Selecting the right screw is like choosing the right rubber for your car. You can drive all year round in the size chosen by the manufacturer and have average figures, or you can choose the right rubber for the type of operation you need. And then it doesn't matter whether you're off-road or annealing on smooth highways. With a screw, the story is roughly the same: You can leave everything as is or choose the screw that is best for your boat, load and use, getting maximum performance with minimum fuel consumption. The wrong screw, on the other hand, threatens not only to underestimate all the essential characteristics, but also to kill the engine quickly: all parts are subjected to high loads, which speeds up their wear and tear.
First of all, you need to understand where the boat will walk, in what activities to participate and with what load to move. If you need speed and maneuverability, we select a screw of increased step and smaller diameter. For towboats, when you need good traction, we prefer screws of larger diameter - like wheels with wider rubber. Diameter and pitch are two indicators that need to be balanced, choosing the best screw for the conditions you need. In addition, screws differ in the number and shape of blades, the material from which they are made, the diameter of the hub, different spline fit. To use a screw for different motors, make a detachable sleeve, but if you select several screws for one motor, the diameter of the hub and the number of splines must match.
More detailed values, e.g. 3×10-3/8×11 R, explain the number of blades (3), screw diameter (10 3/8), pitch (11) and right-hand drive (R). Some manufacturers specify screw and hub types. But the main thing is to determine the diameter and pitch. If the screw itself has a factory marking like this - 323-101-14, the full decoding will be in the attached instructions.
Material: no good and no bad, there is a suitable one for you.
When choosing the material, the pros and cons have to be weighed, understanding where and how the screw will be used. In general, the choice is between aluminium and steel screws. But there are also less popular plastic screws on the market, as well as derivatives of various alloys and compounds.
If you walk peacefully in fresh shallow water without the urge to fly and periodically meet snags and other unwanted objects, an inexpensive aluminum screw will be a good choice for you. If it is slightly deformed, you can fix it with at least a stone, and if you do meet a strong destructive effect, aluminum screw will give its inexpensive life in the name of saving the expensive gearbox, taking the blow. Ideally, you should always have a second one with you - in case of a change of activity or an unexpected incident. It is easy to replace it yourself with a suitable tool. Aluminium is a soft metal, its shape can be disturbed even by a small bottle or snag, and in shallow water sand will quickly cover the blades with mackerel, which will naturally reduce efficiency. However, its low cost and the protection of a more important gearbox make us turn a blind eye to such trifles.
If your only dream is speed, then a strong steel screw is what you need. It is, of course, more expensive than aluminum, but its efficiency is much higher than previous versions: the strength of the material allows you to greatly reduce the thickness of the blade, increase the mirroring of the surface, cavitation on it is not so significant. Due to this, the speed increases by 5-7% compared to aluminum analogues. Steel screw is not afraid of sand and fine abrasion - it will not rub off and even withstand a heavy blow to the top or bottom without changing the geometry of the blades. But when you meet the stone, a strong screw will not extinguish all the force of impact - it will move to the gearbox and shaft, which is much more painful in terms of repair and purse. Sometimes rescues the plastic bushing that takes the blow, but it is better to avoid unfamiliar shallow water at high speed and carefully monitor the echosound.
So, if you like to drive and enjoy speed, acceleration and efficiency, if your route is a familiar fairway and good depth, a steel screw will meet all your expectations. Not afraid of corrosion, it will also be ideal for walking on the sea. If the risk of underwater trouble is great, and the speed of a few kilometers per hour is just a personal ambition, an aluminum screw will be a more sensible solution. Still, a screw is an expensive, but consumable screw. And always with a spare version with a repair kit on board, it is not difficult to replace a suddenly broken device yourself. It is cheaper, simpler and faster than repairing a gearbox. Most importantly, be sure to lubricate the shaft and make sure the mesh is available when replacing.
Number of blades
Having decided on the material, it is necessary to decide what number of blades will be most optimal for you. This directly affects the efficiency and smooth running. The larger the number of blades, the lower the speed, the lower the vibration and the greater the traction.
Double bladed screws are very rarely used. They are usually used in swamp drives and electric motors. The most common and optimal properties are the three-blade screws.
As the stop area increases, both the area of the pushing forces and friction increase. If you need speed lightly, choose three blades. This will give you high efficiency, maximum speed and the chance that the screw will stick to two longitudinal blades and the third will stick out from above, without damage.
