I confess, I’m not a marine electrician, and my understanding of boat electronics is at best rudimentary, but I’ve now been managing my boat’s power generation and consumption for three seasons and have developed an approach which seems to work well. Hopefully, you will find some of the following advice useful when it comes to managing your own yacht’s energy needs.

Every boat has its own particular balance of power generation and demand, so my system may not work perfectly for you, but it might help you start managing your precious amps a lot more effectively.

My wife Debbie and I sail up to nine months a year around the Med, predominantly lying at anchor. We avoid marinas and use our generator infrequently, to help save the planet and our budget.

I won’t address power needs whilst under sail here, as it’s not relevant to our cruising profile. Clearly, if you’re taking on an Atlantic crossing, then you’ll need to consider the power demands from your navigation equipment, lights and auto pilot.

We bought our pride and joy, Spirit, a Bavaria C57, brand new in 2021, and made several upgrades in an attempt to achieve the holy grail of self-sufficiency at anchor. We opted for 800 amp hours (Ah) hours provided by lithium batteries that weigh less than a single 150Ah lead acid battery.

Because lithium offers roughly twice the capacity of lead acid, that’s equivalent to 16 100Ah lead acid batteries with a combined weight of just over a third of a ton. Given Spirit’s generous beam, we’ve been able to accommodate four 420W solar panels, giving a potential maximum of 1,680W.

State of charge

The amount of electrical power you have available is all about batteries. Boat batteries are usually measured in amp hours (Ah) – the total number of amps devoured in one hour of use. So, a 120Ah battery will, theoretically, deliver 120A for one hour or 1A for 120 hours. But, of course, this is a little simplistic.

If you have lead acid batteries, you must never completely discharge them, unless you want to replace them regularly. Lead acid batteries should never be discharged below 50 per cent, so the practical Ah they really offer is half the theoretical Ah rating. In the above example, 60 hours at 1 amp would be the limit before you had to recharge the battery. Typically, the state of a battery’s charge is monitored by volts or a shunt battery monitor.

Knowing the state of charge of your batteries is critical to managing your power needs.

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I replaced our Bavaria’s standard 240Ah of domestic lead acid batteries with 800Ah of lithium batteries. The advantage of a lithium battery is its light weight. And, unlike a lead acid battery, it can be run down to a much lower charge. The downside is that lithium batteries have been known to explode.

To avoid this, they need to be paired with a battery management system, which is best left to a professional, who knows what they are doing, to install.

Charging sources

A standard ‘off the shelf’ mid-size production boat is likely to be fitted with a 60A battery charger which is used by the boat’s generator, if it has one, or when shore power is plugged in. The engine will also have an alternator which will typically produce between 35 and 60A of charge, depending on the size of the engine.

Assuming a 60A charge source, the batteries will receive 60A of charge in one hour. So, to put it very simplistically, if you have, as I do, 800Ah of lithium batteries and they are at 50 per cent, to charge them up to capacity would take 6.6 hours (400 amps divided by the charging source of 60 amps equals 6.6 hours).

Unfortunately, it’s not quite as simple as that. For example, there are various charging states – bulk, absorption and trickle (also known as maintenance or float) – and different types of batteries with different ratings, but I have kept it as simple as possible here for the purposes of explanation.

If you don’t have shore power (when at anchor, for instance), or don’t have a generator, the alternative is to run your engine for six hours under light load, which is not good for the engine and won’t win you many friends nearby who are trying to relax and enjoy a peaceful sundowner.

Battery charger

Increasing the size of your battery charger will speed up the process of charging from both a generator and shore power. I opted to fit a 120A charger and a 3kW inverter for my 240V appliances. There is always the option to beef up the engine alternator to feed a hungry family of batteries, but again, this means the boat engine needs to run for prolonged periods of time.

The 9kW Paguro 9000 generator I fitted produces a lot more power than we ever need. The generator powers both the 240V ‘ring main’ and the battery charger. However, the battery charging will be limited by the power rating of the charger, which in my case is 120A.

