Our goal is to be able to sit for long periods at anchor, without having to resort to the use of the engines or genset to recharge the batteries. We also want a system which has high reliability and longevity.
Our requirements for the FPB series at anchor are exactly the same as with our sailing designs. The difference, to the extent there is one, comes under way. With sail we want to see electrical systems which will give us sufficient power to complete contemplated passages under the assumption that no recharging is available, I.e. that we’ve had some form of breakdown in our charging gear or engine. In a strictly power regime, this is not required.
Powering, with its constant running of engine, eliminates concerns about electrical consumption under way. This means we can turn on the air conditioning if it is a little warm, use large RADARs, and generally forget about the type of power management that goes naturally with sail. This ability to consume large amounts of electricity when passaging, for air conditioning and electronics, is one of the big comfort advantages of this type of boat.
The majority of power yachts are run on AC current. This has the advantage of smaller, less costly motors, lighter wiring, and the ability to use household appliances for the fridge and cooking. However, a strictly AC boat also means significantly higher power requirements at anchor, which is why most powerboats seem to run there generators many hours a day.
We prefer to have a quiet boat onboard and be a good neighbor to others in the anchorage, so we’ve stayed with our sailing based DC approach to systems.
AC Power Requirements
We have modest AC power requirements. The biggest user is air conditioning, which as we’ve mentioned is rarely used at anchor where the boat has excellent natural ventilation. However at the dock (where we spend little time) and at sea, we are finding the air is in use a fair bit.
Next in line is the watermaker, and then the washer and drier – both of which are full sized units. Finally we have the domestic appliance and tool requirements, which are very modest. The biggest consumer on most powerboats is in the galley. Since we’re cooking with propane, this is not be a problem for us.
At sea our 4.0 kW DC alternators (one on each engine for up to 8kW output) provide power for the big inverters which make our AC current. At anchor, the inverters run off a huge battery bank and for light loads, provide for our needs. The genset comes into play during the wash and dry cycle.
We wash every other day, so we’ve gotten into the habit of running the genset every other evening. This provides power for the washer and drier, offsets the DC loads, and usually takes care of the TV and sound system at the same time. To keep the genset properly loaded we’ll also do a little battery charging and run the watermaker.
There are as many ways to set up an AC system as there are electricians. Our goal, as always, is to keep the system as simple and efficient as possible. Towards this end we use a single phase, 230VAC circuit for all the heavy loads (washer/drier, air conditioning, watermaker). The genset is wired single phase, which means we do not have to worry about balancing loads. To get 115V for the smaller appliances around the boat, we feed a step-down transformer off of the 230V panel, which supplies the 115V needs.
It has taken us a while to get the inverter system sorted out to meet our requirements. At present we have three Victron units running in parallel, giving us a total continuous output rating of 7500 Watts, with 210 amps of DC charging ability. These are set up so that they will support shore power or the genset if there is a momentary overload. They are also smart enough to sense genset load and to take up available with the battery chargers.
For backup at anchor and at sea, and for use on wash day as mentioned in the introduction, we’ve got an 8 kW genset, made by Northern Lights.
We put 300 hours on this unit since leaving New Zealand (as of fall 2007) which works out to about average of about an hour per day.
The genset originally lived in a large sound shield, which we did not like because it blocks our view of what is going on with the unit. So, we removed the sound shield. Because of the location of the genset in the aft engine room and the engine room insulation it is very quiet in the saloon and forward cabin.
We always bring shore power aboard via an isolation transformer. These devices break the connection between the boat and the power company’s grounding system, reducing the risks of shock and electrolysis risks in the process. We’re using Charles Industries isolation transformers for this purpose.
On the dock, the normal service is 230V, hopefully 50 amp. However, the transformer also has a second set of “taps” which allow us to step up 115V to 230, should the 230 not be available. We used these taps in Hawaii. This transformer also is happy to run on 60- or 50-cycle current.
We like a quiet boat at anchor. We do not want to hear a generator running (nor do our neighbors), and we want enough power stored to enjoy our cruising lifestyle. This means lots of indirect lighting in the evening, more computer time than should be the case, and a large fridge and freezer(s).
Our basic systems are 24 volts DC-based.
At the heart of the DC systems is our battery bank. This is an area where the switch to power is a big advantage. We’ve got lots of space under the saloon sole, in the “basement”, for as large a bank as we need. As we were under budget in terms of our target displacement, we could afford the weight of the ultimate battery bank.
On our sailing designs we look at the house bank in two contexts. The first is on the assumption that we lose the ability to generate power in the early stages of a long passage (this has never happened). In this situation, we want enough power to run the pilot, critical electronics, and masthead light for at least 10 days. The other requirement is the ability to sit on the hook – usually for a week – without needing to charge.
On Beowulf this brought us to a 1000-amp hour 24-volt bank of traction batteries. Traction batteries can be cycled up to 80% – in fact it is good to hammer them in this manner on occasion as it increases their usable capacity. This gave us 800 usable amp hours from the 1000-amp hour bank.
