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More power! It’s an increasingly common need on boats, new and old alike, as they add more and more systems with high power demands, including bow thrusters, stern thrusters and stabilizers. Imtra hydraulics expert Prentice Weathers reviews the pros and cons of hydraulic and electric options aboard boats. 

Boats aren’t attached to “the grid” so any electrical device needs to get power from batteries or a generator. The boat’s main engines can also sometimes be used as a power source for things other than basic propulsion. For most smaller vessels, economical DC electric power is a great choice to power most devices onboard, and in many cases, it’s the only choice. Larger boats typically turn to hydraulic or AC electric power because it is better suited to higher power demands or the need to run equipment continuously for longer periods of time.


It’s not that DC power can’t do the job of powering larger electrical consumers – after all, some passenger and freight trains run on DC power – it’s just that the size of the battery cables and the required battery capacity can be unwieldy on larger boats. The crossover from DC power to AC & hydraulic typically occurs in boats 60 to 70 feet long, but sometimes owners of boats under 60 feet will find that a hydraulic system suits their boat better than DC power. This is especially true in the commercial market where one might find 30’ers using hydraulic systems.


Determining Factors in Choosing AC, DC or Hydraulic Power

The decision to use AC, DC or hydraulic power is almost always dictated by the size of the bow and stern thrusters, which are the biggest consumers of energy after the main engines and generator(s). If the boat is small enough, and doesn’t need to run its thrusters for long periods of time, DC electric power is a great choice. This is the case for the vast majority of recreational boaters with boats under 60 feet in length.

Hydraulic systems are most efficient at moving and holding high loads, particularly in harsh environments (like bilges and forepeaks), and in applications that require frequent or constant running. Bow thrusters, cranes, steering systems, and stabilizers fit into this category as do fire pumps, crash pumps and winches. A hydraulic system also has the advantage of using a central power source, a hub if you will, which can power almost unlimited numbers of functions anywhere on the boat. It is also cost effective when the powerful main diesel engines can be used as the power source.

Fishing boats, for example, have many hydraulic applications such as bow thrusters, pot haulers, reel winches, which operate in harsh environments and in virtually continuous operation. Other boat types, such as larger motoryachts, may run windlasses, steering, stabilizers, cranes, bilge pumps, fire pumps and more using hydraulics. From a cost perspective, once the core components of a hydraulic system are in place, it’s relatively easy and cost effective to add additional powered functions.

Francis Dawn

For a vessel like a ferry that runs regularly, docks frequently, and doesn’t have other power-hungry systems that would require a hydraulic system, AC electric power may be the best choice. AC can also be a good choice if the power demand of a new piece of equipment is high, like an added stern thruster, and the vessel’s hydraulic system is already at its capacity with other equipment.


Requirements of a DC Electrical System

The recreational boat market has long relied on DC power, and demand continues to grow for DC products due to its economy and simplicity. Even for boats with power-hungry thrusters, DC power can do a great job thanks to innovations in some devices such as motor design and proportional controllers, which allow extended run times, among other advantages. DC-powered thrusters also work well for what is normally low duration usage when leaving and arriving at the dock. But as boats scale up in size and demands increase for longer thruster run times and other power-needy devices, the draw on batteries becomes greater, so more or larger batteries, chargers and cables are needed.

Looking at it simply, a DC-powered piece of equipment like a thruster, windlass, winch, etc. requires a central power source (a battery), as well as ways to: replenish the power (a charger or alternator), use the power (a motor), protect the user (fuses), control power flow (switches and joysticks), and transmit the power (cables/wires).

There are two basic electric systems; 1) the raw power to the motor and 2) the control system of that power. The DC motor system uses heavy, high amperage cables between the battery and the motor, and the control system uses relatively small, low amperage power. The high amperage components are the battery, alternator/charger, main fuse, main battery switch, cables, motor and the contactors to connect the power to the motor. The low amp control components are the DC breaker on the boat’s DC panel, the joystick or button panel – perhaps more than one – and the control system wiring.

Every electric thruster, windlass, capstan, crane, powered gangway etc. will have its own individual electric motor, fuse, contactor and set of cables. They will usually share the power source with other devices powered by the same battery bank. Control systems get a little more complex with the advent of modern CANbus systems, such as C-Zone or Octaplex and similar, but the distinction is still the same: power system versus control system.

Requirements of a Hydraulic System

Just as with DC powered systems, where battery capacity is the limiting factor, hydraulic systems require a central power source of adequate capacity. As with virtually any power system, the basic laws of physics apply: You can’t use more power than you can produce. To understand the hydraulic system better, let’s explore what’s involved in a hydraulic system in a little detail.

