Yanmar vs Cummins fuel mileage???

[Ironworker]

I have Cummins QSB 5.9 380HP electronic engines in my boat. I've been pretty happy with them. However, I've talked with some other Bertram 31 owners who have had Yanmar engines. I've asked about their fuel mileage. They claim fantastical mileage results. One stated he got 4 mpg (I assumed Statue miles). Another told me at cruise he burned 9 gph total. I find this hard to believe but I like to be educated.

In contrast my engines at 2200 rpm, roughly 27.0 kts GPS speed, I am getting roughly 1.06 NMPG and 1.24 SMPG
At 2300 rpm, roughly 28.5 kts gps speed, I am getting 1.067 NMPG and 1.24 SMPG

Again, the sea conditions, clean bottom and load are factors. I measured this yesterday using fuel burn data from the Smartcraft controls, which I have some doubts about the sensor calibration. My STB engine shows a Fuel Per hour rate about 0.4 gph higher than the Port engine. Both engines are sync'd via the Glenndinning Controls.

What are you guys seeing with the Cummins and Yanmars?

[Carl]

You can pull the fuel flow diagrams to get a good idea what the motors burn at every RPM. Sure a lighter boat, or a bit under propped will do a lil' better, but will be in the ballpark. See where 4.5 gph rpm's fall to get 9gph cruising.

I'm thinking some savvy sailors using idling and trolling in the equation.

[Amberjack]

Mixed use maybe 30% @ 1200 rpm and 70% @ 3000+ rpm delivers about 1.67 NMPG. 90% cruise at 3200 10% idle at 1200 = 1.5 NMPG. It doesn’t seem to make much difference cruising between 2500-3300, it still averages about 1.5 NMPG. Yes, the mileage goes way up at idle even on two engines but I’ve never measured it. These are numbers calculated after fueling to a full tank usually 100 gallons or so. I. Could probably get 4-6 NMPG idling on one engine (through the La Connor Slough for 10 miles due to gale winds or back to Friday Harbor to replace a R/W pump) but would pay for it by banging my head against the console in frustration.

Yanmar 6 LP-STP’s 1200 hours installed 2002. Bertram 31 FBC stocked for 30 days cruising but stripped of unnecessary accessories.

[Yannis]

I have a relatively constant burn of 2.6-2.7 liters per nautical mile at 3000 rpm with my 4 LHA-STP 240hp Yanmars, at 24-25 knots.
Any attempt to run at lower rpms resulted in higher consumption!
The 31 with the same engines might be even more economical than my 28, possibly due to a longer waterline and a lesser deadrise.

[Tony Meola]

Rick

Two different engines different fuel burns.

The torque curve is very different. The Yanmar's turn up more like a gas engine the Cummins have more low-end torque.

Yanmar's tend to be more fuel efficient, but you pay more for parts. There is always a tradeoff.

Pick your poison.

[mike ohlstein]

Yanmar 230's. 1.4NMPG at cruise. Maybe 7 NMPG at 1100 rpm (7.2 knots)

[Ironworker]

This is good information for those looking to repower in the future. It looks like about a 0.25 to 0.4 mpg improvement for the Yanmars. I think they are a little lighter engines than the Cummins as well. Dropping a 1,000 to 1500 #s or so will certainly improve performance.

Of course the best average is to use a day of boating with typical use, check the mileage against your track, vs fuel consumed. I wasn't able to do that on Tuesday when I crossed back to Florida. Its a 203 NM trip and I typically use somewhere between 193 to 197 gallons of fuel on average.

I did a little analysis when I was repowering in 2022. The reasons that I chose the Cummins QSB 5.9's vs Yanmars were as follows...

Quietness
Availability of parts
Run at lower RPMs

I chose electronic engines because of fuel efficiency and less smoke.

Of course my decision was made with $4.00 per gallon of Diesel fuel. I just paid $8.35 at Georgetown, Great Exuma a couple of weeks ago.

[Carl]

Rick what Yanmar motors are you comparing to your 380hp QSB Cummins?

If using Mkes numbers, he is being pushed by Yanmar 230's. Doug has Yanmar 315's powerplants, sure both less motor weight but less hp too. HP is made by burning more fuel. I think you also added some weight in the build...sure lots of coosa, but also lots of glass, batteries and...I could be wrong. Just thinking to compare things have to be accounted for which are not equal comparisons.

