Choosing the right outboard motor isn't just about buying an engine, it's about finding a key element of safety and comfort on the water. One of the critical parameters that many beginners overlook is the length of a lower unit known as the "Dadewood" motor-legIt's the size that determines how your boat will behave on the wave, whether the engine will stall in the corners and how efficient the efficiency of the propeller will be. Yamaha, Mercury or TohatsuThe letter "S" stands for standard length, which is the basic solution for most small and medium-sized craft.
The wrong length can lead to cavitation, overheating of the engine or, conversely, to excessive water resistance. S leg In the context of your particular vessel, avoiding costly purchase mistakes, we will discuss in detail the technical details, the differences from long modifications and the installation algorithm of the unit below.
What does the S marking on the outboard motor mean?
In technical documentation and manufacturers' catalogs, letter designations serve as a shorthand code for the length of the lower unit. S (Short) This indicates that you have a short or standard foot motor. For most low-power two-stroke and four-stroke gasoline engines (up to 15-20 hp), this size is the industry standard. The distance from the mounting surface of a lower unit to an anti-cavitation stove in such models is usually about 381 mm (15 inches).
This is important because the transom geometry of the boat has to be very consistent with the length of the leg. S a boat designed for a long leg (LThe propeller will be too high above the water, resulting in air capture, loss of traction and risk of combustion of the piston group due to disruption of the coolant circulation, and vice versa, too long a lower unit on a small trans transom will create excessive drag.
Manufacturers use a single classification system to make it easier for owners of PVC and aluminum boats to choose from. boat-engineSince different brands tolerances can vary within a few centimeters.
⚠️ Attention: Never focus only on the letter designation when buying a used motor. Measure the actual length of the lower unit ruler, as the previous owner could install a non-standard bracket or replace the lower unit with a non-original one.
Technical specifications and dimensions of lower unit
The physical parameters of the foot directly affect the hydrodynamics of the vessel. S It's designed to have an anti-cavitation stove that's 1-2 inches below the bottom of the boat when it's static, and it allows for a stable water draw-out for the cooling system, even in abrupt maneuvers. Minn Kota or HaswingThe gradation may differ, but the principle remains the same: matching the depth of the screw immersion.
It's important to consider not only the length, but also the angle of inclination of the lower unit. differentiatorIf the leg is too short for your boat, even the maximum trim won't lower the screw deep enough to work efficiently. The table below shows the comparative specifications of common leg types.
| Type of leg | Designation | The length of lower unit (mm) | Typical power (hp) | Application |
|---|---|---|---|---|
| Short. | S (Short) | 381 (15") | 2 - 20 | PVC boats, small aluminum boats |
| Long one. | L (Long) | 508 (20") | 15 - 300+ | Boats, yachts, big boats |
| Ultra-long | UL (Ultra Long) | 635 (25") | 100+ | Large boats, commercial fleet |
| Electric S | S | 760 (30") | 0.5 - 3 | Inflatable boats, trolling |
It's worth noting that for electric motors, the dimensions are often in inches whole, including the barbell, so a 30-inch leg for electric traction is the equivalent of a short leg for an ICE, but given the lack of a classically based lower unit. dimensions in the specification.
Differences between the S leg and the L and XL modifications
The main difference is in geometry and purpose. S It is designed for low-transcent vessels where water clearance is minimal. L (Long) and XL They are used on boats with higher sides or a specific bottom shape that requires deep propeller immersion, the difference in length between S and L is about 127 mm, which is a critical parameter.
If you try to use an L-foot motor on an S-foot boat, you'll either have to lift the motor up the transom (reducing the angle of attack of the screw) or put up with the fact that lower unit will drag itself in the water, creating parasitic resistance, which is especially true for high-speed planing boats, where every extra centimeter of submerged part reduces the maximum speed.
On the other hand, putting the S-leg on a large boat will cause the propeller to constantly bare in the wave, which causes a phenomenon of cavitation, where the vapor bubbles collapse near the surface of the blades, destroying metal and drastically reducing traction, and the engine will start operating at high speeds without accelerating the boat, which is fraught with overheating.
⚠️ Attention: Avoid using the extension pads (deidwood extension) of home-made production. Violation of the geometry of the exhaust channel and the cooling system can lead to instantaneous failure of the engine.
Compatibility with types of craft
Motors marked S These are the most popular recreational boating boats, ideal for low-transom PVC inflatable boats, and owners often choose engines ranging from 2 to 9.9 hp, where a standard foot is the only solution.
For aluminum boats ("kazak", "crowonezh"), the situation is similar if we are talking about small size. However, if your aluminum boat has a high transom (more than 45-50 cm from the keel), you may already need a leg. LAlways measure the height of the transom from the bottom of the keel (or imaginary bottom line) to the top cut.
