How to find the right screw to the motor

Shape, size, material — all matter. How to squeeze maximum performance out of your boat and not “kill” the motor

The selection of the right screw — it is a choice of rubber on the car. Available all year to go on the vsesezonka size chosen by the manufacturer, and have averages, and you can pick up the rubber, exactly matching the desired type of operation. And then it doesn’t matter — overcome if you off-road or moves on smooth highways. With screw are the same: you can leave it as is or choose a screw that is optimal for your boat, load, and use, getting maximum performance with minimum fuel costs. Wrong well matched screw threatens not only to understate all essential characteristics, but also quickly kill the engine, all parts are subjected to high loads, which accelerates wear.

First of all, you need to understand where the boat will go, what activities to participate and how download of to move. If you need speed and agility, then pick up a screw increased pitch and smaller diameter. For towing, when you need a good pull, prefer screws with a larger diameter as the wheels with the wider rubber. The diameter and pitch — two measures that need to be balanced, choosing the optimal screw under the necessary conditions. In addition, the screws differ in the number and shape of the blades, the material from which made, hub diameter, different spline fit. To use the screw for different motors, make a removable sleeve, but if you select a few screws under one motor, the hub diameter and the number of slots must match.

Key indicators marked on each screw. More detailed values, for example, 3×10-3/8×11 R, explain number of blades (3) diameter of screw (10 3/8) step (11) and drive on the right (R). Some manufacturers indicate the type of screws and hub. But the main thing is to determine the diameter and pitch. If the screw factory markings like this — 323-101-14, the full transcript is sure to be in the instructions.

Material: there is good and bad, there are suitable for you

When choosing the material you need to weigh the pros and cons, knowing where and how it will be operated the required screw. As a rule, the choice stands between aluminum and steel screws. But in the market there are also less popular plastic, as well as derivatives of various alloys and compounds.


If you can go for fresh shallow water without trying to fly and meet periodically with snags and other undesirable objects, the cheap aluminum screw is a good choice. For small deformations it can be corrected though with a stone, and if it does there is a meeting with a strong destructive effect, aluminum screw give your cheap life for the salvation of expensive gear, taking the hit. And it definitely is less of a pain, especially when you remember that the screw is a consumable. Ideally you should always have a second — in the event of a change of activity or unexpected incident. With a suitable tool to replace it yourself is not difficult. Aluminium is a soft metal, its form to violate the may meeting even with a small bottle or a snag, and in shallow water sand will quickly cover the blade with visitbinary, which naturally will reduce efficiency. However, its low cost and security of more significant gear make a close eye on such things.


If your only dream is speed, durable steel screw — what you need. It is, of course, more expensive than aluminum, but its efficiency is much higher than the previous versions: the strength of the material allows to greatly reduce the thickness of the blade, to increase the reflectivity of the surface cavitation, it acts not so much. In this regard, the speeds increase by 5-7% compared to the aluminum counterparts. The steel screw is not afraid of sand and fine abrasive — it will not be erased and will even withstand a strong blow on the snag or the bottom, without changing the geometry of the blades. But when they met with stone strong screw will repay the full force of the blow — it will go to the gearbox and the shaft that much harder in terms of repair and purse. Sometimes pulling the plastic bushing that accepts the hit, but still better to avoid unfamiliar shallow waters at high speed and closely monitor the performance of the sounder.

So if you love to drive and show care for the speed, acceleration and efficiency, if your route is a familiar channel and good depth, steel screw will justify all hopes. Not afraid of corrosion, it will also be ideal for walking on the sea. If the risk is to stumble upon an underwater nuisance is great and rates of speed of several kilometers per hour — just personal ambitions, aluminum screw would be a more reasonable solution. Still, the screw is expensive, but it is consumable. And always, having on Board a fallback with the kit, it is easy to replace yourself suddenly broken device. It’s cheaper, easier and faster than repairing the gearbox. Most importantly, don’t forget when replacing lubricate the shaft and make sure the presence of the grid.

Size matters
The number of blades

Having defined the material, you need to decide how many blades will be the most optimal. This directly affects the efficiency and smoothness. The more blades, less speed, less vibration and more traction.

Two-bladed screws are used very rarely. Their scope is typically the buggies and motors. The most common and optimal in their properties — three-blade propeller.

With increasing area of focus and increases the area of action of the pushing force, and friction. If you need the speed of light — choose three of the blade. It provide you high efficiency, the maximum speed and the chance that, vitkovskis in the sand, the screw will form two longitudinal blades, and the third will stick out from the top, without damage.