As with a DC system, there are two core components: 1) The primary power source and the consumer of that power, and 2) the control and distribution of power. Instead of moving electrons between the power source and the power-consumer, as in a DC or AC system, a hydraulic system moves oil. Instead of fuses, switches and battery cables, a hydraulic system uses valves and hoses. Interestingly, the control circuits used in a hydraulic system are very similar to an electric system, and in fact share many of the same components. For instance, Side-Power proportional DC, AC, and hydraulic thrusters, and stabilizers for that matter, all communicate through the same basic “S-link” central control system, using the same joystick panels.

In a vessel’s hydraulic system, the primary power source is almost always a hydraulic pump attached to the main engine or transmission PTO (power take-off). Sometimes it’s clutched to the generator. Think of the pump as the engine’s alternator or the battery charger. If retrofitting a hydraulic system, you need to ensure that your engine/transmission has a PTO of sufficient size or that you can install one. Sometimes there is a physical constraint, like an exhaust for instance, positioned so you cannot fit the pump in place. In such cases, an alternative may be to mount the pump on the front of the engine off the flywheel.

The next component is the hydraulic oil reservoir. The reservoir stores oil, just like a battery stores electrons, and has filters to keep it clean. The reservoir is also equipped with internal or external heat exchangers to keep it cool – allowing for unlimited run times – as well as visual and digital sight and temperature gauges.

Next is a control valve which, like a contactor on a DC thruster, directs the oil to the motor to run it one direction or the other: port or starboard in the case of a thruster, up or down in the case of a windlass motor, and so on. Of course, the hydraulic motor is the next component, and finally, the hydraulic hoses which run from the oil tank to the PTO pump where the oil gets pressurized, then to the hydraulic valve, then to the motor.

Hydraulic system

Those are the big power components. As mentioned before, the control system is very similar, to a DC or AC system: Joystick/control panels, DC control circuit breaker, hydraulic controller and CANbus complete the system.

For all intents and purposes, whether you’re running a single hydraulic thruster, or ten hydraulic functions, the basic central hydraulic system (tank, pump, filters, cooler) is the same. Once you have a central hydraulic system, you can run multiple devices from this central power source – a bow thruster, stern thruster, windlasses, capstans, a steering system, gangway, crane, emergency bilge pump, fire pump – you name it. With the core components in place, it is relatively easy and cost-effective to add the second, third…tenth component. You basically just add a valve and hoses. It’s a bit more complex than that, but the point is that expansion of the system is pretty easy. There are marine hydraulic systems serving over 25 functions, all using a central reservoir and power source. In those types of applications, hydraulic power was the only logical way to go. On the other hand, installing a hydraulic system to power just one device may be the most expensive option. In this case, DC or AC electric may be the right answer.


Requirements of an AC Electrical System

AC power is the third common power source for power-hungry equipment on board, and is found mostly in commercial applications. Having said that, there are certainly situations where a larger vessel, a 90’ motoryacht for example, has a hydraulic system already in place, but wants to add something like a stern thruster. Running both the bow & stern thrusters simultaneously would require a lot of hydraulic power, perhaps more than the main engines can provide. You could still install a hydraulic stern thruster, but wouldn’t be able to run it simultaneously with the bow thruster. In this instance, the AC thruster alternative may be the answer. Again, there needs to be sufficient power available to run it – AC power in this case – which comes from the generator(s). Therefore generator size is critical: do you have enough excess capacity to power a thruster?

Advantages of AC motors are that run time is virtually unlimited (some caveats apply such as ambient temperature and power output), smaller electrical cables are needed than a 12 or 24v DC application, and obviously you’re not using up DC battery capacity. As with the other options, there are two basic sets of components – 1) power source and consumer and 2) power control.

The power source is the generator, and the consumer is the electric motor. For thrusters and many other devices, a Variable Frequency Drive is installed between the power source and the motor. An EMC filter which filters out electronic “noise” is also typically installed, and, of course, a fuse is needed to protect the system from short circuits. The control system is comprised of the joystick, a proportional controller interface, and the CANbus. The beauty of the CANbus system is that an AC thruster, stabilizers, DC thrusters and hydraulic thrusters can all share the same control “backbone”.

The variable frequency drive (VFD) is a device that conditions and regulates the power to the motor. Side-Power uses these on AC thrusters and “any-speed” stabilizer systems where the stabilizers need to operate when the other power source (the main engine) is turned off.

As with the other two power systems, DC and hydraulic, available space is a consideration. AC motors are huge and very heavy compared to hydraulic motors, and the boat needs space – protected from the elements – for the VFD and EMC filter. On the other hand, hydraulic and AC motors are essentially the only option for bow and stern thrusters over about 20 horsepower. The reason goes back to available power source. It’s not that there is no such thing as a 60 hp DC motor, it’s just that a boat typically doesn’t have sufficient battery and charging capacity to power it.

So, the answer to the question “Which system is better for my boat” is usually made for you based on available power source, how the device is used, available physical space, weight, and many times, price – the price of the components and the price of installation.


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