Choosing the right size of a lower unit, or so-called "leg," is one of the most critical moments when buying an outboard motor. Many beginners, without knowing the intricacies of the labeling, buy the standard version of the outboard motor. Short Shaft It's a deep hull, which causes the propeller to suck the air constantly and lose traction, and conversely, putting a long leg on a shallow boat creates excess resistance and the risk of hitting the bottom of maneuvers, which is why understanding what it is. L leg on outboard motor sizeIt is becoming a key factor in the safety and efficiency of swimming.
In the modern classification of manufacturers labeling L (Long) stands for a standard long leg, which usually has a lower unit length of about 15 inches (381 mm) from the mounting surface to the water intake hole or anti-cavitation stove. However, these figures can vary depending on the specific brand, whether it is a mounting surface or a water intake hole. Yamaha, Tohatsu or MercuryA mistake in choosing even a couple of inches can drastically change the behavior of a vessel on the water, causing the engine to run in abnormal mode or, worse, lead to its destruction due to overheating.
In this article, we will discuss in detail how to correctly determine the required length of the lower unit for your vessel, what standards exist and why marking is required. L It doesn't always mean the same thing from different manufacturers, but you'll learn how to avoid common installation errors and why transom geometry is important.
What does L marking and other length standards mean?
The outboard motor industry has developed a certain coding system that helps customers quickly navigate the model range. S, L and X or XL) indicate the length of the lower unit measured from the plane of the motor installation on the transom to the critical point of the water intake. L Traditionally, it is 15 inches long, which is the standard for most modern fiberglass boats and high-sided boats.
But don't just rely on the letter. Other manufacturers may put slightly different meanings into the standard leg, for example, some Japanese brands may have 382 mm in length, and some American brands may have 380 mm in length, which may seem small, but when mounted on a tilt transom or when additional add-ons are used, they can be decisive.
⚠️ Warning: Never rely solely on lettering when ordering a lower unit online. Always check the actual length in millimeters or inches with the seller, as tolerances can vary.
In addition, there are intermediate and specialized dimensions that do not fit into the classic three S-L-XL. Some manufacturers produce models with a length of 17.5 inches or 20 inches for specific tasks, such as sailing catamarans or double-bottom boats. cavitation-plate It will be at perfect depth relative to the waterline.
Why leg size is critical for engine operation
The main function of the correct timing of the length of the lower unit is to ensure that the engine is uninterruptedly cooled and the propeller works efficiently. If the leg is too short, the anti-cavitation plate is above the water level, especially when rolling or turbulent, as a result, the screw begins to capture air, cavitation occurs, and the engine loses its speed dramatically, turning to "squeal".
On the other hand, a too long leg increases the surface area to be wetted, which creates additional hydrodynamic resistance, the boat becomes less manageable, it gets worse for planing, and fuel consumption increases, and in fact, in a deep landing, the risk of lower unit hitting underwater obstacles increases, which can lead to failure. gearbox Or the screw itself.
Special attention should be paid to the cooling system, the water enters the engine through holes in the lower unit, located directly above the gearbox. If these holes are above the waterline, the pump starts pumping air, which instantly leads to overheating. Even short-term operation in this mode can deform. block-head Or jam the piston group.
To check the water level, use a transparent hose attached to the lower unit to visually assess the position of the water intakes relative to the waterline when the boat is fully loaded.
How to measure the required length of lower unit
Determining the right leg size is a process that requires precision and a few factors to consider. There is no universal formula that works for everyone, because each boat has its own transom geometry and different draft, but there is a proven algorithm of actions to minimize error in the selection.
The first thing you need to do is to determine the height of the transom. This is the distance from the top plane where the motor is attached to the bottom of the body (keel). The standard heights are 381 mm (15 inches), 508 mm (20 inches) and 635 mm (25 inches). If you change the motor on an existing boat, just measure the length of the old engine's lower unit from the mounting surface to the top of the anti-cavitation plate.
☑️ Measurements for leg selection
It is important to consider that when the boat is fully loaded (passengers, fuel, equipment), its draught increases, and the transom sinks deeper. So it is better to take measurements in a static manner, taking into account the estimated weight of the cargo. If the boat is heavily buried by the stern, you may need a leg marked with a marking. XL (Extra Long) even if the standard L It's technically the height of the transom.
Also worth paying attention to the angle of inclination of the transom. If the transom has a reverse tilt or, conversely, is heavily littered back, this changes the effective depth of diving of the lower unit when moving, in such cases, it is recommended to make a margin of 1-2 inches upwards to ensure that the water intakes will always be under water.
Table of correspondence of transanse sizes and types
To make the choice easier, manufacturers and experienced boatmasters use summary tables that link the type of boat, the height of the transom and the recommended leg length, which are reference data but allow you to immediately cut out the obviously inappropriate options.
| Motor type/Designation | Length of lower unit (inches) | The length of lower unit (mm) | Typical application | Transcent height |
|---|---|---|---|---|
| S (Short) | 15" | 381 mm | PVC boats, inflatable boats | 381 mm (15) |
| L (Long) | 20" | 508 mm | Glass-plastic boats, aluminum boats | 508 mm (20) |
| X / XL (Extra Long) | 25" | 635 mm | High-sided boats, sailboats | 635 mm (25) |
| XXL (Ultra Long) | 30" | 762 mm | Specialized vessels, large yachts | 762 mm (30) |
It is worth noting that low-power engines (up to 5-6 hp) are often unusual, manufacturers can only produce them with a short leg, even if they are formally designed to be installed on higher transverses. lower unit It will be lower than the calculated speed, which is acceptable for low speeds.
