Outboard motor lifetime fuel usage and emissions

[Jammer]

Lifetime fuel usage and emissions for dinghy-sized outboard motors are considerable and are often overlooked considerations when determining size and 2-stroke vs 4-stroke.


Current production 4-stroke outboards, at wide-open throttle, burn 0.1 gallon of gasoline per hour per horsepower. This is true across all brands and sizes up to about 25 horsepower, within about 10%, with some slight improvements in fuel economy in larger sized outboards. The useful life of the powerhead is approximately 2000 hours at which point the rings are shot and many other components heavily worn. Outboards in dinghy service are ordinarily operated at wide-open throttle most of the time.


Some arithmetic gives us a lifetime gasoline consumption figure of 1980 gallons for a 9.9 hp motor and 3000 gallons for a 15 hp motor.


The Yamaha Enduro E15DMH, a widely used 2-stroke 15 hp dinghy outboard in the Caribbean, uses 7.7 lph at WOT (see for example https://yamaha-marine.cami-cfao.com/...MI_en_BD-1.pdf), just over 2 gallons per hour. Over a similar 2000 hour lifetime it will burn 4000 gallons of gasoline.


The lifetime difference in fuel consumption between a 2-stroke and 4-stroke engine, at 15 hp in each cases, is therefore around 1000 gallons.


Oil wise at 50:1 the 15 hp Enduro will burn 320 quarts of oil (4000 gallons fuel divided by 50, 80 gallons = 320 quarts) over its useful life. A 15 hp 4-stroke will require 20 oil changes (one every 100 hours), at 1 quart each, a total of 20 quarts of oil over its useful life. Just looking at the economics of it, oil is around $5 a quart at Walmart and oil filters are around $10. So the 4-stroke will need $100 of oil and $200 of filters for a total of $300 over its useful life. The 2-stroke will need $1600 of oil, a $1300 higher maintenance expense.


If we combine the oil difference, and the difference in gasoline use of around 900 gallons (reduced from 1000 gallons because 80 gallons of the fuel burned is actually oil in the two-stroke) at $4 a gallon (=$3600), we end up with a $4900 higher lifetime operating cost for the 2-stroke.


This is not a small difference, and is often overlooked in discussions of 2-stroke vs. 4-stroke . The difference in operating cost far exceeds the dollar cost of any conceivable differences in maintenance costs or initial purchase price.


There is also the environmental impact as the 1000 gallons of extra gasoline and oil used by the 2-stroke ends up going out the exhaust unburned, into the ocean or whatever other body of water you're operating on.


At reduced throttle, there is some loss of efficiency with any outboard with the newer EFI 4-stroke motors having relatively little loss of efficiency and the 2-stroke motors having a nearly 2:1 loss of efficiency at trolling speeds.


As a point of reference, a typical small sedan gets 35 MPG and has a useful life of 200,000 miles giving a lifetime fuel consumption of about 5700 gallons.

[Benz]

That was super-interesting. Thanks for posting. I ran my 4-stroke, 8hp Yamaha OB on my big boat (I row the dinghy), and it lasted 14 years cruising nearly as many thousand miles. At $2500 new in 2009, I reckon I got good value out of it.
Just replaced it with an identical engine but 9.9 hp (they no longer make the 8) for $3600, so I need to stretch it out a little more to get my value.
One day the diesel outboards I'm starting to see around might get small enough to use on my boat. I'd love to see the efficiency breakdown of those....

[Jammer]

I had a 2010 Yamaha 8hp for four years on my previous 24-26 foot sailboats. Great outboard. I sold it with the second boat, and when I went to get an outboard for my 14' aluminum utility boat, there were no Yamaha 9.9s in stock anywhere, so I ended up with a Honda. So far, it's been fine, too.


Small diesels (~10 hp) are around 0.06-0.07 gph per hp. Most normally aspirated sailboat auxiliaries (inboards) are 0.05-0.06 gph per hp. Turbocharged diesels, including the 150 hp diesel outboards that have recently come on the market, burn around 0.045 gph per hp in the top half of their output range.

[rslifkin]

0.1 gal/hr per HP or 10 hp-hr per gallon is about a worst case for a modern 4 stroke. Depending on the engine, you may see closer to 12 hp-hr per gallon, especially at a bit less than WOT. I know my 6hp Tohatsu (actually a Nissan NSF6C) is in that ballpark. Manufacturer rated fuel consumption is 0.5 gal/hr at WOT, and I've seen a couple of tests online that put the actual burn at WOT around 0.55 - 0.58 gal/hr, so closer to 10.5 - 11 hp-hr per gallon at WOT (the manufacturer figure would give 12 hp-hr / gal, but that's probably optimistic).