A four-blade screw is considered to be a cargo screw, you should not expect maximum speed from it. But you'll get a faster start and lower speed on the glider - and this is a good bonus for water sports enthusiasts. A very important point, which often becomes crucial, is that the large blade area will save fuel on cruising. Thanks to an even number of blades arranged opposite each other, the vibration load is reduced and the boat runs smoother. All these advantages are relevant as long as you use the propulsion system at 70-80% of full power. When you reach maximum speed, the four-blade propeller loses all its advantages. So it is important to understand which of your goals can be called the first priority. If you are a tractor, if speed and fuel economy are not important to you, then four blades are the deciding factor. In other cases, it is better to have a high-speed three-blade propeller.
Diameter of the screw
The diameter of the screw measured at the outer edge of the blade is also variable. The larger the diameter - and therefore the area - the higher the traction properties of the motor, the lower the speed.
The most common are the «round ear» and the ellipse, which provide an optimal ratio of traction to speed. For high speed vessels, blades are usually placed narrowing to the tips - this structure reduces friction and increases the speed of light vessels. Slanting screws are ideal for driving in overgrown waters, whose blades are screwed in the direction of motion and do not have the habit of twisting algae.
Higher speed is more of a step.
The most important indicator, which often has to be selected experimentally - the step of the screw. This is the distance in inches that the boat passes for a full 360 degrees turn of the propeller. By changing the propeller pitch by one unit, we get a difference of 150-200 engine revolutions in one direction or another. When changing the propeller pitch, a simple rule applies: more pitch - more speed, less pitch - more load. Despite the seeming simplicity, it is very important to select a screw for a particular engine, a particular boat and an average planned load, as both size and weight matter. The challenge is to ensure that a fully loaded boat has maximum engine speed at full throttle to the values recommended in the engine manufacturer's manuals.
To select the step of the screw experimentally, you need a normal tachometer and a boat loaded under normal operation, which has passed twelve hours running-in. Let's assume that the operating mode of the motor installed on the boat is 5800-6000 rpm. Let's take the loaded boat to maximum speed and see the tachometer reading. If the value is above the operating range of the motor, the screw is twisted and the motor is subjected to an increased load. It is necessary to lower the rpm by increasing the step at the rate of 1 step - 200 rpm.
If the speed reaches 6,000 - that means increased maximum speed, it is the best way to planing, the engine runs smoothly - the screw is optimally matched.
If the tachometer shows 5700-5800, the maximum speed will increase by several more, but the output will be more prolonged and the engine will feel tensioned. But if you get rid of the cargo in the form of a pair or three people, the indicators will immediately enter the optimal norm.
Strongly underestimated rpm - 5400 and below - speak about too heavy screw: the maximum speed will fall, exit on a glider will become very difficult, fuel consumption and loading on the engine will increase. Turns are increased by reducing the propeller pitch. At the same time, different screws of the same diameter and pitch will have different values. For example, a four-blade propeller will have 100 fewer turns than a three-blade propeller of the same size due to its large stop. And if we change the aluminum screw to steel, the pitch value of the second screw should be lower by one.
If the pitch is chosen correctly, you will get a good value of maximum speed, fast output on the glider, and also will save the engine from excessive wear, and yourself - from unnecessary troubles. For those who love variety and experimentation, walk with different boat loads and change travel goals from towing a skier to quiet fishing and transporting heavy loads with fuel-saving mode, you should have on board an interchangeable screw with a repair kit designed for different tasks.
Or to choose a screw with variable, variable pitch, which is certainly more expensive, but less troublesome and allows you to change the characteristics of the screw depending on the conditions. The screw pitch is changed by a simple movement of the hand - by turning a button located directly on the screw.
The choice of manufacturer is left to personal preferences. But if you decide to save money on a non-original Chinese screw, be sure to pay attention to the edges of the blade (they can be initially curves), the size of the hub on both sides, the thickness of the blade (the thicker, the worse it works). Also marking can be different from reality, that is - do not be surprised, if suddenly, having bought a screw one step more, you do not get the necessary speed reduction.
Screw selection is an important aspect of the future quality operation of the vessel, so it is better to approach the process with all the attention and care to the engine. Properly selected screw will allow to get optimal performance, save gasoline and most importantly - to extend the life of the engine. Therefore it is better not to save time or money. Cheaper will come out.