The new boat does not have the passaging DC power needs of our sailboats. With twin engines, each with its own special charging alternator, in addition to a genset which can charge through the inverters, we are not going to run out of power at sea. This leaves the at-anchor requirements. You could argue that the same size bank as we had on Beowulf would be more than enough. However, the alternators and inverter chargers have the ability to efficiently charge a larger bank than this, so we went with a 1600-amp hour bank (24 volt – 20 amp hour rating). This weighs in at just over 3600 lb. (1610kg)- leaving us still below our weight targets.
We are using the sealed traction type batteries. The image above shows half of the bank installed (on the port side) before the boat was completed (hence the protection to keep them clean).
The larger bank offers several advantages. We get a faster recharge from any given state of discharge. Second, the batteries will not be cycled as deeply, so they will last longer. At anchor, we’ve been able to sit for as long as a week without moving the boat or running the genset.
We do have the battery capacity to run our air conditioning for one of the sleeping cabins all night, for several nights in a row. This was one of the benefits of this large bank. However, as discussed elsewhere, we’ve found that on the occasional hot, still night at anchor where we want air conditioning, it works best to run the genset for an hour, with all hatches and vents closed, lowering temperature and humidity. We are well enough insulated that this will keep us comfortable until the sun comes up.
We’ve been using Electrodyne alternators since the early 1980s (when the company was originally called Maremount). Over the years we’ve tried other brands, but always come back to Electrodyne.
Most alternators are designed not to charge batteries, but to maintain them against a given load. Take an emergency vehicle, as an example. It might have 100 amps of load with various devices in operation, with the engine running. So the battery never really gets discharged and the alternator loafs along, relatively speaking. To keep its output down, the field current typically cycles on and off, which helps keep the alternator cool.
Now come on board a boat with a large battery bank and an intermittent charging cycle. When the alternator starts up, it has to go to full output, sometimes for hours on end, to get the batteries back up to their charged state. This means the current flowing to the alternator field is on constantly – what is called full field operation. There is no chance for the alternator to cool down, except via the air being pulled through it by its own fan.
The majority of the heat comes from the diodes, which convert the alternator’s AC power to DC. Most alternators get so hot, you could fry an egg on them. Heat reduces output, and shortens alternator life.
There are a couple of ways around this problem. When we first started using our big traction battery banks, we worked with Bob Sampson at Electrodyne to come up with a unit that would give us good service life. After trying a variety of approaches – and burning up a lot of alternators – Bob came up with a design that eliminated the diodes from the alternator body. He put them in a fan-cooled aluminum box, which is remotely mounted – so the heat source is removed from the alternator body. That was 20 years ago. There’s been a lot of tweaking in the ensuing period. Bottom line is that we’ve not fried one of Bob’s alternators in many years, and we’ve worked them pretty hard!
The output curve above is for the model of alternator we’re using. It is very conservative and we are actually getting about 10% higher output at cruise RPM than the curve shows.
There is one of these alternators on each engine, and a spare – just in case in the basement. We’ve got a 1.7-1 overdrive between the alternator pulley and engine crankshaft pulley. This means we will run at 3200 alternator RPM when the engines are turning over 1900 RPM. That’s a little over 3.5KW per alternator, when they are running at, and our normal cruising speed. At this level of output the alternators pull about eight HP per engine – not a lot compared to what the engine can give, but if we’re running the air conditioning off the inverters, plus some other appliances, the fuel consumption jumps about three quarters of a gallon (3 liters) per hour.
These Electrodyne alternators are not cheap. They cost about double what other big units go for. But if you look at them as an investment in reliability, and consider that odds are you’ll be using two or three conventional alternators during the life of Electrodynes, they turn out to be economical in the long run (Beowulf has her original two units after 40,000+ miles).
One last thought if you’re thinking about big alternators. The high horsepower requirements put big strains on the drive belts, alternator brackets, and pulley tensioning devices. Add in the power pulses which are inherent in all diesel engines, and it takes muscular engineering for all of this gear to stand up over time. If you are using V-belts, be sure that belt wrap, pulley diameter, and belt specifications are up to the task. And be certain that the alignment of the pulleys is spot on. Any dusting of the belts is an indication of alignment problems. Belts which slip create huge quantities of additional heat. With these John Deere diesels we’re using “poly-V” belts or ribbed belts as they are sometimes called. This is the new thing in drive belts – more reliable and better at power transmission with less belt tension. It took us some time during sea trials to get the drive belt scenario just right, but we put 1500 hours on the first set before changing them on a preventative basis.
As we mentioned at the beginning we also have an AC genset (8KW). This genset is primarily onboard as a back up, and for use for short periods at anchor when we need to run the drier, or feel it necessary to use a air conditioning. The inverter chargers give us a total of 210 amp charging capacity at 28 volts. If we’re using the full output of both inverter/chargers, it leaves a couple of KW on the genset for the drier, making water, etc. So along with back up we also have the ability to put a bit of juice back into the batteries during our weekly clothes washing cycle.
The last category of power consumption is the watermaker. This uses a 2HP single-phase AC motor. On our sailboats we typically used one or two smaller DC units, and ran them off the batteries when charging or just while sitting at anchor – and kept water in the tanks to a minimum while at sea (this is a big plus on performance). On this boat we typically run the water maker each day to replace the weight of the diesel fuel we are consuming – we find we are more comfortable heavy.