Many small boat owners, in their quest to get the speed out of a boat, are looking at the propeller's hydrodynamics. One of the popular but often misunderstood elements of tuning is the so-called "bullet" -- a fairing mounted on the hub of a propeller -- that not only changes the appearance of a lower unit, but also significantly affects the operation of the entire propulsion system. In this article, we'll discuss the physical workings of the bullet, its real impact on the boat's performance and the need to install it on your engine.
The efficiency of a propeller depends on how smoothly the water flows around its blades and hub. A standard propeller hub often has an uneven surface, slit projections, or just a flawed geometry, which creates a flux just behind the propeller. The bullet is designed to smooth these flows, turning the chaotic movement of water into laminar, which reduces water resistance and improves the overall efficiency of the engine, especially at high revs.
But don't think of bullets as a panacea for any motor, there are situations where installing them doesn't add any value or even make them worse, and it's important to understand that hydrodynamics is a science of precision, and every gram of weight, every millimeter of shape, matters. Before you buy an accessory, you need to evaluate your propeller's condition, the type of boat you're using.
Physics of the process: how the fairing of the screw works
The principle of operation of the bullet is based on the fundamental laws of aerodynamics and hydrodynamics. When the screw rotates, it creates a zone of thinning and turbulence. The central part of the screw, where the hub is located, is often flat or stepped in standard design. Water passing through the center of the screw encounters these obstacles, forming vortices. These vortices create additional resistance, which the motor has to overcome, expending excess energy.
FairingIt's a drop-shaped, conical shape that directs the flow of water smoothly, without any sudden change in direction. It reduces cavitation, which is the formation of vapor bubbles that collapse with great force, destroying metal and creating noise. Reducing cavitation is one of the main arguments for installing a bullet, and it's important that the shape of the bullet fits the profile of the screw blades.
There's a misconception that a bullet pushes a boat, and in fact, its job is to minimize energy loss, and if you think of a motor as a pump, the bullet optimizes the outlet of that pump. Hydrodynamic resistance The hub can make up to 5 to 10 percent of the total propeller drag at high speeds, and by eliminating this factor, we get cleaner water exhaust and, as a result, more efficient traction.
- 🌊 Reduced turbulence of the water flow behind the propeller.
- 🔇 Reduced noise of the engine at high speeds.
- ⚡ Increased efficiency of the screw by reducing cavitation.
- 🛡️ Protection of the central part of the screw from mechanical damage and algae.
Impact on speed and fuel consumption
The question of real speed gains is a concern for every boat owner. Practical tests show that placing a bullet on a correctly selected propeller can give an increase in speed from 0.5 to 2 km / h. The figure seems small, but in the world of boating technology this is a significant indicator. The effect is most noticeable on sleeving vessels, where every extra kilogram of thrust or decrease in drag is critical for entering planing mode.
As far as fuel-flowThe bullet itself doesn't magically save gasoline, but because of the lower drag, it takes less than the engine to keep the same speed. If you're used to cruising at 3,500 rpm, you can keep the same speed at 3,200-3,300 rpm with the fairing installed, and that's the mode that saves fuel, which can be a measurable amount in terms of a full season.
It should be noted that on heavy displacement boats, the effect of placing a bullet can be almost invisible, in which case the resistance of the hull is so great that optimizing the flow after the screw does not give a meaningful result. The greatest effect of the fairing installation is observed on light aluminum or PVC boats with engines with a capacity of 9.9 to 60 hp. It is in this power range that the balance between weight, thrust and resistance is most sensitive to changes in hydrodynamics.
The real speed increase is 1-3%, but the main advantage is a smoother and more predictable boat ride on the planing.
Design features and materials of manufacture
The modern market offers many options for bullets for outboard motors, which differ not only in the diameter of the landing hole, but also in the material of the execution. The material depends on the durability of the product, its weight and, importantly, the price. Cheap plastic models often deform when hitting floating objects or when transported improperly.
The most common material is stainless steelThese are heavier bullets, which is a plus in this case -- the extra weight at the end of the lower unit works as a counterweight, lifting the boat's nose a little bit. Steel fairings are virtually indestructible and last for decades. Aluminum alloys are also popular, they are lighter, but more susceptible to corrosion in salt water if they don't have a quality anode coating.
There are also composite and plastic options, which are often bundled with expensive screws, which have the advantage of being complex and low-cost, but when exposed to sand or stone at high speed, plastic can quickly become unusable, and when you choose to do so, you have to pay attention to the quality of the surface treatment: it has to be perfectly smooth, without the injection seams that create vortices themselves.
| Materials | Weight. | Durability | Price. |
|---|---|---|---|
| Stainless steel | Highly | Very high. | Medium/High |
| aluminum | Medium. | High. | Medium |
| Plastic/Composite | Low. | Low. | Low. |
| brass | Highly | Medium (oxidized) | High. |
Effect of bullet weight on differentiation
A heavy steel bullet weighing 300-500 grams, located at the back and bottom of the boat, creates a moment that lifts the lower unit. This can help to lift the boat's nose a little when going to the plane, but being overweight can lead to too early a tear of the bow and loss of course stability.
Compatibility with screw types
Not every bullet will fit any screw. The key is the diameter of the landing hole and the profile of the hub. Boat propellers are divided into several types according to the design of the hub: with slits, with a cone, with threads. The bullet should sit tightly on the hub without dangling. Luft will cause vibrations and rapid wear of both the fairing and the screw shaft.
