The right length of a lower unit is a critical step when buying a new engine or replacing an old one. Miscalculations can lead to lower thrust, overheating, and even engine failure. Boat owners often face confusion about markings and the actual physical dimensions of equipment.
The length of a lower unit is the distance from the motor attachment to the anti-cavitation plate, and that's the parameter that determines how deep the propeller will run, and if the propeller is too high, cavitation will occur, and if it's too low, water resistance will increase. Accuracy of measurements This is crucial for the efficiency of the boat.
In this article, we'll take a step-by-step measurement algorithm, look at manufacturers' standards, and learn how to translate inch values into centimeters, and understanding these nuances will help you avoid costly mistakes when you complete a boat.
⚠️ Attention: Never rely on the "S", "L" or "XL" label on the box. Other brands can actually have sizes that vary by a few centimeters.
What is lower unit and Why Its Length Matters
lower unit is the bottom of the outboard motor, which contains a propeller shaft, a gearshift system, and often a cooling water pump, and the length of this part determines the depth of the propeller relative to the bottom of the boat. Anti-cavitation plateThe stain above the screw must be at a certain level to create the correct flow of water.
If the lower unit is not matched correctly, the water to the pump impeller can come in with vortices or air, which leads to local overheating of the cylinder block, and too short a leg will not provide any additional energy. optimally deepeningThis will cause a loss of thrust at high speeds.
On the other hand, too long a lower unit increases the sailing of the underwater part and the risk of hitting the bottom, especially in shallow areas, so the question of how to measure the length of the leg of a outboard motor is a basic one for any water motor.
When buying a used motor, always check the integrity of the lower unit for dents that may have occurred from impacts on stones or logs.
Standard lower unit Dimensions: Conformity Table
Motor manufacturers use lettering to indicate the length of a lower unit, but there is no single global standard in millimeters. Each brand can have its own tolerances. Below is a table that will help you navigate the basic sizes.
| Marking | Name of name | Length (inches) | Length (cm) |
|---|---|---|---|
| S | Short (Short) | 15" | 38.1 cm |
| L | Long (Long) | 20" | 50.8 cm |
| XL | Extra Long | 25" | 63.5 cm |
| XXL | Ultra Long | 30" | 76.2 cm |
As you can see from the table, the step between the standard sizes is 5 inches (about 12.7 cm). However, modern engines can have intermediate values. For example, some Tohatsu or Mercury models can be 17 inches long. Measurement in centimetres It always gives a more accurate result for the selection of a particular boat.
It is important to understand that the "S" marking does not always mean exactly 381 mm. It can be both 390 mm and 375 mm. So if you change the engine, it is better to take a roulette and measure the old unit physically.
Instruments for accurate measurement
To get reliable data, you need a minimum set of tools, the main requirement is that it is rigid and can be fixed, and a soft tape cyst can give you an error if it sags.
The ideal tool is a metal roulette or a rigid construction meter, and you may also need a straight metal angler or a flat rack if you want to control perpendicularity. rod It is not suitable here because of the small length of the sponges, but it can be useful for measuring the diameter of the lower unit.
Make sure you have a notebook to record the results. You should take the measurements twice to eliminate human error. If the motor is removed from the boat, make sure it's on a flat surface.
☑️ Preparation for measurements
Step by step: how to measure the length of the lower unit
The most accurate way to measure is when the motor is removed from the transom, in which case you have access to the top of the mounting plate, and if you can't remove the motor, you have to use indirect methods that are less accurate.
Here's the correct algorithm for the motor you're removing: Put the motor vertically on a flat floor so that the anti-cavitation stove is parallel to the surface. Use the level to make sure that the lower unit is vertical. Vertical position It is critical for the correctness of measurement.
1. Set the motor vertically on a flat surface.2. Apply the "zero" of the roulette to the top edge of the mounting plate.
3. Lower the roulette along the lower unit to the top edge of the anti-cavitational plate.
4. Record the result.
The measurement is from the mounting plane (where the motor is pressed against the transom) to the top of the horizontal plate above the screw. Don't measure to the bottom edge of the screw or to the base of the lower unit, that would be a mistake. anticavitational It is the industry standard.
⚠️ Attention: When measuring, don't consider the thickness of the gasket or rubber dampers if they are outside the plane of the metal. Measure only the metal part of the mounting site.
If you have the motor on the boat, it becomes more complicated, you have to measure the distance from the top of the transom to the anti-cavitation plate, and then measure the height of the transmunist, and the sum of these values will give you an approximate length of the lower unit, but the error can be up to 2-3 cm.
Measurement nuances for hydraulic-lift motors
If the motor is equipped with a hydraulic lifting system (tilt), make sure that it is fully lowered into the operating position before measuring. When elevated, the lower unit shifts, and you can not get accurate data. Lock the engine in the lower position with a locking screw or ask an assistant to press it against the transom.
The influence of transence height on the choice of length
The length of the motor leg is inextricably linked to the height of the transom of your boat. Standard transom heights for inflatable and plastic boats vary. For a short lower unit (S), the height of the transan is usually 381 mm (15 inches). For a long (L) it is 508 mm (20 inches).
If you put a short-leg motor on a high transom, the propeller will be too high in the water, which will lead to air capture and loss of power, and in the opposite situation, when the long lower unit is on a low transom, the water resistance will increase, and the motor will work with overload.
Optimal entrapment The anti-cavitation stove should be 25-50 mm below the bottom of the boat (or the keel line), which ensures a clean flow of water to the propeller. When planning to buy a new motor, be sure to measure the height of the transomper from the upper plane to the conventional line of the bottom.
Frequent errors in measurement and selection
One of the most common mistakes is to measure from the bottom of the lower unit (where the drain hole or bracket is located). This is wrong. The reference point is always the plane of the transom. Also, the length of the lower unit is often confused with the total length of the motor.
Another mistake is ignoring the tilt of the transom. In inflatable boats, the transom is often tilted. When measured "by weight," the roulette can go at an angle, not vertically down. This distorts the result. Use a plumb or a level to control the vertical.
Don't forget. boat-likeFor planing boats, length requirements are stricter than for displacement boats. If you plan to use the engine on different boats, choose the length for the boat with the highest transom, using the spacer for the second boat.
⚠️ Attention: Never try to lengthen the lower unit yourself by welding pipes or inserts. This will disrupt the cooling system and can lead to depressurization of the lower unit and water entering the gearbox.
A properly chosen leg length ensures that your motor will last a long time and will give the declared power.
Keep a photo of your motor nameplate and measured dimensions in your phone, which will help you quickly pick up parts or a new engine if necessary.
Can a long leg motor be used on a low transance boat?
It is possible to use a long leg (L) motor on a boat designed for a short (S) motor, but it is undesirable. This will lead to increased water resistance, lower maximum speed and fuel consumption. However, if there is no exit, the motor will work, just less efficiently. The reverse situation (short motor on a high transom) is categorically unacceptable.
Does the type of screw affect the choice of the length of the lower unit?
The type of propeller (step, diameter, number of blades) does not affect the length of the lower unit. The length is determined by the design of the boat and the height of the transom. However, if you plan to install a large diameter propeller, make sure that the lower unit is long enough that the propeller does not protrude above the anti-cavitation plate.
How to convert inches to centimeters when buying a motor?
To translate, use the factor 2.54. One inch is 2.54 cm Standard lengths: 15" = 38.1 cm, 20" = 50.8 cm, 25" = 63.5 see Round the values in the big way when choosing slots, but strive for an exact match when buying a motor.