Select Page

Have a look around any boatyard and you will notice quite a variety of propellers – some have two blades, some have three and others have four or more. While most propellers are completely rigid some have blades that fold. Propellers can be right handed, where they turn clockwise as they move forward, or left handed where they turn anti-clockwise. All these variations come about because different types of propeller are suited to different types of boats, engines and transmissions.

Propeller theory basics

The scientific theory behind how a propeller works is quite complex and if you are contemplating buying a replacement propeller for your boat then it would be advisable to read up about propeller theory and hydrodynamics in some detail – what follows here are just a few basics.

A ship’s propeller is often referred to as a “screw”, but since water is a liquid rather than a solid this analogy is a little misleading as a propeller can also be described as a type of foil or pump, which are nothing like a screw.        

A propeller works by converting torque (a force that causes something to rotate) into thrust. A turning propeller moves water downwards and behind the blades, in other words an action which produces a thrust of water from the blades. Each blade has a distinctive curved shape which as it turns helps to move the water down and behind it, acting like a foil in the process and then pumping the water out behind. It acts like a foil because the angle of the blade creates lift as it moves through the water, in a similar way to how a wing creates lift through the air, with a positive pressure, or pushing, on the underside and a negative pressure, or pulling effect on the top side. This accounts for why a propeller blade is also twisted – its shape helps it create lift.

Going back to the screw analogy, if a propeller was screwing its way through a piece of wood, the distance it would move in one revolution is equivalent to the propeller’s pitch, so for example 20cm. The diameter or width of the propeller might be 40cm. Diameter and pitch are the two key dimensions given to describe a propeller and are usually marked on the hub of  a propeller, for example in this case 40 x 20.

Two other factors to be aware of are “advance” and “slip”. Advance refers to the actual distance a propeller moves through one rotation and slip refers to the difference between the pitch and the advance.

Number of blades and folding props

Why the different number of blades? What about props that fold? The quick answers are as follows:

  • Two blade propellers are the best option for sailing boats under 10m in length, giving adequate performance under power and causing less drag through the water when under sail.
  • Three blade propellers give greater thrust than two blades but increase drag. Three blade props are more commonly used for power boats but some larger sailing boats use them. Three blades usually give a slightly better top speed performance than four blades.
  • Four blade propellers tend to be quieter and vibration free. They produce more lift at the stern which can help with acceleration.
  • Folding or feathering props are designed for sailing boats and minimise drag when the boat is under sail. These are very effective but a lot more expensive than standard propellers.
  • Boat type, engine power, displacement and desired boat speed are all factored in when choosing the exact type and specification of propeller.

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60...

First Aid at Sea – strains and sprains

Strains and sprains respond well to rest and cooling. Wrap ice in a tea towel before applying. First Aid at Sea Strains and...

Sector lights, directional lights, leading lights – how do they differ?

Sector lights, directional lights and leading lights guide vessels safely through hazardous waters or narrow channels at...

Repairing a leaking hull-to-deck joint

If you suspect a hull-to-deck joint has failed, then being absolutely sure where the actual leak is occurring is of prime...

Galvanic and electrolytic corrosion

Galvanic corrosion is an electrochemical reaction between two or more different metals, in the presence of an electrolyte (note salt water is a good electrolyte).

A simple guide to understanding tides when passage planning

Understanding tides when passage planning When planning a trip in tidal waters, check the tides before going afloat. Use...

Points of Sailing

The course on which a boat is sailing can be described by its angle to the wind, not to be confused with its compass...

Essential Knots: Clove hitch

Essential Knots: Clove hitch Use: Tying a rope to posts, bollards, rings or a guardrail. Step 1. Make a turn around the object and lay...

Passage Planning Advice & Safety for skippers

Passage planning helps you to: • Decide where to go • Calculate how long it will take to get there • Avoid bad weather •...

Sail care and maintenance – Part 1

When thinking about the care, maintenance and repair of sails it helps to have some understanding of the properties of the ever growing range of modern sailcloth and the fibres they are made from, as opposed to the traditional canvas sails of the past.

Propeller care and maintenance

Propellers are complicated and repairs should be done by specialists but owners can carry out checks and some routine maintenance themselves when the boat is in the boatyard. A propeller is critical to a boat’s performance, fuel consumption and ride, so it makes sense to keep a propeller in good working order.

Top Ten Tips For Learning The ColRegs Boating Rules Of The Road

Colregs Boating Rules Of The Road Skippers struggle to learn and remember the ColRegs Yachtmaster and Day Skipper course...

Understanding marine sealants & adhesives

Sealants, adhesives and adhesive sealantsThere is a bewildering variety of sealants, adhesives and even adhesive sealants available for...

Common marine electrical problems

Most problems with marine electrical systems arise from four possible sources, a lack of maintenance, a poor standard of initial installation, insufficient battery capacity, or ineffective charging systems.
Water ingress is a frequent issue – salt water can corrode contacts very quickly. If connections are not scrupulously clean – or are loose – resistance will be increased, resulting in progressively reduced power.

Essential Yachting + Power Boat Safety Briefing

Yachting Safety Briefing   Down below Lifejackets and harnesses - fitting, when to wear, clipping on Gas - risks,...

Safety Briefings – leave nothing to chance

Before giving your crew a safety briefing, it is worth considering the specific circumstances of the planned trip, the...

The VHF DSC Radio jargon buster

AIS - Automatic Identification System This system is used by shipping. It allows another vessel or coast station to use...

Essential Knots: Sheet bend

Essential Knots: Sheet bend Use: Joining two ropes together. A sheet bend is particularly useful for joining two ropes of different...

Sail boat rig checks – Part 2

In part two of Sail boat rig checks we run through some useful rig maintenance tips and then finish with a brief look at what a professional rig check involves.

Marine toilets – care and maintenance

There are a number of different types of marine toilet, or heads. They fall into one of three categories – manual, electric and vacuum, the most common being the manual, hand pumped type. These have double acting piston-pumps which both discharge the waste and flush the toilet with sea water.

Sail boat rig checks – Part 1

Sailing boat rigs need to be checked regularly to reduce the risk of rig failure at sea. In part one of Sail boat rig checks we run through a series of useful checks that owners and skippers can carry out.

Boat Improvements

My Boat - practical improvements Author - Mike Rossiter Most boat owners who have had their craft for any length of time will have made what they...

Common medical emergencies at sea

A medical emergency aboard a boat at sea requires immediate attention to ensure the safety of the casualty and the crew in general. The skipper needs to know which crew members, if any, have had medical training or have a first aid qualification. All boats should carry first aid handbooks to help an untrained crew cope with a medical emergency.

Avoiding collisions at sea – how to stay safe on the water

Boats have many blind spots, including the headsails of sailing boats. Always keep a lookout, stay safe and remember that...

Getting a tow for your sail or power boat at sea or on inland waterways

FREE tips from the Safe Skipper App for iPhone/iPad/Android: Getting a tow for your sail or power boat Plan how to secure a...