Modern 35hp marine diesel engine fuel economy ?

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Boo2

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Hi,

I recently asked about diesel fuel tanks (thanks to those who replied) but I had a sudden thought that modern diesels will be more fuel efficient and I might not need one.

I am going to re-engine my UFO 34 with either a Beta, Yanmar or Nanni in the 30 - 38hp range and I wondered by how much I can expect it to outperform the Thornycroft 90 (marinised BMC 1.5) in terms of fuel economy ? Anyone done this and have the figures to hand ?

And while I am here has anyone struggled through the engine manufacturer's data and have any ideas regarding the comparative fuel economy of these manufacturer's engines ?

Many thanks,

Boo2
 
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Boo2 said:
Hi,

I recently asked about diesel fuel tanks (thanks to those who replied) but I had a sudden thought that modern diesels will be more fuel efficient and I might not need one.

I am going to re-engine my UFO 34 with either a Beta, Yanmar or Nanni in the 30 - 38hp range and I wondered by how much I can expect it to outperform the Thornycroft 90 (marinised BMC 1.5) in terms of fuel economy ? Anyone done this and have the figures to hand ?

And while I am here has anyone struggled through the engine manufacturer's data and haev any ideas regarding the comparative fuel economy of these manufacturer's engines ?

Many thanks,

Boo2
Click to expand...

consumption will be much the same
general rule of thumb is 1ltr per hr per 10hp used
so a 30hp engine only using 20hp will consume 2lts per hr
 
Very little different. Fuel consumption is almost (but not quite) linear to power used, and if you are using 25hp to cruise with your current engine and use 25hp with the new one consumption will be the same.

The best rule of thumb is 1 litre per 10hp used, so if your cruising speed requires 25hp you will use 2.5 litres per hour. Spec and prop your installation so that you achieve maximum hull speed at maximum revs (or just below) and cruise at around 70% maximum power. If you look at the power and consumption curves this is usually max torque and lowest specific fuel consumption. With your boat this should give you a max speed of around 7 knots and cruising at 2500 and around 5.5 knots.

You will find this subject comes up regularly here and the consensus of those who monitor such things supports the RofT in practice.

Just from my own experience of monitoring my 30 hp Volvo, I crusie at 2400, 5. 5knots and average fuel consumption 2.2 litres an hour. Obviously distance travelled will vary according to sea state, tides etc. My 150 litre tank gives a comfortable range of well over 60 hours.
 
I must admit when I looked at the possibility of updating my T90 I was dissapointed to see that similar sized engines seemed to consume about as much as the T90, perhaps 1/2 a litre less an hour at best. but then I could have read the graphs wrong.

It is worth noting that after a couple of overnight motor sails the T90 behaved like a new engine starting from cold with no preheat and minimal throttle. The only real problem is that the T90 like all BMC B series blocks needs a good dose of radflush every 9-12 months.
 
Boo2 said:
Hi,

I recently asked about diesel fuel tanks (thanks to those who replied) but I had a sudden thought that modern diesels will be more fuel efficient and I might not need one.

I am going to re-engine my UFO 34 with either a Beta, Yanmar or Nanni in the 30 - 38hp range and I wondered by how much I can expect it to outperform the Thornycroft 90 (marinised BMC 1.5) in terms of fuel economy ? Anyone done this and have the figures to hand ?

And while I am here has anyone struggled through the engine manufacturer's data and haev any ideas regarding the comparative fuel economy of these manufacturer's engines ?

Many thanks,

Boo2
Click to expand...

As others have said squat all difference............... Just remember your old BMC was producing a world changing amount of Nox for its displacement, modern engines you mention will be Tier II so they will have a huge slug of timing retard in there to meet emissions. The mere fact that they give nothing away is of huge credit.

Beware of purchasing any engine on the basis of spec sheet data, they are riddled with wobbly data, start reading fuel off the prop curve and you may find somebody is using a different propeller law curve?? We all try to work to a fuel density of 0.840 @ 15 Degrees C. Some naughty marketing people slip in 0.860...(Yes out there, I know who you are!!).

Unless you are a complete spec sheet anorak just assume they are all the same. Purchase your motor from whoever will give you the best warranty package and long term support.
 
Am I right in thinking that the only thing likely to improve economy in small marine diesels is when manufacturers switch to common rail injection?

Even then I think any improvement is likely to be less than 10% as common rail engines in cars don't seem to have done any better, and the big gain in fuel economy in some of the most modern cars has been the fitting of stop-start systems to reduce fuel waste in traffic; hardly useful in a boat engine.
 
Norman_E said:
Am I right in thinking that the only thing likely to improve economy in small marine diesels is when manufacturers switch to common rail injection?

