Choosing the right attachments for a boat is often a stumbling block for boating newcomers, especially when it comes to choosing a lower unit, because it is the geometry that determines the efficiency of the propeller and the safety of the unit itself. The wrong length can lead to the motor constantly capturing air or, conversely, experiencing tremendous water resistance.
In the catalogues of manufacturers and on the price tags of stores you often find labeling. S and LThese letters represent the length of a lower unit, the part of the motor that is submerged, and the difference between them is measured in inches, and it directly affects which translat and body type a particular model is designed to use, and understanding this difference is critical before you buy it.
If you make the wrong choice, you'll have problems in the first few minutes of operation, and it could be either a growl from cavitation or an inability to reach full speed, and let's figure out how to avoid these mistakes and choose the perfect configuration for your boat.
What do the S and L markings mean in the specifications
The abbreviations used by manufacturers come from English words. Short and LongThis is the standard international classification that all major brands, such as Yamaha, Tohatsu or MercuryBut behind these simple letters, there are specific physical dimensions that cannot be ignored.
Leg like that. S The short has a standard 381 mm lower unit length, the most common variant, considered universal for most inflatable boats and small transom boats of standard height, and is the most commonly sold motor and most popular with anglers and tourists.
Option L (Long) implies an increased length of the lower unit, which is 508 mm. Such modification is necessary for boats with a high transom. Using a short leg on a high transom will cause the anti-cavitation plate to be above water level, which is unacceptable for normal engine operation.
⚠️ Warning: Never attempt to install a short leg (S) motor on a high transom boat using wooden linings. This disrupts flow fluid dynamics and can cause the lower unit to break due to vibration.
There is also an intermediate option, which is sometimes found in specifications, but it is less common in the mass market. However, the basic division into two main types allows you to cover 95% of the needs of water engines.
Physical dimensions and geometry of lower unit
The difference of 127 millimeters between the S and L models seems negligible only on paper. anticavitational It is this horizontal plane above the screw that must be parallel to the surface of the water or slightly submerged.
If you look at the structure in more detail, the L leg has a longer shaft that transfers torque from the engine to the propeller, which increases the weight of the entire structure, which can be felt when carrying the motor along the shore, but for sea walks or high-speed swimming, the long leg provides better stability.
The geometry of the lower unit also affects the angle of attack: When mounted on the right transom, the long leg allows the motor to lower, which reduces the risk of air capture during sharp maneuvers or excitement.
To properly set up, you need to know the height of the transcent of your boat. The standard height for the S leg is from 350 to 380 mm. For the L leg, this parameter varies from 480 to 510 mm. Small deviations can be compensated by adjusting holes on the transom board of the motor.
When buying a long leg (L) motor, be sure to check whether it will touch the bottom of the boat when lifting the lower unit into the transport position.
Effect of Length on Hydrodynamics and Speed
The hydrodynamics are directly dependent on how deep the screw is, and if the foot is too short for a transom, the anti-cavitation plate protrudes out of the water, and the result is that air enters the zone of thinning behind the screw, and the screw is released. cavitationAnd the thrust drops dramatically.
On the other hand, the excessive sloshing that you can do when you put a long leg on a low transom increases the wet surface area of the lower unit, and it creates additional water resistance, which causes the motor to run at higher revs to achieve the same speed, and the result is higher fuel consumption.
The optimal position is when the top of the anti-cavitation plate is 1-2 cm below the water level when moving, in which case the flow of water is stable, the screw operates in a dense environment, and the resistance is minimal, and this is the state to strive for when choosing between S and L.
At high speeds, especially planing, the difference becomes even more pronounced. A long leg provides better course stability, as a large area of the underwater part works like a keel. A short leg makes the boat more maneuverable, but less stable on a straight line.
Compatibility with types of boats and transom
The choice between the S and L is primarily dictated by the design of the boat. For classic PVC inflatable boats, which make up the lion's share of the market, 90 percent of the time a type of foot is required. SBoat manufacturers are mass consumer-oriented and make transoms to the most popular standard.
