Saltwater Boat Angling — November 2017

Saltwater Boat Angling 75

L

ast month, we looked at the basic dimensions of propellers, this

month, we shall go into more depth about what differences there

are and how they affect your propeller choices.

Rake is the term applied to describe how far from perpendicular,

to the hub, the blade is angled. Rake can be flat, which is a constant

angle, or progressive, which is a curved, gentle increase in the angle of rake

towards the tip of the blade. Rake provides grip in turns and gives bow lift as

well increasing the ability to operate in surfacing situations. Higher power

outboards and sterndrives will use propellers with some degree of rake,

usually around 15°, whereas very high-performance propellers can go as high

as 30° of rake at the tips.

BLADE RUNNER

The number of blades is an important consideration and will be directed by

the type of hull, overall weight and horsepower available. The most efficient

propeller has a single blade, but it is very difficult to balance, and so the

amount of vibration would be intolerable. Therefore, in practical terms, a

two-bladed is the most efficient with even balance. As blades are added,

so the efficiency is reduced due to drag, but smoothness is improved. Most

propellers are given three blades as a good compromise between cost,

efficiency and smooth running. More blades are added to surface-piercing

propellers to increase the blade area in the water and to suppress the massive

vibration caused by the difference in loading of the blades in the water

compared to those in the air. For constantly-submerged running, the number

of blades and their shape dictates how much power can be transmitted

through to the water before the propeller ‘lets go’ or loses grip. Cavitation is

the result and that can lead to damage to the propeller if uncorrected, but

poor performance will be noticed first.

CAVITATION AND VAPOURISATION

Cavitation is caused through the water boiling and vapourising on the face

of the blade due to the lowering of the pressure. At normal atmospheric

pressure, water boils at 100°C and vapourises into steam but, if you lower the

pressure, boiling and vapourisation occurs at much lower temperatures. This

is why cooling systems in engines are pressurised. Increasing the pressure

raises the temperature at which boiling occurs. The pressure is lowered by the

shape of the blade moving through the water, the faster the blade spins, the

lower the pressure becomes in the hole in the water left by the blade. When

this pressure drops to the critical level, the water on the face of the blade

boils and vapourises into steam. As this steam moves back along the blade

the pressure increases again, the steam condenses back to water, releasing

the latent heat of vapourisation as it changes state again. As the bubbles

collapse they act like miniature implosions, eroding the face of the blade

with tiny pock marks, which looks very much like spark erosion, this is called

cavitation burn.

TRANSMITTING POWER

The ability for the propeller to transmit the engine power, requires sufficient

blade area to spread the load and maintain the pressure on the blades. Most

cavitation is felt either on initial holeshot or during tight turns. The engine

revs, but poor forward acceleration occurs, to improve the holeshot, as this

acceleration is termed, more blade area is required, either by using a bigger

propeller, or by increasing the number of blades to spread the load. This

PROPELLERS EXPLAINED

STANDARD OUTBOARD

ALUMINIUM PROPELLER

cavitation phenomenon can sometimes be felt on a normally well -behaved

boat that is overloaded, the extra weight has to be accelerated and the

propeller simply doesn’t have the ability to do it as well.

Heavy boats require more blade area than lighter ones, and the propeller

will be required to turn more slowly too. As the propeller speed is reduced,

so the pressure increases and cavitation is prevented. Cavitation can also

be caused by nicks on the edge of the propeller blade, which is why it is

important to maintain your propeller in good condition. Just small nicks

caused by impact can create blade-eating cavitation, simply file the nick

smooth to prevent it.

BLADE SHAPE

The shape of the blades also changes according to the required use, the

majority of general- purpose propellers will have rounded blades, set apart

without any overlap. The higher the performance requirement, the

straighter the trailing edge becomes. Modern, high-thrust propellers can

have overlapping blades, especially on heavy displacement hulls, this is to

increase the blade area. In heavily weeded areas skewed blades are used to

prevent fouling because the blades can throw off the weed as it cuts through.

The next consideration is the material the propeller is made of. Plastic

blades can be used on low-power outboards, such as trolling motors or

small auxiliary motors, but they are not very efficient due to the flex in

the blades, they are also prone to breaking and erosion in sandy conditions.

Aluminium is probably the most popular outboard propeller material,

they are light, easy to repair and relatively cheap to buy. Stainless steel

gives increased strength and durability over the equivalent aluminium prop,

allowing the blades to be thinner for greater efficiency and fuel economy.

Stainless steel is better in saltwater, as corrosion resistance is higher,

the material also allows different blade configurations to be used which

is where the higher performance comes from, rather than the material

in itself.

Next month we’ll be looking at torque and transmission.

NUMBER OF BLADES AND VIBRATION

CAUSES OF CAVITATION

VARIETIES OF PROPELLER BLADES

CONVENTIAL OR

ROUND-EARED BLADE CLEAVER BLADE WEEDLESS BLADE

1- BLADE PROPELLER 2- BLADE PROPELLER 3- BLADE PROPELLER 4- BLADE PROPELLER 5- BLADE PROPELLER

EXCESSIVE VIBRATION VERY SLIGHT VIBRATION