Choosing the right size of a lower unit is a fundamental decision that determines not only the efficiency of movement on the water, but also the durability of the power unit itself. Long leg outboard motor (designated L or 20 inches) is the standard for most modern boat hulls, but many beginners are often confused in dimensions, which leads to serious hydrodynamic problems.
In this article, we will discuss in detail the technical nuances behind the term “long leg”, and explain why this parameter is often decisive when buying a hanger unit, you will learn how the physics of water affects the operation of the propeller and why manufacturers strictly regulate the distance from the transom to the anti-cavitation plate.
Ignoring the geometry of the engine installation is a direct path to losing fuel and lowering top speed. We will look not only at the theoretical aspects, but also at the practical steps of installation to make your car more efficient. Tohatsu, Yamaha or Mercury They were working at their optimum.
What is lower unit and Why Its Length Matters
lower unit, or "leg," is the bottom of an outboard motor, inside which the transmission shaft and the cooling system are located, and the length of this structure determines how deep the propeller will be relative to the transom cut. Long leg. It is usually about 508 mm (20 inches) long, allowing the propeller to sink deeper into the water compared to shorter models (S or 15 inches).
The main physical task of a long lower unit is to ensure that the screw is immersed in dense, saturated layers of water, where there are no air bubbles and suspension, and if the screw works too close to the surface, it begins to capture air, which leads to a sharp drop in thrust. Cavitation This is a process in which in the flow of liquid cavities filled with steam are formed, and their collapse causes shock waves that can destroy the propeller blades in a matter of hours.
⚠️ Note: Operation of the engine with an excessively high rotor leads not only to loss of speed, but also to local overheating of the engine due to a violation of the circulation of coolant in the pump.
In addition, the long leg allows the center of gravity of the power unit to be lower, which has a positive effect on the stability of the vessel. In conditions of strong lateral wind or high wave, a low center of mass helps the boat to maintain stability and less prowl. For heavy hulls and high-speed sliding boats, this is a critical parameter that affects safety.
When buying a used motor, always check the integrity of the lower unit for dents and deep scratches, as they can hide violations of the shaft geometry.
Length standards: S, L and XL – what’s the difference?
Global outboard manufacturers adhere to uniform standards to make it easier for boat owners to choose equipment. However, even within the long leg category, there are nuances to consider when selecting a motor for a specific transom. The basic sizes are marked with letter codes, and a one-letter error can cost you expensive repairs or buying a new boat.
The most common classification is as follows:
- 📏 S (Short) Short leg, about 381 mm (15 in) long, designed for PVC inflatable boats with a hard or inflatable transom of low height.
- 📏 L (Long) Long leg, with a lower unit length of about 508 mm (20 inches) standard for most aluminum and fiberglass boats with a transomine height of 510-530 mm.
- 📏 XL (Extra Long) - an ultra-long leg, with a lower unit length of about 635 mm (25 inches) and is used on large boats and high transomania vessels to reduce noise and improve hydrodynamics.
It's important to understand that the sizes can vary slightly between brands. Honda and Suzuki They can have tolerances of several centimeters, which is noticeable when installed on boats with a minimum permissible transom height.
The choice between these categories is dictated by the design of your boat. If you put the L engine on a boat designed for S, the propeller will go too deep, creating excessive resistance ("parachute effect"), which will lead to over-fuel consumption and the inability to get on plane. Otherwise, the engine will constantly grab air.
The effect of leg length on boat performance
Hydrodynamics is a complex science, but its basic principles directly affect how fast and economical your boat will move. Long leg outboard motor It allows you to move the propeller into a more calm water flow, and unlike the surface layer, which is saturated with air bubbles and turbulence from the movement of the body, the deep layers are denser and more homogeneous.
When working in a dense environment, the propeller transfers more energy to movement rather than whipping water, which is especially noticeable at high revs, when the difference in speed between a properly installed motor and a short-leg motor (on the wrong boat) can be as high as 10-15%.
| Parameter | Short leg (S) | Long leg (L) | Superlong (XL) |
|---|---|---|---|
| The length of lower unit | ~381 mm (15) | ~508 mm (20) | ~635 mm (25) |
| Transcent height | 380-400 mm | 510–530 mm | 640–660 mm |
| Type of boat | PVC, small boats | Aluminum, fiberglass | Boats, yachts |
| Water resistance | Above (in large boats) | Optimal. | Minimum |
But the long leg has its limitations: the increased lever creates a greater torque on the transom plate, so the stern design must be strong enough. On light inflatable boats, the installation of a heavy motor with a long leg can lead to excessive differentiation on the stern when the boat's nose is raised too high, impairing visibility and handling.
⚠️ Warning: When the XL engine is installed on a boat not designed for this purpose, a critical shift of the center of gravity may occur, making the vessel dangerous for operation on waves.