Diesels will typically be in the 15 - 17 hp-hr/gal range for small non-turbo diesels. Turbodiesels will typically be in the 18 - 20 hp-hr/gal range with some modern stuff (and some really big, slow turning engines) pushing a little above that 20 mark. Diesels also have a fairly flat efficiency curve in many cases, while gas engines lose a lot of efficiency when lightly loaded.

Interestingly, I've done some math on fuel economy for our dinghy (a 12 foot aluminum Starcraft with a 6hp Tohatsu). All weights listed are additional weight beyond the ~250 lbs base weight for the dinghy, outboard, fuel, oars, and anchor.

With a ~180 lb load (just me), it'll do ~12 kts at slightly less than WOT (WOT with 1 person will hit the rev limiter with the current prop), so fuel burn is likely right around 0.5 gal/hr. That's 24 nautical miles / gallon. Going up an inch of prop pitch gets it up to 13 kts with just me, but leaves it a little overpropped for heavier loads. That wouldn't change fuel economy noticeably it looks like.

Increase the load to ~400 lbs and speed at WOT is down to ~9.5 kts, so we'll go with the worst case burn of 0.58 gal/hr. That gives a little over 16nm / gal.

With a heavier load, the worst speed I've seen at WOT was about 8.5 kts, so worst case is about 14.5nm / gal (this is already slightly over the rated weight capacity for the dinghy).

I don't have good fuel consumption data for lower speeds, but based on the little bit of data I've found, slowing down to ~4 kts should get fuel economy up to at least 30nm / gal.

In general, I'd call that pretty efficient as boats go.

[Jim Cate]

Outboards in dinghy service are ordinarily operated at wide-open throttle most of the time.

Jammer, thanks for an interesting analysis of outboard fuel consumption and costs.

The problem with it for me is the above statement, for it is really far from what my usage and that of other cruisers that I have observed over the years really is and it is pivotal for your analysis.

Perhaps for those folks with small motors on their dinks, operating well below planing speeds it might be so. However, with the common 10-12 foot inflatable/RIB and 10-15 hp motor WOT operation is infrequent (and in my case virtually never approached) for it results in speeds that are excessive for harbour usage, safety and economy.

For us, our Yamaha 15 two stroke is now 21 years old and has been in near daily use for all that time, for we are full time liveaboards who anchor out (or use our newly purchased mooring) nearly 100% of the time. I couldn't estimate the number of hours on the motor, but would guess that it exceeds your 2000 hour lifetime estimate by a considerable margin. I suspect that if an engine will last 2k hours at WOT that its life will far exceed that when operated at 1/4 of that output, which is what I guess our normal cruising throttle might be. We have found that minimum planing speed offers decent progress, quiet operation and good (relative) economy. Around the anchorage we mostly go below planing speed... rules specify <6 knots when near moored/anchored vessels.

I'm not sure about how that influences engine efficiency, but I am sure that it reduces the specific fuel consumption (gal/hr) enormously, and hence the annual totals. The lifetime totals... well, I dunno if that is a useful metric for most of us, for I suspect that few actually run an o/b for its entire life, especially those who are not full-timers.

At any rate, I appreciate your inputs on this subject. Most yotties don't really consider the impact of their activities nor the true costs of what they enjoy doing... so Well Done!

Jim

[Kettlewell]

I'm also in the camp that rarely operates at WOT, and I also row our 10-foot hard dink a lot. This makes our current motor, a 4HP 4-stroke Yamaha, just as useful to us as the previous 8HP 2-stroke Johnson that served us faithfully for about 17 years. The Yammy burns a lot less gas than the Johnson, though I haven't measured how much. Another cost factor is repairs and maintenance beyond oil and fuel usage. The Johnson never required any repairs other than changing the spark plugs a few times. Prior to that I owned an early 9.9 4-stroke Yamaha that was the main auxiliary for our 32-foot cat. That engine did operate a lot at WOT for the more than 12 years we owned it and it routinely burnt about 1 gallon per hour for 5 knots pushing a lot of boat around. But, doing all that hard work also meant more expensive maintenance, like having to replace the seals on the prop shaft that gave out for some reason, possibly fishing line getting in there. Also went through several carburetors and other stuff. Bottom line for me now is that I prefer the fuel savings and environmental benefits of 4-strokes, but more and more I hope to be paddling kayaks and rowing.

