When choosing an outboard motor, many drivers pay attention solely to power and two-stroke or four-stroke cycle, forgetting the critical parameter - the length of the lower unit. It is this node, often called the "foot", that determines the efficiency of the propeller and, as a result, the overall dynamics of the vessel. One of the most common options for small craft is the so-called "flying" ("the "foot"). leg SIt's short, and understanding what the S foot is in a outboard motor will help you avoid cavitation, overheating and loss of traction on the water.
The standard length of a S-category diewood is typically 381 millimeters (15 inches), although some manufacturers may vary slightly depending on the design of a particular model. This length is designed to fit the motor on the transom of boats with a height of 38-40 centimeters. If you plan to use the motor on an inflatable PVC boat or a small metal flatbed, it is the short leg that will be the most likely choice. However, it is important to consider not only the height of the transom, but also the expected speed of movement, as when planing the boat is raised, and the correct sinking factor.
lower unit It's not just a pipe that runs a shaft inside, it's a complex mechanism that transfers torque from the crankshaft to the propeller, and it has gears, lubrication, and cooling channels inside it. Choosing the length of the leg incorrectly can lead to the water intake being above water level, which can cause the engine to overheat instantly. In this article, we'll take a closer look at the technical nuances, benefits, and limitations of using the short leg, and answering some oft-asked questions from beginners.Technical specifications and standards of the length of lower unitThe design of the lower unit outboard motor is strictly regulated by international standards to ensure compatibility with various types of boats. The S leg, as mentioned, belongs to the Short category and has fixed dimensions adopted by most global manufacturers such as Yamaha, Mercury, Tohatsu and Honda. The standard length from the mount flange to the anti-cavitation plate is 381 mm. This value is the “gold standard” for the industry, allowing you to easily match the engines to existing boats without the need for trenching or the use of additional slots.
However, it is worth noting that not all manufacturers adhere to the strict inch standard in millimeters. Some brands may specify a length of 395 mm or 375 mm for their models labeled S. Such differences are due to the design of the gearbox and the location of the gearbox. tailpipeIn any case, the difference of a couple of centimeters is offset by adjusting the height of the suspension of the motor on the transom, the main thing is that the screw is in dense layers of water when driving, capturing air only in the minimum amount necessary for emulsifying exhaust gases.
In comparison, there is also the L leg (Long), which is 508 mm (20 inches) long, and the X leg (Extra Long) is 635 mm (25 inches). The short S leg has a more compact gearbox, which positively affects the overall weight of the motor and its dimensions in the folded or transporting position. The lower weight of the lower unit also shifts the center of gravity of the entire structure, which can be critical for light inflatable boats, where every kilogram on the transom affects the differentiation of the vessel.
- 📏 The standard length of the S is 381 mm (15 inches).
- ⚙️ The design of the gearbox is optimized for high revs with a smaller load shoulder.
- 🚤 Ideal for boats with transom heights up to 40 cm.
- ⚖️ The lower weight of the leg makes it easier to install and dismantle the engine by one person.
Which boats are the short leg for?The main application of S leg motors is small vessels with a flat or slightly rounded transom. These include classic PVC inflatable boats, which are widely used by anglers and tourists. The transom height of these boats usually varies between 36-39 cm, which is ideal for the geometry of a short lower unit. When installing the motor on such a boat, the anti-cavitation plate should be approximately on the level with the bottom of the boat or slightly lower (by 1-2 cm), which ensures the optimal flow of water to the propeller.
The S leg is also used extensively on small metal flat-bottom boats, often called Kazankas or collar-bottles, if their trawler is of the appropriate height. Owners of such vessels often face the choice of putting a lighter motor with a shorter leg or a more powerful one with a longer one. For planing under full load on small craft, the lighter option is often preferred, since it loads less stern and allows the boat to go into mode faster.
