The choice between twin diesels and hybrid electric (electric motors with diesel generator range extender) was our most difficult decision and it actually gave us one or two sleepless nights.

Fundamentally, the CM46 is light, easily driven and able to sail most of the time, so overall fuel consumption by the standard, twin 30hp diesels will be pretty low. 

But hybrid, or even full electric, systems are the talk of the town and of course appealing, in principle. And if they’d work on any boat, surely it would be on a light cat. 

We wanted a hybrid system to work for us (all electric would definitely not give the range and battery charging required for our cruising) so built up a spec with one of the well-known electric propulsion brands: 

• Twin 13kW electric motors 
• 15kVA diesel generator (generator output is the limiting factor to sustained speed under power – but higher output gensets are too large and heavy)

• 20kWh 48v Lithium propulsion battery bank, feeding a smaller 12v house bank
• Run time on batteries: About 2 hours depending on speed – circa 10 to 14 miles
• Max speed in short bursts: Approx. 6.5 to 7 knots (max speed with diesel engines: Circa 10 knots)
• Cruising speed (using generator to charge batteries): Circa 5 to 5.5 knots (cruising speed with diesel engines: Circa 7 knots)
• Diesel consumption at cruising speed using generator: Circa 3lt / hr (consumption by diesel engines: Circa 2lt / hr)
• Total system weight (with above battery capacity): Similar to the diesels 

This is our summary of the pros and cons of the hybrid system:

Pros:

• A “forward looking” solution 
• Short periods (about 2 hours) with zero diesel consumption and quiet motoring at approx. 5 knots
• Large propulsion battery bank which would also feed the house system at anchor 
• Two props providing regen under sail, meaning we wouldn’t need a separate hydrogenator
• Only one diesel engine (the generator) to service

Cons:

• Added complexity – 48v motor system – 12v house system – generator – fast chargers etc
• Diesel generator required (in our view) for motoring range and for recharging batteries as solar / hydro will not always sufficient 
• Diesel consumption by the generator at 5.5 knots would (potentially) be higher than diesel engines 
• Propulsion props are less efficient for hydro regeneration than specific regen props, leading to lower charging rate
• Higher cost
• Uncertain reliability factor with younger technology  

Our hearts said go for the hybrid and we very nearly did. 

However… we became concerned that system complexity was rising. DC generators (able to directly charge the batteries) are larger and heavier than AC. Generator output defines the power / speed available for motoring (once the batteries are depleted after two hours) and the 15kVA minimum required meant we’d have to go for an AC unit, to retain the low, centralised weight by mounting it in place of one of the diesels. This would have required several fast chargers to convert the AC output to quick enough DC battery charging… in addition to the need for two battery banks (48v propulsion & 12v house) and voltage conversion between the two.

We just couldn’t settle on the hybrid option because the uncertainties and speed limitations were not significantly offset by reductions in diesel consumption. Motoring periods over two hours would use the same (or more) diesel – even sub two hour motoring would probably need to be followed by generator time to speed up battery recharging and allow electric house systems (inc cooking) to be used.

Decision…

We finally decided to opt for twin diesels and felt pretty disappointed on one level. But, to be honest, we also felt a sense of relief. 

We put in the time, researched the technical aspects and the benefits (to the planet and to our practical use) of hybrid versus diesel and we’re now comfortable with our propulsion choice. 

I don’t think overall diesel consumption will be meaningfully higher, thanks to the CM’s performance with small engines. It’s a shame we won’t have the huge battery bank that goes with the electric motor system but we’ll increase the house bank capacity which, coupled with a large solar array and a Watt & Sea hydrogenator (one of our favourite bits of kit on the Pogo), should mean the engines are generally only needed for calm passage-making and parking.

Crucially, we always wanted to create a simple, effective boat.

Hybrid v Diesel – Our Summary

A hybrid system on boats like the CMs could be great if the boat was being used for weekending, racing, or even long-term cruising provided the crew were prepared to accept a “wait for the wind” approach or make slow progress against currents. It suits short spells of motoring in and out of harbour / anchorage with long periods of re-charge from solar, shore power (hopefully green sourced) or regen under sail before motoring again.

We may be thinking of our time-limited, cruising past, but passages such as a 36 hour, flat-calm motor out of the Med against the current, with the alternative being over a week’s wait for breeze, are familiar to us. We envisage those situations being part of our cruising life – multiple coastal and inter-island passages between long ocean legs. With weather getting more extreme we see the ability to motor efficiently in calm conditions as a major bonus of the cat. 

We couldn’t ignore a safety consideration as well. We, particularly Amanda, have been closely involved in a tragic MOB situation (albeit in very different, inshore racing, circumstances). The thought of limited power impacting the task of finding and recovering someone in bad weather offshore is not something we can ignore.

CM diesel engine installation

The CMs are designed with the engines mounted relatively well forward to centralise weight. They are, therefore, dry and won’t get splashed by sea or rain water when underway as can happen if you have aft deck hatches for access. No corrosion issues – easier maintenance. Maybe there will be more noise in the boat but hopefully that’s offset by lower noise in the cockpit, not sleeping directly above an engine and, of course, the boat’s minimal need to motor makes the biggest difference.

We’re happy with Current Marine’s recommendation of Nanni diesels, which are based on Kubota engines – internationally used in all sorts of applications such as agriculture. We replaced the Volvo engine in one of our older cruising boats with a Beta (a British Kubota derivative) and were impressed by its smooth efficiency. Parts should be widely available – and maybe less extortionately priced than some marine brands?!