Well-chosen propeller It can dramatically change the behavior of your boat on the water, turning sluggish swimming into confident gleaming. two-stroke Japanese-branded engines Tohatsu This is especially important for 5 horsepower, as this model is often used as a traction or cruising model. Mistake in the choice of step or diameter can lead to overheating of the engine or, conversely, to the inability to go on the operating mode.
Many beginners mistakenly believe that all screws are universal, and buy the first detail, ignoring the specifics. transporter In fact, the market offers a variety of options, from lightweight aluminum models to heavy composites, and understanding the technical details will help you save fuel and extend the life of the powertrain without costly repairs.
In this article, we will go into detail all aspects of the choice, including the exact size of the shaft, the impact of the step on speed and traction, and the algorithm for correct replacement. You will learn why a regular propeller is not always the best solution for your particular boat and load. propeller This is a guarantee of safe and comfortable operation of your boat.
Technical specifications of the shaft and landing dimensions
The first thing you need to decide before buying is the geometry of the shaft of your motor. Tohatsu The M5B and MFS5B series have specific seating requirements, and the shaft is not standard for all outboard motors in the world, so eye fitting is strictly prohibited. A mismatch in diameter, even by a fraction of a millimeter, will cause a beat, a breakdown of the lower unit and a potentially dangerous situation on the water.
The diameter of the shaft in this model is strictly 13 mmIt's an important parameter that distinguishes it from more powerful cousins or motors of other brands, where you can find 15 mm shafts or inch sizes. In addition, the length of the hub and the presence of slits play a critical role. If you install a hub too wide, you risk not closing the nut completely, which can lead to the loss of the screw.
⚠️ Warning: Before ordering a new part, be sure to measure the diameter of the shaft with a rod. Production tolerances can vary, and relying only on the marking of the engine model sometimes leads to errors, especially if the engine is not original or has been previously repaired.
The key element of the fastening is slab-lineOn the motors. Tohatsu 5 hp is often a dual function: it fixes the screw from the twist and serves as a protection element (cut off on impact). The design of the tailing may vary depending on the year of release, so visual comparison of the old and new screw before installation is a mandatory procedure. Do not ignore this step, as installing the wrong part can damage the gearbox.
Influence of step and diameter on driving performance
The main parameters that determine the behavior of a boat are the diameter and pitch of the screw. Step is the theoretical distance that the screw will travel in one full revolution in a solid environment. 5-6 inchesIncreasing the pitch will allow for a higher top speed, but will require more power to accelerate.
If you plan to use a boat for heavy-load fishing or towing, you may need a screw with a smaller pitch. traction At low speeds, allowing the motor to more easily lead the vessel to planing, but excessive reduction of pitch will lead to the fact that the engine will "unwind" above the permissible speed, which is dangerous for the piston group of two-stroke.
- 🚀 High pitch: Suitable for lightweight single-passenger boats, provides maximum speed on the straight.
- ⚓ Low step: Ideal for heavy PVC boats, trolling or swimming against the current, gives a confident start.
- ⚖️ Mid-step: A versatile solution for mixed operation, balancing between speed and traction.
The diameter of the screw also matters, although on low-power engines the choice is limited to the design of the lower unit. Increasing the diameter (within reason) increases the efficiency of the rotor's emphasis on water, which is especially noticeable in glides. However, too large a diameter can cause cavitation and air to enter the blades, which will lead to a drop in speed and overheating.
The optimal pitch of the screw allows the engine to reach the maximum recommended speeds at full load of the boat, without exceeding their idle.
Manufacture materials: aluminum, steel or composite
Material choice is always a trade-off between cost, weight and strength. Tohatsu 5 is aluminum-alloyThese propellers are lightweight, cheap and efficient enough for low speeds, and their main advantage is predictable impact behavior: they bend to protect the gearbox, but they can be easily edited or replaced.
Steel screws are less common and are usually offered as a tuning option. They are significantly stronger and thinner than aluminum counterparts, which theoretically improves hydrodynamics. However, for a 5 hp motor, the weight of the steel screw can be excessive, creating an unnecessary inertial load on the crankshaft when the throttle is opened abruptly.
Composite screws are gaining popularity because of their ability to deform when hit by an obstacle and return to their original shape. This is ideal for rocky rivers or shallow waters. They do not rust or require painting, but may have less efficiency than metal due to the lower rigidity of the blade.
⚠️ Warning: When switching from aluminum to steel or composite, be sure to check the condition of the lower unit washer. Other hardness of materials can lead to accelerated wear of the shaft seat, if you do not use protective bushings.
