Choosing a sailing catamaran engine is not just about buying hardware, it is a strategic decision that determines the comfort and safety of the entire crew, and many inflatable sailboat owners mistakenly believe that any low-powered engine will do the job without having to worry about the specific hydrodynamics of dual-hulled boats. catamaran It has a much higher water resistance due to the wide stern part and the presence of a bridge connecting the cylinders, which requires a special approach to calculating thrust.
Wrongly selected squirrel-engine Unlike monoholed boats, where the keel provides stability, the catamaran is more sensitive to the shifting of the center of gravity and the location of the thrust point, so the question of which motor is better here requires a detailed analysis of technical nuances, not just a comparison of horsepower.
In this article, we will discuss all aspects of motorization of sailing catamarans, from the calculation of the required power to the intricacies of the installation of a lower unit. You will learn why some models are critical to the weight of the engine, and for others – maximum thrust at low speeds. We will also touch on the issues of legal compliance, because the installation of the engine on a sailing boat has its own legal peculiarities in different jurisdictions.
Calculation of the required power and thrust
The first thing that a catamaran owner has to do when they want to install a motor is to determine the power they need. The more the better rule doesn't work here, because excess power can cause the transom or cylinders to collapse when they start out abruptly. The main parameter for calculating is the displacement of the vessel at full load, including crew and equipment. For inflatable catamarans, the optimal ratio is that allows a speed of about 5-6 knots per calm.
There is an empirical formula often used by engineers: for every 25-30 kg of total weight, 1 horsepower is required. catamaran With their high sailing and shape resistance, this ratio is often increased. If your catamaran weighs 400 kg in running order, the 10 hp engine may be "back-to-back", especially if there is a headwind or current.
⚠️ Warning: Never exceed the maximum power specified by the catamaran manufacturer in the boat's passport. Exceeding the limit can cause the transom board to tear off and the cylinders to flood.
It is also important to consider the length of the lower unit. For catamarans, where propellers often operate in the turbulence zone between cylinders, a standard lower unit leg can be short. Long lower unit (Long Shaft) is preferable, as it allows you to lower the screw deeper, into calmer layers of water, increasing efficiency and reducing the risk of cavitation.
Types of engines: gasoline, electricity or gas?
The market offers three main powertrain options, and each has its own advantages for sailing boats. Gasoline engines They remain classics because of their high power density and fast refueling anywhere in the world, and they are ideal for long-distance crossings where autonomy is crucial.
However, for sailboats, increasingly popular is gaining electric outboard motorsTheir main trump cards are quiet, no vibrations and no exhaust. On a sailboat that values silence and unity with nature, the electric motor becomes not just an alternative, but the preferred choice for maneuvering in port or sailing in protected areas.
Gas engines (LPG/CNG) are a niche solution that has not yet been widely adopted, but has the right to life: they are cleaner than gasoline counterparts and often cheaper to operate, but require a complex system of mounting cylinders, which on a compact catamaran can be problematic.
The comparison table will help to determine the choice:
| Parameter | Gasoline two-stroke | Petrol four-stroke | Electrical. |
|---|---|---|---|
| Weight. | Light. | Heavy. | Medium/Heavy (battery) |
| Noise. | Highly | Medium. | Missing. |
| Expenditure/charge | Highly | Low. | Depends on the capacity of the BAC |
| Service | Frequent (oil in gasoline) | Rare. | Minimum |
When choosing an electric motor, pay attention not only to the power in Watts, but also to the voltage of the system. 24-volt systems are more efficient than 12-volt ones, since at the same power the current is lower, which reduces losses in wires and heating.
Features of the installation of the motor on the catamaran transom
The engine installation on a sailing catamaran is very different from the installation on a motorboat, the main problem is the lack of a rigid, vertical transom in the classical sense. On most inflatable catamarans, the engine is attached either to a special inflatable cushion (outrigger) or to a removable transom board attached between the cylinders.
It's critical to make sure that the mounting is rigid, and when the motor is running, it creates vibrations that quickly destroy the soft elements. cross-plate Sea aluminum or laminated plywood that distributes the load over a large area, and the fastening should be done through through bolts with wide washers, rather than just being clamped with rods that can slip.
Another nuance is the angle of the differentiation: on a catamaran, because of the wide stern, the screw can "grab" air from the insides of the cylinders when rolling, you need to experimentally adjust the height of the diewood installation so that the anti-cavitation plate is 2-3 cm below the bottom line when moving.
☑️ Checking the engine installation
For safety, always use a safety rope (storm trap), which is attached to the handle of the motor at one end and to the rim on the catamaran body at the other end, which will save the engine from loss in the event of a transom board breakage or sliding of the rods.
