Hybrid boat: How to equip your boat with an electric motor and combustion engine

How does a standard boat with an outboard motor become a hybrid boat with a combustion engine and electric drive? And is a conversion even worthwhile? We have tried it out.

A text by Andreas Mengen

Anyone considering the purchase of a new or used boat should be aware of a few points beforehand: What is the intended use, which cruising areas are in focus, should it be more leisurely or faster? Does the specification include extended trips, weekend trips or just day trips? The answers have an impact on the individually preferred boat type - and therefore on the type of drive.

Some time ago, we were looking for a new motorboat to take on extended trips on European inland waterways. Looking to the future, we asked ourselves whether we should still rely on a combustion engine today. Or whether what has long been recognised in the automotive sector might not happen: electric drives are slowly but inexorably gaining ground.

Combustion engine versus electric drive

That would come as no surprise. On the water, an electric drive can score points with its emission-free operation, the absence of lubricants and its whisper-quiet running. The ongoing operating costs are characterised by low maintenance and low energy costs. If the electricity is also generated from renewable sources, the machine is even climate-friendly.

The rapid advances in battery technology driven by the automotive industry are also reducing not only costs but also energy consumption for production while simultaneously increasing energy density and charging speed. Last but not least, the skipper of a boat with an electric drive is likely to be deeply relaxed about the increasing regional driving bans on combustion engines. So far, so good.

Read more about this:

• Interview: Why outboard boats are ideal for electric conversions

However, the combustion engine still has the edge when it comes to fast travelling and longer distances. Cruising motorboats in particular need a lot of energy to make rapid progress. And although combustion engines still have a poor efficiency rating, they come into their own when high power is required.

This is thanks to the large amount of energy that nature has packed into one litre of fossil fuel. Those who refrain from long full-load journeys are also rewarded with very adequate ranges. If you wanted to make the same amount of energy available on board in electrical form, you could or would have to design boats as floating batteries - despite the better energy efficiency of electric motors.

We therefore opted for an internal combustion outboard. In addition to the advantages of an outboard in terms of space utilisation below deck, the requirements for sailing on current waters were the deciding factor. If you don't want to be in a neck-and-neck race with pedestrians on the banks of the Rhine when travelling uphill, you need to be able to keep going for at least a few hours under full load and make headway at 15 km/h or more over the ground. If this is important to you, there is still no getting round a combustion engine. But as we know, technical development is continuing and the range of electric drives for boats is constantly increasing. A lot is happening with outboards in particular, as these are often used on smaller boats. Due to their lower weight, these can also be driven at speed with moderate energy input, meaning that reasonable ranges can already be achieved today.

Armed for the future

This is where our considerations come in: If we buy a motorboat with a combustion outboard engine today, we can easily replace it in the future with a suitable electric outboard engine that may then be available. At least for the engine replacement itself, we expect significantly less conversion work than with an inboard solution.

The idea of later electrification had an impact on another aspect of our choice of boat: Our Neon 910 from Nautic Yachts is particularly eye-catching due to its straight, elongated roof, which extends over the wheelhouse and aft cockpit. It not only protects against sun and rain. It also has plenty of unshaded areas for solar panels (PV modules) on the upper side.

The sun in the tank

We fitted these with inconspicuous flat modules from Sunware with a rated output of around 420 watt peak. The large roof hatch in the centre remained unobstructed so that light and air could continue to enter the cabin. The result is impressive. The modules generate more electricity than we need in everyday life on board - as long as no large consumer such as an electric motor is installed. But that should change quickly. Despite being very satisfied with the performance of the 150 hp outboard motor, we took our first steps towards electrification in the first season: the small Torqeedo Travel (approx. 3 hp) from the dinghy was attached to the swim ladder for test purposes using a temporary motor board. And lo and behold: it works!

It does take some time to overcome the inertia of the Neon's four tonnes of displacement. But then the relatively large boat with the small motor really does sail through the water. Snappy manoeuvres and "stretching" don't work, but holding course and steering are possible.

Two realisations emerged: If you stay well below the hull speed, you can get a boat moving with surprisingly little energy. However, the position of the helmsman effectively prohibits even occasional use of a hand-controlled electric outboard motor. Who wants to and can squat at the back of the bathing platform with the motorised tiller in their hand and keep course without reasonable foresight?

This means that any drive system that is to function properly in practice must be controllable and operable from the cockpit. In addition, a certain minimum power is required to achieve an acceptable travelling speed.

Nevertheless, the whole thing was the trigger for a change of plan: How about a hybrid drive? A supplementary electric drive could be particularly useful for canal or river trips downstream. The idea of taking a commercially available electric outboard motor in the power class up to ten hp on board as an additional drive took shape. If this drive could be designed in such a way that manoeuvres could also be carried out without using the main engine, it would kill two birds with one stone. The auxiliary engine could also act as an emergency drive should the main engine fail.

