Outboard motor lifetime fuel usage and emissions

[FlyingScot]

Suzuki offers lean burn engines and to my knowledge are the only outboards to do so. The DF10/15/20 drop the fuel/air ratio from 14:1 to 19:1 increasing fuel economy. I've owned a couple of Honda cars that had this feature and the MPG increase when it drops into lean burn was significant! Honda dropped to 22-23:1 ratio but still the Suzuki engines should get way way better economy at steady state partial throttle settings.

Our old Yamaha Enduro 15 just sucked down the fuel and oil. We had to keep much more on board and ended up keeping at least one jug premixed so you could refuel quickly when needed. Therefore we had to keep even more gasoline jugs on hand. We keep some gasoline for a little Honda EU1000i generator.

For those complaining about weight, the 4 strokes get closer and closer in weight. 5lbs for the new Tohatsu 20 EFI compared to their 18 2T. Our 20HP is about 12-13 pounds heavier than our Yamaha Enduro 15HP. Noticeable for sure, but so far totally worth it. If truly concerned just bring 3 gallons of fuel instead of 6 since it uses half the fuel and you saved more weight combined.

Also, changing spark plugs isn't much different in either. Remove the cover, remove the cap and swap the plug.

[mjgill1]

Originally Posted by 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.

Well said.

[Jammer]

Originally Posted by sailingharry

I have a 9.8 Tohatsu 2-stroke. Using these figures, about 1.2 gallons/hour. Sounds high, but maybe I do only get 2 hours from the 3 gallon tank. When actively cruising, with a dog aboard, we run ashore at least twice a day. Over a 4 month period, I *think* I put in about 5 fillups, so about 15 gallons -- or 10 hours of operation (which seems low). Be generous, call it 40 hours (which would be 10 hours/month, or 20 minutes a day, which might be reasonable). To get to useful life (2000 hours) would be 50 years -- except this past summer we didn't cruise 4 months, and only used one tank. Next summer is looking to be a repeat. The VAST majority of outboards have a useful life well under 1000 hours.

I find that it's really useful to have one of those combination tachometer/hourmeter devices installed, the kind that has its own little coin battery and no connection to the motor other than a sense wire wrapped loosely around one of the spark plug wires. It helps with prop selection and with recording running hours so that maintenance can be performed at proper intervals. In your case, you may find that the reason you're burning less fuel than you might expect is that you're overpropped and the engine isn't developing its rated horsepower.


As for useful life, sure, it varies from one motor to the next and based on use. 2000 hours is IME a useful limit to consider for motors that are used regularly and frequently. Here is some reading material on that:

• https://premierwatersports.net/how-m...utboard-motor/
• https://www.thehulltruth.com/boating...rd-motors.html
• https://www.godownsize.com/boat-motors-endurance/

[Shaneesprit]

Engine wear is much more about Revolutions than it is throttle and or load, I worked for many years for a company that made money out of Engine Output contracts, (fixed speed, so throttle only affected load not RPM) problem is when the client doesn’t need the power and so reduces throttle, service costs actually remain the same, because the RPM is the same or can increase, and revenue decreased.

[Cheechako]

It sure doesn't seem my old Yam 15 , 2 stroke, used anywhere near 2 gallons an hour ~(7.7 lph). But I didn't run fully WOT that much, very near WOT for 45 min runs exploring sometimes though. I would most often go WOT then back off to where it seemed the planing speed was still near max. I think with the 3 gallon tank I would have known though...?
It's true that regardless of maker, these engines get about the same fuel use though... so... hmmm

We tested each new boat at the boat builder I used to manage. A nice plane at 3200 RPM was the most effective and when you pushed for another 5-10 MPH, they really sucked fuel. These were mostly 200-250 HP outboards. Single or twin motors. All brands. 13-15 gal an hour usage was not uncommon.

[Cheechako]

Originally Posted by Shaneesprit

Engine wear is much more about Revolutions than it is throttle and or load, I worked for many years for a company that made money out of Engine Output contracts, (fixed speed, so throttle only affected load not RPM) problem is when the client doesn’t need the power and so reduces throttle, service costs actually remain the same, because the RPM is the same or can increase, and revenue decreased.

Yep. Commercial large diesel makers will rate the same engine at different RPM's based on usage. An engine may be rated at 4000 rpms for Recreational use and rated at 2800 RPM for 24/7 commercial use, with the commercial use warranted longer.

[Lee Jerry]

Originally Posted by MicHughV

Though the engines say 150 hp, I've been told by people in the know, that they are closer to 200 hp...
Yamaha makes a 4 cylinder, 200 hp engine, which is a 2.8 litre....mine are 2.7 litre, can't see where 100 cc makes an extra 50 hp....

If the engines are labeled 150 hp, then that is what they produced at WOT when delivered from the factory. There are any number of reasons that what you say (i.e. "...are closer to 200 hp") is not true:
1. Yamaha is not giving away close to 50 hp for free.
2. The ratings are based on a standard, like ISO or ASTM or similar or both, and are not open to interpretation or leeway.
3. That would open them up to a bunch of potential legal issues. What do think would happen if someone bought what they think is a 150 hp engine that was actually 200 hp and something bad happens?

