Every owner of a outboard motor sooner or later faces a situation when, after hitting an underwater obstacle or prolonged contact with sand. boat-rotor The drop in speed, the vibration at high speeds and the increased fuel consumption are just the first symptoms that the geometry of the blades is broken. If the damage is not repaired in a timely manner, the deformation will lead to destruction. gearbox and expensive overhaul of the entire power unit.
Many people mistakenly believe that the restoration of the screw is a factory procedure that requires complex equipment. However, minor repairs, which consist of removing bullies, burrs and easy editing of the edges, can be done independently in a garage or even on the shore of a reservoir. The main thing is to understand the physics of water movement and strictly follow the sequence of actions so as not to turn a small chip into a fatal crack.
In this article, we will discuss in detail the process of how to properly cock-rotorAnd what tools are needed to do that, and how to avoid the typical mistakes that can permanently put a part out of order, and good surface treatment is not just aesthetics, it's also a guarantee of economical performance of your engine.
Diagnosis of screw damage
Before you can get a file or a grinder, you need to do a thorough visual and tactile inspection. blade-edge And the back side, where you hit a rock or a log, you get the main pressure of the water flow, and you look carefully at the metal for cracks, especially at the base of the blade, where the bending load is maximum.
Running your hand over the blade, and if you feel bullies, or steps, or sharp edges, the laminar flow of water breaks off, creating cavitation bubbles, and it's cavitation that causes the characteristic noise and vibration that is transmitted to the engine's lower unit, and ignoring these signs causes the engine to start operating in an abnormal mode, resetting or overheating.
⚠️ Note: If you find through cracks on the blades or a significant curvature of geometry (the blade is twisted relative to the axis of rotation), self-correction is strictly prohibited.
For an accurate assessment of the degree of damage, it is useful to use markerPaint the damaged area, then wash some of the paint with water, the boundaries of the chip will be much clearer, allowing you to plan the scope of work. Don't try to cut the metal right away, first determine how deep the deformation has gone.
Necessary tool and workplace preparation
The quality of the sharpening depends on the tool you choose. For rough processing, when you need to remove large burrs or level deep dents, you need personal filers with different notches. corner-surfacing (Bulgars) with petal circles or sandpaper on a solid basis.
It is important to ensure that the screw is firmly fixed during operation. If you attach it to the grip of the hub, be sure to use soft sponges or rubber pads to avoid damaging the landing place under the slats. The workplace should be well lit, as glare on the polished metal can hide small defects.
The list of equipment required includes:
- 🔧 Fills (flat and round) for manual fine-tuning of hard-to-reach places.
- 🌀 UCM (Bulgar) with speed adjustment and petal circle of grain 40-80.
- 🧤 Personal protective equipment: glasses, respirator and tight gloves.
- 📏 A bar or ruler for controlling the symmetry of processing.
Remember that grinding produces fine metal dust, so working indoors requires good ventilation. If the process takes place outside, make sure the wind does not blow dust back on you.
Technology for removing bullies and leveling edges
The recovery process begins with the removal of sharp edges, and the movement of the tool must be strictly directed from the root of the blade to the end, along the radius of rotation. In no case do not drive a file or grinder across the blade, this will create cross-sectional risks that will become centers of tension and can cause crack growth under load.
If a metal burrow is formed on the edge, you can not just knock it down. First, you need to carefully straighten the bent part using a soft hammer or special mandrels, and only then start grinding. Edge thickness It should remain sufficient to withstand hydraulic shocks, so do not get carried away by removing excess metal.
☑️ Procedure for handling edges
When working with aluminum screws Be especially careful: this metal is softer than steel, but prone to abrasive adhesion. Periodically clean the tool with a metal brush. Steel screws require a more powerful tool, but forgive more grinding errors due to their viscosity.
Recovery of geometry and angle of attack
Once the obvious defects are removed, the geometry is restored, and the angle of attack of the blade is the key parameter for thrust and speed, and if you run too much metal on the back of the blade, the angle of attack decreases, and the screw starts to slip without developing the right thrust, and it's also dangerous to remove metal on the front side, and it changes the profile of the blade.