Even then I think any improvement is likely to be less than 10% as common rail engines in cars don't seem to have done any better, and the big gain in fuel economy in some of the most modern cars has been the fitting of stop-start systems to reduce fuel waste in traffic; hardly useful in a boat engine.
Click to expand...

my Nanni 4150 has a conventional pump, individual injectors & is normally aspirated
 
Boo2 said:
Hi,

I recently asked about diesel fuel tanks (thanks to those who replied) but I had a sudden thought that modern diesels will be more fuel efficient and I might not need one.

I am going to re-engine my UFO 34 with either a Beta, Yanmar or Nanni in the 30 - 38hp range and I wondered by how much I can expect it to outperform the Thornycroft 90 (marinised BMC 1.5) in terms of fuel economy ? Anyone done this and have the figures to hand ?

And while I am here has anyone struggled through the engine manufacturer's data and haev any ideas regarding the comparative fuel economy of these manufacturer's engines ?

Many thanks,

Boo2
Click to expand...

:D:rolleyes: I , modern 20 years ago lol , boat engines are 20 years behind cars lol
 
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
 
Replaced a Perkins 4108 with a Beta 35...
Better fuel consumption by about 30%, with original prop 15x15
Prop was holding the engine back and was giving 3.5 knots on tickover, so not easy to pick up moorings. Repropped with 15 x10, which resulted in better low speed performance reduced top speed, and fuel consumption increased to marginally better than the 4108
 
We've been through all this lots of times but there seems to be a discrepancy between the 1 litre per 10 bhp.hour group and those such as myself who favour 1 gallon per 20 bhp.hour i.e. 2.27 times as much. My data are supported by the engine graphs in the Vetus catalogue which show min. consumptions of about 190 grammes per bhp.hour for a range of engines. The problem is to know how much power your prop. is absorbing - for my VP2002/Sadler 29 it's usually only 6-7 bhp to judge by my 3-3.5 hours/gallon. [Glazed bores, anyone?]...Or maybe bored gazes.
 
reeac said:
We've been through all this lots of times but there seems to be a discrepancy between the 1 litre per 10 bhp.hour group and those such as myself who favour 1 gallon per 20 bhp.hour i.e. 2.27 times as much. My data are supported by the engine graphs in the Vetus catalogue which show min. consumptions of about 190 grammes per bhp.hour for a range of engines. The problem is to know how much power your prop. is absorbing - for my VP2002/Sadler 29 it's usually only 6-7 bhp to judge by my 3-3.5 hours/gallon. [Glazed bores, anyone?]...Or maybe bored gazes.
Click to expand...

1.5lts per hr on an 18hp engine
 
reeac said:
We've been through all this lots of times but there seems to be a discrepancy between the 1 litre per 10 bhp.hour group and those such as myself who favour 1 gallon per 20 bhp.hour i.e. 2.27 times as much. My data are supported by the engine graphs in the Vetus catalogue which show min. consumptions of about 190 grammes per bhp.hour for a range of engines. The problem is to know how much power your prop. is absorbing - for my VP2002/Sadler 29 it's usually only 6-7 bhp to judge by my 3-3.5 hours/gallon. [Glazed bores, anyone?]...Or maybe bored gazes.
Click to expand...

The problem with many sailboat auxilliaries is that they are rarely run continuously for long periods to enable one to check consumption. You will find those that do long trips under power at constant cruising revs and check fuel used over long periods are the source of the empirical data that underpins the 1 litre per 10hp hour. As an example, my 2.2l hour was based on 150 hours of running time at constant 2400 revs across the middle of the Med.
 
For the past several years I have been monitoring fuel usage on my Beta 13HP. It works out at a boringly exact 1Ltr/Hr time after time. My usage pattern is the usual tickover on the mooring, short time motoring (upto 20 mins) to clear water, sail, short motor to berth/anchor, tickover while we tie up/anchor. Only very occasionally do we motor for a couple of hours.
To be on the safe side I always plan on 1.5Lt per hour.
 
edit Ignore this, I'm talking nonsense /edit
Can I just point out that marine HP and road BHP are not the same thing...
 
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TThere is a perfectly acceptable performance grpah on the BetaMarine website for the engine.

Just like all marine engines the fuel consumption is very dependant on power setting. Max rpm for the engine is 2800 and at that you use 6 litres per hour. Madness to do that.

Conventional wisdom is that you get the most "bang for the buck" at max torque which is about 2100 rpm and uses just under 2.4 litres per hour. Thats according to the graph anyway.

I had a Beta 34 on the previous boat. We did about 750 engine hours over the years and most of the time we were bewteen 2100 and 2300 prm. My average consumption was 2.3 litres per hour. I checked this every time we filled up and it never varied much.

Bearing in mind that this includes the user of our Eberspacher I would say the graph on the website is probably spot on.
 
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