Modern 35hp marine diesel engine fuel economy ?

[Tranona]

Not sure where your 32hp comes from - nothing to do with my engine.

If I understand you correctly, you are saying that at 2400 rpm my engine is only producing half of what the power curves say it is, and is moving 6.5 tons of boat at 5.5 knots. If that were the case then I can junk my engine and replace it with a 12hp engine.

 

[Latestarter1]

Not sure where your 32hp comes from - nothing to do with my engine.

If I understand you correctly, you are saying that at 2400 rpm my engine is only producing half of what the power curves say it is, and is moving 6.5 tons of boat at 5.5 knots. If that were the case then I can junk my engine and replace it with a 12hp engine.

Just a little surprised how little some boaters understand their propulsion systems..

Lets take hypothetical vessel of 6.5 Tonnes and LWL of 28 feet. To drive this hypothetical boat at maximum hull speed of a smidge over 7 knots the propeller will demand say 28.5 Hp or better put 21.25 kW.

If we install say a Yanmar 3YM 30, 29.1 Metric Hp or 21.3 kW @ 3,600 rpm it will just about do the job.

Getting down to the nitty gritty we need to focus on cruising CONTINIOUS power.

Think of the throttle as cruise control on a car, however on a boat you set the desired rpm. At say 5 knots on say a 2.8 exponent propeller will be demanding exactly 10 Hp 0r 7.46 kW and probably consuming around 2 liters/Hr. You have set your throttle at 2,500 rpm and our wee 3YM has the the POTENTIAL to produce 25 MHP or 18 kW at this setting........Where has the 14.2 MHP or 10.45 kW gone? Simple answer is the engine is not producing it. That is the very purpose of the governor on a diesel engine, you set the rpm the governor responds to the load placed upon it.

Other than at WOT forget the full load curve of a diesel engine that is for the birds.

Apologies to posters to whom this is obvious, however it is apparent that there are a few who don't get it.

 

[reeac]

Thank you. Latestarter, at last someone who understands!

 

[Deleted member 36384]

.... Yanmar 3YM 30, 29.1 Metric Hp or 21.3 kW @ 3,600 rpm ...... 25 MHP or 18 kW .....

Wow, you could power a small town from that Yanmar. Metric Horse Power symbol is hp(M). 25 000 000 hp would consume a lot of fuel.

 

[Latestarter1]

Wow, you could power a small town from that Yanmar. Metric Horse Power symbol is hp(M). 25 000 000 hp would consume a lot of fuel.

You are quite correct term is hp (M), metric horespower was staight lift from Yanmar America literature what do expect them to know about the metric system,and brain not properly engaged at 5.30 am.

Exactly the reason I always prefer to work in kW.

 

[Davy_S]

Think of the throttle as cruise control on a car, however on a boat you set the desired rpm. At say 5 knots on say a 2.8 exponent propeller will be demanding exactly 10 Hp 0r 7.46 kW and probably consuming around 2 liters/Hr

I said 10hp consumes around 2 litres/hr.
but in non tech speak

 

[MM5AHO]

"...28.5 Hp or better put 21.25 kW...."

Why is 21.25 kW better put than 28.5 Hp ?

Is that like saying kg are better measures that lbs, or m better that ft?

I was educated entirely on metrics, but had to learn all this imperial stuff on moving south, but don't see that one is "better" than the other.

 

[davidej]

I , modern 20 years ago lol , boat engines are 20 years behind cars lol

Thank God. Do you really want to rely on all that electronic flummery in a salt environment.

 

[doris]

Just a little surprised how little some boaters understand their propulsion systems..

Lets take hypothetical vessel of 6.5 Tonnes and LWL of 28 feet. To drive this hypothetical boat at maximum hull speed of a smidge over 7 knots the propeller will demand say 28.5 Hp or better put 21.25 kW.

If we install say a Yanmar 3YM 30, 29.1 Metric Hp or 21.3 kW @ 3,600 rpm it will just about do the job.

Getting down to the nitty gritty we need to focus on cruising CONTINIOUS power.

Think of the throttle as cruise control on a car, however on a boat you set the desired rpm. At say 5 knots on say a 2.8 exponent propeller will be demanding exactly 10 Hp 0r 7.46 kW and probably consuming around 2 liters/Hr. You have set your throttle at 2,500 rpm and our wee 3YM has the the POTENTIAL to produce 25 MHP or 18 kW at this setting........Where has the 14.2 MHP or 10.45 kW gone? Simple answer is the engine is not producing it. That is the very purpose of the governor on a diesel engine, you set the rpm the governor responds to the load placed upon it.

Other than at WOT forget the full load curve of a diesel engine that is for the birds.

Apologies to posters to whom this is obvious, however it is apparent that there are a few who don't get it.

All hugely helpful and am very grateful for this.

I have a 29 hp that shoves 9 tons along in a flat calm at 6 kts at 2,100 rpm. This uses pretty close to 2 1/2 litres an hour. Max revs, just under 3k gives 7. 5 kts. If that were 2.9 litres per hour then flat out would be more efficient but can't buy that. Have never done sufficient at max revs to have any data. I think I can follow Latestarter's explanation rather better than the blanket 1 litre per 10 hp(M).

 

[JumbleDuck]

I suspect you will never see the improvements in fuel economy that we have seen in road vehicles in marine engines primarily because they are all heavily derated to suit the usage they get. My last Fiat had a 1.9 diesel giving 120bhp, the equivqlent size engine from Nanni gives 40bhp, a third. On that basis it is unlikely to ever reach the levels of efficiency of the car engine

Note, though, that in your car the engine will hardly ever be giving 120bhp, and if you tried to get that out of it continuously it would last next to now time. Sitting at <cough> 70mph on the motorway you are very unlikely to be using even 40bhp.

 

[macd]

They've all gone to the pub, Jumbleduck.

 

[maxi77]

Note, though, that in your car the engine will hardly ever be giving 120bhp, and if you tried to get that out of it continuously it would last next to now time. Sitting at <cough> 70mph on the motorway you are very unlikely to be using even 40bhp.

Even with 1,5 tons of caravan on the back, and as a comparison my normal cruising revs on both car and boat were 2500 to 3000

 

[lynall]

I never quite get the different bhp/kw readings between marine and truck diesel engines.
You are saying the marine bhp is lower, whereas the ones i work on are roughly 1/2 to 2/3 the bhp of the marine version, point being the volvo 12 ish litre truck engine 450 ish bhp, in the marine version just about doubled!
Was looking at the cummins marine section site the other day, duty cycle of the marine version at max bhp was funny, i think it was the little 5.9 or the 6.7 ltr straight six, 470bhp max power 1 hour in every 8, down to 220/50 bhp for trucks with a 100 percent duty cycle.


Lynall