Propeller Questions and Answers

Rudder Questions and Answers

How is a propeller defined?

The general notation for a propeller size is in the form of: Diameter x Pitch x No. Blades : Blade Area % (DAR).

E.g. a 28 x 30 x 5:92 propeller has a 28” diameter and 30” pitch, with 5 blades and a DAR of 92%.

What is the maximum diameter I can fit?

Teignbridge Propellers recommend that there should be a minimum clearance between the propeller and the hull of 15% of the propeller diameter.

What is pitch?

Pitch is the theoretical distance a propeller would move forward in one revolution if it were moving through a solid. A 30” pitch propeller would theoretically move forwards 30” in one revolution.

What is Slip?

When rotating in a fluid a propeller is subject to slip. Slip causes the difference between the geometric pitch and the actual distance travelled in one rotation.

What is DAR:

DAR stands for Disc Area Ratio. The DAR is the ratio of the total area of the blades to the area of a circle the same diameter. A DAR of 92 means that the total blade area is 92% of the area of the equivalent circle. Large blade areas are achieved with overlapping blades.

The above propeller has a DAR of 92%.

The total area of the circle is
PI x (Diameter/2)2 = 3.141 x 3002 = 0.283m2

The total blade area would be:
PI x (Diameter/2)2 x DAR= 3.141 x 3002 x 0.92 = 0.26m2.

This produces a DAR of 0.260/0.283 = 0.92

What determines the number of blades?

The number of blades is chosen to suit both the required DAR and the vibration requirements. Theoretically a 1 blade propeller would be the most efficient propeller but this is not practicle. As the blade number increases vibration decreases but in theory the efficiency decreases.

What is rake?

Rake is the angle of the propeller blades relative to the hub (viewed side on). If the face of the blade is perpendicular to the hub then the propeller has zero rake. If the blades angle aft the propeller has positive rake. If the blades angle forward the propeller has negative rake

The diagram above has positive (aft) rake.

What is skew?

A blade that is radially symmetrical is said to have zero skew. A blade that is swept back from the direction of rotation is said to be skewed.

As a blade rotates the skew ensures that the radial sections of the blade do not pass through the same section together, but their entry is delayed slightly (from hub to tip) due to the skew of the blade. A skewed blade form can help reduce vibration.

Is my propeller left or right handed?

If the propeller is still fixed to the boat: When viewed from the aft a right handed propeller will rotate clockwise. A left handed propeller will rotate anti clockwise.

If the propeller has been removed:
Place the propeller on a surface ensuring the large end of the bore (diameter 1) is on the surface.
Look at the right hand blade.
If the blade slopes down towards you it is a right handed propeller. If the blade slopes away from you it is a left handed propeller.

How do I measure my propeller taper?

There are three common propeller tapers – 1:10, 1:12 and 1:16.

To calculate the taper of the above boss divide the boss length by the difference between dimension 1 and dimension 2.
E.g. for a boss with a length of 150mm, dimension 1 of 50mm and dimension 2 of 40.63mm: 150/(50–40.63) = 16.01. Therefore the taper is a 1:16.

What propeller taper should I choose?

If your taper has not been set it is best to use a 1:16 taper for high performance applications. This will allow the blade section lengths (chord lengths) towards the root (Near hub) to be as long as possible. A longer chord length allows us to use a thinner section which reduces the risk of cavitation.

A 1:16 taper also has the highest contact area between propeller boss and shaft.

What is bore relief?

Bore relief is the provision of area within the propeller hub machined out to a larger diameter as shown in the diagram below. It has the effect of increasing surface pressure between the propeller hub and shaft for a given fitting force. This will reduce the likelihood of frettage occuring between the hub and shaft. It can also make the removal of the propeller easier.

It is important that the length of the bore relief is calculated accurately to ensure that an adequate contact area (between the boss and the shaft) remains.

How does a propeller work?

The mechanism for generating thrust from a propeller blade is the same as that for generating a lift force on an aircraft wing. Motion of fluid over a blade establishes a pressure difference between the two sides i.e. fluid on one side is moving more rapidly than the other. The integral of the pressure differential over the area of the blade gives a force acting on the fluid – Thrust on the vessel and torque necessary to rotate the propeller.

How do you calculate the propeller for my boat?

If you complete our enquiry form we will be able to calculate your required propeller. Teignbridge Propellers have several propeller sizing programs suitable for the various types of propellers we supply. Boat builders should also supply a lines plan. A propeller sizing can only be as good as the information we are supplied. It is essential that the information you provide is as accurate as possible. An incorrect gear ratio or an over optimistic top speed can have a dramatic effect on the accuracy of the propeller sizing.

What is cavitation?

As a propeller blade rotates a pressure distribution is created.

The magnitudes of this pressure distribution are dependent of several factors including lift coefficient (for required thrust), section shape and section thickness.

On the back (suction) face of the blade an area of low pressure is formed. If the peaks of the low (suction) pressure falls to the vapour pressure of the liquid then vapour filled cavities are formed. The forming of these cavities is called cavitation. The effects of cavitation include:

Erosion due to the collapse of cavitation bubbles as they move into areas of higher pressure.This is also know as cavitation erosion.
Vibration
Effective profile of the blade is changed due to the disturbed flow along the surface. This can cause thrust and torque reductions and a decrease in efficiency.