[rslifkin]

We're in a bit of a different situation for how much time the dinghy outboard spends at WOT. Our 6hp is a little on the underpowered side, so for the most part our dinghy has 2 throttle settings: putting along slowly near boats, docks, etc. or WOT. Actual time spent at WOT depends on where we are. I won't blast along on plane after dark, so a night time dog run is always at low speed. But otherwise, if it's a short run to shore near boats, etc. we're just putting. When it's a half mile to get where we're going, then it's a slow putt until we're clear of the surrounding boats and WOT until it's time to slow down on approach to the destination.

The only time it can really be on plane at less than WOT is with only 1 person. With 2 of us and the dog, minimum planing speed and WOT are pretty much the same thing, as it just barely makes it onto plane with that load.

I might enlarge the trim tabs a bit this year to see if I can get some improvement in lift/drag ratio, but I still expect that without more power, the combo of minimum planing speed increasing with added weight and maximum speed decreasing will mean that cruising on plane when loaded means running at WOT.

[thomm225]

My 2011 Mercury 5 HP 4 stroke outboard was very good on gas.

I used it to push my 6600 lb. Bristol 27.

I normally have 2-4 gallons of gas onboard in two containers and can motor in from 20-35 miles out if necessary.

It's best when the seas are calm but can be used in chop.

I got 12 years out of my first one that I paid $1550 for.

The motor still ran well but the main shaft had some how welded itself to the power head.

The motor was always near sea level so maybe the salt etc. caused it.

I didn't put too much time into trying to get it apart and just bought another Mercury 5 HP 4 stroke for $1600 in 2023.

I rarely ran that motor at full throttle but in the video below the tide was so strong I had to as I was very near the mouth of the Chesapeake Bay in a deep channel with a strong outgoing tide.

Normally it will push the boat near 5 knots but due to the tide I was around 2 knots on this day.

Soon as I got past the fish trap pilings I headed in closer to shore out of that channel.

Later that night the weather came in and I had 2'-3' waves coming into my anchorage with a strong 20-25 knot onshore breeze. I was anchored about 70 yards off the beach using an old CQR anchor. At 1 am, I was somewhat praying the anchor would hold. Anchor line was moving all over the bow deck.

Second video much better conditions.

3rd a bit choppy but I was in a hurry so used the outboard to round some anchored tankers

https://www.youtube.com/watch?v=q6sHpweobRM

https://www.youtube.com/watch?v=cpZhq35etcw

https://www.youtube.com/watch?v=ri0c5GnZ1I8

[thinwater]

Jammer stated all of his assumptions clearly, so three stars for him. Yes, many will run the engines less. Many won't make it to 2000 hours.


That doesn't negate his math or change the point very much. 2-strokes are considerably more expensive to operate and you need to lug more gas around. Then there are all the weight, noise, and pollution arguments.



A good post.

[Jammer]

Thank you all for the comments and engagement.

Originally Posted by Jim Cate

Jammer, thanks for an interesting analysis of outboard fuel consumption and costs.

The problem with it for me is the above statement [that most operation is at WOT], for it is really far from what my usage and that of other cruisers that I have observed over the years really is and it is pivotal for your analysis.

My cruising grounds are the Great Lakes and inland rivers and so to some extent I'm always extrapolating from that. For short distances (most anchorages) I row or use the 3hp. For longer distances (upriver excursions and trips to the next island) I use the larger boat and 9.9 hp. I suppose if I had a 15 hp on it I would throttle back.


But WOT operation isn't pivotal to the analysis, on the contrary, the cost difference for 4-strokes is weakest at WOT and much better at lower throttle settings.



On engine life, to a good approximation, piston engines will produce some certain number of HP-hours before they wear to the point where they need overhaul or replacement. So if an engine will last 2000 hours at WOT then it would last approximately 4000 hours at 1/2 power.


On fuel usage, specific fuel consumption (the value I quoted upthread as 0.1 gallon per horsepower-hour for four strokes, it's 0.133 for the 15hp 2-stroke enduro) is lowest at WOT. It increases as engine power is reduced, but the extent of the increase depends on engine type:

• Fuel injected four-stroke engines have very flat specific fuel consumption curves, that is, they burn roughly the same amount of fuel per hp-hour at reduced throttle as they do at WOT.
• Carbureted four-stroke engines have moderate curves and do have some efficiency loss particularly below 1/2 power.
• Carbureted two-stroke engines have the steepest curves, and have significant efficiency loss at lower speeds.

