The right selection lower unit It's not just a formality when buying machinery, it's a fundamental condition for safety and efficiency of navigation. A miscalculation can cause the engine to run idle or choked with water, which can eventually overheat and repair expensively. For owners of inflatable boats, this parameter is critical, because the geometry of the transom and the behavior of PVC tubes in water are different from rigid hulls.
Many beginners rely solely on standard values, forgetting to consider the actual landing of a boat under load. squirrel It should be at an optimal level relative to the waterline, and if you ignore this, even the most powerful engine will not be able to develop the declared thrust, and fuel consumption will increase significantly.
In this article, we will discuss in detail how to determine the required length, what standards exist, and why a “short” leg is sometimes better than a “long” one, and you will learn how to measure yourself and understand the technical documentation of manufacturers to avoid annoying misses when completing your boat.
What is lower unit and Why its Length Matters
lower unit It's the bottom of the outboard motor, the vertical shaft, which is the control, the fuel lines and the propeller shaft itself, and it's the length of the propeller that determines how deep the propeller will be while driving. For inflatable boats, this varies, and manufacturers offer several modifications for different types of boats.
The main function of the correct length is to ensure full immersion. propeller And if the propeller is too high, it will start to capture air, which will lead to cavitation, which is the collapse of air bubbles that destroy metal and reduce efficiency. If you lower the engine too deep, the water resistance will increase, which will adversely affect speed and handling.
It's important to understand that the length of a lower unit is not just a physical size, it's an engineering trade-off: Too long a shaft increases the weight of the entire structure and makes the motor more vulnerable to impacts on bottom obstacles; too short a shaft makes it impossible to use engine power efficiently, especially in planing.
Always check the marking on the lower unit: the letter S stands for Short (short), L for Long (long), and X for Extra Long (long).
Standard lower unit Sizes: Classification and Labeling
Global manufacturers of motors such as Yamaha, Tohatsu or MercuryIt's a simpler choice, but it requires careful study of specifications, and basically there are three main sizes, each of which is tailored to a specific operating environment and transomper height.
The most common option for small inflatable boats is the short leg, designed for boats with a transom height of up to 381 mm. However, it is worth remembering that under load the boat sags, and the real height can change. The long leg is usually complete with medium-power engines and is designed for transoms about 508 mm high.
- 📏 Short (S): The length of the lower unit is about 381 mm (15 inches), ideal for lightweight PVC boats with low transceiver.
- 📏 Long (L): The length is about 508 mm (20 inches), suitable for heavier boats and high-transe boats.
- 📏 Extra Long (X/XL): Lengths of 635 mm (25 inches) and more are used on large boats where maximum propeller entraction is required.
There are also intermediate options, which are often labeled as 17.5 inches or 22.5 inchesThey were designed for specific boat models where standard sizes were not perfect. When buying a used engine, be sure to measure the lower unit with a ruler, as the markings may be erased or missing.
How to measure the height of the transom of an inflatable boat
Before you go to the store for a new engine, you need to take accurate measurements of your boat. For inflatable boats, this process has its own peculiarities, because the soft transom can deform. Measurements must be made from the very top of the transmutation board to the conventional line of the bottom, but with loading in mind.
The best measurements are when the boat is pumped to working pressure, use a rigid ruler or building level, attach it to the outside of the transom, and if the transom is beveled (which is often the case to improve driving performance), the measurement is done vertically, not along an inclined surface.
☑️ Pre-measurement checks
It's important to consider that when you install a heavy motor, the transom can bend inside the boat and the boat itself can go deeper into the water. So the resulting value of the transom height is often added to a small margin or, conversely, you choose a motor with a slightly shorter leg if the boat is light and flat. Measurement accuracy A few centimeters can play a crucial role.
⚠️ Warning: Never measure the height of a transom on a deflated or half-descended boat. Soft materials will give distorted data, leading to the purchase of inappropriate equipment.
Anti-Cavitation Plate Rule and Depth of Immersion
The key element in the leg length equation is anticavitationalIt is a horizontal plate located directly above the propeller, and its upper edge should ideally be on the same level with the waterline or slightly lower (by 1-2 cm) when the boat is moving in plane.
