The right selection screw-step This is a critical step in the configuration of the outboard motor Tohatsu 9.8Many owners are faced with a situation where the engine does not develop maximum power or, conversely, runs at prohibitive speeds, without accelerating the boat to the planned speed, and it is the geometry of the propeller that determines how efficiently the engine thrust will be converted into the movement of the boat on the water surface.
Owners Tohatsu M9.8B newer versions of the series FS The standard product that comes with the kit is designed for average operating conditions, but real life makes its own adjustments: different loading of the boat, type of hull (PVC or aluminum) and desired driving mode require individual approach to the choice of diameter and step.
In this article, we will discuss in detail the technical nuances that affect the performance of your motor, you will learn how the screw pitch interacts with the gear ratio of the gearbox and why the wrong choice can lead to serious breakdowns. crankshaft Understanding these processes will allow you to make the most of your Tohatsu 9.8.
Physics of the process: how the screw's pitch affects speed
First, you need to understand the basic definitions. Screw pitch This is the theoretical distance that a screw will travel in one full revolution in a solid medium. If you imagine a screw turning into a tree, you'll see how far it goes in one turn. In water, it'll always be smaller than the theoretical one because of a slip called a slide. slip.
Increasing the pitch directly affects the boat's potential speed, but simultaneously puts additional strain on the engine. Tohatsu 9.8 It has limited torque, and if you install a too heavy propeller, it simply cannot spin to the working speed, which leads to inefficient operation, overheating and increased consumption of the fuel mixture.
On the other hand, a small-step propeller allows the engine to reach maximum speeds easily, but the boat cannot reach high speed, in which case you get excellent traction at low speeds, which is useful for trolling or transporting heavy loads, but you lose at maximum speed. The balance between thrust and speed is a key task in tuning.
⚠️ Attention: Engine operation Tohatsu 9.8 with a screw that does not allow the engine to reach the operating range of revolutions (usually 4500-5500 rpm for this model), can lead to detonation and burnout of the piston.
It's important to remember that a pitch change of just 1 inch changes the engine's speed by about 150 to 200 units. This one-inch rule helps you navigate the initial selection. If your engine doesn't get 500 rpm to maximum, you need a screw with a smaller pitch. If it exceeds maximum rpm, you need to increase the pitch.
When measuring the screw pitch, note that the marking indicates the pitch in inches, even if the screw itself is produced in a country with a metric system. 1 inch is 25.4 mm.
Technical specifications of the Tohatsu 9.8 engine
Two-stroke engine Tohatsu 9.8 (often known as the M9.8B2) has unique specifications that set it apart from its competitors: a 169 cubic centimeter displacement and 6.2:1 compression ratio require careful consideration of load selection. The gear ratio is 2.08 (27:13), which is a sufficient traction indicator for engines of this class.
The maximum power of 9.8 hp is achieved at 5,500 rpm. This is the target when set up. The standard screw, which is often bundled or offered as a primary option, is 8.8 x 9. This means 8.8 inches in diameter and 9 inches in pitch. However, for different conditions, this is not always the ideal option.
Exhaust system design Through Propeller Hub (through the screw hub) also affects efficiency. The exhaust gases that come out through the center of the screw create a gas cushion that can reduce resistance, but if you step wrong, leads to cavitation. Cavitation is the formation of vapor bubbles that, when they collapse, break down the metal of the blades and reduce efficiency.
When analyzing the technical specifications, you should pay attention to the torque curve. Tohatsu 9.8 It has good traction on the bottom, which makes it an excellent choice for heavy boats, but only if the propeller is correctly selected. Exceeding the permissible load instantly "strangles" the engine, preventing it from reaching its potential.
How to determine the current pitch of the screw
Before you buy a new propeller, you need to know exactly what the current one is. Tohatsu 9.8The digits are usually knocked out or cast, such as "8.8 x 9" or "8.8 x 10".
If the marking is erased or painted with antifouling paint, you can use a rod and formula, but this requires high accuracy and knowledge of the geometry of the blade. The easiest way to refer to the documentation for the outboard motor or to the catalogues of spare parts by serial number of the screw, if it is preserved.
Visually, the screws with different pitches may look similar, but the angle of inclination of the blades will be different. The screw with a large pitch has a more "palled" edge. When you compare two screws next to each other, the difference becomes obvious even to the naked eye.
⚠️ Warning: Do not attempt to change the geometry of the blades (bend the edges) to change the pitch. This will disrupt the rotor's balancing, causing vibration and destruction of the engine's lower unit shaft. Tohatsu.
It is also worth checking the screw for chips and cracks. Even minimal damage to the edge of the blade reduces efficiency by 10-15%. For a 9.8 hp engine, every lost horsepower is noticeable, so the condition of the screw should be perfect.
Calculating the optimal step for different conditions
The choice of propeller pitch depends on many factors: the weight of the boat, the number of passengers, the type of hull and the desired speed. For light inflatable boats up to 3 meters long, a standard pitch of 9 inches is often enough. However, when increasing the length to 3.4-3.6 meters or at full load, a standard propeller can become too "heavy".
If you plan to use a fishing boat with a lot of equipment, it makes sense to consider propellers with 8 or even 7.5 inches increments, which will allow the engine to more confidently put the boat on the plane. For high-speed walks together on a light boat, you can experiment with 9.5 or 10 inches increments, if the engine allows you to gain working speed.
