The correct timing of the length of a lower unit, or as water-engineers often say, the length of a "leg," is a critical step in the assembly of a boat kit. A mistake in the calculations here can cost the owner not only loss of speed, but also expensive repairs of the power plant. boat-engineThey do not attach importance to the L, XL or XXL markings, relying on assurances from sellers or standard dimensions, which often leads to fatal consequences for the engine.
The main function of lower unit is to deliver torque from the crankshaft to the propeller to the required depth of immersion. cavitationIt's a blade-shattering system that keeps the engine from reaching full power, and if you lower the engine too low, you'll have a higher water resistance, and the water pump can start to grab the bottom silt, which will cause you to overheat, and that's why understanding the geometry of landing the motor on a transom is a basic skill for any boat owner.
In this article, we will discuss the technical nuances that will help you avoid common mistakes when choosing the length of a lower unit for your home. fuel-fuel or electric engine.
Physics of the process: why altitude matters
The propeller's operation depends on the pressure of the water above it, and to work effectively, the propeller must be submerged to a depth that prevents air from being sucked from the surface. cavitationWhen the anti-cavitation plate (the horizontal plate above the screw) is too close to the surface of the water, air enters the zone of thinning, the engine begins to "reset speeds", the speed drops, and the screw runs idle.
On the other hand, excessive entanglement lower unit It also causes more sailing in the underwater part of the motor, which creates additional resistance to movement, and in fact, in shallow water, there is a high risk of a screw hitting the bottom, which can lead to a failure of the gearbox or the propeller itself. Water pump The cooling system also has its own water intake height limits.
⚠️ Note: Operation of the motor with signs of constant cavitation (breakdown of full-gas revolutions, characteristic noise) can lead to overheating of the piston group in a matter of minutes.
The optimal position is when the anti-cavitation plate is at the same level with the conventional line of the bottom of the boat or slightly lower (15-25 mm), but this rule only works for a flat bottom.
The main task is to immerse the screw deep enough to avoid cavitation, but not to lower the engine below the required minimum, so as not to lose efficiency.
Standard Classification of lower unit Length
Manufacturers outboard (Yamaha, Mercury, Tohatsu, Honda) have a unified notation system to make it easier for consumers to choose from, measuring the length of a lower unit from the mounting surface (the upper plane of a lower unit pipe where the motor is attached to the transom) to an anti-cavitation plate. Standard sizes vary depending on the power and class of the motor.
Small power (up to 5-6 hp) is often the same size as most inflatable boats. However, in the 8-15 hp class, there is a division into short and long legs. Powerful motors from 20 hp and above are almost always produced in long-legged versions, as they require more rotor depth for efficient operation at high speeds.
Below is a table showing the dependence of marking on the actual length of the lower unit in inches and millimeters:
| Marking | Length (inches) | Length (mm) | Typical application |
|---|---|---|---|
| S (Short) | 15" | 381 mm | Small boats, inflatable boats |
| L (Long) | 20" | 508 mm | hull boats, large PVC |
| XL (Extra Long) | 25" | 635 mm | High-transage boats |
| XXL (Ultra Long) | 30" | 762 mm | Specialized courts |
It's important to note that labeling can vary slightly between brands, for example, some manufacturers. electric motors Minn Kota or Haswing may be its own scale, but the principle remains the same: the larger the letter, the longer the leg.
How to measure the height of your boat’s transom
Before you buy a motor, you need to know exactly the height of the transom, which is measured strictly vertically from the top edge of the transmuner (where the motor will lie) to the conditional line of the bottom (kilson). For accuracy, use the building level and roulette, the boat should be on a flat horizontal surface, and the keel should be parallel to the ground.
There is a common misconception that the height of a transom is always 381 mm (15 inches) or 508 mm (20 inches). In reality, especially for homemade boats or boats of the old years, this size can be arbitrary. If you buy a 381 mm lower unit engine for a transom height of 450 mm, the screw will hang around in the air, and the engine will instantly go into cavitation.
☑️ Testing the height of the transan
If your transom's height doesn't match the standard size of the motors, you have two options: making a transom liner or ordering a non-standard lower unit length motor (which is often expensive and time-consuming).
⚠️ Note: When using the expanse, make sure that the motor fasteners pass through it and have a sufficient thread length for reliable nut fixation.
