Small genset or additional fuel tank?

[RaymondR]

Originally Posted by rslifkin

Yup, that's exactly the reason. Throttle induced pumping losses are killer for gas engines under light loads.

Yep, time to move to wood fired Stirling cycle.

[Adelie]

If you compare similar sized gas/petrol & diesel engines at comparable loads, the diesel will be more efficient.

But that's not what the OP asked, he wanted to know if a small gas/petrol genset would be more efficient than pulling power off his main engine, a Beta-50. Thus you are comparing apples to oranges.

There are 3 ways to approach this, test the 2 under load, find a data set that you can make comparisons with or a combination of the 2.

Honda has supplied the test data for their EU2200i
Continuous rating of 1800W
0.95gal tank. Lets call it 1.0gal to account for converting the AC power back to DC
Will run 3.2hr at rated power.
3.2hr * 1800W / 1.0gal = 5.76kwh/gal

Post #24 (https://www.cruisersforum.com/forums...ml#post3079298) had data for a Westerbeke 7.6kW diesel genset. I assume 7.6kW is electrical output, not motor output which would result in a lower electrical output.
25% of 7.6kW is 1.9kW which is comparable to the Honda.
At that load it is using 0.36gal/hr diesel.

1.9kW / 0.36 gal/hr = 5.28 kWh/gal.

At that load the Honda is more efficient.

Here's the thing about engine, gas or diesel, larger tends to be more efficient at full throttle, but at lower outputs the smaller engines generally become more efficient.

An 18-25hp engine generating 15hp will produce 13-16 hp-hr/gal diesel.
A 45hp engine will be under 10 hp-hr/gal
See 2nd graphic.

At idle the larger engine will be using much more fuel to overcome friction to keep the engine turning over than the smaller engine.

My expectation is that a small gas generator will be significantly more fuel efficient and cost effective than running the main engine to recharge batteries.

The exception is if the vessel is under motor propulsion anyway, then the main engine would be the way to go, the diesel's fuel efficiency advantage when operating near peak output would tip the scales that way, especially a large engine such as the OP's Beta-50.

[alamoana]

I think that 550 AH batteries will rarely take over 50 Amps at 14 V = 700 watts = 1 HP plus inefficiencies, ñ will pull 2 HP. Without AC I would not put a genset nor a wind generator. I would concentrate all my efforts on rigid solar panels. I am sure 500 watts will fit and with 4 PSH it yields 140 AH per day. I would I insulate the fridge and freezer a little more, LED lights and it will work out. Any deficit should be covered by main engine running sometimes.

[sailingharry]

Originally Posted by alamoana

I think that 550 AH batteries will rarely take over 50 Amps at 14 V.

My last boat had 220Ah of lead acid (Trojan GC) that could take 50A for at least 1/2 hour.

My current boat with 800Ah of AGM can take 150A for over an hour.

[hpeer]

Originally Posted by alamoana

I think that 550 AH batteries will rarely take over 50 Amps at 14 V = 700 watts = 1 HP plus inefficiencies, ñ will pull 2 HP. Without AC I would not put a genset nor a wind generator. I would concentrate all my efforts on rigid solar panels. I am sure 500 watts will fit and with 4 PSH it yields 140 AH per day. I would I insulate the fridge and freezer a little more, LED lights and it will work out. Any deficit should be covered by main engine running sometimes.

That is about my set up. Plus I have wind, which helps here. I have a Honda but typically only run if I am using power tools a fair bit. Winter in USA is different, then I need some charging.