APITVLO XLV

6.1. The fixed pitch propeller

The propeller is the more classic way to drive a vessel, but it’s not the only one. Propellers are such a huge topic that whole books are dedicated to their study: this handbook only gives basic information. The paragraph is dedicated to “fixed pitch propellers”, which means that there are also

“controllable pitch propellers” (CPP). We’ll see them later.

6.1.1. The propeller’s structure

The propeller is built with metal: sometimes it’s stainless steel but more often it’s casted of an alloy of copper, magnesium, aluminium and other metals. In other words, bronze. The propeller is made of a hub and of a certain number of blades: motor vessels generally have three, four or five blades (see figure # 31). The hub is hollow and the cavity is conical: in the inner face there’s a slot for a steel key. The shaft holds the same slot, so that propeller and shaft are integral. Abaft the propeller there’s a nose cone, screwed to the threaded shaft end. The cone is made of zinc, which is used as sacrificial anode. In other words: sea water, which is salty, is a natural electrolytic solution.

The propeller is made of bronze, which is an alloy of copper, aluminium and magnesium: the

• Platinum;

• Gold.

The so called “nobility” of a metal stands for its major or minor aptitude to lose ions during an electrolytic passage. Gold is on top, lithium is the “poorest”. Zinc is in between. It’s therefore plain that copper would lose ions to the advantage of stainless steel, unless there’s a less noble metal to sacrifice: aka the zinc anode.

Fig. 31

6.2. The pitch

The main characteristic of a propeller is the pitch (P), aka the progress of the propeller in one revolution. Modern propellers have a progressive pitch: in other words, near the hub it’s higher than the nominal pitch while it’s lower near the blade’s tip. The nominal pitch is measured at 70% of the blade length.

6.3. The slip

The propeller works like a screw and its geometrical pitch corresponds to its progress for each revolution: in theory. Practically, the propeller is not a screw turning in a solid wood plank. It works in water: there’s therefore a difference between its geometrical, or nominal, pitch and its actual progress for each revolution. We call this difference “slip” (S), it’s a variable, influenced by many parameters such as the rate of revolutions per minute, the type of hull, the vessel’s speed, the position of the propeller, the appendages (shaft, bracket…) and more. As a rule of the thumb we can assume the slip to be roughly:

• 20% for high speed planning hulls;

• 25% for planning cruising vessels;

• 30% for displacing hulls;

• 40% for sailing boats.

6.4. The pitch calculation

It’s a wise idea to leave the final decision about the propellers characteristics to their builder: it’s a complicated matter and it’s prudent to let the last word to who knows better. Yet we need to set up a preliminary calculation, at least of pitch and diameter. Now, between the engine and the shaft line there is a gear box. It changes the rotation of the propellers from forward to aft and reduces the engine revolutions: the reduction ratio depends from the type of vessel. Let me highlight that the pulling force that drives every vehicle, including boats, it’s not just the power (Ne) but mainly the “torque” (Nm).

The torque formula is Nm = (K * Ne) / RPM where K is a parameter and RPM (Revolutions Per Minute) the revolutions, either of the engines or of the shaft, depending what we’re looking at. RPM is a numerator: therefore the higher the revolutions, the lower the torque. Torque is therefore a force, and in fact its value unit is Newton x metre: it could be defines as the aptitude of an engine to generate work.

It’s plain that a vessel having a low weight/power ratio, therefore able to achieve high speed, doesn’t need high torque values. On the contrary a tug boat, slow but powerful, needs high torque to do her work. These simple criteria shall help the choice of engines for every type of vessel.

Thereafter we shall use a reduction ratio between 1: 1.5 and 1: 2 for a planning vessel, while we shall use a higher reduction ratio (i.e. 3: 1 or 3.5: 1) for a displacing, slow motion ship.

Let’s now calculate the propellers pitch.