Special attention should be paid to screws with a rubber torsion sleeve. In such models, the hub is often hidden inside the rubber. The bullet is only possible if the screw design provides for this (the presence of a thread or a special mount on the end).
Step and diameter of the screw For high-speed propellers with a large pitch (for example, 21-23 inches), the shape of the bullet must be more elongated to effectively flow around the powerful flow of water. For traction screws with a large diameter and small pitch (11-13 inches) used on heavy boats, the shape of the bullet can be more flattened. Mismatching the shape of the bullet to the type of screw can negate the entire hydrodynamic effect.
- 🔩 Check the type of screw attachment (slits, cone, thread) before buying.
- 📏 Measure the diameter of the screw hub with a rod.
- 🚫 Make sure the screw has a rubber sleeve if you plan to put a bullet on it.
- ⚙️ Adjust the shape of the bullet to the pitch and diameter of your propeller.
When installing a bullet on a rubber sleeve screw, use a special lower unit lubricant to prevent metal from sticking to rubber and facilitate future dismantling.
Installation and configuration instructions
The process of installing the bullet does not require complex equipment, but requires accuracy and adherence to technology. Installation errors can lead to the loss of a screw on the water, which is a critical situation. Before starting work, be sure to turn off the engine and make sure that the boat is securely secured.
First, you have to dismantle the propeller itself. This uses a special key that comes with the motor. It's important to lock the shaft from the rotating so that the locking mechanism is not damaged. After removing the screw, inspect the shaft for blows and corrosion. Clean the thread and lubricate it with anti-corrosion lubrication.
If you put a bullet on the screw hub, make sure it sits down and is fixed with a locking ring or a splint. If the bullet has its own mount on the shaft, follow the manufacturer's instructions. screw-nut It should be tightened at the recommended moment. A short nut is the main cause of screw loss.
☑️ Post-bullet check
⚠️ Warning: Never use regular wire instead of a regular splint. When vibrating, the wire may burst, and the screw nut will spontaneously unscrew, causing the screw to lose and damage to the lower unit.
After assembly, you need to run a test run. Go out and gradually increase your speed. Listen to the engine, new vibrations or a hum may indicate a wrong fit or imbalance of the bullet. If everything went smoothly, you can move on to speed tests.
Procedure:1. Remove the locking puck and nut.
2. Remove the propeller.
3. Clean the shaft of dirt and algae.
Place the bullet on the screw hub (or shaft).
5. Set the screw with the bullet on the shaft.
6. tighten the nut at a moment of 30-50 Nm (depends on the motor).
7. Lock it with a new splint.
Typical errors and possible problems
Despite the simplicity of design, users often make mistakes that reduce the benefit of a bullet to zero. The most common of these is placing a bullet that is too heavy on a low-power engine. This shifts the center of gravity of the lower unit and can overload vertical shaft bearings. For motors up to 5 hp, the use of massive steel bullets is often unjustified.
Another mistake is ignoring the surface condition, and if you buy a cheap bullet with roughness or injection defects, you have to grind it down before you install it, and a rough surface creates more resistance than a smooth screw hub without a bullet, and hydrodynamics doesn't tolerate carelessness.
There is also a common problem of anti-cavitation stove incompatibility. If the bullet is too large in diameter, it can hit the stove when the engine is tilted or on the wave, which will cause a strong vibration and a characteristic knock. Before buying, be sure to try on the bullet or check the compatibility table for your model motor. Yamaha, Tohatsu or Honda.
⚠️ Warning: If you notice a drop in maximum speed after installing the bullet (the engine does not spin), it may be that the bullet creates excessive resistance or the screw is wrongly selected, in which case the fairing should be removed and measured again.
What to do if the bullet starts vibrating?
The vibrating of the bullet is a sign of imbalance or a bad fit. Take off the screw, check if there is a gap between the bullet and the hub. If there is a gap, you can use a thin layer of epoxy glue or Teflon tape to seal, but only if it does not disturb the balance of the screw.
Frequently Asked Questions (FAQ)
Will the bullet give a speed gain on the engine 5 hp?
On motors with a capacity of 5 hp and less, the speed gain will be minimal, rather within the margin of error (0.1-0.3 km / h). The main purpose of installation on such engines is to protect the screw hub from algae and debris, as well as reduce noise.
Can I use a bullet on two-blade propellers?
Yes, you can. Two-blade screws are often less optimized than three- or four-blade screws, so setting up a fairing can even have a more noticeable effect on smoothing the flow.
Do I need to change the settings of the carburetor after installing the bullet?
In most cases, no. The bullet doesn't change the load on the engine, it just optimizes the efficiency of the propeller. The engine will operate in the same temperature regimes. However, if you change the propeller to a heavier or lighter one at the same time you install the bullet, then you may need to adjust the carburetor.
How often do I need to remove a bullet for maintenance?
It is recommended to remove the bullet along with the screw at least once a season to clean the shaft from winding lines or algae that may fall under the fairing, and this will check the condition of the glands and lubrication of the shaft.
Does the bullet affect the boat's handling?
If properly installed, the bullet does not affect handling; on the contrary, due to a more stable flow of water to the steering wheel (which is located just behind the screw), the course stability can improve slightly, but too heavy a bullet can make the motor more inert when turning the ramp.