Renewable energy

Renewable energy for boats is either wind, hydro or solar. I opted for solar on a custom-made stern arch. I fitted four 400W panels, which produce a theoretical maximum output of 1,600W at 12V or, in amp speak, 133A.

Like lithium batteries, the voltage and charge from solar panels needs to be managed, so each panel is equipped with an MPPT (maximum power point tracking) controller to ensure the batteries are getting the right amount of charge when the sun is shining. The next conundrum was to get all this in perfect balance based on the boat’s power consumption.

Background noise

I started by building a spreadsheet to calculate the various power ratings of my many onboard electrical appliances. However, this isn’t simple as you must calculate the power demand of each appliance, estimate how long you will run it for, and when you will run it.

You then need to map this over time to calculate a theoretical power demand and how much power you need to be generated. Estimating the power output from my solar panels alone became a headache: calculating the assumed number of ‘sunny’ daylight hours, the angle of the sun, solar panel efficiency, and when the next solar eclipse would be! So, like all people with limited brain capacity, I gave up.

I decided, instead, to take a more pragmatic approach. The first thing I did was to measure the ‘background noise’ of my boat; that is, the power being consumed whilst at anchor and not running any major appliances.

It turned out to be around 200A, which is very high; but then I do have three fridges, a deep freeze and more internal lighting than Blackpool Illuminations.

The time it takes to recharge my batteries once the sun has dragged itself up to the right angle differs dramatically based on which way the boat is lying. If my stern is exposed to the sunny side my batteries will fill to the brim in a few hours. However, if my bow is facing the sun, it takes a lot longer.

On an average day, we are at 100 per cent by midday or early afternoon, leaving a good four or five hours of surplus power generation for running more critical systems.

Power draw

We have a lot of power consuming paraphernalia on board, for example the kettle, coffee maker, hair dryer, microwave oven, and so on, but I ignore these when it comes to power management as they are used randomly and are never on for long.

However, we do have several appliances that are critical to our power management, including a washing machine, water maker, ice maker and water heater. All of these have high demand and can run for long periods of time. I also have air-conditioning which can be run off the inverter, but I prefer to use fans and open hatches to keep the temperature tolerable when we are at anchor. Air-con is a battery power killer and is best left to when shore power is connected or the generator is running.

For this equipment I simply use a rota and allocate a specific day to run either water production, laundry, ice making or water heating. We do laundry once a week without using the drying function and relying instead on nature’s outside dryer, which does tend to lower the tone of an idyllic anchorage.

The water maker produces 60 litres an hour and I typically run it for around four hours, which will then keep us going for several days. I fit in ice-making and heating the water at other times.

Having guests on board who insist on having a shower every time they go for a dip off the swimming platform requires the water maker to be run most days. Inevitably under these circumstances I lose the battle of consumption versus generation and will need to resort to running the generator.

I always delay this until my battery charge is showing 30 per cent or less in the morning. At that point I will run the generator for three to four hours, which is enough to get my batteries back up to around 70-80 per cent, and then let the solar panels take over.

When I run the generator, I take advantage of the surplus power it produces by running as many devices as possible: I make water, run the air-con and heat water. Never waste any of those precious amps! I find that I run the generator, on average, every eight to 10 days when we’re on our own and every four to five days when we have guests.

How to improve power management on a yacht

Before increasing the capacity of your service battery bank you need to calculate your total power requirement by multiplying the amperage of all the equipment by the period of time it will be run over a charge cycle (usually 24hrs).

Tally up the amp hours and then double the result (to allow for not going below 50 per cent of your charge capacity). Then add another 20 per cent to ensure you will always have enough to spare.

If you already have separate engine start and service batteries but want to add further service batteries, they should all be of the same age, type and capacity (Ah rating) to the first. It’s best to create your service bank from a number of smaller batteries and then link them together to achieve the total voltage and capacity you require.

If you’re planning to install a large bank (500Ah or more), it is often better to use 6V cells for this as these allow a large deep-cycling bank to be created, while still having the ability to move them around easily or distribute them evenly over a greater area.

