I/O vs. Outboard Fuel Consumption

[Cherokee-Runabout]

I had a couple quick questions about I/Os vs. outboards. I've read up on outboard fuel consumption, and from my own observations, it seems that an outboard will burn approximately 10% of the rated HP in gallons per hour (GPH) at WOT. So does this hold true for an I/O? Or is the fuel consumption closer to that of a car with a similar engine? How much difference in consumption could I expect between a 4cyl and 6cyl? I understand that a good deal of this depends on a hull design and shape, but for comparison we'll say that they are all attached to the same hull. Thanks for any input you have.

[NYBo]

There are far too many variables to give you a simple answer. But one thing IS true: The best fuel economy in a boat will be near the worst fuel economy of any car. (Think Lamborghini and 12' tinny with a 10 HP four-stroke outboard.)

[smokeonthewater]

If you do a search this gets asked at least once every 3 months.... The 10% rule of thumb carries to pretty much all engines with some variance + or- Even your car, if it makes 100 hp will use close to 10 gallons of fuel in an hour if it is operated at full throttle for the whole hour such as if you were towing 5 more cars up a steep hill the entire time... the huge difference between cars and boats is that a car can run 60 mph all day at 15-30% throttle and a boat of about the same size as the car with the same engine might do 45 mph at 100% throttle.

For a comparison that you can feel, walk 100 feet on a side walk, then walk twice as fast 100 feet.... next do the same in a swimming pool.... you'll find that the fast walk is more than you can reasonably do

On dry land, under about 50 mph, double speed takes double hp.... in the water double speed might take 10x hp

[Don S]

Gallons Per Hour (GPH) tells you nothing but how many gallons you used in an hour.
For it to mean anything, you have to turn it into miles per gallon. If you burn 10 gph at 4000 rpm, that doesn't tell you much. But, if you go 40 miles in that hour at 10GPH, that is a lot better than if you only go 20 miles in that same hour at 10 GPH.

Now you get to take a boat with an outboard at 10 GPH and see how many miles you get compared to a IO running 10 GPH.
Oh, and different size engines on different sized boats and different rpm, all makes a huge difference in the MPG, while all still burn that same 10 GPH.

Clear as Mud, right????

[MH Hawker]

That`s one of those things you have to get past real quick. A boat will suck down a lot of fuel period. The bigger the motor the more it burns.

[steelespike]

I usually say a marine engine uses fuel at a rate of "about" 10% of the hp at wot whether pushing a houseboat or a runabout.
mpg is achieved by getting the best speed for the setup.

[Mischief Managed]

On dry land, under about 50 mph, double speed takes double hp.... in the water double speed might take 10x hp

Actually, the rules for aerodynamic drag and hydrodynamic drag on a planing hull are pretty much the same. Double the speed takes 4 times the HP.

[cyclops2]

I do not think a passenger 5.0 L Chevy car engine can EVER come near a 5.0L Mercruiser engine on fuel consumption.

Cars simply do not get a high enough LOADING of the engine long enough............The transmission would die.

Boats do get the engine fully loaded.

To answer your topic. The testing done by marine engine companies are PROBABLY as truthfull as automobile companies.

Is a 2 stroke better or worse ? Who COULD perform a honest unbiased test ?

2 strokes do NOT have a true camshaft. So they can not vary the valve timing for better power or mileage.............4 strokes can vary the camshaft timing. & most do.

[smokingcrater]

Actually, the rules for aerodynamic drag and hydrodynamic drag on a planing hull are pretty much the same. Double the speed takes 4 times the HP.

Except at slower speeds the losses in the tires and transmission are bigger factors than aerodynamic drag. At some point the graphs cross over and aerodynamics become the major influence. (I'd have to guess its probably below 50 mph, but its going to be vehicle specific.)

[smokeonthewater]

Except at slower speeds the losses in the tires and transmission are bigger factors than aerodynamic drag. At some point the graphs cross over and aerodynamics become the major influence. (I'd have to guess its probably below 50 mph, but its going to be vehicle specific.)

what he said.... from say 5 mph to 10 mph is a negligible difference in a car but in a boat it is HUGE

[smokeonthewater]

2 strokes do NOT have a true camshaft. So they can not vary the valve timing for better power or mileage.............4 strokes can vary the camshaft timing. & most do.

not entirely true... ever hear of a power valve? I don't know first hand of one on a marine engine but two stroke engines can vary "valve" timing

So far I have only ever owned one 4 stroke engine with varible valve timing... it is in my bmw.... I've never even seen any boat up close with variable valve timing

Anyhow for the O/P's question... there isn't enough difference from engine to engine w the same hp to get excited.... sure a brand new 4 stroke outboard with fuel injection will use less gas than a 20 year old 2 stroke or I/O of the same HP but the cost of the engine will far outweigh any fuel saving for about the first 80 years of ownership for the average user.

[Tail_Gunner]

A few thing's to consider...a outboard engine has had a lot of RD that be research. Each mfg is highly competitive so the bench mark is always being moved and the price point..better econ and power yes. In additon the leg on a OB is far more hydrodynamic and can be placed to a Optimum setting to each individual boat. Combine those both and you have your answer..but at a price sometimes outrageous ..but that's a opinion.