Installation rules: height and angle of inclination
The right way to set the motor is not just to pin it to a transom, but to balance the height of the anti-cavitation plate with the angle of inclination. For long-leg motors, the top plane of the anti-cavitation plate should be at the bottom of the boat or 10 to 25 mm lower for flat-bottomed boats. For deep-V hulls, the bottom line can be higher, allowing the engine to be lifted a few more centimeters.
The process of adjusting the height requires a practical approach. Start by mounting the engine so that the anti-cavitation plate is parallel to the bottom line. After the first step, run a test: if the engine starts to growl and lose momentum with a sharp addition of gas, and there is a lot of white foam behind the screw, then it captures air and it needs to be lowered lower. If the boat does not go into the planing and the socket burrows into the water, perhaps the engine is too deep.
☑️ Checking the engine installation
Trim is also key. By adjusting the angle between the lower unit and the transom, you change the thrust vector. To get to the plane, the nose is usually lowered (by deflecting the motor from the boat), and to move at cruising speed into calm water, your nose is raised to reduce the area of wetting the hull. On modern motors, this is done using electro-hydraulics or mechanical support.
Don't forget to check the condition of the transom bolts and the clamping bar. long-dadewood Regular stretching of threaded joints is a mandatory procedure before each season.
Problems of cavitation and ways to eliminate them
Cavitation is the main enemy of any propeller, and owners of long-leg motors encounter it less often, but the consequences can be more serious because of the high speeds that such units are designed for. Cavitation erosion destroys the metal of the blades, and cavitation noise reduces comfort. The main features are a characteristic crackling, similar to the noise of rolled pebbles, and a drop in speed at full revs.
Often the cause of cavitation is not only the height of the installation, but also the condition of the screw itself. chips, burrs or the wrong pitch of the screw disrupt the laminar flow. For long-leg motors, it is recommended to use propellers with increased pitch, as they work in denser layers of water and can use the power of the engine more efficiently.
What is screw ventilation?
Ventilation is the process of sucking air from the surface of the water into the propeller zone, and unlike cavitation, where water vapor is formed, air gets into the work here, often when you turn abruptly or hit a wave, when the propeller temporarily hits the surface.
To combat this, lower units are equipped with anti-cavitational plates that cut off the air flow along the hull. If the standard stove fails, some craftsmen put extra side-cheeks on lower unit to stabilize the flow of water going to the propeller, which is especially true for flat-bottom boats where the flow of water is very turbulent.
Service of lower unit and Reductor
A long leg means a longer distance between the engine and the propeller, which means a longer shaft and more bearings that require lubrication and control. Regular maintenance of the bottom of the engine is the key to its long life. First of all, you need to monitor the condition of the shaft glands, as their wear will lead to water entering the gearbox.
The oil change in the gearbox should be done annually or every 100 hours. For long legs, the oil volume can be slightly larger than that of shorter ones, so always have a margin. When draining, pay attention to the color and consistency of the fluid: the presence of a “condensed” (emulsion) indicates water entering, which requires immediate repair.
Also worth paying attention to the lower unit belt (if you have one in your motor design, although modern ones use direct gear or gears) and the condition of the lower unit pipe. Corrosion is another enemy. The anode treads installed on the lower unit take on the electrochemical effects of water and must be replaced as they break.
Timely replacement of anodes and control of oil color in the gearbox can prevent 90% of serious breakdowns of the lower unit part of the engine.
⚠️ Warning: When changing the oil in the gearbox, always start filling from the bottom hole, which ensures that air is completely displaced and there is no air traffic jam in the gear lubrication system.
FAQ: Frequently asked questions
Can I install a long leg motor (L) on a short leg boat (S)?
Technically, this is possible with a special extension bracket, but it's not recommended, and it shifts the center of gravity back, increases the load on the transom, and can degrade handling, and it's better to choose a motor of the appropriate dimension.
How to measure the height of the transom to know which leg is needed?
Take a straight slat or level, put it horizontally on the top of the transom. Measure the distance from the bottom point (in the plane of the transom) to the top cut. The result is about 380 mm for the short leg, about 510 mm for the long leg.
Does the length of the leg affect fuel consumption?
A properly fitted long leg on the right body reduces drag and cavitation risk, allowing the engine to operate at optimal speed, reducing fuel consumption. The wrong length (too short or too long) always leads to overruns.
Why is it harder to transport a long leg motor on a small boat?
Because of the longer length of the lower unit and the overall weight of the motor, it is harder to lift and lower by hand. On boats without electric winches, this can be a problem.
Should I change the screw when switching from short to long?
Not necessarily if the diameter of the barrel of the lower unit and the shaft slit are the same, but for the long leg, large pitch propellers are often recommended to power the engine at higher speeds, which are made available thanks to better hydrodynamics.