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.

Pleasure craft safety equipment recommendations

Safety equipment is an important part of boat preparation and it is advisable for all pleasure craft skippers to check their vessel is...

Boating emergency – how to broadcast a MAYDAY emergency call

How to broadcast a MAYDAY emergency call   How to broadcast a MAYDAY emergency call if a vessel or person is in grave...

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.

Rudders and steering systems – Part 3

In the third of our three blog articles on rudders and steering systems, we look at how to replace rudder bearings and repair a water-saturated core.

The Boatyard Book – a boat owner’s guide to yacht maintenance, repair and refitting

The Boatyard Book is a fully illustrated 224 page practical reference manual that provides advice for boat owners on planning and carrying out annual maintenance, repairs, upgrades and refits of sailing yachts and motorboats, up to 20 metres in length.

Repairing chips and dings in gelcoat

The gelcoat topsides of a GRP boat can be pampered and restored to their former glory relatively easily when it is ashore. Gelcoat is only a very thin outer layer of the hull, often less than 1mm thick, so you should avoid cleaning it with highly abrasive cleaners, or an-ything that could potentially damage its surface.

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...

Boat electrics inspection checklist

With the boat ashore, here are some recommendations for carrying out a boat owner electrics inspection. Safety is always paramount so remember to do the checks with the batteries off. Wearing a head torch helps, make notes as you go and only tackle a repair if you are 100% sure you know what you are doing:

How to trim a genoa sail

I recently had two new sails made by Sanders Sails, based in Lymington UK. The first to arrive was the new genoa and it took me a little while to get to know it and learn how to adjust it correctly. Here is an aide memoire for getting to know how to trim a genoa so that it will deliver the best performance.

Sail care and maintenance – Part 2

At the end of the sailing season sails should be washed and inspected carefully for damage, including small tears, stitching failure, ultraviolet damage, stains and mildew.

Electric motors and hybrid systems

In recent years there have been considerable advances with the development of electrically powered propulsion in the leisure marine sector. This includes developments with inboard and outboard electric motors, hybrid systems, lithium-ion battery technology as well as solar, wind and hydro powered generators.

Hull inspection – the annual checks

With the boat ashore for the winter it is time to do a hull inspection - the annual checks. Are there any scratches and chips in the...

Man Overboard Drill

How to respond to crew overboard under sail • Keep the MOB in sight • Tack into the heave-to position, do not adjust the...

Sterndrive maintenance

Sterndrives are a popular form of propulsion in the powerboat market, but require a fair amount of care and maintenance. The main factors to be aware of are salt water corrosion, lubrication and regular inspection of the bellows, the condition of which is vital to prevent water from entering into the hull.

Antifouling for leisure boats – Part 1

Boats that are kept afloat can very quickly become a home for small marine organisms such as barnacles, weed and slime. Applying an antifouling paint to your hull is necessary to protect it from these micro-organisms, as a fouled hull can cause problems and will slow down a boat’s maximum speed considerably if left unchecked.

Boating Rules of the Road – International ColRegs

    International ColRegs Rule 7: Risk of Collision Anyone who is responsible for a vessel at sea, from the...

Marine diesel exhaust checks

You should inspect the exhaust system for corrosion damage regularly, especially around the injection bend. If you have noticed the engine exhaust smoking a lot during the sailing season this can also indicate a number of potential problems.

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...

Essential Boat Spares for Safety

  Boats Spares Tool kit What you carry in the boats tool kit will be useful for many boat repairs, but you might want...

Medical Emergency at Sea

How to deal with a medical emergency afloat   If you are planning a boating trip, it is important to have at least one...

Antifouling for leisure boats – Part 4

Applying antifouling. Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.

Diesel engine winterisation

An inactive boat engine needs to be protected from corrosion during the winter, caused by the rising humidity levels through the cold months and the salty coastal air. This applies whether the boat is left afloat or hauled out over the winter. Read here about the two important stages of winterisaton for a diesel boat engine.

Boat maintenance below decks

While most interior maintenance work can be done when a boat is afloat, some jobs such as servicing the seacocks have to be done ashore. It makes sense to do any major interior repairs and improvements with the boat hauled out in the boatyard.


Liferafts should be stowed where they are ready for immediate launching. All crew should know the location of the liferaft and know how to launch, inflate and board it. They should also know what equipment it contains.

An explanation of the IALA maritime buoyage systems – IALA A and IALA B

What are the differences between the two IALA buoyage systems, IALA Region A and IALA Region B, and where are they used?   As recently as the 1970s...