75 HP MERCURY OPTIMAX $6,875 200 HP MERCURY OPTIMAX COUNTER $13,425
90 HP MERCURY OPTIMAX $7,325 225 HP PRO XS MERCURY OPTIMAX $14,850
115 HP MERCURY OPTIMAX $7,650 225 HP PRO XS MERCURY OPTIMAX TORQUEMASTER 20" $14,900
115 HP PRO XS MERCURY OPTIMAX $7,975 250 HP PRO XS MERCURY OPTIMAX 20" $15,650
125 HP MERCURY OPTIMAX 20" $8,325 250 HP PRO XS MERCURY OPTIMAX TORQUEMASTER 20" $15,725
125 HP MERCURY OPTIMAX 25" $8,550 250 HP MERCURY OPTIMAX 25" $15,675
150 HP PRO XS MERCURY OPTIMAX $10,375 250 HP MERCURY OPTIMAX COUNTER 25" $15,825
175 HP PRO XS MERCURY OPTIMAX $11,675 250 HP PRO XS MERCURY OPTIMAX 25" $15,775
200 HP PRO XS MERCURY OPTIMAX 20" $12,450 250 HP MERCURY OPTIMAX 30" $16,075
200 HP MERCURY OPTIMAX $13,075 250 HP MERCURY OPTIMAX COUNTER 30" $16,225.............................

[smokeonthewater]

Yup... $15,000-$20,000 will buy a WHOLE FREAKING LOT of gas........

[Cherokee-Runabout]

First off I love the banter on these boards. Second off, comparing effeciency between say a '77 Johnson 70 and an '86 170 Mericruiser I understand that obviously the 70 will burn less fuel per hour. But, in the 80's did outboards not become more efficent? I feel that with the energy crisis of the late 70's people were more focused on more economic engines and less gas consumption coinciding with higher fuel prices. Also "Gallons Per Hour (GPH) tells you nothing buy how many gallons you used in an hour.
For it to mean anything, you have to turn it into miles per gallon. If you burn 10 gph at 4000 rpm, that doesn't tell you much. But, if you go 40 miles in that hour at 10GPH, that is a lot better than if you only go 20 miles in that same hour at 10 GPH."
I realize the difference between MPG and GPH, MPG is determined by the speed and efficency of the engine, and the efficency of the engine can effectively be rated by its GPH consumption. But this isn't an economic forum nor a pissing contest, so to cut to the point you're all saying that the 10% rule of thumb will still apply to an I/O regardless of whether it's a four-banger or a straight six? Sorry I'm 23 and have somehow developed this money-eating hobby.

[smokeonthewater]

Sadly yes.... the engines got more efficient... that let a certain boat get 3 mpg instead of 2.8 mpg

[Cherokee-Runabout]

Lol, they got us there eh?

[JoLin]

Forget any comparison of a boat to a car. There's simply no commonality between the two. Having run three very different types of boats, the simple answer is that whatever you run you'll use a lot of fuel. Here are some real world examples (mine) with engines in good state of tune. My normal, comfortable cruise speed regardless of the boat I'm running always seems to be the same and is GPS-based, so it's a good basis for comparison. I never had a fuel flow meter so calculations are approximate, but they're very close. I'm kinda geeky and spent a lot of time on this :-)

21' ProLine Walkaround cuddy with carbed 175 hp Johnson 2-stroke outboard
- normal cruise speed app. 25 mph @ 4000 rpm
- consumption at cruise 9-10 gph.

24' Chris*Craft express cruiser with single fuel injected 5.7 V-8 (280 hp) and duo-prop I/O
- normal cruise speed app. 25 mph @ 3200-3400 rpm
- consumption at cruise 10-11 gph

27' Four Winns express cruiser with dual fuel injected 4.3 V-6's (205 hp each) and single-prop I/O's
- normal cruise speed app. 25 mph @ 3200-3400 rpm
- consumption at cruise app. 16 gph

The big surprise for me was when I went from 21' to 24'. The increased efficiency of the fuel injected 4-stroke and duo-prop outdrive in the bigger and much heavier boat made it a wash in terms of fuel cost. I spent months double checking my calculations.

Hope this helps a little.

[jigngrub]

Quad carbed 50 hp Merc 4 stroke @ 32 mph divided by 5 gph = 6.4 mpg.

[coolbri70]

fact, 2 stroke engines are not fuel effecient by design. fact, 4 stroke engines are heavy by design. in a boat weight cancels out hp and uses more fuel to do it so i'm thinking either way boating aint cheap

[cyclops2]

Well we all seem to agree.

Marine engines of all types are never going to deliver anything close to a car on MPG.

They also have such horrible shapes that EAT horsepower.

They usually are overloaded with people & gear having tons of fun. Which no car can come near to doing.

So do not bother with a MPG test.

If it carries your friends safely & laughter is plentifull ? The engine is delivering fantastic MPG.

Rich

[Bubba1235]

Boats and fuel economy?


You're kidding right?

[NHGuy]

Cyclops hit it right on the head. In a boat or your cool Saturday night cruise car you are looking for smiles per gallon.

ps. Happy Holidays & Merry Xmas

[dingbat]

Fuel is the cheap part of boating. If you're worried about the fuel cost, you better look for a different hobby

[cyclops2]

Ding bat has me pegged.

I use MAYBE 40 to 50 gallons of regular a year in the Chaparral SSI 5.0 L.
The 16' Lowe with a 9.9 hp & electric start. 8 gallons a week. May 10 thru September 15. Weather permitting. No typing error.

[smokeonthewater]

Fuel is the cheap part of boating. If you're worried about the fuel cost, you better look for a different hobby

Strange... fuel has always been the single biggest cost for me...... from my first 12' tinny 25 years ago to my 30' cruiser now, fuel has always been the one thing that hurts the wallet more than anything else.

With my current boat, if I just ease out to the island that I can see from the ramp, and spend the day before easing back, it still costs me $50 and if I pull tubes it's $200- $500...... I pretty much never pull tubes.