When choosing or upgrading an outboard motor, many boating owners are faced with confusing terminology related to the geometry of a lower unit. One often hears the question of how to measure the length of a boot, and why this parameter is so critical for the performance of a vessel.
The wrong length of a lower unit leads to serious problems, from the banal overheating of the engine to cavitation and loss of traction. If the lower unit is too short, the screw will capture air even on small waves, which instantly raises the speed of the engine and can lead to its failure. On the other hand, an excessively long boot increases water resistance, overgrown with algae and makes the boat sluggish at the start.
In this article, we will discuss how to correctly determine the required length of the lower unit for your transom, what standards exist in different manufacturers and whether it is possible to replace this unit yourself. Understanding these nuances will help you save fuel and extend the life of the power unit.
lower unit length standards and their marking
The world's outboard motor manufacturers have long standardized the size of their lower units to make life easier for customers, and there are three main outboard sizes that cover 95 percent of civil water transport needs. Short lower unit.The PVC, often marked with the letter S (Short), is 381 mm (15 inches) long and is the standard for most PVC inflatable boats and small low-transcent aluminum boats.
For larger vessels with high transenses long-dadewoodThe longest motor is 508 mm (20 inches) and is most commonly found on transom-engine-engineed Kazanka or Volzhanka boats, and there is also an ultra-long variant, the 635 mm (25 inches) XL, which is used on large cruise boats and pontoons to ensure better streamlinedness and reduce noise.
⚠️ Warning: Never attempt to install a short-dadewood (S) motor on a high transom trunk using wooden stand-ups, which creates dangerous vibration and the risk of losing the motor overboard when hit by a wave.
You can determine the type of your motor not only by documentation, but also by visual inspection of the lower unit pipe. Yamaha or Tohatsu, can have its own nuances in the marking of the model range, where the length can vary within a few centimeters depending on the year of release.
How to measure the length of the boot correctly
If you lose your technical data sheet or buy a used motor, you have to measure the length of the lower unit yourself, which is a critical procedure before you buy spare parts, and it is measured from the plane of the motor attachment to the transom (the top of the lower unit pipe) to the anti-cavitation stove directly above the propeller.
For accuracy, use a construction tape measure or a hard meter, and it's important not to confuse the total length of the motor with the length of the lower unit. Anti-cavitation plate This is the horizontal shelf above the screw, which is the point of depth of immersion, and this is the line that the upper slice of the lower unit should reach in a standard installation.
The obtained data are compared with the standard values:
- 📏 381 mm (10 mm) is the standard short lower unit (S).
- 📏 508 mm (10 mm) is the standard long lower unit (L).
- 📏 635 mm (10 mm) is an extended lower unit (XL).
When measuring the length of a lower unit, be sure to remove the propeller so as not to mistake the lower reference point. Focus strictly on the horizontal plate above the place where the screw is installed.
There are also transitional options, especially for Chinese manufacturers or specialized trolling motors, where the length can be 444 mm (17.5 inches). In such cases, it is important to look not at inch roundings, but at actual millimeters, since a difference of 2-3 cm can be critical for hydrodynamics.
The Effect of lower unit Length on Hydrodynamics
Why don't engineers make all the dayswoods as long as possible? It's about the physics of a boat. The longer the boots are, the larger the area they're washed by water. It creates additional drag that the motor has to overcome by using up excess fuel. In addition, a long lower unit pipe creates a stronger moment of inertia, which can impair the boat's handling at high speeds.
On the other hand, insufficient screw enclosure leads to cavitation, which is the process of forming and collapsing vapor bubbles in local pressure-lowering zones. Cavitation Not only does it reduce the efficiency of the screw, it also causes erosion of the blade metal and noise that scares the fish away, and the right length of the boot strikes a balance between minimal drag and sufficient rotor depth.
The table below compares the specifications depending on the length of the lower unit:
| Parameter | Short (S) | Long (L) | Extended (XL) |
|---|---|---|---|
| Length (mm) | 381 | 508 | 635 |
| Water resistance | Minimum | Average | High. |
| Depth of the screw | Basic | Optimal for the move | Maximum. |
| Application | PVC boats, shallow waters | Boats, trolling | Big boats, yachts. |
For anglers using sonars, the length of the lower unit also matters: Too close to the surface, the screw creates a bubble curtain that can shield the signal of the sounder sensor mounted on the transom, in which case it is preferable to use L-series lower units or special elongated lower units for trolling motors.
