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Do you own a boat? Then you need an outboard motor to sail. But your outboard motor also needs maintenance sometimes, such as replacing the screw (propeller). If you want to do this yourself, it is important to know what size screw you need. In this blog we tell you how to calculate the propeller size of your boat.

The importance of a boat propeller

A boat propeller is the same as a propeller. This is an important part of the boat’s engine. A boat propeller transfers the horsepower of the engine to the water. It is important that you choose the right boat propeller for your boat. The propeller must give the right speed to get your outboard motor to the right power. To calculate the correct propeller for your boat, we look at the pitch.

Pitch of boat propeller

The pitch is the most important characteristic of a boat propeller. It refers to the curvature of the blades of the propeller. It indicates the forward distance (in inches) that a boat propeller would travel during a full rotation. A greater pitch means more curvature and therefore a higher top speed. If you make too many revolutions, it is better to choose a screw with a higher pitch. If the speed is too low, it is better to go for a screw with a smaller pitch. This ensures less curvature and therefore better acceleration.

Tips for choosing ship propellers

When choosing marine propellers, you should also consider the material and number of blades. Most screws are made of stainless steel, aluminum or, in some cases, plastic. For smaller engines up to 40 hp, an aluminum propeller is used as standard. Aluminum also bends more, limiting damage from rocky soils. The advantage of stainless steel screws is that they are stronger and less vulnerable than other materials. This makes a stainless steel screw durable, but more expensive. Plastic propellers are only used for very small outboard motors of 2 and 2.5 hp.

You can also choose from a 3-blade, 4-blade or 5-blade propeller. Most boat propellers have 3 blades, because this improves speed the most. A 4-blade propeller provides better acceleration, but a lower top speed. 5-blade propellers have a lot of grip on rough water and very good acceleration, but clearly less top speed. 4-blade and 5-blade propellers are usually used for water sports.

How to calculate the boat propeller

Your current screw size and pitch size can be found on the outside of the propeller hub. For example, it says 11 ¼ X 13 or 11.25 X 13. The first number is the diameter (11 ¼ & 11.25) of the screw, the second number is the pitch (13). Some brands have a different indication for the pitch size.

  • Yamaha: 11 1/4 X 13 – G. Where ‘G’ is the blade type.
  • Mercury Mercruiser: 48-832832A45 21P. The last number 21 is the pitch size.
  • Solas: 5111-093-10. The last four numbers are the dimensions (9.3 is the diameter and 10 is the pitch size).
  • Volvo Penta: 814629. This brand uses its own numbering system.

Would you like to have the propeller of your boat calculated? Please feel free to contact us. We are happy to help you choose the right screw.

Maintenance of your outboard motor

Regular maintenance of your outboard motor is important. It significantly increases the lifespan of your engine. You are welcome with us for repairs and maintenance. With us you can choose between a minor maintenance service and a major maintenance service. Is there something broken on your motorcycle that needs to be repaired soon? Please contact us and our technicians will ensure that you can get back on the water as quickly as possible.

More news & tips

Most popular outboard motors in horsepower

Preparing your outboard motor for winter

Frequently asked questions

Here you will find answers to common questions. Is your question not listed? Please feel free to contact us. We are happy to help you.

Contact us
  • Which propeller do I need for my boat?
    Should the speed of your engine block be increased or decreased? Then remember the following rule of thumb: for every 2 inches of a screw, the revolutions per minute change by 400 revolutions. If you choose a smaller pitch, the speed will increase. If you choose a higher pitch, the speed will decrease.
  • How do you calculate the pitch of a propeller?
    Your current propeller size and pitch size can be found on the outside of the propeller hub. For example, it says 11 ¼ X 13 or 11.25 X 13. The first number is the diameter (11 ¼ & 11.25) of the propeller, the second number is the pitch (13).