Specificity of PVC inflatable boats
Owners PVC They face a unique problem: the trunk of such boats is often variable in height or can bend under the weight of the engine. In addition, inflatable tubes create specific aerodynamics and hydrodynamics. For inflatable boats up to 3.2-3.6 meters in length, the standard is the short leg (S). Installing a long-legged motor on such a boat will lead to excessive nose differentiation and loss of handling.
For PVC boats with a length of 3.8 meters or more, especially those with an inflatable keel (NDND) or hard floorboard, a long leg (L) is often required. This is due to the fact that when entering the stern planing of such a boat is pressed against the water, and a short lower unit may not be enough to preserve the propeller's immersion.
When installing a PVC motor, it is important to consider the elasticity of the transom board. Under load, it can bend down by several centimeters. Therefore, when choosing between a short and a long leg for a border size (for example, a transom of 430 mm), it is better to lean towards a longer daydvud, but use tilt adjustment to compensate for the height.
Adjustment of slope and depth of immersion
Even a well-chosen engine with a lower unit length requires fine tuning, and there are holes on the hub of the outboard motor (or on the transom plate) to adjust the angle of inclination (differentiation), and by rearranging the tilt pin, you change the angle of attack of lower unit relative to water.
If you lift the nose of the motor (deflect the lower unit backwards), the anti-cavitation plate will rise, which is equivalent to reducing the length of the dive. This can be useful if you feel that the motor is digging and pulling the bow of the boat down. If you lower the nose (press the lower unit forward), the stove will go deeper, which will improve the propeller's emphasis, but increase resistance.
⚠️ Warning: Never change the angle of inclination of the engine at full speed. Stop and adjust only when the engine is turned off.
Optimal position is considered when the anti-cavitation plate moves parallel to the bottom of the boat, and the stream of water from the control hole of the cooling system goes smooth, powerful stream without air bubbles.
What is a transom angle?
The transom angle is the angle between the lower unit and the vertical. The standard angle for most boats is 0-2 degrees (vertically or slightly tilted backwards), and the wrong transom angle leads to yaw, poor planing output, and increased fuel consumption.
The effect of length on speed and fuel consumption
The wrong length of the lower unit has a direct impact on economic performance. Engine efficiency You give more gas, the speed goes up, but the speed doesn't go up. fuel-mix It can become too rich or poor (depending on the type of carburetor or injector), leading to the formation of soot on candles and pistons.
Excessive slogging also increases flow, and the boat has to drag an extra mass of water around its long leg, and at low speeds (trolling mode), it's not as noticeable, but when moving on a sleeve, the difference can be as much as 10-15% of fuel.
In addition, the long leg increases the risk of damage when swimming in overgrown algae places or on a rocky bottom. The screw, which is deeper, more often comes into contact with bottom sediments.
When buying a used motor, pay attention to the condition of the anti-cavitation plate. If there are chips or impact marks on its edge, then the previous owner used the engine with too low immersion.
Frequently Asked Questions (FAQ)
Can I shorten or lengthen the engine’s lower unit?
It's theoretically possible, but it's technically very difficult and risky. lower unit is a complex structure that includes shafts (rowing, vertical), fuel supply tubes, gearshift cables and cooling channels. Changing the length will require digesting the body, lengthening or shortening all internal communications and shafts. It's almost impossible to do it in a home that's tight and reliable. It's easier and cheaper to buy a tray or sell a motor and buy the right one.
What is the difference between a lower unit S and an L for 9.9 hp engines?
The difference is 5 inches (127 mm). The motor with leg S (381 mm) is designed for transoms about 38-40 cm high. The motor with leg L (508 mm) requires a transom height of about 50-52 cm. Installing the L motor on a low transom will cause the screw to be above water level, and the motor will not be able to push the boat.
Why does the engine stop when you turn abruptly?
Most often, it's a sign that the lower unit is too short for the boat or the angle of inclination is set incorrectly. When the boat rolls on board, the screw bares and captures air. The engine loses load, the revolutions jump sharply, the limiter is triggered or the detonation occurs, and the engine stalls.
How does the length of the lower unit affect electric motors?
For electric motors The principle is the same, but the requirements are milder. Because electric motors are not afraid of the "spin" (no carburetor and no complex ignition system), short-term cavitation is less scary for them. However, for the efficiency of the screw, the depth of immersion is critical. Electric motors often have a telescopic leg that allows you to adjust the depth of immersion manually in wide limits.