Choosing the right length of a lower unit, or as the popular saying goes, "legs," is a critical step when buying an attachment motor. The wrong length can lead to cavitation, overheating and even failure of the power plant, especially in shallow waters or inflatable boats. short-leg It often becomes the optimal solution for owners of light PVC boats and shallow-sitting hulls.
In the marine engine industry, sizing standardization helps you navigate hundreds of models, but beginners are often confused in inches and centimeters. Standard classification divides lower units into short (S), medium (M) and long (L). Understanding what size you need will save money on repairs and ensure comfortable navigation in all conditions.
In this article, we will discuss in detail the geometric parameters of the short leg, the features of installation on different types of boats and the technical nuances that must be considered before buying, you will learn how to determine the required length and understand why there are no universal solutions to this issue.
Standard lower unit Dimensions: Classification S, M, L
Global manufacturers of motors such as Yamaha, Mercury and TohatsuThey have a single system of descriptive lengths for a lower unit, a short leg marked with a letter. S (Short)It has a very specific size from the installation site to the end of the lower unit, which is usually 15 inches, which is about 381 mm in terms of the metric system.
However, the actual length can vary slightly depending on the specific series of motors and their power. For example, low-power "spots" and "nine" often have a lower unit slightly shorter than the powerful 50-horsepower units, formally referred to as Short.
For comparison, the average leg (M) is 20 inches (about 508 mm) and the long leg (L) is 25 inches (about 635 mm). There are also extra-long legs (XL and XXL), but they are used mainly on suspension boats or on specialized vessels.
⚠️ Attention: Never install a short-leg motor on a high-transe boat without the use of supercharges, which will cause water flow to swirl and the pump to overheat instantly.
Geometry and physical parameters of the short leg
Physically. lower unit It's a tube that runs through the propeller shaft, cooling hoses and control cables, and in a short leg, it's a tube that's minimal in length, which reduces the overall weight of the motor and reduces the lever on the transom of the boat, and this is especially true for lightweight inflatable boats, where every kilogram counts.
The distance from the mounting plane to the anti-cavitation plate (horizontal plate above the screw) is a key parameter. For a short leg, it should be approximately 38-40 cm. If the stove is immersed too deep, water resistance will increase; if too high, air suction will begin.
The slope of the lower unit also plays a role. On short legs, the trim angle allows the screw to be raised higher as it passes over obstacles. The S-type Dadwood design often has reinforced stiffness ribs at the bottom, because the risk of hitting the ground is higher than with the long versions.
Technical tolerances for dimensions
The exact dimensions can vary by +/- 10-15 mm depending on the manufacturer. For example, the Suzuki DF9.9B has a 382 mm lower unit length, and the similar Tohatsu M9.8B has 385 mm. These differences are not significant for installation, but are important when selecting accessories.
Selection criteria: for which boats is suitable short leg
The main purpose of engines with lower unit type S A classic example is PVC inflatable boats, old-model aluminum "cossacks" and flat-bottom boats, which typically have transom heights of 40-45 cm, which is ideal for a short leg.
If you plan to use the boat primarily in shallow water, in grassy bays or on rocky rivers, a short leg will be your main advantage. The ability to lift the screw higher allows you to skip critical areas without risking damage to the blades or the rod.
But there are limitations: on high-speed glitter boats with a high transom (over 50 cm), the short leg will not work effectively; the screw will constantly grab air on the wave, which will lead to loss of traction and "growl" of the engine, in which case it is better to consider the option of medium length or use of a transition plate.
- 🚤 Ideal for PVC boats up to 4 meters long with an inflatable or hard bottom.
- ⚓ Suitable for aluminum flat-platelets and traditional "Kazanki".
- 🌊 It is recommended for fishing in shallow waters and overgrown waters.
- 🏗️ Not suitable for boats with high transceiver without special improvements.
