Which screw is better for the Tohatsu 18 outboard motor: a complete analysis

Choosing an 18 horsepower propeller is a critical step in tuning, because it is the component that determines whether the power unit can reach its potential. Incorrectly selected blade geometry often leads to the boat not being sleeved and the engine running inefficiently, consuming excess fuel. Tohatsu Often faced with the dilemma: to leave the standard version or to choose a more productive alternative for specific operating conditions.

The main task in the selection is to ensure that the engine reaches the maximum recommended speeds at full load of the boat. If the screw is too "heavy", the tachometer hand will not rise above 4500-5000 rpm, which is fraught with overheating and detonation. Conversely, too "light" option will make the engine roar at the limit speed without real speed gain, which is also detrimental to the resource of the piston group.

In this article, we will look at the technical details that will help you find the perfect combination of pitch and diameter for your boat, we will analyze the effect of fabrication material on driving performance and analyze the typical errors in the modernization of the propulsion system.

Basic specifications and staffing solutions

A standard propeller, which is installed in the factory, usually has optimal parameters for an average boat about 3.6-4.0 meters long with two passengers. Tohatsu M18E2S Or its two-stroke predecessors, the standard propeller is often 9 or 10 inches increments, which allow medium-sized PVC inflatable boats to be confidently glided, providing a balance between thrust at launch and maximum speed.

But the concept of "best screw" is always relative, because it depends on the hydro-aerodynamics of the hull and the weight of the crew. The regular three-bladed version of aluminum alloy is a compromise solution that ensures reliability and maintainability. Aluminum is soft, it extinguishes blows against underwater obstacles, saving lower unit from breakdowns, but its efficiency is lower than that of more advanced alloys.

It is important to bear in mind that blade-geometry different manufacturers may differ even with the same declared step figures, so when replacing the brand of the screw (for example, by the Solas or Michigan Wheel) a test speed measurement shall be carried out as the actual pitch may differ by 0.5 to 1 inch from the factory pitch.

For heavy boats or when operating with full load (fish, gear, gasoline) regular pitch may be lacking. the engine begins to "choke" at full gas, without developing maximum power, in which cases owners are forced to look for options with a smaller pitch or increased diameter to return the engine to spin in the operating range.

Influence of step and diameter on driving performance

A screw's pitch is the distance that a screw theoretically travels in one rotation in a solid medium. A 1-inch increase in pitch usually results in an engine speed change of about 200 to 300 units per minute. If you want to increase the top speed on a light boat, you'll need a big-step propeller, but you'll also have a drop in acceleration dynamics and planing ability.

The diameter of the propeller also plays a key role in creating thrust. The larger diameter allows large volumes of water to be moved, creating a powerful persistent force, which is critical for heavy boats or when towing a water skier. However, increasing the diameter requires more power to rotate, which can lead to a drop in maximum speed if the engine does not have enough torque.

When selecting a set, you need to keep a balance: you can not indefinitely increase the diameter without reducing the pitch, otherwise the motor simply cannot spin the screw. The best solution for universal operation is often four-bladed screws. They provide a smoother ride, less vibration and better traction at low speeds compared to three-bladed counterparts.

Experimental selection is the most reliable way to find the ideal. Start by measuring the real speed at full gas with the current rotor. If the motor does not reach its maximum (for example, it shows 5200 instead of 6000), the step should be reduced. If it digests (6300+), the step should be increased.

Manufacture materials: aluminum or stainless steel

Most 18 hp motor owners initially deal with aluminum propellers, a budget and lightweight option that, when hit by a rock or a log, simply deforms itself, taking on the impact energy, and replacing such a propeller is inexpensive and does not require special skills or tools.

Stainless steel (arts.Stainless Steel) This is a material for those who require maximum performance, steel screws are thinner than aluminum screws at the same strength that reduces water resistance and increases efficiency by 5-10%, they perfectly hold geometry without bending when working in grassy bodies of water, and allow the engine to gain speed faster.

⚠️ Warning: Installing a steel propeller on an old or worn motor can be dangerous. If hit by a solid obstacle, the energy will not be extinguished by the screw deformation, but transferred to the gearbox, which can lead to the failure of the gears or even the bend of the propeller shaft.

In addition, steel screws are much more expensive and heavier, which can shift the center of gravity and create additional stress on the thrust bearing. For an 18 hp engine, the transition to “stainless steel” makes sense only if the boat is used intensively, and the conditions of navigation allow you to develop high speeds in clean waters.

There are also composite screws that combine lightness and flexibility, often equipped with gearbox protection, cutting or turning at critical loads, and are great for rocky rivers where the risk of damaging lower unit is highest.

Compatibility of screws with PVC boats and hulls

For PVC inflatable boats with a length of 3.2-3.6 meters, which are often equipped with an engine Tohatsu 18It's a very high sailing and a soft bottom, and it's a very fast plane, so 9 to 10 inches increments are going to be the most efficient, and it's a very rare slack increase in speed, because the soft bottom and sides are very strong.

