 stroke technology there is and as fuel efficient as any. 2 stroke are lighter in weight than 4 strokes and on a 16 ft that might be a consideration.

Answer 4

Deal with it. It's not like putting rubber to ashphalt.

The power transfer is inefficient at best.

It's a decadent pastime this playing about on the water.

answers to your 3 questions...

1. no

2. yes

3. in some cases, yes.