The right propeller is a fundamental condition for the efficient operation of water machinery. Honda BF20 Often faced with the need to replace regular equipment to improve the running specifications of the boat, the wrong choice of the geometry of the blade can lead to overheating of the engine, increased fuel consumption or, conversely, to the inability to develop the design speed.
In this article, we will take a closer look at the technical parameters that are compatible with the two-stroke and four-stroke versions of the Japanese brand’s twenty-stroke units. You will learn how pitch and diameter affect trolling and full-load plane. Understanding these processes will allow you to optimize your boat-motor bundle on your own.
The main challenge is to match the engine power with the water resistance and the weight of the boat. The Honda 20 propeller is often a compromise solution for medium conditions. However, the specifics of operation are different: someone is engaged in fishing with a heavy load, and someone is interested in maximum speed for water skiing.
Technical specifications and compatibility of screws
Series engines Honda BF20The two-stroke and four-stroke versions have similar propeller seating requirements, the key being the shaft diameter and number of slits. Most of the models in this series use a 1-inch (25.4 mm) shaft with 13 slits, standard configuration for low-power outboard motors of the period.
These engines were often fitted with 9-inch diameter propellers, but it is important to consider that depending on the year of manufacture and the market, the configuration of the hub may differ slightly, so when buying non-original products, you need to check not only the diameter of the shaft, but also the length of the hub so that it does not rest on the anti-cavitation plate or lower unit crankcase.
fabrication plays a critical role in durability. Aluminum alloys such as AluminumThey're lightweight and strong enough for most amateur tasks, but when hit by a hard obstacle, aluminum deforms, which can save lower unit from breaking, but will require replacing the screw itself.
⚠️ Warning: Installing a screw with a shaft diameter different from the engine specification will lead to the destruction of the slats joint and expensive repair of the gearbox.
There are also stainless steel screws, but for a 20 hp engine, their installation is economically impractical and technically redundant. The weight of the steel screw will create an extra load on the crankshaft, and the increase in efficiency at such speeds will be minimal. The optimal choice is a quality aluminum screw with a protective coating.
The effect of step and diameter on performance
The geometry of the blade determines how the motor will behave on water. Screw pitch (Pitch) is the theoretical distance that a screw will travel in one full rotation in a solid medium. For a Honda 20, the standard pitch is 8 to 10 inches. Increased pitch allows for higher speed, but requires more engine power to rotate.
If you install a screw with too much pitch, the engine will not be able to reach its working speed (usually 5000-6000 rpm), a phenomenon called "failure", the engine will operate in a low-revv zone under high load, which will lead to candles coking, compression drop and rapid wear of the piston group. Honda BF20.
On the other hand, a small-stroke propeller ("traction") will make it easy to pull a heavy boat into a plane, and the engine will be easy to spin to reach maximum speeds, but the speed limit will be lower because the engine will be limited in speed without realizing its full power performance.
- 🚣 Heavy PVC inflatable boats with hard bottom
- ⚓ Trolling fishing at low speeds
- 📦 Transportation of goods with full load
- 🌊 Operation in high wind or against the current
The diameter of the screw also affects performance. Increasing the diameter increases thrust, but requires more power to rotate. For a 20-horsepower motor, the diameter range is usually between 8.5 and 9.5 inches. Exceeding this value can lead to cavitation and reduced efficiency.
The optimal pitch of the screw is selected experimentally: the engine should reach the maximum permitted speeds (indicated in the manual) with a fully open throttle and an average load of the boat.
Performance analysis: Conformity table
To illustrate, let's look at how the change in propeller parameters affects the behavior of the boat, and the data are averaged for a 3.2 to 3.6 meter inflatable boat with a single operator, and the actual numbers can vary depending on the state of the bottom and the power booster.
| Rotor parameter | Acceleration (0-30 km/h) | Maximum speed | Fuel consumption | Recommended application |
|---|---|---|---|---|
| Step 8” (Traction) | Very fast. | Low. | High at max speed. | Heavy boats, cargo, trolling |
| Step 9” (Standard) | Medium. | Medium | Optimal. | Universal use |
| Step 10” (Speed) | Slowly. | High. | Low on cruise. | Light boats, 1-2 people |
| Step 11”+ (Extreme) | Very slow. | Maximum* | Risk of overheating | Not recommended for BF20 |
As you can see from the table, trying to squeeze the maximum speed by increasing the pitch often leads to the opposite effect on low-power engines. The engine simply "choked" and can not turn the screw at the desired speed. The middle ground For the Honda 20, it is most often in the 9-9.5 inch range.
Seashells or damage to bottom geometry can completely offset the benefits of a properly selected propeller, and regular washing and inspection of the bottom of the boat are mandatory before testing new settings.
Symptoms of an incorrectly selected screw
How do you know if your current propeller is not right for your motor? There are obvious signs that you can ignore, and you should look at the tachometer first, and if you're running at 4,500-4,800 at full gas and normal load, you're going to be running too high. You're running in overload mode.