Replacement of lower unit: can you change the length
Older motorists often wonder if it's possible to replace short lower unit with long ones to put the motor on a higher boat? Theoretically, this is possible, but in practice, it's a technical challenge for the hydraulic motor, primarily the length of the lower unit shaft, which transfers torque from the engine crankshaft to the gearbox.
If you just take off the short gearbox and put the long gearbox on the other model, the shaft just won't reach the shaft, and you have to change the lower unit pipe, the intermediate shaft, and maybe the gearshift system, because the thrust length may be different. Mercury or Yamaha There are official conversion kits, but they often cost up to 70% of the price of a new engine.
Technical risks of self-replacement
If you try to lengthen the lower unit with artisanal methods (brewing pipes, using homemade shafts), you risk disrupting the coaxiality, which will lead to rapid destruction of the ossels, water entering the gearbox and jamming the engine on water.
Moreover, replacing the lower unit often requires reconfiguration of the carburetor or reflashing of the electronic control unit (ECU), as the hydraulic resistance of the screw will change. The engine can begin to "choke" at high revs or, conversely, go into dispersion.
Features of lower unit for Other Types of Boats
The choice of the length of the boot depends on the type of boat. For inflatable PVC boats with inflatable floor (UHD) or sleigh, it is critical to use short lower units. High transom is rare here, and excessive slogging will only increase the risk of a screw encountering the bottom or snags in shallow water.
For aluminum motorboats with a constant transom, the situation is different. Many factory transes are 380-400 mm high, which formally fits a short lower unit. However, when fully loaded, the boat lands, the transom lowers, and the standard short motor begins to grab air. In such cases, experienced water engineers recommend using L-length lower units, even if the boat is formally a small vessel.
- 🚤 PVC boats: exclusively lower unit S (381 mm).
- 🚤 Aluminum boats: more often L (508 mm), less often S for shallow water.
- 🚤 Trolling engines: Often have an elongated leg for quiet running and sonar operation.
☑️ Check before buying the engine
A separate feature is the rotary boot motors, which allow the lower unit to change the angle of inclination to pass through shallow water. Although this does not change the physical length of the pipe, this option allows you to efficiently use engines with a longer lower unit in shallow water, lifting the screw above the bottom without losing cooling.
Typical errors and operating tips
One of the most common mistakes is to ignore the condition of the lower unit pipe. The aluminum alloys from which it is made are subject to galvanic corrosion, especially in salt water. If you notice white oxides or bubbles of paint on the boot, you need to immediately perform anti-corrosion treatment, otherwise the water will get to the inner cavity.
It's also important to monitor the intake holes on the front of the lower unit, which are often clogged with mud or debris. Water intake It has to be free, otherwise the engine will overheat instantly, and many modern models have a pump indicator (the "control jet"), which is used to judge the health of the cooling system.
⚠️ Warning: If the control jet becomes weaker or disappears after replacing the lower unit or screw, immediately shut down the engine. Working without cooling for more than 30 seconds can lead to bullying in the cylinders.
Another nuance is the attachment of the lower unit to the clam bracket, where the bolts that connect these knots are under constant vibration load, and it is recommended to check their puff once a season and treat them with a fixing sealant to avoid spontaneous unwinding and loss of the lower unit along with the screw.
Well-chosen lower unit lengths are a trade-off between the height of your boat's transceiver and the operating conditions. Don't chase versatility, it's better to have two motors for different tasks than one running in off-duty mode.
Frequently Asked Questions (FAQ)
Can you cut down the lower unit if it's too long?
Absolutely not. The lower unit pipe has a complex internal structure for the passage of thrust switching, shaft and exhaust gases, and the breakdown of geometry will cause the engine to break.
Why does a long-dadewood engine make a boat worse off?
Long-dadewood creates more hydrodynamic drag, and the motor uses some of the power to overcome the drag of its own leg, which reduces the overall acceleration efficiency.
How to determine the wear of the lower unit shaft?
The main sign is vibration at high speeds and noise in the gearbox, and accurate diagnosis can only be made after removing the lower unit and checking the beating of the shaft at the centers.
Will the 15 hp lower unit fit the 9.9 hp engine?
Only if it is the same series and year of production. For example, Yamaha’s dayswoods are often interchangeable within the same line, but different brenches (for example, Tohatsu and Mercury) the geometry of the mounts may differ.