You can look at Bob, he has 315 Cummins and he says he is in the 1.8 nmpg range. I think I heard higher with different props.

[Robert lico]

carl that is correct 1.8 I dropped two mph off wot (useless and stupid to talk about ) but still 2 knots faster than 315 yanmars and about .3 mpg better fuel milage then 315 yanmar at 28 knots cruise not going further with 1984 technology fan belts ,belt driven cam fresh water pump, and oil pump let alone ALUMINUM exhaust sprinkler heat and bogus turbos .you have to see them apart on a engine stand to understand nobody in the entire world builts a fan belt driven cam ,oil pump anymore nor 1984 fan belt acesserary drives. does anybody in the world not have a serpentine belt please tell me

[Robert lico]

carl that is correct 1.8 I dropped two mph off wot (useless and stupid to talk about ) but still 2 knots faster than 315 yanmars and about .3 mpg better fuel milage then 315 yanmar at 28 knots cruise not going further with 1984 technology fan belts ,belt driven cam fresh water pump, and oil pump let alone ALUMINUM exhaust sprinkler heat and bogus turbos .you have to see them apart on a engine stand to understand nobody in the entire world builts a fan belt driven cam ,oil pump anymore nor 1984 fan belt acesserary drives. does anybody in the world not have a serpentine belt please tell me

[Amberjack]

I think you also added some weight in the build...sure lots of coosa, but also lots of glass, batteries and...

Carl is right about increased weight. More weight puts more of the hull in the water increasing friction. My last weigh was on a new comporter and gauges showed 13,000 lbs. add maybe 500 lbs cruising gear so we cruise at 13,500 lbs. Rick I think added quite a bit of gear.

Also aerodynamics. I travel with the Bimini furled and have no outriggers so the boat presents a clean profile to the wind—for a Bertram 31. The boat handles entirely differently with the Bimini up so there is some kind of an effect.

Yanmar 315’s. With all those engineering ‘flaws’ Bob they have been clean, efficient and remarkably reliable for 22 years. I don’t think you can ask more of a marine diesel engine.

[Ironworker]

What kind of Yanmars? I don't know. I believe one guy and a 4 cylinder and I never got around to asking the other guy.

My boat has a full tower and 31 ft outriggers. It is not very aerodynamic. I probably lost a 1,000 pounds during the rebuild but made up for it in an additional 120 gallons of fuel in the Aux fuel tanks. When I checked my fuel flow it was strictly off the Smartcraft gauges which I don't know if it accounts for the unused fuel returned to the tank or not? That in itself could be a big factor.

I really need to top off the main tank which I can isolate from the Aux tanks, run the boat for the day, then refill the tank and check the efficiency against a track mileage.

BTW, I'm running the same props as Jimmy.

[Carl]

I don't think 4 cylinder Yanmars being compared to 6 cylinder 370hp Cummins is a fair side by side fuel usage comparison.

I'd be like me comparing my 210 6BT's to 250hp 4BT's... more hp from a lighter motor. Guess whose burning less.

[Priceless]

1971 Bahia Mar 'Priceless" modified with custom tower so added weight. Yanmar 315s. Run to the canyons at 3150rpm- 25-25.5 knots troll all day and run back 1.5nmpg. Straight cruising burn 12-14gph total depending on speed and weight in the boat. Interestingly put Seakeeper Ride automatic trim tabs on before last season. Its a gamechanger. Totally different ride, much more comfortable, and gained 1.5 knots at 3150 cruise. Well worth it. If your thinking of getting them and want to trial I'm at Indian River Marina north in Delaware. I'll be happy to show them off.

[Ironworker]

1971 Bahia Mar 'Priceless" modified with custom tower so added weight. Yanmar 315s. Run to the canyons at 3150rpm- 25-25.5 knots troll all day and run back 1.5nmpg. Straight cruising burn 12-14gph total depending on speed and weight in the boat. Interestingly put Seakeeper Ride automatic trim tabs on before last season. Its a gamechanger. Totally different ride, much more comfortable, and gained 1.5 knots at 3150 cruise. Well worth it. If your thinking of getting them and want to trial I'm at Indian River Marina north in Delaware. I'll be happy to show them off.

Wow, that is unexpected information. Thats worthy of a whole new thread on the Seakeeper Ride system.