When buying a PVC boat, pay attention to the height of the transom. Standard for S-leg motors is 381 mm (15 inches). If the transom is higher, the motor will have to be raised, which will worsen handling.
Also, it is important to consider the type of bottom: flat-bottomed boats require less deep propeller immersion than V-shaped boats, which burrow heavily when overclocked. For the latter, sometimes even a standard S-leg can be a bit small, and experienced boatmasters prefer models with slightly longer lower unit or install hydrowings to press the stern.
Rules for installing the motor on the transom
The right installation is the key to long-term engine life. Installing an S-foot motor requires a strict sequence of actions. First, you need to determine the central axis of the transom. Then the motor is fixed with rods or bolts. The key is the height of the installation. The anti-cavitation plate must be parallel to the conventional bottom line and lowered below it by about 10-20 mm for a flat bottom, and deeper for a V-shaped one.
To fix, use the following steps:
- 🔧 Clean the surface of the transucin from dirt, sand and old paint in places of contact with strubcines.
- 🔧 Set the motor in the center, checking the symmetry of the distances to the sides.
- 🔧 Tighten fasteners or rods with the force recommended by the manufacturer (usually indicated in the manual) Yamaha or Honda).
- 🔧 Check the angle of inclination of the lower unit: at full gas, the bottom of the boat should be parallel to the water.
☑️ Checking the engine installation
After the mechanical installation, be sure to check the cooling system. Turn on the motor on the shore (in a tank of water or connecting a hose) and make sure that a stable stream of water comes from the control hole. If the jet is weak or intermittent, it is possible that the water intake is located too high and captures air.
Common mistakes in selection and operation
One of the most common mistakes is to buy a growth engine without considering the geometry of the boat. Owners often take a powerful long leg motor, hoping to "pin it up" somehow, and end up with a roll, a hard plane, and fuel consumption disproportionately rising.
Another mistake is ignoring the state of the lower unit when buying used equipment. The foot of the motor is not just a pipe, it runs shafts, fuel hoses and cooling channels. Hidden dents, traces of impacts on stones or sand can indicate the displacement of the pipework, which will lead to vibration, the destruction of the glands and water enter the gearbox.
How to check the red light of the used motor?
Remove the oil plum plug. If the oil is black and smells like smoke, the redder is overheated. If the oil has metal shavings (check with a magnetic screwdriver) the gears are worn out. If the oil is emulsified (similar to coffee with milk) the water has entered the redder, the oils need to be replaced.
Also worth mentioning is the screw material choice error: For S-legs in shallow water, aluminum screws are often recommended, which are easier to handle when impacted, but for better speed performance in clean water, it is better to consider composite or stainless options, if budget allows.
Service to lower unit and Editor
The engine leg requires regular maintenance, especially if you operate the vessel in salt water or muddy rivers. After each unloading, be sure to wash the outer surface of the lower unit with fresh water. This will wash away salt, silt and sand suspension, which are the main enemies of anti-corrosion coating.
Once a season or every 50-100 hours, you need to prevent the gearbox, which involves replacing the transmission oil, and the procedure is simple: twist the top and bottom screws, drain the old oil, flush the gearbox, and pour the new one before the oil enters the top hole.
Visual examination is also required.
- 🛠 Cracks on the welds of lower unit.
- 🛠 Luft propeller (checking the veneer connection).
- 🛠 Anode state (if installed) – they must break down to protect the essential details.
⚠️ Attention: When changing the oil in the gearbox, always start filling from the bottom hole. If you pour it from above, there will be air pockets in the case, which will lead to dry gears and rapid wear.
Frequently Asked Questions (FAQ)
Can I install an S-leg motor on a high-transcent boat?
Technically, you can, but you're not recommended. You're going to have to lift the motor above the top of the transom, which requires you to make special brackets, which disrupts the centering of gravity and can make the boat unstable, and you're going to have to buy an L-leg motor or use a special trawler supercharge if it's designed.
What if the S engine screw is constantly getting enough air on the corners?
This is a sign that the motor is too high or the different is in the wrong position. Try lowering the motor down the transom. If that's not possible, consider putting a hydrodynamic plate (hydro wing) on an anti-cavitation plate, which will improve the adhesion of the water flow.
What is the difference between the S-foot for 2x and 4x motors?
The outer dimensions (dadewood length) are usually the same for the same power. However, four-stroke motors are often heavier and may have a slightly different form of lower unit due to the location of the oil crankcase and the block head. Always check the compatibility of the motor model with your boat from the manufacturer's catalog.
A properly selected S leg ensures optimal efficiency of the propeller, protects the engine from overheating and guarantees the boat's handling at high speeds.