Anyone who buys their first outboard motor or plans to upgrade an old boat is faced with mysterious numbers in the spec, which is a parameter that is designated in the catalogs by the letter L, but in reality determines whether you will swim fast or only smoke water. The length of lower unit It is not just a marketing designation, but a critical size that affects the hydrodynamics of the entire boat.
To make a mistake here is to get either a porridge of the air bubbles on the propeller at the slightest wave, or a parasitic resistance that will burn the excess fuel. Wrong choice. lower unit It turns powerful machinery into inefficient load, so let's figure out how to find the perfect fit for your body.
In the technical data sheet of any outboard motor you will find the markings S, L or XL. These letters are not a random set, but a standard classification adopted by most manufacturers, such as: Yamaha, Tohatsu or Mercury. Length of leg is determined by the distance from the installation pad (bracket) to the anti-cavitation plate, which is located directly above the propeller.
It's the anti-cavitation stove that needs to be in the water at a certain depth to ensure that fluid flows to the propeller blades in a stable way, and if the stove is too high, the propeller will start to grab air, which will cause thrust loss and overheating, and if it's too low, the water resistance will grow, and the boat will lose speed.
The standard classification is as follows:
- 📏 S (Short) - Short leg, usually about 381 mm (15 inches).
- 📏 L (Long) - long leg, standard about 508 mm (20 inches).
- 📏 XL (Extra Long) is an ultra-long leg, approximately 635 mm (25 inches).
Always check the specification of a particular model, as different brands can vary in size by 1-2 centimeters.
There are also intermediate options, for example, TL Tiller Long, which is less common and designed for specific tasks, but 95% of the market is in three main categories, and it's important to understand that switching from short to long legs is not always possible without replacing the lower unit itself, as this is a design feature of the engine.
Accurate measurements of the transom: geometry matters
Before you look at the catalogues, you need to arm yourself with a roulette and make accurate measurements of your boat. transom The vertical part of the stern, where the motor will be attached, is measured from the top edge of the transom to the bottom of the keel (or flat bottom) strictly vertically.
And here's the big mistake that beginners make: they measure diagonally or at an angle, which leads to a few centimeters of error. lower unit These centimeters are critical. Use a building level or plumbing to make the measurement line perfectly perpendicular to the waterline.
⚠️ Note: If you have a boat with a deep V-shaped hull, measure the height of the transom at the lowest point where the plane of the mounting of the motor will pass, and not along the edges of the skin.
The resulting value should be checked with the recommendations of the manufacturer of the boat. Usually the optimal height of the transom for most glittering boats is from 380 to 510 mm. If your measurement shows 420 mm, you are likely to fit a standard long leg. If the transom is only 350 mm, you will have to look for a motor with a short lower unit or increase the transom.
Conformity table and configuration selection
To simplify the selection problem, you can use a composite table that correlates the height of the transom with the recommended length of the lower unit. displacement The shape of the feed also makes adjustments.
| Translation height (mm) | Recommended length | Marking | Type of boat |
|---|---|---|---|
| 381 (15") | Short. | S | Inflatable boats, fine-seated aluminum boats |
| 508 (20") | Long one. | L | Boats with deep V and deep V boats |
| 635 (25") | Super-long | XL | Pontoon boats, large yachts, sailboats |
| 762 (30") | Special. | XXL | Specialized boats, commercial fleet |
As you can see from the table, the scatter is quite large. Using a short leg motor on a high transom will require a sub-motor plate, which will change the center of gravity. Conversely, a long leg on a low transmolator will force you to keep the rompel in the "gas to the floor" position to lift the screw, which is uncomfortable and unsafe.
Special attention should be paid to boats. PVCThey often have soft transoms that can bend under the weight of the motor, in which case the actual height of the installation in the water will be less than when measured on land, so inflatable boats are often recommended to take a motor with a slightly shorter lower unit or use adjustable suspension height.
The effect of length on speed and controllability
Why can't you just put the longest leg in order not to get the air? The answer lies in the physics of motion. frontal resistanceWater is a dense medium, and every extra centimeter of metal sticking into the stream takes away precious horsepower.
If the lower unit is too long for a given speed and shape, a turbulent trail forms behind the propeller, which strangles the boat, the top speed drops, and fuel consumption rises, and the long leg makes the motor more vulnerable to impact on underwater obstacles: the lever on the transom increases, which can lead to a breakage of the mount.
The optimal length is a compromise between guaranteed water supply above the screw and minimum resistance of the body.
On the other hand, too short a leg leads to cavitation, and when the anti-cavitation stove comes out of the water, the propeller starts to work in a mixture of water and air, which not only reduces efficiency, but also causes erosion of the blades and overheating of the engine due to insufficient coolant intake. Cavitation The main enemy of your engine’s life.
Specificity of inflatable boats and keel boats
Owners PVC The soft transom of these vessels, with a load of 30-50 kg (motor weight), bends back and down, and if you pick up the engine upside down, it's too deep in the water, and the boat's nose will pick up and the speed will fall.
For inflatable boats, S-marked motors are often the ideal solution, even if the transom is formally closer to 400 mm high. The solution is also adjustable suspensions that allow you to change the angle and height of the mount directly on the water, which allows you to find the “golden mean” experimentally.
- 🚤 For boats without a keel (flat bottom), it is important to take the maximum lower unit back so that the screw works in a clean stream.
- 🚤 Keel boats are more stable, but require precise compliance with the depth of the propeller relative to the keel.
- 🚤 Aluminum boats with a hard transom allow you to use standard long legs without the risk of deflection.
Remember that inflatable boats often use a transom board, which also has its own thickness. Measure the height of the transmunist already with the board installed if you plan to use the motor constantly.
The secret of setting up
If the engine “snarls”, but the boat does not get on plane, try to raise the lower unit a little or install an anti-cavitation plate.
Adjustment and installation: practical advice
Installing the motor is a process that requires patience. After the initial attachment, you need to run a test run. Notice the jet of water coming out of the control hole (the urinal) that needs to be powerful and confident. A weak jet is the first sign that the air is not moving. water-intake It's taking air.
When you're driving, you should pay attention to the humming wheel. If the motor is constantly trying to jump out of the water at full speed, then the leg is short or the angle of the differential is too large. If the engine is sitting in the water dead weight and the boat is sluggishly accelerated, the lower unit is too long or lowered too low.
⚠️ Warning: Never start the motor out of water or with a partially submerged propeller to check the operation - this is guaranteed to overheat and deform the impeller pump.
To fine tune the engine, use the sequential approach: set the motor at an average height, walk 500 meters, record the time and flow, then lower or raise the motor one division and repeat the test, the difference can be up to 10-15% of the speed.
Frequently Asked Questions (FAQ)
Can you build up lower unit on your own?
Theoretically, it's possible, but it's not recommended without the factory parts, and if you break the geometry of the exhaust channel and the cooling system, you lose power and you overheat, you'd better buy a motor with the right leg length.
What to do if the boat is sloping?
The transom angle (usually 10-15 degrees) is already factored into the outboard motor design. Measure vertically, ignoring the bevel, and use transom wedges or diewood tilt adjustments to compensate.
Does the length of the leg affect maneuverability?
Yes, the long leg increases the moment of inertia when you turn the bumper. The boat becomes more lazy in handling, but it goes more stable in a straight line. The short leg makes the control more sharp and responsive.
How often should I check the height of the installation?
It is recommended to check the tightening of the fasteners and the height of the installation before each season. On inflatable boats, it makes sense to adjust the height depending on the load (number of passengers and cargo).