Small genset or additional fuel tank?

[Saltyhog]

I'm refitting our Cabo Rico 38 for some extended cruising. I have just enough room under the cockpit where I can fit *either* a small diesel genset, or an additional diesel fuel tank of about 25 gallons. Currently the boat has a 55 gallon fuel tank. My anticipated energy budget is 150-200 AH per day. I have a new Beta 50 with a 200 A large frame alternator that will make 120 A all day long at 1250 RPM (engine speed). I'll have about 300 watts of solar on the bimini, and maybe a wind generator. Plan is to use either main engine/alt or the genset for bulk charge, and solar/wind to top off. Batteries are 550 AH of LifeLines for now, lithium in the future.



So my options are:

1. Main engine / alt with additional 25 gallon fuel tank. I'm not sure what the engine will burn at 1250 rpm running only the alternator. I calculate I'd be pulling about 3.5-4 HP off the engine. The fuel curves from Beta Marine are no help as they assume turning a currently sized prop. Also concerned that the engine won't get to temperature.
2. Genset. Burns about 0.25 Gals/hr. at my bulk charging loads.

So my question is whether the additional efficiency of the genset would outweigh the additional fuel tankage gained without it. I calculated that the the main engine would need to burn 0.36 gal/hr or less for that option to win. Of course this assumes *only* charging batteries. It gets far more complicated if motoring and charging at the same time. Then the additional fuel load for charging via the alt would be far less. So I figured I'd ask here it there was any wisdom or experience. I know folks don't like to run the main engine for charging only, but my specific case is a little different.

[cabo_sailor]

I too have a CR 38 that has recently been refitted. I sure don’t know where you can squeeze a genset under the cockpit. Remember you need to access the genset for service. I put a 3.5 kw Nextgen in the lazarette and it was tight. It burns about a qt per hour while the Westerbeke burns about 3 qt/hr. For relatively short trips of 200-300 nm the 55 gal tank and a couple of jerry cans is more than adequate. For longer trips I can add a 37 gal fuel bladder on deck giving me approximately 100 gal. I can’t imagine needing to motor and use the genset at the same time. I’ll know more in the future when the electrician finishes the wiring.

[a64pilot]

Depends on how often you think you will need power, if it’s often go the generator route.
Fuel is too easy to carry on deck in my opinion to be building in a 25 gl tank, but that’s just my opinion.
I have a built in generator, a Nexgen 3.5, and my go to generator is my little Honda portable.
Why? Cause it’s so easy to service and I just don’t often need the almost 30 amps the Nexgen can make.
Rainy overcast days is when I’ll run the Nexgen, cause of course you don’t want the Honda out in the rain.
I’d hate to rack up hours on my big money motor just to charge batteries, when a cheap suitcase generator can easily do that.

[Saltyhog]

Originally Posted by cabo_sailor

I too have a CR 38 that has recently been refitted. I sure don’t know where you can squeeze a genset under the cockpit. Remember you need to access the genset for service. I put a 3.5 kw Nextgen in the lazarette and it was tight. It burns about a qt per hour while the Westerbeke burns about 3 qt/hr. For relatively short trips of 200-300 nm the 55 gal tank and a couple of jerry cans is more than adequate. For longer trips I can add a 37 gal fuel bladder on deck giving me approximately 100 gal. I can’t imagine needing to motor and use the genset at the same time. I’ll know more in the future when the electrician finishes the wiring.

Thanks for your input. I had originally considered the Nexgen 3.5. It was a snug fit in the Laz as you say. No way would fit under the cockpit sole. I bought a Mastry M4000, which used to be sold as the Entec West. A bit smaller than the Nextgen in footprint, but also a bit taller. It will fit under the cockpit if I custom build my own enclosure sides. There is room for service and is actually easier than in the laz. Question to you: You say your Westerbeke burn 3 qts./hr. Is that when motoring, ie propeller engaged, or when just charging batteries via the alternator. My question is what is the fuel burn of a modern diesel propulsion engine when only charging batteries via large frame alternator. Many quote poor fuel performance for this vs a small genset, but I wonder if they are considering that the fuel burn of the main engine will be less when only charging and not motoring. A diesel engines fuel consumption is directly related to the power used when loaded plus the base necessary to just keep the engine turning. I'll bet that my new 50 HP Beta will burn a lot less at 1250 RPM just running the alternator at 120 A than it will if the prop is engaged and just propelling the boat. Much more HP used for that second task.

