Outboard motor lifetime fuel usage and emissions

[rslifkin]

45 gal/hr sounds very high for a pair of 150hp engines. I wouldn't expect WOT fuel consumption to be more than 30 gal/hr for the pair.

[MicHughV]

Nope, at WOT, them engines suck it down.

[rslifkin]

Originally Posted by MicHughV

Nope, at WOT, them engines suck it down.

That's very odd. You're only getting ~6.7 hp-hr per gallon out of those at WOT if that consumption is accurate. That's like 1940s low compression flathead level of inefficient. Even my low-ish compression carb-ed gas big blocks from the 1980s will achieve 10+ hp-hr per gallon and you're dealing with much more modern engines.

[Tedd]

That BSFC figure of 0.1 gal/hr/hp seems on the high side. Where did it come from? I know smaller engines are generallly going to have slightly poorer BSFC than larger engines but, by comparison, the GM Saturn I4 car engine's BSFC is only 0.0652 gal/hr/hp. That would give the four-stroke outboard engine more than 50 percent higher BSFC.

[MicHughV]

Just telling you what the gauges read...it's a heavy boat...carries 150 gallons of fuel, two live bait wells, etc, etc...and there are usually 3 people on the boat.

Engine trim is another factor...when it's rough, boat rides better with the bow down, which means the engines are pulled all the way in...

[rslifkin]

Originally Posted by MicHughV

Just telling you what the gauges read...it's a heavy boat...carries 150 gallons of fuel, two live bait wells, etc, etc...and there are usually 3 people on the boat.

Engine trim is another factor...when it's rough, boat rides better with the bow down, which means the engines are pulled all the way in...

Your 13 gal/hr at cruise number is entirely reasonable and would be impacted by all of those factors. But I'm not convinced that 45 gal/hr is accurate at WOT. The engines just shouldn't be capable of consuming that much fuel in any situation.

[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....

[MicHughV]

I obviously don't run it at WOT throttle much due to the excessive fuel consumption...

[MicHughV]

This video compares the twin 150's vs a single 300. They claim 32 gal/hr at WOT, but I think they are running a lighter boat.

https://www.youtube.com/watch?v=gG_-YmGN8SY

[rslifkin]

Boat weight shouldn't impact fuel consumption (in gal/hr) at WOT, it'll just determine how fast you go at WOT (and therefore miles/gal). Assuming both examples are propped correctly.

If your engines are actually making 200hp then the total burn of 45 gal/hr would be much more reasonable. Still a little high for modern EFI engines, but not crazy. The displacement difference isn't necessarily the only thing between those 2 engines that could impact power output, but it's entirely possible that the 150s are under-rated.

[MicHughV]

I've never been able to get more than about 45 mph out of my boat.....not even on flat water.
Yamaha mechanics at my marina tell me that the 150's are closer to 200 hp...I think it's a marketing thing...ie, Yahama wanted to offer a 150 and a 200 hp engine without making too many changes.
I had a fishing boat previous with a single 200 hp Yamaha on it and the engine looked identical to my eye.

I also think the 150's are underrated and have been told that by several people.

Regardless, the fuel burn is high.

There are fishing boats at the marina that run twin, or even triple 450 hp engines. I can't even imagine the fuel burn on those things.

[rslifkin]

Originally Posted by MicHughV

There are fishing boats at the marina that run twin, or even triple 450 hp engines. I can't even imagine the fuel burn on those things.

A lot. There's a guy here that has triple 450s on a 40-ish foot center console (and for a while there was one in the area with quad 400s). The hourly burn must be eye-watering (WOT should be something like 130 gal/hr for the triple 450s). But at the same time, they're going fast enough relative to that burn that they get better mpg than my boat does when running on plane (which is why we rarely cruise on plane).

[MicHughV]

Marina fuel comes at a premium too, being the non-ethanol variant..it's nothing for me to burn $200 of fuel for a quick trip out to the ocean...it costs me $60/lb to catch a fish and $6/lb to buy fish at the fish market....go figure..

We have several of those triple/ quad engine center consoles at the marina, but I never see them go out anywhere...

My 15 hp Suzuki 2-stroke on my dink could also suck it down, as could the 15 hp Yamaha 4-stroke...my 8 hp 2-stroke remains my sweetheart engine for a dink

[P3sailor]

Words of wisdom from an accountant, no doubt. Wow!

[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.


As far as speed. I rarely anchor in speed limit zones (and usually in zero-population-areas). I usually am WOT (my dink won't plane at anything less than WOT). Low speed is reasonable for how others think of you (why you going so fast?), for low wake (but my dink makes FAR more wake at 6 kts than at WOT/12 kts), and for traffic/safety reasons (and yes, if there are other moving boats, this is a real issue).


But this analysis is really interesting, and even if assumptions don't match individual use patterns, it easily scales. Although, over a 50 year useful life, the cost difference isn't detectable -- and over that 50 year useful life, the weight difference is!