Many small boat owners, especially those who recently purchased their first aluminum or hard-bottom PVC boat, come across a confusing term when studying outboard design: the so-called “crinoline,” a horizontal plate located directly above the propeller, which is often left unaddressed until thrust problems or an underwater obstacle hits.
Actually, crinoline It's a critical part of the hydrodynamics of the rotor-motor complex, which not only shields the fasteners from the algae, but also creates the right flow of water to keep the rotor at an effective stop, and without that, the engine's planing would be impossible or extremely inefficient.
In this article, we will discuss in detail the physics of the process, explain why this part saves the engine from overheating and how its correct setting affects the speed of your vessel. anticavitational This will help you avoid common mistakes when installing the motor on the transom.
Design features and location of the element
The crinoline is a horizontal plate that is an integral part of the lower unit foot of the outboard motor, and it is located exactly parallel to the plane of attachment of the motor to the transom, and is in line with or slightly above the axis of rotation of the propeller. Yamaha, Tohatsu or MercuryThis plate is often integrated into the body of lower unit, but can be removable.
The main purpose of this design is to separate the flow of water. When the boat moves, the water flows around the body of the lower unit, creating vortices. Krinolin cuts off these flows, preventing them from entering the zone of thinning created by the propeller blades. If this barrier did not exist, the propeller would begin to capture air from the surface of the water, which would lead to a sharp drop in efficiency.
In addition, on the surface of the crinoline are often located zinc anodes And this is a very clever engineering solution that allows you to use one piece of corrosion protection and engine control, and it's not a coincidence that the sensor is here, because the water is stable and has no air bubbles, so it's accurate.
- ⚓ The main function is to prevent cavitation and air sucking.
- ⚓ It serves as a platform for the installation of sensors and sacrificial anodes.
- ⚓ Protects the upper blades of the screw from impacts on floating objects.
- ⚓ Improves the boat’s handling at high speeds.
⚠️ Warning: Never ignore the condition of the zinc anode on the crinoline. If it is completely destroyed by corrosion, the electrolytic reaction will begin to destroy the aluminum alloy of the stove itself, leading to costly repairs to the lower unit.
Physics of the process: the fight against cavitation
The main enemy of the propeller is cavitation, which occurs when the pressure in the fluid stream drops below the pressure of saturated vapor, and the water begins to boil, forming vapor bubbles. Cavitation Not only does it reduce the efficiency of the propeller, turning the useful energy of the rotation into noise and vibration, but it also causes the metal of the blades to erod. Krinolin creates a barrier that prevents air from entering the low pressure zone behind the propeller.
When a boat is moving in a plane, the propeller is operating at the top of its described circumference very close to the surface of the water, and without a horizontal barrier, the propeller would start to grab air, creating an air cushion. screw-ventilatorAs a result, the engine speed increases dramatically (as the load is lost), and the boat speed drops. crinoline physically cuts off the surface layer of water with air bubbles.
It's important to understand the difference between cavitation and ventilation, although crinoline fights both. Cavitation is the physical process of boiling water due to pressure drops, and ventilation is the sucking in of atmospheric air. Anti-cavitation plate minimizes both risks by providing stable flow of water to the blades even in conditions of strong wave or sharp turns.
The efficiency of crinoline depends on its position relative to the waterline. If the boat is overloaded with the nose or the engine is too high, the efficiency of the plate decreases. Under ideal conditions, the lower edge of crinoline should be on the same level with the bottom of the boat or slightly lower, creating a single plane of slip.
Effects on the engine cooling system
Most modern outboard motors use off-board water to cool, which is sucked through water intakes, and these holes are almost always located at the bottom of the lower unit, often directly under or in front of the crinoline, and a steady flow of water is vital to prevent overheating.
If the crinoline is damaged or missing (in the case of artisanal refinement), the flow of water becomes turbulent. pumperThis causes cavitation wear of the rubber impeller, which reduces the pressure in the cooling system and, as a result, overheats the engine, and crinoline provides a laminar (calm) flow of water to the water intake.
Also worth noting is the hydrodynamic effect of the pentacus, which, when the water hits the front edge of the crinoline, creates a high pressure zone, which drives water into the intake holes, allowing efficient circulation even at low speeds when the centrifugal force is not yet great, without which the engine launches to dry or underpressure would be more likely.
- 🌊 Provides laminar flow of water to the cooling pump.
- 🌊 Prevents air from entering the cooling system.
- 🌊 Creates a high pressure zone for effective water withdrawal.
- 🌊 Protects the rubber impeller pump from cavitation wear.
There are motor models where the design of the crinoline has been specifically modified to improve cooling. Suzuki or Honda You can find additional guide ribs on the bottom of the plate, and they serve to direct the flow of water strictly into the water intake, eliminating vortices on the sides.
