Most modern boats are full of proudly shown state-of-the-art gadgetry but could not fulfil their reason to be without an element that has not changed much in the last 240 years. The propeller.
Let’s start with a definition: a propeller is a type of fan that transmits power by converting rotational motion (turning) into thrust (pushing).
Propeller’s ancestor is the Archimedes screw, used for bailing boats and helping irrigation since 250 BC. The maritime industry had to wait until, in 1775, when Yale student and inventor David Bushnell designed and built “Turtle”, the first war submersible ever, which mission in 1776 was to attach mines to the enemy British fleet in New York Harbour. The “Turtle” propulsion was by a hand-driven, two blade bow propeller made of wood.
By 1827, Czech-Austrian inventor Josef Ressel invented a screw propeller which had multiple blades fastened around a conical base. His ship “Civetta” reached a speed of about six knots in 1829. This was the first ship successfully driven by an Archimedes screw-type propeller. From there on, the synonym “screw” was forever assigned to the noun “propeller”.
Props have come a long way, and nowadays are mostly made of an alloy containing, nickel, bronze (copper+tin) and aluminum (nibral). But there is one thing that has not changed for centuries and will never do: their vulnerability to submersed objects, from wood adrift to uncharted rocks, or the sea bottom.
Modern yachts request not only to be fast but to be extremely quiet, vibration free, able to keep an efficient cruising speed allowing a long range, and to maintain fuel consumption efficiency. Propellers are a critical part of the propulsion system needed to achieve this.
They have to be precisely shaped and balanced in order to transform the engine power in the strongest possible thrust with minimal vibration, in a similar way a tire does for a car. The two most important non-hull design factors affecting a propeller performance and smoothness are: pitch (angle of blades) and balance (homogeneous distribution of mass).
Pitch should be homogeneous in all blades and can be altered by blade deformation when hitting an underwater body like a submersed log. Even getting the prop tangled in a large fishing net or plastic sheet can deform the tips of the prop blades to a point that affects performance and vibration. Forces as small as the ones generated by sea growth on the blades alter the flow of water on them, and cause engine monitoring systems to activate alarms and the propeller performance to diminish noticeably.
Propeller balance can be affected by blade repairs, pitch adjustment, and corrosion. Sometimes it is very difficult to notice vibration if these defects are small, but when tested properly, the anomalies show up. An unbalanced propeller never improves, on the contrary: both uneven pitch and unbalance can cause fast erosion of cutlass bearings, shaft misalignment, increased fuel consumption and vibration.
Corrosion can be of three main types: galvanic, electrolytic and by cavitation. I will spare you from hi-tech jabber on this, as it does not affect the article context.
Propeller blade tips are very sensitive to exterior pressures as they are also the thinnest part . Even rotating through sand in shallow waters, often unnoticed by crew, is enough to create a deformation.
And to make the situation worse, adequate logistics are needed to repair them: in most cases the yacht must be hauled out at a boatyard (it is possible but not advisable to extract and fit back propellers with the yacht in the water), and the few places that can repair propellers, or the factories themselves, have often long delays to supply or repair, especially during the summer season when propeller accidents increase proportionally to the increased use of yachts.
Once the propeller reaches the workshop, it is visually examined to verify signs of corrosion. Bronze alloy propellers can be bend back to their original form without applying heat, by pressure (hydraulic pistons) or impact (hammering). That is the first step in propeller repair. With the blades (or what is left of them) straightened, the broken and corroded sections are refilled by welding, using alloy electrodes melted electrically in presence of an inert gas (TIG welding). The third step, if the welding area is large, is to wrap the blade in thermal mats (electrically powered) and rise the temperature to 500ºC, keeping it for at least one hour. This will release the tensions the bronze suffered while welding in one area and having normal temperature in the rest of the blade. The subsequent cooling down cannot be faster than 50ºC per hour.
After the prop is at ambient temperature the coarse grinding of the welded parts starts, followed by fine grinding.
It is now time to check visually the shape of the blades and correct by applying pressure or hitting. Repairing propellers is a craftsman’s work aided by machinery and not the other way around. The prop is fitted in the Prop Scan unit, which makes a numeric model of the blades by using a physical probe.
The result is a detailed chart reflecting each blade shape at 9 different distances from the prop hub. With this information, the blades angle (pitch) can be corrected to fabrication standard (all propellers have their diameter and pitch engraved in the hub for reference).
The exact shape of each blade respecting the others is only achievable using robotics to fabricate them. To measure pitch precision, the ISO standard 484-2 :2015 defines tolerances ranging from wide tolerance to very high accuracy, being used as reference and incorporated in the Prop Scan software to be reflected in the information graphics. The wide tolerance (less precision setting) is good enough to avoid vibration and achieve a smooth navigation.
The very high accuracy will guarantee maximum fuel savings by full efficiency of the prop thrust, total absence of propeller vibration and minimal wear of cutlass bearings at P bracket (the hull attached shaft holder next to the prop).
The process is not finished. Once trimmed to the desired level of accuracy, the prop is fitted in the shaft which has the flange connection to the engine gearbox attached. This is part of what is called the propulsion train (engine, gearbox, shaft flange connection, shaft, propeller). The set is mounted in the dynamic balance unit, connecting the flange to its driving head and supporting the shaft in two or three points depending on its length and diameter, the first next to the driving unit and the last one very close to the propeller. The DBU spins the train to approximately 500 RPM in most cases (an average rotation speed for props from 700mm to 1500mm) and measures the eccentric mass of the train at the prop and at the flange. Correction of such eccentricity is made by adding or eliminating weight at certain distances and angles in both ends, either grinding or adding material by welding or, in case of the flange, small pieces of steel.
Once the train is balanced, there is 100% assurance of no vibration. This process is very similar to a car tire balancing. The propeller itself can be balanced without shaft or connecting flange, but vibration could arise from combined eccentricity action of the last two.
If any action has been done to a blade (adding or subtracting material), the prop has to go back to the Prop Scan unit to verify the pitch. If it must be corrected, back to the DBU, and so forth until Prop Scan and DBU wished values are achieved without further intervention.
Only now is the propulsion train, or the propeller, ready to leave the workshop. When the elements are delivered, they have gone through hours of testing and repairing using different equipment and both ancient and top edge technologies. The props will immerse in the water and spin endlessly out of sight aiding the boats to sail to both nearby and far away destinations. They will be forgotten until the next accident or maintenance check. The craftsman who played the magic with the machines help will be busy with the next prop project paying attention to the priorities, always different: accuracy, fast work, price. He will be applying tons of power to bent blades, pounding the edges with copper hammers, and always, to every propeller he treats, caressing the ready blades with an open hand, giving an emotional farewell after a work well done!!
By Oscar Siches of Metalnox