[Saltyhog]

Originally Posted by a64pilot

Depends on how often you think you will need power, if it’s often go the generator route.
Fuel is too easy to carry on deck in my opinion to be building in a 25 gl tank, but that’s just my opinion.
I have a built in generator, a Nexgen 3.5, and my go to generator is my little Honda portable.
Why? Cause it’s so easy to service and I just don’t often need the almost 30 amps the Nexgen can make.
Rainy overcast days is when I’ll run the Nexgen, cause of course you don’t want the Honda out in the rain.
I’d hate to rack up hours on my big money motor just to charge batteries, when a cheap suitcase generator can easily do that.

Thanks for responding. I'm building the boat for long range cruising. We plan to cross oceans. The South Pacific is in the plans. So although I truly would love to be able to put 1kW of solar on the back as you have, I can't do that for this boat's purpose. Too great a chance of major failure. We'll be limited to about 300 W on the bimini. Likewise a honda. Makes perfect sense for many, but not practical on deck mid ocean in any real weather. I'm just curious as to the fuel burn in gallons / HP used is for a modern 50 HP marine engine. I think this question is relevant to many here. We're told that fuel efficiency will be poor if the main engine is used to "just charge". I'm sure that's true if we're talking absorption for lead batteries. But what about full out bulk charging lithium batteries? I think the question has gotten more complicated in the context of modern batteries/high charge rates and with large case high output alternators.

[Dockhead]

If your concern is fuel efficiency, then forget the generator. It will make little difference and certainly not enough to significantly reduce your fuel requirements.


The purpose of the generator is not to save fuel, but to save main engine hours and potentially harmful running of the main engine with too little load, and to provide redundancy in power generation.



You might want to do your own research, but I GUESS that 200 amps at 1250 RPM for a 50hp engine will be reasonably healthy.


Besides that, you will generate a lot of power while you are using the main engine anyway for propulsion, and that is definitely more fuel efficient than using a generator.



You could keep a suitcase generator for times when you just don't want to use the main engine, like especially when you need AC power but the batteries are already out of bulk phase. I would personallly not sacrifice fuel tankage. 55 gallons is only 200 liters; not nearly enough in my opinion for anything remote. Another 100 liters is still marginal, in fact. I would make tankage a priority if I were you.

[Pizzazz]

Most of the time, diesel fuel will not be a limitation on your cruising plans unless you have very specific cruising patterns away from refueling stops. The way to think about gensets is that they are more efficient at higher loads. For example, the nextgen 3.5 will consume 0.4 gph at full load but if you are operating only to bulk charge your batteries (say about 120A), its efficiency will be lower, say 0.2 gph. In general constant speed generators trade-off speed, noise, max power, etc. to provide a relatively good average experience but they are not optimized for charging the batteries (a relatively low load). For example, the cheap gas powered inverter generators are much more efficient because they vary the speed to match the load. With a diesel generator, you need to maintain the speed to achieve the required frequency, which means the pistons, the flywheel, everything has to be spun up to 3,600 rpms (or 2,800 or 1,800) and then vary the fuel to match the load. It is less efficient and there are curves on the web that show that variable speed generators are nearly twice as efficient as constant speed generators at low loads.

Another way to look at this is, if you get a nextgen 3.5, that spins at 2,800 rpm, some part of the energy will go in maintaining the rotational speed, the rest will go into power generation and heat. The electrical part of the generator could be 90% efficient but at low loads you still need some horsepower for the high rotational speed. If using the main engine, you have much lower rotational speed (1,250 rpm stated) but then the alternator efficiency is probably less than 60%. These two curves (modes) will intersect at some load point, may be around 20-30% of load. If you charge at higher loads, the genset is more efficient, at lower loads, the engine will be more efficient.