⚠️ Attention: Installation of the motor with foot S on a boat with a high transom (more than 45 cm) without the use of an extension line (tiller extension and lower unit) is strongly discouraged. This will lead to the screw working in "rare" water with air bubbles, which will cause severe cavitation, loss of speed and rapid wear of the blades.
In addition, a short leg is often found on electric trolling motors, although the gradation may differ. In the context of gasoline outboard motors, the S leg is the de facto standard for boats up to 4-4.5 meters long. If you own a larger vessel with high sides, you probably will already need an L leg. Always measure the height of the transompen from the lowest point of the keel (or bottom) to the top cut of the transmuncle before buying the engine.
Critically This is a fine tuning that often requires experimenting with the height of the suspension on the transom board.Advantages and Disadvantages of Type S lower unitLike any engineering design, a short leg has its strengths and weaknesses. The undeniable advantages include compactness. The S leg motor is easier to transport in the trunk of a car, takes up less space when stored in a garage. A shorter lower unit length means less water resistance when driven, although at low speeds this difference is almost imperceptible. However, at high plane speeds, a shorter lower unit creates less drag, which theoretically can give a small increase in speed.
On the other hand, there are disadvantages, and the main one is the increased risk of getting air in abrupt maneuvers or in agitation. Because the distance from the water to the anti-cavitation stove is smaller, any rolling can lead to short-term exposure of the propeller, which causes a sharp surge in engine speed, which is dangerous for a motor without a speed limiter, and it is more difficult to implement complex exhaust systems on the short leg, although this is not a problem for small power.
Another nuance is the depth of the propeller, and in shallow water, a short leg has the advantage of being able to lift the motor higher without risking damage to the screw, but it also reduces the cooling efficiency if the engine is running at high revs with a shallow depth of intake.
- ✅ Compact and convenient transportation in the car.
- ✅ Lower weight, which is critical for light inflatable boats.
- ❌ There is a higher risk of cavitation on excitement compared to a long leg.
- ❌ Limited applicability only for low-transcent vessels.
S and L leg comparison: what to chooseChoosing between a short (S) leg and a long (L) leg often becomes a dilemma for owners of boats with a transient height of a transom (about 40-42 cm). A long leg (508 mm) provides a deeper dive of the screw, which guarantees stable water intake for cooling even when heeled or excited. Motors with a leg L is quieter, as exhaust fumes go a longer way in the water and are better dampened.
However, the long leg is heavier and creates more resistance. For a lightweight inflatable boat, the L-foot motor can become an anchor on the transom, disrupting the balance. The boat can start pecking with its nose or, conversely, picking up the stern if the engine is too heavy. While the S leg allows you to maintain lightness and maneuverability. If your boat has a transom height of 38 cm, you can only put the L leg using a spacer, which increases the overall length of the structure and sailability.
The table below compares the main specifications for clarity:
| Parameter | Foot S (Short) | Leg L (Long) |
|---|---|---|
| Standard length | 381 mm (15) | 508 mm (20) |
| Recommended transance height | 40cm | 52 cm |
| Weights of lower unit | Less. | More. |
| Risk of cavitation | Higher. | Below. |
| Application | PVC boats, flat-bottoms | Boats, keel boats |
Can I convert the leg S into leg L?
It's theoretically possible, but it's almost never done in practice: it requires replacing the lower unit, lengthening the steering thrust, lengthening the exhaust system, and often replacing or refining the cylinder block itself, and it's easier and cheaper to sell the S engine and buy the L engine.
Features of operation and height settingThe proper height setting of an outboard motor with an S leg is an art that requires an understanding of hydrodynamics. As I said, the anti-cavitation stove should be aligned with the bottom of the boat. If you set the motor too low, the water resistance will increase, the boat will be less plane, and lower unit will collect algae and debris. If you are too high, cavitation will begin.
To adjust the height, use a multi-hole transom board or an adjustable transom. Run a test run. If you hear a characteristic roar when you add gas abruptly (the engine is gaining speed but the speed is not increasing), then the screw is enough air. In this case, the engine should be lowered lower. If the boat sluggishly accelerates and "grows" at low speeds, perhaps the engine is buried too much.