Four-bladed propeller is considered to be freight, it is not necessary to wait for the maximum speed. But a quick start and reduce the speed boat is provided for you — and it’s a good bonus for are fond of water sports. A very important point, which often becomes crucial — a large area of the blades will save fuel consumption at cruising course. Due to the even number are located opposite each other of the blades of weapongrade reduced and the progress of the boat becomes more fluid. All these advantages are relevant, as long as you use the propulsion system to 70-80% of full power. Is to achieve maximum speed — and four-bladed propeller loses all its advantages. It is therefore important to understand which of your goals can be considered a priority. If you tractor, if you do not care the speed and fuel economy for you is a crucial factor, then four blades. In other cases, it is better to take the high-speed three-bladed propeller.

The diameter of the screw

A variable value is the diameter of the screw measured at the outer edge of the blades. The larger the diameter and, consequently, the area of the higher traction characteristics of the motor, the — as a consequence — a lower speed.

The most common: “round ear” and the ellipse providing the optimal balance of traction and speed. For high-speed ships usually put the blade, tapering to the tips — such a structure reduces friction and increases the speed of moving light vessels. For movement in the overgrown ponds are ideal slant the screws, the blades are twisted in the direction of movement and do not have the habit to wind the algae.

Higher the speed — the bigger the step

The most important indicator that is often necessary to select empirically — step screw. This is the distance in inches that runs the boat during a full rotation of the screw in 360 degrees. Changing the pitch by one unit, we get the difference at 150-200 rpm of the motor in one direction or another. If you change pitch of the screw operates a simple rule: the bigger the step — higher speed, smaller step is more cargo. Despite its apparent simplicity, it is very important to choose the right screw for a specific motor, specific boat, and an average projected load because both the size and weight are important. The challenge was to have loaded the boat at full throttle maximum engine speed aspired to the values recommended in the manual of the engine manufacturer.

To pick up the pitch empirically, need for conventional tachometer and loaded under normal operation, held a twelve-hour run-in boat. Suppose the working mode of the engine installed on the boat is 5800-6000 rpm Output loaded the boat at maximum speed and watch the readings on the tachometer. If the value is above the range of operation of the engine means, the screw overwinds and on the motor is increased. It is necessary to understate the speed by increasing the step rate of 1 step is 200 rpm.

If the revs reach 6000 — so, increased maximum speed, this is the best planing, the motor operates smoothly — screw is matched optimally.

If the tachometer shows 5700-5800, the maximum speed will increase by a few indicators, but planing will be more protracted, and motor work will be felt with interference. But if you get rid of the burden in the form of a pair of three people, the figures will immediately be included in the optimal norm.

Greatly underestimated the speed — 5400 and below — talking about too heavy for the screw: fall maximum speed, the yield on the boat will become highly difficult, increase the fuel consumption and the engine load. Turnover increased by reducing the pitch of the screw. The values of the different screws of the same diameter and pitch will be different. For example, the momentum four-bladed propeller due to a large focus will be 100 units less than a three-blade is the same size. But if we change the aluminum screw-on steel, the value of the second step should be lower per unit.

If the step is chosen correctly, you will get good value for the maximum speed, quick release on the glider, and protect the engine from excessive wear and themselves from unnecessary troubles. For those who like variety and experimentation, walks with different loading of the boat and changes the destination of the towage of the skier to the quiet fishing and transportation of heavy loads with the mode fuel economy, worth having on Board replaceable screw kit, intended for different purposes.

Or choose the screw with a variable, adjustable-pitch, which is certainly more expensive, but less troublesome and allows you to change characteristics depending on the conditions. The pitch changes with a simple hand movement by the rotation of the button located directly on the screw.

The choice of the manufacturer remains with the personal preference. But if you decided to save the stodgy Chinese screw, be sure to pay attention to the edges of the blades (they may be initially crooked), the size of the hub on either side, blade thickness (the thicker, the worse it works). The same marking may differ from reality, i.e., don’t be surprised if suddenly, buying a screw on step more, you will not receive the necessary decrease the rpm.

The choice of propeller is an important aspect of future high-quality vessel operations, therefore, to approach the process better with all the attention and care of the engine. The correct propeller will allow you to obtain optimum driving performance, save petrol and most importantly — extend the life of the motor. Therefore, it is better not to save neither time nor money. Come cheaper.

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