We start from the design speed (V): let’s figure out it’s 30 knots. Remember that a knot equals one nautical mile per hour, aka 1,852 m/h. 30 knots equals 30 * 1,852 = 55,560 metres per hour. Let’s imagine that our vessel is equipped with two marine diesel engines, with a maximum rotation speed of 2,300 RPM, and that the reduction ratio (Rr) is 2: 1.

The propellers therefore rotate at 2,300 / 2 = 1,150 RPM.

The propellers do 1,150 * 60 = 69,000 revolutions per hour.

Provided that, during that same hour, 69,000 rotations of the propellers shall cover a distance of 55,560 metres, every revolution should cover a distance of 55,560 / 69,000 = 0.805 metres. This would be the necessary pitch to run at 30 knots, were not for the slip. We must thereafter add 20% to the value above said, which becomes 0.805 * 1.20 = 0.966 which we round up to 1 metre. We can transform all this long and boring calculation in a formula: P = (V * 1,852) / [(RPM / Rr) * 60] * S that can be simplified into P =[(V * 30.8666) / (RPM / Rr] * S.

6.5. The diameter calculation

The propeller diameter is function of its design, its number of blades, of the shaft revolutions. The diameter of a four blades propeller can be gathered from the chart of figure # 32.

It correlates (right) the propeller revolution (let me highlight: not the engines revolutions but the

shaft speed) with the power (left). Draft a line joining the revolutions with the power: the reading on the middle column gives, with good approximations, the propeller diameter. Using the same data of 6.4 we join the shaft revolutions (1,150) with the power (let’s say it’s 2,300 Hp), we read a diameter value D of 1.13 metres.

The propellers of pleasure crafts might have five blades: in this case we can reduce the diameter about 10%.

Fig. 32

6.6. The cavitation

Every point of the blade surface rotates with the same angular velocity, but it’s plain that the blade tip has a linear speed much higher than it has near the hub. The local pressure drops as the blade tip cuts through the water at high speed: the drop of the pressure below the vapour pressure produces bubbles of vapour. These cavities quickly explode, generating noise, vibrations, local corrosion of the blade and a reduction of the propeller’s performance. The theoretical calculation concerning cavitation is quite complex and we shall leave it to experts: knowing the basics is enough for our purpose. The cavitation is influenced by the propeller diameter: the larger the propeller, the higher the linear speed of its tip; by the number of revolutions per minute: the higher, the worse; by the blade design, by the shaft angle, by the draft of the propeller, by the appendages in front of the propeller and by tens other parameters.

6.7. The As/Ad ratio

There’s another parameter which influences the propeller performance: it’s the ratio between the expanded surface of the propeller (As) and the disc surface (Ad), meaning the circle which inscribes the propeller. Figure # 33 shows how As might be considerably higher than Ad (drawing on the left),

where each blade “covers the” other.

Fig. 33

6.8. The clearance

The propeller tip should not be too near to the hull surface. There is a laminar water flow below a running hull: the water in this area has a different speed compared to the surrounding, still water. In case the propellers blades cross this laminar flow they get a stroke which, multiplied by the number of blades and by the revolutions per minute, builds up a strong vibration, which might shake the whole vessel. Besides, the propellers lose efficiency. Thereafter, there must be a space, or

“clearance”, between the blade tip and the hull. As general rule, this span must equal 15-20% of the propeller diameter D. The higher the vessel’s speed, the greater the clearance must be.

6.9. The shaft line

The propeller is connected to the gear box by means of a cylindrical shaft, made of stainless steel.

Both ends of the shaft are threaded: the aft end is bolted to the propeller, while the forward end is bolted to the coupling flange, which in turn is bolted to the gear box flange (figure # 34), and also here there’s a slot with a key. There is a stuffing box where the shaft crosses the hull: it allows the shaft rotation and is tight to the sea water. The shaft needs at least one support, called “bracket”: in case it’s very long the brackets could be two or even three. The bracket (built either in bronze, steel or light alloy) is bolted to the hull bottom. Its lower end is a cylinder, inside which there is a rubber bushing, which holds the shaft. The upper face of the bracket is shaped like the hull (see figure #35).