⚠️ Attention: When operating the motor with foot S in shallow water, always use a tilt system. However, remember that with the engine raised, starting the engine is prohibited, since the water intake will be above water level, which will lead to dry pump and impeller destruction.
Also, consider the trim angle. When you adjust the tilt of the motor, you change the angle of attack of the propeller. For the S leg, proper trimming is especially important to compensate for less depth and provide better plane output. Experiment with the position of the stops of the deflection to find the optimal balance between speed and stability of the course.
☑️ Checking before getting on the water
Frequent malfunctions and reduction gear maintenanceThe gearbox on the S leg, like any other, requires regular maintenance. Since lower unit is in the water all the time, the main enemy is corrosion and water entering the gearbox through the glands. Maintenance consists of regularly changing the oil in the gearbox, which is recommended every 50-100 hours or once a season, depending on the intensity of operation.
The oil must be transparent and not milky. If the oil turns white or cream, this is a sure sign that the gearbox has got water. This can happen due to wear of the glands or microcracks in the body. Operating a motor with water in the gearbox will lead to corrosion of the gears and bearings, which will eventually require expensive repair or replacement of the entire assembly.
It is also worth paying attention to the state of propeller. Locks on rocks or snags can cause the shafts to deform. On a short leg, the load on the shafts may be higher due to more rigid work on the excitement. Regularly lubricate the screw slag joint with a special lubricant to avoid fixation.
When changing the oil in the gearbox, always start filling from the bottom hole until the oil comes from the top one, which ensures that air is completely displaced and there are no air traffic jams.
Effects of Length on Speed and Fuel ConsumptionMany owners wonder if leg length is a factor in efficiency. There is no direct relationship, but there is an indirect link. The S leg motor on the right boat is operating optimally, providing the declared fuel consumption. If the motor is not selected correctly (for example, the S leg on a high transom), the engine will run in overload or, conversely, "twist" that will lead to overfuel consumption and resource reduction.
At planing speed, a properly set foot S achieves the maximum performance set by the manufacturer. The reduced resistance of the lower unit gives its small bonus. However, if the boat is heavy and the engine does not pull due to cavitation, the speed will be lower and the flow rate higher. So the key factor is not the length itself, but the correspondence of the leg length to the geometry of the particular vessel.
Fuel savings and top speed are only achieved with the perfect combination of the boat's transceiver height and the engine's lower unit length. Don't chase power if the geometry is wrong.
Frequently Asked Questions (FAQ)
Can I use a motor with foot S on a boat with a transom height of 45 cm?
The only way to use a 45 cm S leg motor is to use a special extension (extension) that will increase the distance from the transom to the lower unit. Without the insert, the screw will work in the surface water layer, leading to cavitation, overheating and loss of traction. However, the use of pads is not always recommended by manufacturers, as this changes the center of gravity and load on the transom.
How do I measure the height of my boat's transceiver?
To measure the height of the transom, place the boat on a flat horizontal surface. Take a ruler or a roulette. Measure the distance from the bottommost point of the bottom (keel) vertically up to the top cut of the transmuner (the horizontal plane where the motor is attached), which is the value of the transmuneral height along which the motor leg is selected.
Why does the engine with leg S warm up at low speed?
When you're not going fast enough, the flow rate of water through the intake is lower, if the motor is set too high, or if the intake is partially clogged, the water circulation is not sufficient to cool, and it may be a malfunction of the thermostat (if any) or impeller pump, and make sure that the control stream is running freely.
What is the difference between a lower unit S and a lower unit for electric motors?
Although the S (Short) marking is used for electric motors, the length standards may differ. For electric motors, the length of the lower unit is often indicated as the total length of the shaft (shaft) from mount to screw, and it can vary (30, 36, 42", etc.). Gasoline outboard motors have a more stringent standardization (15, 20, 25").