It's hard to find good data on this. Tohatsu has recently announced a lighter weight fuel-injected four stroke 9.8 horsepower outboard, that is supposed to be available summer 2024, and the announcement literature includes curves for a couple of their 4-stroke outboards.



But the upshot of all this is that if you run at lower throttle settings, your engine will last longer, the % fuel consumption difference between 2 and 4 stroke will be larger, and the difference in oil usage will be slightly smaller. The overall cost picture remains roughly the same.

[Tupaia]

I think the analysis you made is excellent, well done.

I do however have a couple of practical observations, that do not detract from your conclusions, but have not been taken into account.

As dinghies tend to cross the displacement/planing speed threshold. The analysis should really be based on distance per litre/gal, not litres/gal per hour as the results are not the same.

I have two Tohatsu 2 strokes a 4hp and 9.8hp.

The 9.8 is required to use WOT momentarily to bring the dinghy up on the plane and then runs at around 1/2 throttle to maintain it. WOT, rarely used.

The 4hp will not get the dinghy on the plane and needs to run at close to WOT to maintain even a moderate displacement speed.

I would estimate that the distance travelled per litre/gal for the two is about equal although the time taken is very different.

When I replaced my old 9.8 Tohatsu, 2 stroke (circa 26kg) the dealer let me test a 9.9 - 4 stroke (circa 43kg). I should add that even after 12 years of full time use the outboard still ran perfectly, but did have a mole grip for a gear change lever. I was offered a good p/x on a nice shiny replacement, the old one made an Asian fisherman very happy.

With a full load, two persons and full dive gear, the 4 stroke was not able to get up on the plane. The 2 stroke was and is JUST able to get up on the plane, not and instant event, but once on the plane can maintain it at 3/4 throttle. The extra weight of the heavier outboard was the limiting factor. The choice would have been between a 15hp 4s that was even heavier compared to a 9.8hp 2s. A diminishing rate of return.

Based on these circumstances any analysis should take account of what is effectively the power to weight ratio and balance the fuel burn of a 2 stroke running at 3/4 throttle doing 9-10 knots to the same power 4 stroke at 1/2 throttle doing 6 knots. Apples with apples in all respects.

[pcmm]

Originally Posted by Benz

That was super-interesting. Thanks for posting. I ran my 4-stroke, 8hp Yamaha OB on my big boat (I row the dinghy), and it lasted 14 years cruising nearly as many thousand miles. At $2500 new in 2009, I reckon I got good value out of it.
Just replaced it with an identical engine but 9.9 hp (they no longer make the 8) for $3600, so I need to stretch it out a little more to get my value.
One day the diesel outboards I'm starting to see around might get small enough to use on my boat. I'd love to see the efficiency breakdown of those....

Yamaha still makes an 8hp outboard

https://www.yamaha-motor.ca/en/water...le/F8-Portable

[SailingHarmonie]

It's a simplistic analysis, but certainly has truth to it. But it doesn't matter to me. I did not buy a 2-stroke because it was cheaper.

The significantly lighter weight (15 kg) of the 2-stroke means I need to use less power to run the boat, almost surely not enough to make up the difference, but it is there for sure, especially since the dinghy can plane at significantly lower speeds. My dinghy+outboard weigh in at 86kg. Changing to a 4-stoke would increase the weight by 17%. If you are not counting that in your fuel consumption numbers, you're missing something significant.

I would bet you that on a mile/liter basis (which is the ONLY measure that counts) my 2-stroke with its optimized prop does way better against a 4-stroke with a stock prop than those numbers would suggest.

The 2-stroke's lighter weight also means we can haul the dinghy up on the beach, which is where it ends up on at least 80% of our excursions. For this every pound counts, and counts more as we get older! If you are the kind of cruiser who only takes their dinghy to the dock-n-dine restaurant, this doesn't matter.

There is a significant difference in "maintainability and repairability" between the two and the balance of the scale really depends on where you are, and the skills of the owner, and the kind of cruising you do.

In the USA you can get parts for the 2-stroke, but they are not likely to be on the shelf. Out cruising "the islands" this situation is exactly reversed. This is especially true for engines like the Yamaha 15 2-stroke which has been exactly the same engine, using the same parts for over 15 years. The parts infrastructure for these is huge. No single 4-stroke model has this kind of market dominance in the cruising grounds that allow a small shop to profitably stock a wide range of parts.