If the stove is too high above the water, the screw starts sucking air from the surface, a phenomenon called cavitation. The engine is gaining speed, but the thrust is falling, and the characteristic noise and vibration signal a problem, and prolonged operation in this mode is dangerous for lower unit.
On the other hand, if you turn the engine too far, you increase the area of contact with the underwater part, which creates additional resistance, which the motor has to overcome, using excess fuel, and in maneuvers in shallow water, the risk of hitting the bottom or grass increases significantly.
The optimal position of the anti-cavitation plate is parallel to the bottom of the boat and 10-20 mm below the water line when moving at full speed.
Table of conformity: choice of motor for the type of boat
To make the choice easier, you can use reference data. Below is a table that will help you navigate according to your type of boat and your desired speed. Remember, these are averaged values, and individual design features can make adjustments.
| Type of boat | Translation height (mm) | Recommended leg length | Note |
|---|---|---|---|
| Small inflatable (2 seats) | 350 - 380 | Short (S) | Standard for light PVC |
| Medium PVC boat (3 seats) | 380 - 420 | Short (S) / Long (L) | Depends on the load. |
| Hard-bottomed boat (HDB) | 400 - 450 | Long (L) | It takes a lot of power. |
| Boat / Heavy Boat | 500 - 520 | Long (L) / XL | For powerful engines |
When choosing between two adjacent sizes, such as a short leg and a long leg for the boundary value of the transom height, it is better to lean towards the longer option if there is an adjustable tiller gas or the possibility of installing a drawer. However, for inflatable boats, the rule is usually: slightly higher than too deep to minimize resistance.
Some boat manufacturers indicate the recommended length of the lower unit in the product passport, and if such information is available, it should be trusted first, since engineers have already calculated the center of gravity and driving performance of this particular model.
What if the engine is not a little high?
If the motor is slightly longer than it should be, you can use the washers to hold the rods up to lift it, and if the motor is shorter, there is a risk of cavitation, and the only solution is to replace the lower unit or use a boat with a lower load so that it is less submerged.
Consequences of Wrong Dydewood Length Choice
Ignoring the rules of leg length can lead to a number of technical problems. engine-heatingThe cooling system of most outboard motors draws water through holes located on the lower unit, and if the motor is set too high, these holes can be above the water, and the pump will start pumping air.
The second negative effect is loss of control: cavitation causes the propeller to lose traction with the water, and the boat becomes less obedient when cornering, and in an emergency, it can cause an accident, and the constant vibration destroys the fasteners and can damage the transom of the inflatable boat.
- 🔥 Overheating: Working without water leads to a jamming of the piston group in a matter of minutes.
- 📉 Speed drop: The wrong angle of attack of the screw reduces the efficiency of the propulsion system.
- 🔨 Mechanical damage: The screw strikes the bottom at too low the location of the engine.
⚠️ Warning: If you notice that water has stopped flowing from the control hole (the "urinal"), immediately turn off the engine. Continuing to work even at idle speeds without cooling is guaranteed to bring the engine down.
FAQ: Frequently asked questions
Can a long leg motor be used on a short transom boat?
Technically, you can install it, but it's not going to work, it's going to create too much drag, and it's going to hit the bottom, and it's going to shift the center of gravity, which is going to make the boat less stable, and it's going to require special spacers to correct, which is not always safe.
How often should I check the height of the engine?
It is recommended to check the position of the engine before each season, as well as when changing the configuration of the boat (addition of equipment, change in the number of passengers). Under weight, the boat can sit deeper, and you will need to adjust the angle of inclination or height of the attachment.
Does the material of lower unit affect the choice of length?
No, the material (usually aluminum alloy or stainless steel) affects weight and corrosion resistance, but not geometric dimensions. Length is determined by the S, L, XL standards, regardless of what the lower unit case is made of.
Why do some engines have the ability to adjust the height?
This is the vertical movement of the motor on the transom, which allows you to (finely adjust) the position of the anti-cavitation plate relative to the waterline depending on the load of the boat, which is a useful feature that allows you to adapt the same engine to different conditions.