When you choose a motor over 40 hp, the scatter becomes even more significant. You can no longer rely on a "rough" match. An error in choosing between 20 and 25 inches for a powerful motor will result in a significant loss of efficiency of the screw. A screw operating in a surface layer of water with many air bubbles will not be able to realize its power, no matter how much horsepower the engine produces.
The rule of the golden mean: anti-cavitation plate should be 1-2 inches below the bottom line of the boat at full load, but not deeper, so as not to create unnecessary resistance.
Problems of cavitation and overheating with incorrect choice
The most common consequence of putting a leg that is too short is cavitation, a physical process whereby bubbles of steam form behind the blades of the propeller in local pressure-lowering zones, and when they collapse, they create a shock wave that not only makes a characteristic noise, but also destroys the surface of the propeller and the lower unit, which for the wearer is expressed in vibration, loss of speed, and inability to plane.
The second, more dangerous scenario is overheating. As mentioned earlier, water is drawn through holes in the lower unit. If the motor is set too high, the water intake is exposed during the wave or roll when you turn, the engine starts to run dry. Modern motors have temperature sensors that can save the situation by turning off the ignition, but old models or engines without protection burn up in minutes.
⚠️ Warning: If you notice that water has stopped flowing from the control hole, immediately release the gas and turn off the engine. Continuing to work even at idle speeds without cooling is guaranteed to lead to major repairs.
Sometimes the problem is solved by installing a propeller with increased pitch or using screws with specially designed blades, which are less susceptible to cavitation. However, these are only half measures. If the geometry of the installation is fundamentally broken, no tricks with the propeller will help. You either change the lower unit to a longer one, or lower the motor down the transom, if the design of the boat allows.
Specifics of choice for PVC boats and rigid hull
PVC boat owners often face a dilemma: Which leg to choose? On the one hand, inflatable tubes create a high landing, and it seems that you need a long leg. On the other hand, a soft bottom (NDND) or an inflatable keel (NDK) can bend when moving, changing the angle of attack. For most PVC boats up to 3.6 meters long, the standard is a short leg. S (15 inches).
For boats with a rigid fiberglass or aluminum hull, the situation is different: the keel is rigid, and the height of the transom is strictly fixed. If you own a boat with a transom height of 508 mm, you can physically install a motor with a short leg (381 mm) (the tray allows), but it will be extremely inefficient. The screw will "grab air" even in calm water.
Can we build up lower unit?
There are theoretically some diewood extensions, but they're not recommended by manufacturers, and they disrupt the alignment, increase the vibration, and can cause a lower unit pipe or a tiller to break, and you'd better buy a motor of the right length.
Special attention should be paid to boats with double bottom or complex stern geometry. In such cases, the estimated depth of the engine installation can shift. It is recommended to conduct test runs using temporary pads for an outboard motor to empirically find the height at which the boat is going smoothly, without yawing and at maximum speed.
Frequently Asked Questions (FAQ)
Can a motor with a long leg (L) be mounted on a short leg (S) transom?
Technically, this is possible if the height of the transom of the boat allows the engine to lower lower. However, if the transom has a standard height of 15 inches (381 mm), then installing a 20-inch engine will cause lower unit to rest in the bottom or be too deep, creating a huge resistance.
How to know if the engine leg is not selected correctly while on the water?
The main signs: the engine does not develop full speed (maximum 4000-4500 instead of 5500-6000), the boat does not go out for planing or comes out with great difficulty, the characteristic noise of the rotor's "growl" is heard, a mixture of water and steam is coming from the exhaust hole or, conversely, water does not go at all. Also, a sign of too long a leg will be poor handling and the feeling that the stern is "pulling" down.
Does the type of screw affect the choice of leg length?
Indirectly, yes. If you use a Speed screw, the demands for clean water flow increase. It's critical for these screws to have no vortexes or air, so it's often recommended for high-speed propellers to choose a leg slightly longer than standard to ensure that water flows to the propeller even at high speeds.
What if my motor with a short leg is constantly getting air on the wave?
The simplest solution is to put a hydrofoil on an anti-cavitational stove, which is a small wing that presses the stern and creates an extra water cushion over the propeller, preventing air suction, and if that doesn't work, all that remains is replacing the lower unit with a longer model or replacing the entire motor.
Is there a difference in leg sizes in 2 and four-stroke motors?
Within one manufacturer and one model line, the difference is minimal or absent. However, four-stroke motors are often heavier and may have slightly different lower unit geometry due to the placement of the valve mechanism in the cylinder block. Always check the specific drawings (dimensions) in the manual to the selected model, without relying on general rules.