Krinolin as an element of the adjustment of the differentiator
One of the hidden functions of crinoline is its role in tuning the angle of the boat's trim. The position of the horizontal plate relative to the flow of oncoming water creates a hydrodynamic force. If the front edge of the crinoline is raised relative to the flow, the emerging force tends to lower the lower unit down, which in turn raises the boat's nose.
Conversely, if the lower unit is tilted forward and the crinoline crashes into the water, it causes the boat's nose to peck, and if you adjust the angle of the motor on the transom correctly, you can use this effect to get the boat to plane with minimal fuel consumption. hydrodynamic substrate Under the stove helps to "pull" the boat from the displacement mode.
For fine tuning, there are special hydrofoils that are mounted on top of the standard crinoline, which increase the area of the horizontal surface, enhancing the effect described. However, even the standard crinoline, if it has an adjustable angle (found on some old or specialized models), can serve as a balancing tool.
☑️ Adjustment of the position of the engine
The screw will be too high, and the risk of air capture will increase many times over, and the balance between the plane rate and the stability of the course is key.
Typical damage and recovery methods
Although made of strong silomin or aluminum alloy, crinoline often suffers from underwater obstacles, hitting rocks, logs or concrete piers primarily on the front edge, and the deformation of this edge disrupts hydrodynamics, creating additional vortices.
Cracks in the body of crinoline are a more serious problem, and if a crack passes through the water intake hole or near the diewood mount, the engine becomes dangerous to operate, and water can get inside the lower unit or, worse, into the engine cylinder through the cooling system if the ducts are leaky.
The geometry of crinoline can be restored by argon welding and then machining, but if the deformation is severe, it is easier and more reliable to replace the lower unit leg or the plate itself if it is removable. cold-welding It is not recommended to restore large chips, as vibrations and water pressure will quickly destroy the repair structure.
| Type of damage | Impact on operation | Method of decision |
|---|---|---|
| Paint chips and micro-scratches | Absent, risk of corrosion | Painting, anode installation |
| Front edge bending | Deterioration of planing, yaw | Richt, grinding. |
| Cracking at the water intake | Pressure drop in the cooling system | Welding or remodeling |
| Completely broken stove | Impossibility of exploitation (cavitation) | Replacement of a lower unit leg |
⚠️ Attention: If you find a crack on the crinoline, don't just try to brew it without removing the stresses of the metal. Vibration of the motor will cause the seam to break again. Complete digestion with chamfer and reinforcement is required.
Modernization: installation of hydrofoil
For boat owners who often walk full load or in shallow water, the need to install an additional hydrofoil over the standard crinoline is an additional plate with profiled edges that is bolted to an existing anti-cavitational stove.
The hydrofoil can do several things at once. First, it significantly reduces the boat's yaw on the wave, making the course more stable; second, it helps the boat to go plane faster, reducing the load on the engine; third, it provides additional protection for the propeller from impact from the top.
When choosing a hydrofoil, it is important to consider compatibility with your engine model. Some universal models require drilling holes in the standard crinoline, which is not always desirable. There are models with clamping that do not require violation of the integrity of the factory design. The installation must be carried out strictly according to the instructions, with respect to the timing of the tightening of the bolts.
It is worth noting that the installation of an additional element changes the hydrodynamic profile. After installation of the hydrofoil, it is recommended to conduct test runs and possibly re-calculate the angle of the engine trim, since the area of the horizontal projection of the lower unit has increased.
Can you operate a boat if the crinoline has a small chip?
Operation is possible if the chip is not in the water intakes and does not cause strong vibration. However, the chip site must be painted with an anti-corrosion compound to prevent corrosion deep into the metal. If the chip is on the front edge, it can create noise, but the thrust will affect minimally.
How often should I change the zinc anode on crinoline?
The frequency of replacement depends on the salinity of the water and the intensity of use. In seawater, the anode can dissolve in 50-70 motor hours; in fresh water, the process is slower, but the condition of the anode should be checked every season; if more than 50% of the mass of the anode is lost, it must be replaced to protect the main components of the motor.
Does the shape of the back edge of crinoline affect speed?
Yes, it does. The back edge needs to be sharp and smooth so that the flow of water goes off without swirling. If the edge is rounded or has growths (for example, from paint or corrosion), this creates additional resistance and can worsen the outflow of water, which indirectly affects the efficiency of the screw.
Is it true that crinoline protects against grass?
Part of the truth is, a horizontal stove cuts the algae off, preventing them from winding themselves into a lower unit above the screw, but the main defense against winding the grass is the oil padding and the design of the lower unit, and crinolin only prevents the grass from rising to the top of the motor, but it doesn't completely solve the problem.
Should the surface of the crinoline be lubricated?
Lubricating the surface with antifriction compounds is not necessary or even harmful, as it can disrupt the flow fluid dynamics. However, fastening bolts, if they pass through the body of crinoline, must be reliably sealed and lubricated with lithium lubricant to prevent acidification.