fuel useage comparison i/o vs ob

[22E6441]

Trying to figure out if an 1990 Mercury 115 ob would be much worse on gas than a similar year small mercruiser i/o, say 130 or so. Probably carburated i/o but vs a 2 stroke ob would there realy be a huge difference? typical usage is tubing wakeboarding and cruising.

Thoughts? would an i/o be drastically significantly different?

 

[NYBo]

Re: fuel useage comparison i/o vs ob

The I/O will weigh more, which will offset most of the advantage of 4 stroke over 2 stroke in this area. A fuel injected outboard, however, will likely be more fuel-efficient than a carbed I/O.

 

[UncleWillie]

Re: fuel useage comparison i/o vs ob

A 2 strobe is less fuel efficient than an equivalent 4 stroke.
However the 2 stroke will be a smaller and lighter package.

Here is a comparison of an 135hp OB vs. a 135hp I/O. Both 4 stroke.
It isn't Apples to Apples though.
The OB boat was heavier and fuel injected, the I/O is lighter and Carbed.
Compare the GPH columns




Pick different boats and the winner will change.
There is really no really glaring difference. 135hp is 135hp no matter how it was produced.

Comparing a 115 OB 2 stroke to a 130hp I/O 4 stroke is comparing Apples to Meat Loaf!

 

[Bamaman1]

Re: fuel useage comparison i/o vs ob

The 4 cylinder 140 hp I/O will run more like an 80 hp 2 stroke outdrive. It won't have much power off an idle, but most I/O's will cruise pretty fast. They'll also get very good fuel mileage--vs. the 2 stroke 115 hp motor. It might be the difference between 5 mpg vs. 6.5 mpg, but the 2 stroke will also require 2 stroke outboard motor oil.

But if I had a choice, I'd probably go with the 2 stroke if I was doing some water sports or skiing. My last I/O lasted 10 years, where my 115 hp Yamaha 2 stroke is running good @ 26 years old. Marine salvage yards are full of junked I/O's--many of which the hulls are perfectly good. You can re-engine a hull with another outboard.

 

[dozerII]

Re: fuel useage comparison i/o vs ob

The 4 cylinder 140 hp I/O will run more like an 80 hp 2 stroke outdrive. It won't have much power off an idle, but most I/O's will cruise pretty fast. They'll also get very good fuel mileage--vs. the 2 stroke 115 hp motor. It might be the difference between 5 mpg vs. 6.5 mpg, but the 2 stroke will also require 2 stroke outboard motor oil.

But if I had a choice, I'd probably go with the 2 stroke if I was doing some water sports or skiing. My last I/O lasted 10 years, where my 115 hp Yamaha 2 stroke is running good @ 26 years old. Marine salvage yards are full of junked I/O's--many of which the hulls are perfectly good. You can re-engine a hull with another outboard.

You can't re-engine a hull with a new I/O????
I think so!! and if your talking new, then the I/O wins hands down on price

 

[H20Rat]

Re: fuel useage comparison i/o vs ob

But if I had a choice, I'd probably go with the 2 stroke if I was doing some water sports or skiing. My last I/O lasted 10 years, where my 115 hp Yamaha 2 stroke is running good @ 26 years old. Marine salvage yards are full of junked I/O's--many of which the hulls are perfectly good. You can re-engine a hull with another outboard.

???

Repowering an I/O is FAR cheaper than repowering an outboard... I can find a reman marine 350 with 300 horse or so for about $3000, give or take $1000 depending on model and options. I challenge you to find a comparable new outboard for that price, not gonna' happen!

 

[Chris1956]

Re: fuel useage comparison i/o vs ob

I traded my '88 18 foot bowrider with a 4.3LV6 Mercuriser for a 21 foot bowrider and a 150HPV6 carb outboard. The 21 footer has lots more room in it(longer and wider, plus no motorbox), and performance is about the same at WOT, but the OB has better acceleration, but burns about twice the fuel.