If you have a boat that is 3.8 to 4.2 meters long with an inflatable keel or hard bottom (HDB), that changes, and the large plane area allows you to use screws in increments of 10.5 to 11 inches more efficiently, and this is especially true when you're going against the wind or current when you need a thrust reserve.

For small hull boats (season, salute) weighing up to 300 kg with two passengers, the 18-horsepower engine is borderline. It often requires maximum thrust, and owners sometimes even reduce the pitch of the screw to 8 inches to ensure they get on the plane at full load, sacrificing maximum speed.

The effect of the transom on the rotor

If the boat's transom is too low or has a large angle of the kilvater flow, the propeller can capture air (cavitation), in such cases, the installation of a propeller with an increased diameter or the installation of a hydrofoil on a lower unit helps.

Sharp cheekbones require more speed to enter mode, so the screw must be heavier at the start (smaller pitch). Tube-nosed boats go to the glise easier, but have high drag at high speeds.

Table of selection of screws for different conditions

Below is an indicative table that helps you choose the initial version of the screw depending on the type of boat and the desired specifications. Tohatsu M18 series.

Type of boat / Conditions / Conditions	Recommended step (inches)	Number of blades	Expected impact
PVC 3.2m (1 person)	10 - 10.5	3	Maximum speed
PVC 3.6m (2 persons)	9.5 - 10	3	Balance of traction and speed
PVC 4.0m + load	9	4	Confident exit to plane
hull boat (light)	10.5 - 11	3	Economical move

The numbers in the table are averaged. Real matching always requires customization. For example, a screw labeled 9.25 may behave like 9.5 from another manufacturer because of the shape of the blade.

When using four-blade screws (4-blade) often there is a decrease in the maximum speed of 1-2 km / h compared to the three-blade, but the time to go to the plane is reduced by 20-30%.

Diagnosis and adjustment of the propeller

After installing the new propeller, you need to conduct a running test. First of all, check the absence of vibration at idle and working turns. Any vibration indicates an imbalance of the screw or damage to the blades.

Make sure to use the tachometer for fine tuning. Tohatsu 18 It's usually 5,000 to 6,000 rpm. If you see 6,200 rpm, the propeller is too light, you lose traction and engine life. If 4,800 rpm is heavy, the engine is overloaded.

⚠️ Attention: Long-term operation of the engine at speeds below the minimum recommended (below 4500-4800 for 18 hp) leads to candles coking, overheating of the piston group and accelerated wear of the crank-shaking mechanism.

And also, notice the phenomenon of cavitation, which is sucking air into the propeller, and if you turn abruptly or into a straight motor, you start to growl and lose traction, and you see air bubbles behind the propeller, you're either wrongly picked up or too high up, and in these cases, you can install an anti-cavitation stove or replace the propeller with a larger diameter model.

Regularly inspect the edges of the blades. Even small bullies and chips disrupt the flow of fluids, reducing efficiency and causing vibration. Aluminum screws can be adjusted and grinded, steel screws require professional repair in the workshop.

Frequently Asked Questions (FAQ)

Can I put a Yamaha or Mercury propeller on a Tohatsu 18?

You can physically determine if the number of slits (usually 13) matches the diameter of the shaft. However, the diewoods of different brands have different geometries. Yamaha screws can work inefficiently or cause vibration due to differences in the flow of water created by Tohatsu's lower unit. Better use screws specially designed for Tohatsu or universal brands like Solas with compatibility.

How do I know if the screw is damaged?

The main signs: the appearance of vibration on the hull of the boat, a decrease in maximum speed, the inability to go to the plane with normal load, visual serrated or cracks on the edges of the blades, and the engine may begin to work louder than usual.

Which is better: 3 or 4 blades for fishing?

For fishing, where quiet running, stability at low speeds and fast exit to plane with a heavy load, a four-blade screw are important (see below).4-blade) better. Three-bladed will give you a higher top speed, but it will be worse to pull from the spot.

Do I need to balance a new screw?

Factory screws are usually already balanced, but cheap Chinese screws can be imbalanced, and if you feel a strong vibration after installation, it is advisable to take the screw for balancing or replace it with a better one.

Does the height of the engine installation affect the choice of the screw?

If the motor is set too high, the propeller will grab air, and no geometry will help. If it's too low, the water resistance will rise. The optimal height is selected experimentally, and only then the screw will change for fine tuning.

In summary, we can say that for the motor Tohatsu 18 There's no single "best" propeller. There's a solution for your particular boat-cargo-condition bundle. Start by analyzing your current speed situation, determine what you're missing, whether it's traction or speed, and use pitch and diameter. A well-chosen propeller will turn your motor into an efficient and reliable tool for conquering the waters.