The reverse is torsion, where the tachometer hand is going over the top of 6,500-7000 rpm, and the speed is not growing very much, which means that the screw is too light for the engine, the motor is not resisting enough to unleash its power, and it is idle, which is also harmful, because it can lead to the inserts turning.
⚠️ Attention: Prolonged operation of the motor in the "torsion" mode (above the maximum passport speed) can cause the destruction of the rod due to increased inertial loads.
Another sign is cavitation: if a sharp addition of gas or turns the screw begins to "break" (revolutions increase sharply, and the traction disappears), perhaps the diameter of the screw is too large, or the anti-cavitation plate is located incorrectly, and cavitation can also cause damage to the edges of the blades.
Visual inspection of the propeller can tell you a lot: If there are signs of erosion (small holes) or bends on the edges of the blades, it is a signal of problems. Curved edges reduce efficiency by 10-15%. Vibration on the steering wheel and hull of the boat also often indicates an imbalance of the propeller or shaft beating.
What is screw cavitation?
Cavitation is the formation of vapor bubbles in local pressure-lowering areas. When these bubbles collapse, they create shock waves that destroy the screw metal and create noise. For the Honda 20 engine, cavitation is dangerous because of the overheating of the lower unit, because the water circulation in the cooling system is disrupted.
Instructions for replacement and maintenance
Replacement of the propeller on the motor Honda BF20 The procedure is simple, but requires a certain sequence of actions. To work, you will need a front head (usually 13 or 14 mm), a hammer, a bump (or wooden bar) and a consistent lubricant. Before starting work, be sure to turn off the motor and disconnect the candle wire to avoid accidental start.
☑️ Preparation for rotor replacement
The first thing you need to do is lock the shaft from the twist, and some models have a stopper that fits into the lower unit hole, and if there's no stopper, you can ask the assistant to hold the blades tightly, but you need to do this with gloves so you don't get hurt on the sharp edges, and then you unscrew the screw attachment nut.
Often the nut can be "stiff" due to corrosion. In this case, do not use excessive force immediately. Treat the connection with penetrating lubricant (WD-40 or analog) and let you stand for 10-15 minutes. After unscrewing the nut, remove the puck and the persistent washer. If the screw is sitting tightly on the shaft, gently beat its hub with a hammer through a wooden spacer so as not to damage the metal.
When installing a new screw, be sure to lubricate the shaft with a thin layer waterproofThis will prevent the swelling and make it easier to replace the screw, put on a thrust washer (it is important not to confuse the sides if they are different) and tighten the nut. The nut is fixed using a splint or a locking plate, depending on the modification of the shaft.
The main rule of replacement is never to leave the shaft without lubrication and always use a new locking washer or splint to prevent spontaneous unwinding.
Care for propeller and lower unit
Regular maintenance of the propeller prolongs the life of not only himself, but the entire engine. After each release to the water, especially in salt water, it is recommended to wash the lower unit with fresh water.
Once a season or after 50 hours, you need to remove the screw for inspections. Check the shaft for winding fishing line. The line caught between the shaft and the omentum can damage the cuff, which will lead to water entering the gearbox. Removal of the fishing line is a mandatory procedure.
- 🔍 Check the edges of the blades for chips and burrs
- 🧹 Clean the screw hub of algae and shells
- 💧 Lubricate the shaft before each screw installation
- 🔩 Check the nut tightening and the status of the stopper
If you find serious damage to the blades (broken pieces, deep cracks), such a screw must be replaced. Balancing the destroyed screw in artisanal conditions is impossible, and vibration will quickly disable the bearings of the propeller shaft. Honda They are highly reliable, but they are sensitive to rotating mass imbalances.
How often do I need to change the screw?
The propeller has no strict replacement time and lasts until physical damage or loss of efficiency. When operated carefully, an aluminum propeller can last for years. However, if you often walk on shallow water or rocky shores, the spare propeller should always be with you in the boat.
Can the geometry of the screw be restored?
Minor edge curves can be carefully straightened with a file and a hammer, trying to preserve the profile, but if the deformation has affected more than 10% of the blade area or there are cracks at the hub, the screw cannot be repaired, and the imbalance will lead to vibrations.
Does the shape of the blades affect the noise?
Yes, the shape and number of blades directly affect acoustic comfort. Rotors with more narrow blades are usually quieter, but have less efficiency at low revs. Standard 3-blade propellers for the Honda 20 are a compromise between noise, traction and cost.
What to do if the screw is scrolling on the shaft?
If the screw is twisted, it means that the slime is slithered on the shaft of the motor or in the screw hub. In the case of aluminum screw, the screw itself (wear of the hub) suffers more often. If the motor shaft is damaged, the propeller shaft or the entire lower unit will need to be replaced.
Which screw is better for fishing?
For fishing, especially with sonar and downriggers, stable running at low speeds and good traction are important. It is recommended to use propellers with increments of 8-9 inches, which will ensure a confident course against the current and allow you to accurately control the speed when trolling without jerking.