Of course maintence of the OB is a whole lot less than maintenance on an I/O, especially in salt water. Maintenance on an I/O can be $200/year more than an OB, if you amortize the cost of replacement manifolds, risers, thermostats and the other things that break or wear out on an I/O versus the few things that need replacement on an OB.

 

[Silvertip]

Re: fuel useage comparison i/o vs ob

Do not compare gallons/hour at xxxx rpm. Doing so skews the real economy number. Always compare fuel consumption at the same SPEED. Once on plane, the I/O is 8 MPH faster than the outboard at the same RPM. Therefore, look at the I/O at 3000 RPM which provides 23.2 MPH and at that speed is buring 4.4 gallons of fuel per hour. The outboard needs to spin nearly 4000 RPM to make the same speed and at that RPM is burning 4.5 gallons per hour. If you look at the I/O at 4000 RPM it is pushing the boat at 33.5 MPH and buriing 6.8 GPH. At 5000 RPM the outboard is at 32.6 MPH (less than 1 MPH slower) but is burning 8 gallons per hour. So RPM vs GPH is not a good comparison. Fuel consumption at the same SPEED is a valid comparison. The long and short of this is not about EFFICIENCY -- it is about ECONOMY, and they are not the same. A boat that burns 5 gallons per hour but only goes half as fast as a boat that burns 10 gallons per hour will consume the same amount of fuel over the same distance because it takes twice as long to get there. Your cost is the same either way.

 

[UncleWillie]

Re: fuel usage comparison i/o vs ob

Re: fuel usage comparison i/o vs ob

Do not compare gallons/hour at xxxx rpm. Doing so skews the real economy number. Always compare fuel consumption at the same SPEED. Once on plane, the I/O is 8 MPH faster than the outboard at the same RPM...

+1 Agree. Which in a convoluted way is saying compare MPG at the same speed.

The I/O at 33.5mph is getting 5.03 mpg.
The OB at 32.6mph is getting 4.08 mpg.

The economy cruise speed is just after the hull comes on plane.
The I/O planing at 23.2 mph is getting 5.26 mpg.
The OB planing 15.6 mph is getting 6.54 mpg.

This is mostly a matter of weight and hull design.
Add Tabs to the i/o and you can lower the planing speed and improve mpg.


FWIW. The OB is a 25ft toon and the I/O is a 20ft bowrider.
Finding stats on the same hull with I/O and OB options with the same HP is hard to come by.

 

[greenbush future]

Re: fuel useage comparison i/o vs ob

My experience is that overall 2 stroke outboards are the cheaper route in just about every aspect, except buying new. They are pricey, but I dont buy new ones, and really enjoy the quick winterization that takes me 10-20 minutes. No shop work ever and my outboards just keep on running, they dont break often, at least none of mine do. Only I/O's I have owned would run a few hundred every year on small but important items. Bellows, Gimble. winterizing every year!

 

[flightdoc]

Re: fuel useage comparison i/o vs ob

Comparison based on speed doesnt make any sense. Of course different boats are going to be more economical at different speeds, due to hull design and the prop on the back. RPM is accurate for comparison of the motor it's self. That is the point where they are performing at the same level. Output is always going to be different. Same as a car. A V8 in a mustang will be economical at a higher speed due to different gearing and power output but sucks in the city. Where as a car with a small 4 cyl engine is far more economical in the city but try to get it up to a high speed and that car has to work harder to maintain a higher speed.

Now looking at the RPM of the V8 and the 4 cyl. the 4 cyl will always be more economical.

That being said you will want to see what motor is going to perform more to your liking. I have a 120 outboard and for pulliing little tubes it works just fine but when I get my big 4 man tube out it really has to work hard. Where as a bigger motor will definitly pull better. Personally I would look at what motor is going to perform better for what you want to do and then look secondary at what gas consumption is going to be. The same motor is going to have different consumption figures depending of you are just out cruising, pulling a skier, tuber, wake boarder.