Stay in charge

Once you’ve decided on the battery type, make sure you have enough charging power to fully charge them between cycles. As a rough guide you will need to be able to bulk-charge the bank at a minimum of 10 per cent of its rated capacity (ie. 20A for a 200Ah battery).

However, 20 per cent is a better figure to aim for if you’re looking to fully recharge over one night in a marina. Modern AGM (absorbent glass mat) style batteries can usually take a greater charge than wet lead-acid type, although gel cells require a more particular regime if they are not to be damaged.

Chargers (both mains and alternator regulators) should be of the multi-stage type, with bulk, absorb and float stages. This allows the batteries to be rapidly charged until they reach around 90 per cent charge, then the charge voltage drops to attain the final part of the charge more slowly, keeping temperature (and hence internal resistance) down, and eliminating gassing.

Temperature noticeably affects a battery’s ability to give out and absorb charge. The colder a battery gets, the greater the power required to charge it fully. For this reason, always fit a charger or regulator with a temperature sensor that will automatically compensate for these differences.

Most power devices produce a trickle charge, and are used to keep the engine battery topped up. However, if you’re planning to install a powerful (5A+) wind or water generator, or a large solar array, then you’ll need to install some sort of voltage regulator to prevent overcharging. This can vary, from a small solid-state switch for small solar panels, to a large dump resistor that dissipates excess charge from a wind generator through heating up a wire-wound resistor.

Monitoring your batteries

The easiest way to ensure your batteries are kept in tip-top condition is to observe their state of charge every day you’re on board, using a modern ‘smart’ battery monitor. This will give you a real-time display of the current going in and out, the state of charge (SOC), and the remaining capacity available. They also often have alarms to warn you when the voltage is dropping dangerously low, or if too high a charge is being applied.

A rough idea of the SOC can be attained using a voltmeter, but this is not particularly accurate and can indicate a false condition when recently charged or under a heavy load. It’s far better to install a monitor that has a shunt, which measures current flow over time and can calculate the available charge capacity remaining much more precisely.

Optimising solar power

The efficiency of solar panels can be compromised by saltwater and long-term exposure to UV and high temperatures. Good regular maintenance will improve a solar panel’s performance.

Clean your solar panels early in the morning, while they are at their coolest, as cleaning them when they are warm or exposed to direct sunlight can cause internal thermal stresses.

Use distilled or deionized water to avoid the formation of mineral stains or deposits on the surface of the panels, and avoid using harsh chemicals or abrasive solvents that could scratch the photovoltaic cells. Let the panels air-dry or use soft cloths, and make sure no water residue is left. Check regularly for cracks, breaks or loose connections.

Hydrogenerators

Hydrogeneration has become a great deal more efficient in recent years. It’s a very simple concept: the yacht’s motion through the water turns an alternator on the transom-mounted hydrogenerator which generates electricity to recharge the boat’s batteries. Achieving 300Ah each day is a realistic expectation when cruising at 7-8 knots.

Main prop regeneration

You can also use your main propeller to ‘regenerate’ electricity whilst under sail by using a parallel hybrid propulsion system where an electric motor is installed alongside the engine. Lynch Motors in Devon has supplied its systems to Vendée Globe boats for years, purely as a re-generator, and now produces a Red Snapper electric motor for cruising yachts.

The only problem with a regeneration system is that the pitch required for the propeller to drive the boat efficiently through the water may not always be the same as the pitch for optimum regeneration. Manufacturers have tackled this in different ways.

Oceanvolt has developed its ServoProp for saildrives, which electronically adjusts its pitch depending on speed and function. The latest incarnation allows total 360° blade mobility and faces forwards, increasing efficiency: at six knots, it produces an eye-watering 1kW of power.

Bruntons has another solution with the cleverly engineered Autoprop, which automatically pitches up to match the boat speed. Its Ecostar version of the prop can generate 200W at five knots and up to 1kW at 10 knots when connected to an electric motor.

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