In both cases, though, you will use much less fuel than you think. 10 gallons of fuel at 0.25 gph will give you 40 hours of operation, which in bulk charge will produce 40 x 120A = 4,800 Ahrs of energy which at your daily rate of 200 Ahrs will give you 24 days of power reserve. On top of this you have solar.

By the way, a Honda 2000i will give you better fuel efficiency for charging the batteries (despite gas being less energy dense than diesel) because it is a variable speed generator. I would definitely start with just using the main engine (plus a spare alternator) as my bulk charging source, then consider a gas inverter generator and only after a couple of years I would go for a diesel genset. If you are running AC, electric cooker or electric heat then it makes a bigger difference to have a diesel generator.

Regarding your fuel reserve, I think 55 gallons is plenty. If you are planning on some crazy motoring runs (for example, sail to Hawaii then motor back through the High) then your consumption will be enormous (you will need 1,200 nm+ range) but in a situation like this 20 gallons will not make a difference. A bladder would be the way to go.

SV Pizzazz

[rslifkin]

Originally Posted by Pizzazz

Another way to look at this is, if you get a nextgen 3.5, that spins at 2,800 rpm, some part of the energy will go in maintaining the rotational speed, the rest will go into power generation and heat. The electrical part of the generator could be 90% efficient but at low loads you still need some horsepower for the high rotational speed. If using the main engine, you have much lower rotational speed (1,250 rpm stated) but then the alternator efficiency is probably less than 60%. These two curves (modes) will intersect at some load point, may be around 20-30% of load. If you charge at higher loads, the genset is more efficient, at lower loads, the engine will be more efficient.

You're forgetting that the genset is a smaller engine, so even spinning faster, it'll likely take the same or less power to spin.

[hpeer]

If it were me I would know the answer.

1. 25 gallons of fuel
2. A GOOD wind generator, not one of those 3 bladed things, way too noisy.

The 25 gals of fuel can be used to motor or produce electricity or both. The gen can only use fuel, which is rare for you, to produce electricity. So the extra fuel is a a more flexible use of the space.

The wind generator is IMHO a no brained, do that no matter what you do with the tankage space. Even here in Antigua we have just had a run of days with very little solar power but good wind. The wind generator has tombs capacity to produce power 24 hours a day, the solar about 8. So it can be less efficient and still provide as many amps. And if in dire straights, turn off the fridge, and when truly low crank up the engine. But that will only occur if you have no wind AND no solar.

That’s my fault analysis anyway.

[Saltyhog]

I just wanted to thank everyone for their input. My thinking at this point is to go with the additional tankage. I don't think the additional efficiency of the genset will makeup for the added complexity and cost of maintenance. My general sense is that I'll be running the main engine for charging more than I like while I still have the AGM's, that will be reduced considerably once I switch to lithium. The flexibility of charging schedule is one of lithium's advantages.

[stormalong]

Rather than a diesel generator consider a drag prop generator. I did a lot of ocean passages with refrigeration and an electric autopilot. Solar and wind generator was not enough and I had to run the main engine every couple of days to top the batteries. Then I got a drag prop generator. Producing about 6 amps at 6 knots it gave me power to burn.

Advantages: little noise, no maintenance. Disadvantages: No fishing with it deployed, Have to stop the boat to retrieve and line gets twisted up, Sliding a cone over the line prior to retrieval helps.

There is also the Duogen that has no line - just mounts like an outboard - more expensive but easier to use still less expensive than a generator and works 24/7.

[Cheechako]

What range do you need? Where are you likely to cruise?
With the 55 gal tank you have.... you have maybe.. 550- 600 mile range now at 2/3 gal per hour and 7 knots?
Maybe carry a small gas gen you just pull out into the cockpit for emergency?

I added two additional tanks to my catamaran and just emptied them once and never used them again.