To check the symmetry, use a rod. Measure the distance from the edge to the base of each blade at the same distance from the center. The spread should not exceed 0.5-1 mm. If one blade is shorter than the other because of the chip, you will have to carefully grind the others to bring them to the common denominator, sacrificing diameter, but maintaining balance.
| Parameter | Norma. | Permissible deviation | Consequences of violation |
|---|---|---|---|
| Radius of the blade | Same for everyone. | ± 1 mm | Vibration, beating. |
| Edge thickness | 0.5 - 1.0 mm | Not thin 0.3 mm. | Scotch, cavitation |
| Bending angle | Symmetrical | Visually exactly | Loss of speed |
| Surface. | Smooth. | No steps. | Noise, erosion |
When you re-establish your profile, try to repeat the factory contours. If you're not sure about yourself, you'd better leave a small amount of metal and leave the final refinement to a specialist. balancing The screw directly depends on the uniformity of the distribution of the mass of the blades.
How to check the balance of the screw at home?
There is an easy way to statically balance: slid a flat metal rod through the screw hole (like a long bolt) and place the ends of the rod on two parallel horizontal ribs (you can use two chairs or racks). The screw should rotate freely. If it stops in one position each time, then one of the blades is heavier. The heavy blade should be carefully polished on the inside at the base until the screw stops in an arbitrary position.
Surface polishing and anticorrosion protection
Finish polishing isn't just a matter of beauty: A smooth, polished surface reduces water resistance and prevents algae and shells from fouling. Use metal pastes and felt circles or fine-grained sandpaper (P400-P800) to polish, gradually moving to zero.
Pay special attention to the transition from blade to hub. This corner often accumulates dirt and corrosion begins. High-quality polishing allows you to identify hidden microcracks that may not be noticeable when roughing. After polishing, the surface should have a mirror shine.
After machining is complete, a protective coating is required. Aluminum screws are great for special corrosion sprays or graphite lubricants that create a protective film. Steel screws require more protection, perhaps epoxy soil or screw paint.
Typical mistakes in self-sharpening
The most common mistake is symmetry breaking, where owners often work hard on one damaged blade, forgetting that the rest need to be symmetrically treated to balance, and this causes the boat to shake so that it is impossible to hold the steering wheel.
The second mistake is overheating the metal. When using the CMM at high speeds, the metal can heat up to blue. This means changing the structure of the metal (vacation), it becomes softer and collapses faster under load. Work with short touches and let the metal cool down.
⚠️ Warning: Never heat the screw with an open flame (gas burner) for editing! Aluminum alloys lose strength when heated, and tempered steel can become brittle as glass.
The third mistake is ignoring the state of the lower unit, and often the gearshaft suffers along with the screw, and if the shaft is bent, even a perfectly balanced screw will create vibration. Always test the shaft for the beat before installing the rebuilt screw.
The main rule of repair: it is better to remove less metal, but maintain symmetry and balance, than perfectly align one blade to the detriment of the rest.
Frequently Asked Questions (FAQ)
Can I grind a screw file without removing from the engine?
It's not recommended. To be properly processed, the screws must be removed, cleaned of algae and securely fixed, weight treatment will not give you the necessary quality control and can damage the lower unit seals.
How often should I check the condition of the screw?
Visual examination should be done after each outing, especially if you have been swimming in unfamiliar places, and full diagnostics with groping edges should be done at least once a season or after any strong impact.
What if the screw still vibrates after the correction?
Most likely, the static balancing is broken. Try to check on parallel rulers (as described in the spoiler above) and run the excess from the heavier blade at the base, also check if the sand has fallen between the screw flange and the gearbox.
Do I need to lubricate the shaft before installing the screw?
Yes, it is, and the sludge part of the shaft and the inside of the screw hub must be waterproof (usually with the engine or bought separately) to prevent the screw from attaching to the shaft and corrosion.