Hybrid Power: Myth or magic?

Following a recent enquiry from a K50 client to have a hybrid power system fitted, the Kraken technical team explored the pros and cons of such a system, and it soon became clear that the search for the ecological holy grail of a blue water yacht powered entirely by renewable electric energy, that has a zero-carbon footprint, remains exactly that: a search.

To date such a system cannot be built, unless the owner accepts that the yacht – other than at a berth with shore power can only practically be powered by the wind in its sails, other than for just an hour or two. Even then, only if it has been regenerating power whilst sailing for several days and without using the regenerated power for onboard systems or equipment.

This is because most hybrid drive systems are based on boats returning to a marina berth to recharge their batteries, and most blue water cruising yachts do not sail along the nearby coastline where power points exist, but set off across many hundreds of miles of ocean where they do not. The conclusions we have arrived at are focused therefore on the needs of the long distance blue water yacht.

The core issue is the power required to push a yacht through the water when she’s not sailing. Our investigations were based on our Kraken 50, but the conclusions reached apply to a greater or lesser degree to any heavy displacement blue water cruising boat.

The first key factor to note is that it takes approx 28kW to push a K50 (20 tonnes displacement) through the water at six and a half knots for an hour. Surprisingly many familiar brands already established in this field, do not start from this premise. It is, however, fundamental. The two essential factors to establish are these: how much power will be required to run a boat for a given time or distance; what is the yacht’s resistance factor? This is because the origin of the required power must come from one of three sources: 1) stored electricity in the drive side battery bank 2) generated or regenerated electrical power onboard 3) diesel fuel.

Only lithium batteries can be used because the weight, capacity and size of conventional storage banks is impractical on a boat.

Some other issues that must be considered:

1) Since the electric drive motor will require 48v the minimum battery capacity that can realistically be used is 2 x 300 amp x 24v batteries, which must be cabled in series to give 300 amps at 48v.

2) Lithium batteries have a usable capacity of around 80%. Each 300 amp x 24 v battery can provide 10kW of usable power assuming they are full.  

3) At 6.5 knots each battery will only be able to power the yacht for just over 20 minutes.

4) This running time will be reduced further if the hull has any fouling or there is some wave motion on the sea.

5) The power required to drive the yacht is radically reduced at lower speeds: the K50 requires only 5kW to run for an hour if the speed is 3.5 knots, therefore one 300 amp battery will power the yacht for over an hour at 3.5 knots.

6) The regeneration that can be achieved from the prop turning whilst sailing at seven knots is approximately 400 watts therefore to recharge the battery bank sufficiently to drive the yacht for just one hour at 6.5 knots it will take 70 hrs!

7) The output from the size of solar panels that we are able to accommodate is approximately 200 watts, so solar will contribute half that of hydro regeneration and only in good sunlight.

8) While price differs depending on the brand of lithium batteries used, we found the cost per 300 amp x 24 v battery with the battery management unit required is approximately $5,000 USD per battery.

From those conclusions our analysis follows:

Since it would require 36 x 300 amp lithium batteries to power the vessel for 12 hours, having no diesel engine back-up is not viable as over long distances such alternative drive power will be necessary.

Therefore battery cost alone – $180,000 USD – renders this solution impractical: and even if there’s enough room in your wallet for such a sum there is not enough room aboard for such a system. 

Strike one.

We were next recommended a hybrid drive system based on electricity power from an on board diesel generator, and discovered two fundamental problems, as well as some inconvenient truths, in the system that was suggested. 

There is no getting away from the power needed, and considering the inefficiencies there are using a generator based system, which turns diesel power to AC electric power, then AC power to DC electric power, DC to battery power, battery power to electric drive power, which finally turns the prop shaft, it’s no surprise to discover we would need a generator of at least 32kW output or more.

1) This is a massive generator which is far too large to fit into a 50ft sailing yacht.

2) Its fuel consumption will be approx nine litres per hour.

3) The fuel consumption of a Yanmar 80 HP (58.8 KW) diesel engine running at 1600 rpm as required to power the yacht to six and a half knots is less than 3.5 litres per hour.

Strike two.

The Kraken answer is. It’s clear that if we want to power the yacht, other than by wind, and have home comforts such as air conditioning, water maker, refrigeration, auto pilot, radar, GPS plotter, VHF radio plus electric cooker and more, we need to accept that we need a diesel main engine to drive the yacht, but we then need to improve its efficiency, and include a reasonable sized generator that is dedicated to produce electric power for the house battery bank and the onboard systems.

For the specialised use of blue water cruising, the only solution we found is compatible, incorporates Hybrid Marine’s V Twin electric drive motor/generator and a diesel main engine.

So after filtering out all the snake oil, and the dragon’s teeth, here’s the hybrid drive system we have developed for the Kraken 50. It comprises of:

1) Yanmar 4JH80 80HP diesel engine.

2) Hybrid Marine V Twin electric drive generator.

3) 2 x 300 amp x 24v lithium batteries with battery management unit cabled in series to produce 300 amps x 48v (the drive battery bank can be increased to give longer silent power running).

4) 2 x 300 amp x 24v lithium batteries cabled in parallel to provide 600 amps x 24v for the house battery bank (the house battery bank can be increased to provide more power storage if required).

5) A Northern Lights 7kW generator.

6) Victron Quattro inverter charger or chargers according to what onboard systems are installed. 

7) A Bruntons Eco Autoprop to provide maximum drive efficiency at lower RPM and improved electric regeneration whilst sailing.

8) A solar panel array with controller to produce 200 watts of electric power.

This system has many benefits including the fact that it will reduce your yacht’s carbon emissions and save money on fuel consumption.

A) If the main engine fails, the Northern Lights generator can be run to charge the batteries sufficiently to continually power the vessel by the V Twin Drive motor at just under four knots continuously.

B) The V Twin generator will produce up to 12kW of electric power whilst the engine is running and in gear. This will mean that it will be unnecessary to run the NL generator to top up the house bank if you have been running under main engine power for anything over an hour or so (dependant on the house power consumption of course). 

C) The V Twin generator and Autoprop will regenerate up to 500 kW whilst the yacht is sailing again, reducing generator running times.

D) The solar panel will provide top up power to keep the batteries fully charged whilst the vessel is left unattended and will provide extra power for the house bank to reduce generator running time.

E) The lithium battery bank’s lifespan is approximately three times longer than a conventional Gel or AGM battery bank.

F) Lithium batteries can absorb much higher input levels of charge power. This efficiency enables them to take all the charge capacity that is available to the batteries and they will charge faster than conventional Gel or AGM batteries. 

Conclusion

It’s hard to over-emphasize the value of having a backup propulsion system. Although the generator and V Twin drive motor described in the system will only provide a little under 4 knots or so of steerage, anyone that has faced sailing into an anchorage hidden up a dog’s leg of a reef-strewn channel will know that a guaranteed four knots of steerage way are gold dust. 

Whilst this system is more expensive than the conventional alternative, it greatly reduces the carbon footprint of the vessel.

The technology does not yet exist to power a sailing yacht on passages by renewable energy alone. This system however will greatly reduce fossil fuel consumption and carbon emissions.

Assuming that the yacht is used regularly, the costs of the system should be recouped within 5 years or so.