Low-pressure inflatable floor boats (LDPBs) often have a stiffer transom design, but their height rarely exceeds the standard 380 mm. But rigid hulls made of aluminum or fiberglass can have a transom of any height, and here you already need to look at the boat's passport.
- 🛶 PVC boats: They require a short leg (S).
- ⚓ Metal boats: They are often equipped with transoms under the leg L (long) for marine use.
- 🏎️ Sports boats: They may have an adjustable transom allowing both options to be used depending on the task.
If you own a boat with high sides, such as some models of "kazank" or specialized fishing boats, you will most likely suit a long leg, which will allow you to lower the engine lower, keeping the propeller working correctly even when the boat is fully loaded with crew and equipment.
⚠️ Warning: Installing a long-legged motor on a low-transe boat is dangerous by hitting the screw on the bottom or bottom obstacles, as lower unit will protrude below the keel.
specifications comparison table
For quick technical orientation, it's convenient to use a summary table, which will help you match your boat's parameters with the specifications of a lower unit.
| Parameter | Foot S (Short) | Leg L (Long) |
|---|---|---|
| The length of lower unit | 381 mm | 508 mm |
| The height of the transom of the boat | 350–380 mm | 480–510 mm |
| Typical application | PVC boats, small boats | Naval boats, high transom |
| Weight (relatively) | Easy. | Harder. |
As you can see from the table, the differences are systemic, and the choices should not be based on saving or taking with a margin, but on the exact geometry of the motor and the boat, and even a few centimeters of mismatch can cost you comfortable operation.
What if the engine is already purchased and is not suitable?
If you're wrong about the length, there are special transom adapters that allow you to change the height of the engine, but this increases the sailing and weight, so you'd better change the engine or the boat itself.
Problems of wrong choice and their consequences
Ignoring the rules of leg length leads to specific technical problems, the most common of which is engine overheating; many motors' water-cooling system draws water through holes in lower unit; if the foot is too short and the water intakes are above the waterline, the motor will start sucking air.
The second problem is power loss. Cavitation doesn't just make noise, it physically destroys the propeller blades and keeps the boat from going to the plane. You'll hear the engine roar, but it won't accelerate. It's a direct route to overuse of fuel and engine life.
In the case of a leg that is too long, the main danger is mechanical damage, and a screw that is too low will affect floating objects, algae or the bottom, and this can lead to a breakdown of the lower unit or even more serious accidents on the water.
☑️ Pre-launch checks
Practical advice on installation and adjustment
When you put a motor on a transom, it's important not just to screw it up, but also to set the right trim, and that's using adjustment holes, and if you lift or lower the motor within one hole, you can get the angle of inclination, which also affects the performance of the leg.
If you're using a PVC boat, remember that when people are fully loaded, the transom can go down a little bit in the water, in which case the short leg motor can start to "grab" air at full throttle, and in this situation, it makes sense to consider a slightly longer lower unit model, if the transom design allows.
Always check the oil level in the gearbox after the first hours of operation, especially if you have changed the engine or boat.
Properly selected leg length is the foundation of safe and economical operation of the outboard motor, which can not be ignored.
Can I use the L-foot motor on a boat with a transometer under the S-foot?
Technically, this is only possible with a special transom setup that will lift the motor higher, but it will degrade handling, increase the load on the transom and shift the center of gravity, and it is better to avoid such compromises.
Does the length of the leg affect fuel consumption?
Yes, it does. Too long a leg creates excess water resistance, making the motor work harder. Too short leads to cavitation and inefficient power use. In both cases, fuel consumption increases.
How to accurately measure the height of the transom?
Take a flat ruler or level. Apply it horizontally to the top edge of the transom. Lower the perpendicular to the bottom edge of the cutout for an outboard motor (or to the bottom in the plane of the transom), the distance between these points is the height of the transmolator.
Are there motors with adjustable leg?
There are telescopic lower units for specialized equipment, but for conventional outboard motors, the leg length is fixed by the manufacturer.