Hybrid: The best of two worlds

Our plan received an additional boost when the sun-drenched roof of our clubhouse was fitted with a large PV system. This floods the harbour network with plenty of green electricity, especially during the boating season. The original idea of "combustion engine out - electric drive in" was thus shelved, and the search for an electric outboard motor for the hybrid solution began.

Faster than expected, we found a Torqeedo Cruise 4.0, only two years old, with two lithium batteries (100 Ah, 24 V each), which a long-time water sports enthusiast no longer had any use for. Now it was time for the practical realisation. The goal: the additional electric drive had to be easy to install and remove and, as the sole drive, at least provide sufficient driving performance. It must not hinder the existing combustion engine during operation. And, of course, the electric drive must also be operable from the cockpit, just like the combustion engine.

No sooner said than done: the realisation

It was initially unclear where and how the electric motor could be mounted. The initial idea of placing it to the left of the combustion engine had the charm of being able to use the existing mirror for mounting.

The dual motorisation would also have been visually less conspicuous. And both drives would have been positioned close to the existing hydraulic control unit, to which a mechanical connection had to be established.

Although, according to the plan, the combustion engine should be raised in electric mode, it should be possible to lower and deploy it if required at short notice - without restriction from hard to port to hard to starboard. And this is exactly where there were problems: At extreme rudder angles, the two propellers touched each other. So a solution with more safety clearance between the combustion engine and the electric motor had to be found. The offset to the end of the bathing platform solved the problem. However, a suitable and sufficiently stable mount first had to be created at this exposed point.

With the help of a self-created template, a metalworker set about manufacturing the required part from VA. After a trial assembly, the retaining flange was enlarged again and a stabilising rod was added. The whole thing is now held in place by screw bolts with a counter plate on the inside of the bathing platform.

The next step was to create a push-pull connection between the two engines so that the existing hydraulic control system not only moves the combustion engine but also the electric motor at the same time. A customised actuating cable was attached to the combustion engine or the flange of the hydraulic cylinder and routed to the electric motor. A stainless steel tiller additionally mounted on the Torqeedo absorbs the movements of the cable and converts them into rotations that correspond exactly to those of the large motor.

The inner motion cable always needs a fixed holding position in front of the connection point for the immobile outer cable. This point was designed as a wooden bearing and fulfils a crucial additional task: it also serves as a support when the electric motor is taken out of the water when not in use and placed on the bathing platform. Its head then rests there, and the motor housing finds its place in a semi-circular, customised second mounting device. The motor is thus optimally secured against tilting and remains safely in position without any further fastening.

The Cruise 4.0 requires a 48-volt power supply, which can be provided by two 24-volt lithium-ion batteries connected in parallel. These must not only be stored securely in the cockpit to prevent them from slipping, but also require a connection to the motor and charger. The motor cable exits at the stern just above the bathing platform and then runs in a direct line to the motor.

Wiring issues: The tricky power supply

The batteries are charged using two chargers, which are permanently mounted in the cockpit and are supplied either via shore power or 230-volt on-board power. This is generated by the inverter drawing 12-volt current from the consumer batteries and converting it. The consumer batteries, in turn, are permanently recharged via the PV modules.

If time is not an issue, the lithium-ion batteries can be fully charged in this way using electricity generated by the PV modules. You are then not dependent on shore power at the berth. As we are not on board all the time during the season, this is a viable option.

Shifting gears and accelerating

Coupling the electric motor with the hydraulic control of the combustion engine via the control cable ensures that the Torqeedo can be controlled from the cockpit. The only thing missing is the ability to accelerate from here - forwards and backwards. With the Torqeedo, this is achieved either via a motor tiller or via the remote throttle lever.

Two things have to be realised for this: Firstly, it needs a practical position at the helm and a cable connection to the engine is required. A mounting plate above the radio holds the control lever and ensures that it is easy to reach, has a good view of the display and is sufficiently far away from the throttle lever of the combustion engine. From here, the control cable can be routed directly behind the panelling and below deck across the entire rear of the boat into the bathing platform. There it exits at the side of the electric motor.

Conclusion and open questions

Overall, a fully-fledged additional boat drive has been realised, which functions as the sole drive in favourable conditions. As the electric motor is stored in the immediate vicinity on the bathing platform when not in use, the power and data cables can remain connected at all times. They are therefore "permanently" installed. Operation from the helm station ensures complete manoeuvrability, just as with the combustion engine.

However, it remains to be seen what performance can be achieved with a 4 kW motor. How far can you even get with it? And can a lock manoeuvre work completely without the combustion engine? In other words, can the dream of limitless and "free" driving be realised? In the next issue, these and other questions will be answered based on practical experience. Attention, spoiler: Things are going better than expected.

The technology: Mounting at the rear, operation at the driver's cab

The power supply: solar energy from on board or from land

Read more about this:

• Interview: Why outboard boats are ideal for electric conversions