The span between the aft end of the bracket and the forward face of the propeller’s hub should not be more than 50% of the shaft diameter: this avoids vibrations. The shaft diameter is function of the power, or to say better, of the twisting moment (or torque) and depends upon the mechanical

characteristics of the steel, namely its torsion breaking load. The shaft line is seldom horizontal: its angle depends from many parameters, such as the engine dimensions, the propeller diameter, the hull shape. Due to the shaft angle, the propeller pulling force splits into two vectors: one horizontal and forward, the other vertical upwards. The larger the angle, the greater the loss of driving forces:

besides, a too large angle is a cause of cavitation. The maximum allowed angle is therefore 12°. The horizontal distance between the propellers should be at least two diameters: this shall avoid unwanted interferences. Please remind that all vessels, during a quick sharp turn, list on the same side of the turn. This brings one of the propellers near the water surface: in case it’s too far from the centreline it might cavitate.

6.10. The stern tunnels

Some vessels can be equipped with stern tunnels, near the aft end of the hull (see figure # 36).

Tunnels reduce the draft, increase the propellers efficiency and lower the resistance. Practical tests show that tunnels allow roughly 10% speed increase.

Fig. 34

Fig. 35

Fig. 36

6.11. The controllable pitch propeller

Controllable pitch propellers (CPP) are mainly used on workboats. The blades turn around a pin fixed to the hub and the pitch can be positive (forward drive) or negative (backwards). CPP are very complicated pieces of machinery and are seldom used on pleasure crafts.

6.12. The jet propulsion

The “jet” is kind of a high pressure pump: the engine pulls a rotary pump, which suckles water from below the hull and pushes it backwards through a stern drive. It’s one of the more efficient propulsion systems. There are no rudders or gear boxes: on the stern drive there’s a pivoting shell which diverts the water flow, or bends downwards to re-direct the gush forward, to pull the vessel back. It’s quite an expensive system and easily suckles debris when it operates in shallow waters.

Also manoeuvring in restricted water, such as in a harbour, isn’t easy: the crew needs to run the engines lively to handle the vessel. This moves huge amounts of water, builds waves, and other sailors might be very unhappy. See figure # 37.

Fig. 37

Chapter 7 Rudder

7.1. The rudder effect

Examining the rudder effect might sound strange: it’s plain what it does. It steers the vessel. Yet one thing might not be carefully considered: the bow doesn’t turn, when a ship hauls off. It’s the stern that moves aside under the rudder effect. The ship then changes course, yet you’d better remember this while manoeuvring in restricted waters. The rudder is an appendix which increases the resistance:

not much when it’s centred, quite sensibly while it turns to steer the vessel. The rudder use reduces the vessel’s speed. Finally, at high speed, the rudder raises the stern and the vessel lists in the same direction of the turn.

7.2. A design guideline

The rudder surface must be proportionate to the type, dimension and speed of the ship. Giving a general rule is therefore impossible, even if there are empirical and debatable formulas. Experience and good sense suggest that the rudder surface of a displacing ship should equal 2.5 to 5% of the projection of the immersed hull, while for a high speed planning vessel an area of 1 to 2.5% should be enough. The rudders must not be installed right abaft the shafts, but need to be misaligned, with enough offset from the shaft line to allow its disassembly. There should be enough space between the propeller and the rudder to allow the dismantling of the propeller. The rudder stock dimension depends from the material it’s made of, from the rudder dimension, from the number of stands. The vessel’s structure must be well reinforced in the neighbourhoods of the rudder stock: it’s a particularly delicate and stressed area. Part of the rudder blade is ahead of its shaft: it reduces the work load on the rudder machine and it’s called “compensation”. It’s dotted in figure # 38.

Fig. 38

7.3. The rudder machine

The rudder movement is either mechanic or hydraulic, generally backed up by an electric motor.

The rudder machine (see figure # 39) must have a by-pass position, to allow the use of an emergency tiller in case of failure of the main system. The automatic pilot works together with the rudder machine and grants a constant course.

Fig. 39

Chapter 8