There is a simple formula for approximate calculation: the desired speed (in miles per hour) is multiplied by a coefficient (usually 1.1-1.2 to account for slip) and divided by the rotation of the screw shaft. However, in practice, it is easier to use the trial and error method based on the correspondence table and the tachometer readings.
☑️ Checking operating conditions
In fresh water, the load on the propeller is less than on salt, so a propeller that works perfectly on river fishing may be too "light" when going to sea, and the engine will go into a rpm gap.
Comparative table of the screw and revolutions step
Below is a table showing the approximate effect of the screw pitch on the maximum engine speeds. Tohatsu 9.8 The data are averaged, because the real picture depends on the state of the bottom of the boat and the purity of the propeller.
| Screw pitch (inches) | Diameter (inches) | Expected turnovers (rpm) | Recommended application |
|---|---|---|---|
| 7.5 | 8.8 | 5800 - 6000+ | Heavy boats, full load, trolling |
| 8.0 | 8.8 | 5600 - 5800 | PVC boats 3.2-3.4 m, 2 people |
| 9.0 | 8.8 | 5300 - 5500 | Standard equipment, light boats |
| 10.0 | 8.8 | 5000 - 5200 | Single fishing, light housing |
As you can see from the table, a 1-inch pitch change significantly changes performance. If your measurements show 5,100 revolutions with a 9.0 screw, then switching to an 8.0 screw will lift the revs to about 5,300-5,400, which is closer to the optimal power range.
When using the table, always leave a small margin. The motor should operate in the range of 4500-5500 revolutions with a fully open throttle. Working at the limit (above 5600-5700) reduces the engine life, and working below 4200-4300 leads to soaking on candles and pistons.
The optimal pitch of the propeller allows the Tohatsu 9.8 engine to develop 5000-5500 rpm at full load of the boat, ensuring maximum speed and economy.
Effect of screw material on productivity
Spin screws for Tohatsu 9.8 They're made of aluminum alloys or stainless steel. Aluminum screws are standard. They're lightweight, they're cheap, and they can deform when they hit a hard obstacle, and they protect the engine shaft from failure.
Steel screws are significantly stronger and thinner in the blade section, which reduces water resistance. They allow you to increase speed by 5-10% compared to aluminum analogues of the same step. However, their installation on a 9.8 hp engine is often economically impractical due to the high cost and risk of damage to the shaft on impact.
There are also composite screws that combine plastic flexibility and strength, and they often come as standard, with low cost and shaft safety, minus lower efficiency and the ability to change the geometry of the blades over time.
⚠️ Warning: When switching from aluminum screw to steel screw, be sure to check the condition of the persistent washer and shaft slits. Steel screws require more frequent lubrication of the slate joint.
For most owners Tohatsu 9.8 The best choice is the quality aluminum propeller of proven brands such as Solas or Michigan Wheel, which offer a wide range of steps and blade profiles, specially sharpened for low-power engines.
Frequent errors in selection and operation
One of the most common mistakes is to try to compensate for power shortages by increasing the pitch of the propeller. Owners think that a larger pitch will give more speed, but as a result the motor stops pulling, speed drops, and fuel consumption increases. At low power plants such as 9.8 hp, it is more important to have a thrust reserve than a theoretical speed reserve.
Another mistake is ignoring the state of the lower unit, so if the antifouling coating on the foot of the motor or the screw becomes overgrown with algae or shells, the efficiency drops by 20-30%, in which case no replacement of the screw will help until the cleaning is done.
Also, you often forget about the height of the engine. If lower unit is too deep, the screw runs in denser water, which increases resistance. If it's too high, air suction and cavitation begin. The optimal height is when the anti-cavitation plate is in the same plane as the bottom of the boat or slightly lower.
What to do if the screw is constantly growing?
If you operate a boat in waters with rich flora, consider using self-polished screw paints or regularly (once every 2-3 days) remove the motor from the water for drying and cleaning.
The correct adjustment of the carburetor also plays a role: if after the change of the screw the motor began to work unstablely, it may require (fine adjustment) of the quality of the mixture.
FAQ: Frequently asked questions
Can I put a Yamaha 9.9 screw on a Tohatsu 9.8?
No, you can't. Tohatsu and Yamaha Other number of screws per shaft (10 slits for Tohatsu vs 13 for Yamaha in most models) and different nut threads. Installing an incompatible screw will cause damage to the shaft or loss of the screw during movement.
Which screw is better for a 3.2 meter inflatable boat?
For an inflatable boat 3.2 meters long and an engine Tohatsu 9.8 The best is a propeller in 8.5 to 9.0 inches, which will provide a confident exit to the planing with one or two passengers. If the boat is heavy (with a hard bottom), it is better to choose step 8.0.
Why doesn’t the engine develop full speed with a new propeller?
The pitch of the new screw is probably too big for you to boot. The engine can't turn the screw faster because of increased water resistance. You need to reduce the pitch of the screw by 0.5-1 inch to get the revs into the range of 5000-5500.
Should I change the lower unit when changing the type of screw?
No, you don’t need to change lower unit if you stay within compatible screw models for the Tohatsu 9.8However, always check the length of the veneer and the condition of the hole in the shaft with each change of the screw.
How often do I need to change the screw?
The propeller has no shelf life, but it has a resource, and you have to change it when you have inexorable chips, cracks, or when the geometry of the blades is so disturbed that you have a strong vibration. Aluminum propeller, when you're in active use, lasts 3-5 seasons.