Unless...
4. Engines (of all types, whether outboards or inboards, gas or diesel, boat or car, etc.) are built as families based on a common block. The engines are then "tuned" via fuel flow and other tweaks to get that particular engine rating. (For example, in small outboards, 9.9 hp and 15 hp are typically the same engine.) So any given displacement can produce a range of power. In this case, the Yamaha 150 hp and 200 hp engines are the same family. Thus, it could be possible to get 200 hp (or more) out of your engine (maybe fairly easy), but that would have entailed some aftermarket changes (by you or previous owner). Obviously this is very commonly done with cars and trucks, it's a whole market. From the factory your engines produced 150 hp. (I would think you'd know if you modified the engines. )

Originally Posted by 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

If your 2x 150 hp engines are "consuming" 45 gal/hr then there is either a problem with the engine(s) or a problem with the fuel monitor. Those engines are NOT burning all of that fuel properly and turning it into useful work/energy.

[Lee Jerry]

Originally Posted by Jammer

In your case, you may find that the reason you're burning less fuel than you might expect is that you're overpropped and the engine isn't developing its rated horsepower.

That's not my understanding of the proper interpretation.

Over-propped means you can't reach rated speed (rpm) but (probably) do reach rated power.

Under-propped means you (easily) reach rated speed but do not reach rated power.*



The target for proper propeller sizing / matching generally means reaching rated speed at 85%-90% of rated power (in the design condition, which could vary a bit). (While the former is easy to measure, most recreational boats do not know the latter part. Ships and yachts do.)

[sailingharry]

Originally Posted by Jammer

I find that it's really useful to have one of those combination tachometer/hourmeter devices installed, the kind that has its own little coin battery and no connection to the motor other than a sense wire wrapped loosely around one of the spark plug wires. It helps with prop selection and with recording running hours so that maintenance can be performed at proper intervals. In your case, you may find that the reason you're burning less fuel than you might expect is that you're overpropped and the engine isn't developing its rated horsepower.

I've used those. Put one on my 16' bow-rider last summer, and keep meaning to figure out where to mount one on my little 9.8. They are really great. Might buy the 2-pack deal, and put one on the riding mower!


I've been struggling with prop pitch. Even bought a different one, but didn't work out (I forget what I bought now). But, yes, I think I've got problems. Might have solved the problem -- our 40 pound boat dog is in boat dog heaven now, so I think we'll be planing a lot more easily now....

[Jammer]

Originally Posted by sailingharry

keep meaning to figure out where to mount one on my little 9.8.

One way is to velcro mount them to the motor cover. If you run the wire the right way you can still take the cover off far enough to check oil etc. without pulling the velcro mount apart.

[rslifkin]

Originally Posted by sailingharry

I've used those. Put one on my 16' bow-rider last summer, and keep meaning to figure out where to mount one on my little 9.8. They are really great. Might buy the 2-pack deal, and put one on the riding mower!

I keep one on every small engine I have. One on the mower (mounted to the engine cover), one on the snowblower (mounted to the control console), and one on the dinghy outboard (mounted towards the base of the tiller handle). The dinghy tach is a little awkward to read, as you have to look back to read it and it's mounted sideways, but it's usable enough for confirming correct propping and as an hour meter. You can see the location in the picture below.

[sailingharry]

This entire discussion scales to other issues as well. For years, I drove a 1994 Honda Civic CX. Routinely got over 35 MPG. I drove it to work daily -- single passenger vehicle and all that. My second car was (is) a Mercury minivan, used when I needed to haul more than 2 people, or tow, or gear, or whatever.

I drive my cars 200K+. For that Civic, that would be 5700 gallons of gas. Most single passenger vehicles on the road around me were small SUV's. Being generous, most made 20MPG. For that same 200K miles, that would be 10,000 gallons -- 4300 gallons more. Not to mention tires that cost 2-3 times as much, etc, etc, etc.

For sure, there were at least one or two of those vehicles that belong to single car households, and they needed it to be the weekend car too. But I suspect that vast majority had another car very similar to that small SUV (or even bigger!), and that both household wage earners commuted to work in single passenger vehicles getting ~20MPG. They could really, really easily replace one of those with a car getting over 30MPG.

Today, the equation is much harder to analyze, as many are hybrid or EV (with some wild made-up MPGe rating), with no effort to quantify the environmental damage in China to make them, or the disposal costs, or eventual battery replacement costs, or any of that. I'm mostly pro-EV, so don't misread that -- but it is no longer a matter of dividing miles by MPG...

I look at my under-50 gallons a year of gas, and realize I could cut that by 10-15 gallons a year -- and can't get excited.

I'm married now, and my wife just pushed us to a mid-size Volvo wagon. I cringe every time I look at the MPG tracker (right around 27), and then vow to never look again.

America really doesn't care about energy.....

[Sailing Hanse37]

The difference in fuel consumption is irrelevant… we use maybe 10 l of fuel per year for our outboard. Even if the outboard would have twice the consumption, the difference is irrelevant. And almost no outboard will ever reach 2000h of running.

[Kettlewell]

And almost no outboard will ever reach 2000h of running.

Corrosion usually does them in long before the engines are shot, but I managed about 15 years on my last 2-stroke with no major repairs and I once used a 9.9HP Yamaha 4-stroke for 12 years on a liveaboard catamaran. I once estimated that motor had over 2000 hours on it--we did the ICW four or five times at least. 1,000 miles at 5 knots uses up 200 hours right there.

[rslifkin]

In terms of lifespan, outboards are no different than any other marine engine. Outside of commercial use, it's rare to use one enough to actually wear it out. Most either last so long they get retired because nobody wanted it anymore or something else does it in before it's worn out.