Diesel consumption of alternator while charging batteries

[elias]

Hi to all ,
I am trying to find out what is the impact on diesel consumption, in a typical marine engine (15-60Hp) for the normal use on sailboats . I found discussions in caravan car forums but I would like to know if someone has more experience . Personally I notice a slight RPM drop when lifting up anchor ( I use idle around 1300 RPM ) .

[sy_Anniina]

I don't have accurate information, but a suggestion where to start looking.

• Measure or estimate charging current, multiply by voltage -> electrical power output
• Google alternator efficiency for your alternator model - if not available, use conservative estimate of 50% -> engine output for alternator = 2*electrical power
• Find out accurate data or assume average specific consumption of 200g/h for each kW of power output -> Diesel consumption kg/h
• Convert to l/h by dividing [kg/h] with diesel density 0.85

BR,

Tommi
s/y Anniina

[elias]

Thanks a lot !
Actually I have an excel with calculations and my 24h needs are around 1.7kwh. Meaning 3.4 kWh that my alternator has to produce . Let’s say it generates with a rate of 600w per hour means almost 5 1/2 hours to charge . Average specific consumption 0.6*200gr/hour = 120gr/hour =>> 660gr of diesel = 776ml of diesel = 1.1 € per day . Plus the tear and wear of engine .

Thanks again

[Yngmar]

If you're planning to run the engine specifically for battery charging, rather than just charging while motoring, you also have to add the idle fuel consumption of the engine on top.

[elias]

Correct . I just realised the importance of the PV on the engine’s consumption and life circle .

[Clivert]

Of course you will have an rpm drop, you're creating a load on your batteries and the alternator is compensating and that takes energy!!!!!!!

[tiger79]

Hi to all ,
I am trying to find out what is the impact on diesel consumption, in a typical marine engine (15-60Hp) for the normal use on sailboats . I found discussions in caravan car forums but I would like to know if someone has more experience . Personally I notice a slight RPM drop when lifting up anchor ( I use idle around 1300 RPM ) .

The extra consumption is so small it isn't worth worrying about.

[SYJetzt]

If talking about costs of consumption, the diesel consumption is only the minor part. Considering the price of a new engine of approx. 10k€ and a lifespan of 5000 hours the cost of each hour for wear is 2€ (without costs for maintenance like oil, filters,..)
The value of calculated charging current (600W/12V=50A) is far to optimistic. With the common lead-acid or agm batteries you get such a high charging current only at the beginning of the charge cycle, and soon a drop of current later to far lower values to top up the batteries to full state. So it would take much more time as expected and  increase the cost once again.
So it is pretty obvious, that every produced kWh from the alternator sums up to a few €.

[symphony2]

I love these "how many angels on the head of a pin" discussions! It is important to remember that you are using the surplus power available from the propulsion engine to provide the 12v needs of the boat and therefore the key figure is the marginal power demands from the alternator while the engine is fulfilling its primary purpose of moving the boat. Typically cruising power requirements for a 40'+ boat is 25kw, so using even the optimistic 50A alternator output that equals les than 2.5% of the power used to move the boat. Nudge the revs up by 100 will have a similar impact.

Effectively you can ignore wear and tear. Modern engines are lifed to 8000 hours of mixed use, but in reality few engines die because they wear out. Typical weekend leisure use yachts do 100-150 hours a year. My charter boat in the Ionian averaged around 500 hours a year. so anything from around 15 years for an intensively used boat to 50 years life! intensively used engines if properly maintained will exceed 8000 hours but leisure engines can die prematurely because of the pattern of usage.

[elias]

As understand so far , the alternator in its  current factory design , produces bulk energy ( useful when picking up 70m of chain on a deep moor)but the “fine” charge is made by the chargers ( solar or shore ) .
For sure running idle the engine doesn't worth so much and preferable would be the PV array as part of a carefully designed system of batteries.
Also as I see, PV are cost effective and will return the initial cost quite fast comparing to the use of engine alternator .

Thanks all for this thread !