While I appreciate all the advantages that an electronic fuel injection system brings to the table, for me, cruising remote places, far from sophisticated service centers, I avoid them for exactly the same reasons that people avoid common rail diesel engines. If I was cruising on the Chesapeake, my decision would be very different. If I have a carburetor problem, I can disassemble the whole thing, and fix it with a very small inventory of inexpensive spare parts.

The whole cost analysis is based on the assumption that a 2-stoke and a 4-stroke will have the same lifespan. Having maintained a fleet of 10HP outboards (over 2 dozen) for a charter company, that received very heavy use, I can tell you that is just NOT true. All those environmental advantages you credit yourself with, evaporate when you count the environmental cost of producing a new outboard when the 4-stroke needs replacing before the 2-stroke. This is a commonly missed part of environmental decision making, at all kinds of levels.

For me, the dinghy engine is every bit as important to a successful cruising experience as the yacht's diesel. I have never come up against a problem with a 2-stroke that I could not fix on board. To me, that is priceless. If I needed to visit a mechanic every time a sparkplug needed changing, that would be a different story--with a different ending.

[rslifkin]

The weight impact on performance of a heavier outboard may be significant on a light dinghy that's carrying a light load. But if you're heavily loaded (500+ lbs of people and stuff in the dinghy), then 20 extra lbs of outboard weight is hardly significant to how the dinghy performs.

As far as props, any outboard needs a tach to determine if it's propped appropriately for the intended load (and a prop change if it isn't) before it'll be at its best. I tried 3 different props on our dinghy before I settled on the one that's best all-around.

With our setup, the end result was to run the 7.8x7 prop, as that gives the best speed when loaded and the best acceleration. The 7.8x8 (stock) is faster when lightly loaded (the 7 pitch hits the rev limiter before WOT lightly loaded), but it doesn't rev quite high enough to get full power when heavily loaded, so it's slower. The 8.3x6 high thrust prop sits between the other 2 options for RPM at WOT, but it's the slowest in terms of speed. The big, thick, round blades are just too draggy at high speeds. I do keep that prop around, as it's very durable and provides much stronger reverse thrust (and better low speed thrust in general).

I bought an 8.5x7 prop for a Tohatsu 8/9.8 as an experiment to see if more blade area in the same design and pitch would help performance, but I haven't managed to fit it yet. It clears the anti-vent plate and the shaft is the same, but the hub is placed further back in that prop than the 6hp prop, so it would need a big spacer for the thrust washer, and there's not quite enough thread exposed on the shaft to install the prop nut. One of these days I'll come up with a solution to mount it and try it out.

[MicHughV]

I can't speak for the little engines, but I also run a twin engined fishing boat, powered by two 150 Yamaha's. They are 4 cylinder, 2.7 litre engines. Fuel injected.
The boat is equipped with a fuel flow meter, so I can see fuel consumption at glance.

I've learned that outboards suck a prodigious amount of gas. The fuel consumption of my engines is around 13 gal/hr at cruise....this is around 3,500 rpm, giving a speed of about 25 knots.
Pushing the throttles all the way down, increases fuel consumption to around 45 gal/hour at 5,000 rpm more or less, giving a speed of around 45 knots.

I can throttle the engines down a bit to 3,000 rpm, but the boat struggles to stay on plane at that rpm, and fuel consumption actually goes up, despite the lower rpm.

I have watched video's comparing two identical boats, one equipped with two 150's as above, and the other a single 300 hp engine. Fuel consumption varies according to boat speed, but there is little to differentiate between the two setups.

My fishing boat will get up on plane on a single engine, but is clearly not happy there.

I've "experimented" with numerous engine hp ratings on dinks, and it's hard to define what is optimum due to many reasons. More power is always a good thing, but comes at a price, not only in engine costs, but also fuel consumption.

I have come to love my little Yamaha 8 hp two stroke as an optimum all rounder, though I have had both bigger and smaller engines. I can pick it up with one hand with ease and it never fails to start on the first pull.

Outboards generally like to run at 3,500 rpm or more. This seems to be their "sweet spot" and the high rpm likely accounts for the high fuel burn rate.

The 2 stroke vs 4 stroke will likely be a topic of conversation for years to come with proponents on both sides of the fence.

There is an outstanding book authored by Dave Gerr called the "The propellor Handbook" which goes into great detail on this subject.