[a64pilot]

The Honda may in theory use less fuel than a Nexgen, but in actual practice it doesn’t.
I have both, the fuel burn is about the same. The Honda can do anything the Nexgen can, it can make water, charge the batteries and heat water, even run the AC if I had a soft start kit.
However the Nexgen can do all of them at once, the Honda can make 30+ gl of water an hour and charge the bank at 40 amps, or it can heat water, but nothing else.
So to use today as an example, I ran the Honda for two hours this morning, topped off the water tank and shoved 80AH into the bank, Solar has me fully charged by 1PM, this afternoon, I will run it for an hour to heat water, cause we like a hot shower. Water heater draws 11.7 amps, Honda rated continuous output is 13.3 amps, so if your heating water, your not doing anything else.

If I had run the Nexgen the water would already have been hot and I could have shoved much more than 80AH into the bank, so two hour Nexgen run, three hour Honda run. It took two hours to fill the water tank, so your your stuck with two hours based on watermaker output.


If your planning on using your main engine to charge an LFP bank, find out what the power level you can draw from it from the front of the crankshaft is. On another recent thread I found that my 44 HP Yanamar can only safely pull 100 amps at 2000 RPM and it’s a straight line, so at 1,000 RPM it’s only 50 amps, at redline RPM I believe it can safely have 8 HP pulled from it, but who wants to redline their engine battery charging?

Use the engine when your motoring to charge of course, but as far as it being a primary source of high amps to charge an LFP bank, find out what it’s limits are before you plan on that, maybe the Kubota is much more robust and you can?

I have 185 amps of shorepower charger, and that pretty much maxes out my Nexgen 3.5, it’s actually about a 28 amp at 120VAC or so machine continuously, any more and the motor sags in RPM, it’s wide open at about 28 amps.
Just to give you an idea of how much power 200 amps from an alternator actually is, then realize that an alternator is pretty inefficient, about 50% for a small frame, 65% for a School Bus one, so of your really getting 200 amps, that’s a lot of HP off the front of the crank.

[Dockhead]

Originally Posted by a64pilot

The Honda may in theory use less fuel than a Nexgen, but in actual practice it doesn’t.
I have both, the fuel burn is about the same. The Honda can do anything the Nexgen can, it can make water, charge the batteries and heat water, even run the AC if I had a soft start kit.
However the Nexgen can do all of them at once, the Honda can make 30+ gl of water an hour and charge the bank at 40 amps, or it can heat water, but nothing else.
So to use today as an example, I ran the Honda for two hours this morning, topped off the water tank and shoved 80AH into the bank, Solar has me fully charged by 1PM, this afternoon, I will run it for an hour to heat water, cause we like a hot shower. Water heater draws 11.7 amps, Honda rated continuous output is 13.3 amps, so if your heating water, your not doing anything else.

If I had run the Nexgen the water would already have been hot and I could have shoved much more than 80AH into the bank, so two hour Nexgen run, three hour Honda run. It took two hours to fill the water tank, so your your stuck with two hours based on watermaker output.


If your planning on using your main engine to charge an LFP bank, find out what the power level you can draw from it from the front of the crankshaft is. On another recent thread I found that my 44 HP Yanamar can only safely pull 100 amps at 2000 RPM and it’s a straight line, so at 1,000 RPM it’s only 50 amps, at redline RPM I believe it can safely have 8 HP pulled from it, but who wants to redline their engine battery charging?

Use the engine when your motoring to charge of course, but as far as it being a primary source of high amps to charge an LFP bank, find out what it’s limits are before you plan on that, maybe the Kubota is much more robust and you can?

I have 185 amps of shorepower charger, and that pretty much maxes out my Nexgen 3.5, it’s actually about a 28 amp at 120VAC or so machine continuously, any more and the motor sags in RPM, it’s wide open at about 28 amps.
Just to give you an idea of how much power 200 amps from an alternator actually is, then realize that an alternator is pretty inefficient, about 50% for a small frame, 65% for a School Bus one, so of your really getting 200 amps, that’s a lot of HP off the front of the crank.

If you want to make hot water while running a diesel engine, you should be doing it with waste heat, not electrical power.

[Adelie]

25g tank, increased range unless you never going to places well away from fuel supplies.
That’s a personal preference.
If you really want a separate generator, Honda 2200.