Problems with cavitation and air capture
One of the most common problems with a catamaran motor is cavitation and air suction, and the two-cylinder catamaran design creates a tunnel that creates vortices when you move, and if you get in the vortices, you start to idle, lose your focus.
To minimize this effect, it is recommended to use screws with increased pitch or special screws. heavy-boat propellersIt also helps to install additional hydrofoil stabilizers on lower unit, which not only improve the planing output (if possible), but also direct the flow of water to the screw, reducing turbulence.
⚠️ Warning: Constant operation of the motor in cavitation mode (when the screw is boiling with air) leads to rapid burnout of the piston group due to overheating and detonation. Immediately turn off the gas if you hear a sharp change in the sound of the engine.
If your catamaran has a U-shaped stern (between the cylinders), the risk of air capture increases, in which case the only solution is to lower the motor below or move the mounting point further back, outside the cylinders' influence zone, using an elongated humming wheel or cable control.
What is screw cavitation?
Cavitation is the formation of vapor bubbles in the local pressure-lowering zones behind the propeller blades, and when these bubbles collapse, a shock wave occurs that destroys the screw metal and creates the characteristic sound of "rocking."
Control and navigation for an outboard motor
Drive a catamaran for an outboard motor requires addiction. Unlike a motorboat with a keel, a catamaran has a huge sail even with deflated sails. The side wind blows the boat much more than a monohol, so maneuvering moorings must be carried out with due regard to demolition.
To facilitate management, many owners set up cable-type steering This allows you to sit closer to the center of the boat, balancing the catamaran, and better control the situation, and also the cable thrust allows you to place the throttle handle in a stationary place, freeing your hands.
When driving on the waves, the catamaran begins to wiggle (nose wag) To hold the course, you need constant steering, which helps to install an additional stabilizer fin under one of the cylinders or use a lazy steering wheel with a friction brake to lock the position of the pen.
It's important to remember inertia. A light inflatable catamaran picks up speed quickly, but it stops just as quickly. A heavy cruise catamaran, by contrast, has tremendous inertia. Brakeway Such vessels may have several hull lengths, which must be taken into account when approaching the berth.
For comfortable motorized catamaran control, it is critical to shift the control center closer to the vessel’s diametric plane, using cable thrust to be able to balance the boat with body weight.
Maintenance and preservation of the engine on the sailboat
Operating a motor in salt water requires a disciplined approach to maintenance. Sail catamarans are often used in aggressive marine environments, where salt corrodes metal faster than in fresh water. After each outing, the cooling system must be flushed with fresh water.
You can use a simple pump or a special wash bag that you put on the water intake. Pay special attention to the magnesium anode, which can burn out faster on a sailboat because of wandering currents if the boat is in an electric parking lot. Regularly replacing the anode is the cheapest way to save lower unit from corrosion.
When putting the catamaran for winter storage, it is better to remove the engine. However, if this is not possible (for example, when storing in assembled form on a trailer), preserve the engine: drain the fuel (or add a stabilizer), replace the oil (for 4T), treat the exposed metal parts with anticortex and cover with a protective cover to ensure ventilation.
Do I need to lubricate the screw hub?
Yes, periodically (once a season) it is recommended to remove the screw and lubricate the shaft with solidol or special lubricant to prevent the screw from clinging to the shaft due to galvanic corrosion.
Legal aspects and registration
Installing the engine on a sailing vessel changes its status in the eyes of regulatory authorities. If the catamaran was not registered as a motorized boat, formally it remains a sailboat. However, having an engine with a capacity of more than 5 hp (in Russia and many other countries) requires the right to operate a small vessel with the appropriate category.
Power restrictions for unregistered vessels must also be taken into account. In many regions, catamarans less than 3 meters long and up to 5 hp motors do not require registration with the GIMS. But once you put the motor on, you must undergo registration and annual technical inspection.
Keep in mind environmental regulations: In some conservation areas and national parks, the use of internal combustion engines is prohibited year-round or during certain seasons, and electric motors are often the only legal way to get around.
Do I need to register a catamaran with a 2 hp engine?
In most cases, if the length of the vessel is less than 3 meters and the engine power does not exceed 5 hp (in Russia), registration with the GIMS is not required.
Can the engine be more powerful than the factory recommends?
Technically, but legally and safely, it is not possible: Excessing passport capacity removes the warranty, invalidates the insurance and can lead to accidents, and an inspector can also issue a fine for the conversion of a vessel.
Which screw is best for a heavy catamaran?
For heavy catamaras that don't go planing, propellers with larger blade area and smaller pitch are better suited, providing better traction at low revs, allowing you to move more confidently against wind and wave.