Symptoms of an incorrectly selected screw
You can tell if the propeller is wrong, you can tell by the behavior of the boat and the sound of the engine. If the engine can't reach full speed even on a light boat with one person, and the speed is clearly below the passport, the screw's pitch is probably too high. The engine suffocates without drawing the load, which leads to the formation of a swell on the candle and in the exhaust system.
The reverse situation, where the motor is easily spinned to cut-off or even higher, and the boat is not accelerated, indicates too small a step. overheating It comes very quickly because the two-stroke engine is operating in a mode that exceeds the calculated one, and the strong vibration of the boat hull and the characteristic cavitation hum can also be a sign of a problem.
| A symptom. | Probable cause | Decision |
|---|---|---|
| The engine's not gaining momentum. | Too big a screw pitch | Replace with a screw with a smaller pitch |
| Overheating at full gas | Engine torsion (small step) | Increase step or reduce load |
| Vibration and noise | Damage to the blades or cavitation | Correcting the screw or replacing the lower unit washer |
Instructions for replacement and maintenance
Rotor-swapping process Tohatsu 5bhp It's simple enough and doesn't require a complicated tool, but it requires care. Before you start, be sure to turn off the engine and, if possible, remove the cap, make sure the motor is securely mounted on the transom or raised horizontally to access the lower unit.
To dismantle, you will need to remove the locking splint washer (or the puck nut, depending on the year modification). Often the nut can be sour, so use penetrating lubricant beforehand. Remove the old screw, inspect the shaft for winding fishing line that could damage the oils.
☑️ Rotor replacement algorithm
When installing a new screw, make sure that the veneer sat down to the groove. Do not put excessive effort into tightening the nut so as not to break the thread on the thin shaft. After installation, scroll the screw with your hand - it should rotate freely, without jamming. Be sure to check the presence and condition of the lower unit washer, which protects the odule from rotating the screw.
Lubrication nuances
With each change of the screw, it is recommended to lubricate the shaft with a thin layer of water-resistant lubricant. This will prevent boiling and facilitate future dismantling, but do not overdo it - excess lubricant can be sprayed when rotating.
Frequent malfunctions and methods of their elimination
The rotor is subjected to constant stress and shocks during operation. The most common problem is bending the edges of the blades. Even a small defect in geometry leads to a strong vibration that is transmitted to the lower unit and can destroy the bearings. If you notice a vibration, remove the screw and put it on a flat surface: the gap between the edge of the blade and the plane should not exceed the gap between the edge of the blade and the plane. 2-3 mm.
Another problem is the screw sleeve twisting, and inside the hub is a rubber sleeve with sleeves that extinguishes the jerks, and over time, the rubber tans or breaks, and the screw starts to turn around the shaft without passing traction, and it looks like a sharp drop in engine speed when the gas opens, although the screw continues to rotate.
- 🔍 Visual examination: Look for cracks at the base of the blades and chipped paint.
- 🔧 Checking the slithers: Make sure that the slats on the shaft and sleeve are not slime-free.
- 🧼 Cleanliness: Remove the algae and fishing line wound under the screw to avoid overheating the oal.
Repairing a badly damaged screw with welding or welding on low-power engines is economically impractical and technically risky. Disruption of the mass balance of the blades will lead to a beat; it is easier and safer to purchase a new part selected according to the manufacturer's recommendations.
⚠️ Warning: Operating a motor with a damaged screw or without a lower unit washer is strictly prohibited, guaranteed to result in water entering the gearbox through the omentum and costly repairs to the bottom of the motor.
Always carry a spare screw and a full set of tools to replace it. A blow to a heater or a stone can happen at any time, and a spare part will save your fishing or walk.
Questions and Answers (FAQ)
Can I put a screw from Tohatsu 9.9 on the model 5 hp?
No, it's impossible and dangerous. The screws have different landing diameters of the shaft (usually 13 mm versus 15 mm or 1 inch) and different hub lengths. Installing the wrong screw will cause the gearbox to break or the screw to lose.
How often should I change the lower unit puck?
The washer should be changed every time the screw is replaced if it has signs of wear, or immediately when the backlash is detected, which is an expendable material that is inexpensive but protects expensive engine components.
Why is aluminum screw better than steel for a beginner?
Aluminum, when hitting a hard obstacle (stone, log) bends, absorbing the impact energy and protecting the crankshaft and gearbox. A steel screw will transfer the entire impact to the engine mechanics, which can lead to serious breakdowns.
Does the pitch of the screw affect fuel consumption?
Right, right. Wrong pitch makes the engine run outside the optimal rev range, which increases fuel consumption and engine wear, and the right propeller makes it efficient.