The choice also includes the boat's load capacity, where if the boat is heavily loaded, the stern will sink deeper, and the actual height of the transom above the water will decrease, in which case the short leg may be even long, requiring additional adjustment of the tilt of the engine.
Installation problems and height adjustment
The right set of motors on a transom is a science that requires precision. slab-stove If the plate is raised too high, the propeller will start sucking in air, causing cavitation and loss of rev.
A common mistake of owners is to set the engine too low, in which case, lower unit creates excessive drag, drags the stern down and increases fuel consumption, the motor must run freely, and the water must flow around the lower unit without forming a powerful wave in front of it.
☑️ Checking the correctness of installation
To adjust the height on outboard motors, holes are provided in the tiller or rod. By rearranging the pin or changing the position of the clamps, you can vary the landing height of the engine within 5-10 cm. This allows you to adapt the same engine to slightly different loading conditions of the boat.
⚠️ Attention: If you hear a loud roar when you're driving, and you see that your speed is rising and your speed is falling, you can immediately drop the gas, which is a sign of cavitation, which can destroy your propeller blades in minutes.
Comparative table of specifications of lower unit
For the sake of clarity, we'll give you a comparison of standard sizes, and note that the dimensions are in inches, because that's the world standard, but for convenience, we've added centimeters.
| Type of leg | Designation | Length (inches) | Length (cm) | Typical application |
|---|---|---|---|---|
| Short. | S (Short) | 15" | 38.1 | PVC boats, shallow waters |
| Medium | M (Mid/Long) | 20" | 50.8 | Metal boats, station wagon |
| Long one. | L (Long) | 25" | 63.5 | Boats, seagoing vessels |
| Extra long. | XL (Extra Long) | 30" | 76.2 | Sailboats, specifics |
As you can see from the table, the difference between the types is exactly 5 inches (about 12.7 cm). This is a standard step that allows manufacturers to unify production. When buying a used motor, always check with the seller the type of foot, as it can be difficult to determine the difference in 5 inches visually from the photo.
When buying a used boat, measure the height of the transom with roulette. If it is 38-43 cm, you definitely need a short leg. If 48-53 cm, look for the middle one.
Shallow water exploitation and technical risks
Use of use short-leg It involves frequent use in difficult conditions, sand, rocks and algae are the main enemies of the cooling system, and even a small amount of sand trapped in the pump can cause the glands to fail, leading to water entering the crankcase.
Owners of short-dadewood motors should inspect the cooling control hole more often. A weak jet of water is the first sign of problems. It is also recommended to install mesh filters on intake holes if you often walk on overgrown areas.
Mechanical damage to the propeller is another risk: When hitting a hard obstacle at high speed, the impact energy is transferred to lower unit. The short leg, being less protected height, takes the hit more often. cut-off or propeller guards are required here.
A short leg is a choice for specific conditions, it gives you the advantage of patency, but it requires a more careful attitude to the cooling system and the cleanliness of the reservoir.
FAQ: Frequently asked questions
Can a transition plate be used to install a short motor on a high transom?
Yes, it's a common practice. A transient plate (or transom supercharge) allows you to raise the mounting point of the motor, but it increases the overall weight on the stern and can worsen the hydrodynamics of the boat, and only use this method if you can't replace the motor with a longer leg version.
How to measure lower unit if there is no ruler?
Take a rope, apply it from the mounting plane (where the motor is in contact with the transom) to the bottom edge of the lower unit (before the screw). Then measure the length of the rope. If the result is about 38-40 cm, it is a short leg. If about 50 cm is the average.
Does the length of the foot affect the speed of the boat?
Length alone doesn't affect power, but it does affect efficiency. If the leg is wrong (too short for a transom), the motor will cavitate and the speed will drop. A properly selected short leg on a low boat will ensure passport speed.
Is there a price difference between the S and L engines?
Usually the prices are the same for single-power engines, the difference may be that short legs are often in high demand in regions with popular PVC boats, so they may not be available.