ShipShaft Marine Alignment and Mechanical

31/10/2025

Let’s talk marine propulsion drivetrains.

Put simply, the drivetrain of a vessel is the collection of components that deliver, or support the delivery of, the power developed by the engine to the propeller.

Generally, each component has a role in harnessing or managing the resultant dynamic (moving) forces created through the delivery of that power. Some are ‘support’ components – they don’t necessarily directly contribute to power transmission, but they make the power transmission possible.

Whether they are directly or indirectly part of the power transmission, they are all subject to wear or damage due to poor alignment.

Common Components.

Propeller shaft.

The most obvious component in the drivetrain is the propeller shaft.

In most private pleasure craft, these are generally a single shaft with a positive drive feature (such as a keyway or spline) on each end of the shaft where the propeller and the drive hub which connects the assembly to the gearbox or engine output shaft are fitted.

In larger commercial craft or in some compact arrangements, they can be designed as a collection of linear or offset shafts. These types of arrangements add complexity to the power transfer and require careful installation and maintenance practices.

The role of the propeller shaft is self-evident – to transmit the rotational motion of the engine/ gearbox output shaft, over a distance, to the propeller.

During power transmission, the shaft is subject to torsional, linear thrust and bending forces. Due to the dynamic nature of the forces being applied, these are cyclical in nature and contribute to material fatigue over time which can result in distortion (bending) or localised cracking.

A good portion of the design of a propeller shaft in consideration of these forces is in material selection and cross-sectional area of the shaft, however there are practical limitations in material selection.

In order to overcome these limitations and manage these forces, other components are required. These generally fall into two categories; support and force neutralisation (or dampening).

Bearings.

Some bearings are employed radially to support the shaft along its length at certain load points identified by the drivetrain design engineer. They may be plain bearings or bushings, roller-type bearings.

They can also be used to arrest or dampen axial thrust along the drive train created from the resultant forces of the vessel being propelled through the water.

Cutless Bearings.

The Cutless bearing is a submerged bearing which is lubricated by the body of water on which the vessel is operating through radial grooves running longitudinally with the propeller shaft.

They are generally made of rubber with a metallic or non-metallic (composite fibre) casing. Depending on the support application, they may be a straight casing or be fl**ge mounted.

Like other bearings, the Cutless bearing supports the propeller shaft. Its’ rubber construction assists with vibration dampening.
In the drivetrain, they are is generally located immediately before the propeller and is housed by a support bracket and in the stern tube.

Support Brackets.

The most common support brackets are known as ‘P’-Type, ‘A’ or ‘V’-Type (depending on your orientation) and ‘I’-Type (also known as ‘Glassed-In’).

‘P’-Type brackets are a single armed bracket which have a ‘platform’ (hence the ‘P’) fl**ge mounting system which is manufactured to suit the angle of the hull and line of the propeller shaft. These are the type most common support, particularly on non-commercial vessels.

‘A’ or ‘V’-Type are similar to the ‘P’-Type but are manufactured with two arms for greater rigidity.

‘I’-Type are simpler single leg brackets designed to be bonded directly into a GRP (fibreglass) hull and offer flexibility for adjustment during installation. Most common on sailing vessels due to their lower cost of manufacture.

Stern Tube.

Stern Tubes are an integral part of the construction of the hull which pe*****te from water to bilge through which the propeller shaft passes.

On the bilge end of the stern tube, a sealing arrangement called a ‘stern seal’ prevents water from passing through the stern tube into the bilge.

In some ships designs, the stern tube is also used to house the cutless bearing where there is minimal projection of the propeller shaft into the water.

Stern Seals.

Stern seals are generally of two types; Gland or Stuffing Box and Mechanical.

Gland seals are inexpensive and reliable sealing arrangements which are easily maintained and replaced. The packing material, generally a graphite or PTFE infused rope, is a wearing item and needs monitoring for regular adjustment.

Correctly adjusted, the gland should pass a small amount of water (2-3 drips per minute) under operation to lubricate and cool the interface between packers and shaft.

Mechanical seals are manufactured from two part which seal on the shaft and the stern tube. The part mounted on the shaft has a spring loaded graphite ring which runs on a finely polished steel ring mounted on the stationary side. The running interface between these faces forms the seal.

These type of seals also need cooling and are generally cooled by water in either enclosed or open loops. They are more complex and expensive but generally need little or no maintenance.

Why Alignment Is Important.

All of the components in a drivetrain are subject to some form of loading or friction. These are minimised by design through proper selection, installation and maintenance.

Whilst all components in a mechanical system are subject to wear, due to the nature of their service some are considered to be ‘wearing components’ with a finite ‘useful life’.

Maintaining correct alignment is the major contributing action to allowing components to reach their designed useful life.

Component manufacturers go to considerable lengths to understand and be able to predict the ‘life’ of these items under a given set of circumstances and have very specific data on this through observation and failure analysis. Specifically, these are load (forces applied during operation), speed (in revolutions per minute), lubrication and alignment.

Load and speed are somewhat redundant as they are taken into consideration at design with bearing selection being derived from the intended operational parameters. Lubrication and alignment are installation and maintenance factors that need to be monitored in an ongoing basis to help ensure reasonable service life.

The type and method of lubrication is also considered at the design phase. Any motion between two moving objects creates friction which creates heat and wear. The lubricant and application method chosen during design assumes that the component is operating within its specific alignment parameters.

If the alignment is installed and maintained within the manufacturers specifications and the lubrication system is maintained, you can expect to assume reasonable service life.
If the alignment is poor, the forces the components are subject to will be amplified which may result in critical loads being applied creating higher running temperatures which the selected lubricant may not be suitable for.

This, along with the irregular mechanical loading, generally results in premature failure. If this failure is ‘catastrophic’ (seizure/ collapse/ burnout), there is often secondary damage to components that are not usually considered to be ‘wearing’ resulting in significant downtime and expense to rectify.

Some early signs of misalignment are:
• Vibration
• Noise or harmonics
• Clunking or banging on thrust reversal
• Overheating
• Power loss
• Inability to reach operating speeds
• Decreased fuel economy

31/10/2025

Cutless or Cutlass?

'I continue to hear people use the wrong words to describe something nautical. In this case, I refer to whether the bearing that cradles a propeller shaft is called a “cutless” or “cutlass” bearing. Most direct drive boats have one or two of these bearings.

Several years ago, I reached out to a number of professionals in the marine industry, who could neither agree or offer definitive proof one way or the other. But eventually I found the true origin and history of the term. And its roots are not even nautical.

After speaking with Dave Gerr, author of The Propeller Book, I called Mike Schonauer, who was at the time VP of Sales & Marketing for Duramax Marine of Hiram, Ohio. Duramax is a pioneer in marine bearings.

Mike was happy to explain the history of the bearing, in which his company is part. In the 1920s, Charles Sherwood, an engineer responsible for maintaining vertical lift water pumps used in mining operations. The traditional bearing material used in these pumps was lignum vitae, the world’s heaviest and densest hardwood. Given the abrasive nature of this water, sludge, and mud pump application, the hardwood bearings needed frequent replacement, a regular chore for Mr. Sherwood.

Preparing to again replace a pump’s bearings, he found they were out of lignum vitae. A clever engineer, he searched his shop for alternatives and found a sheet of rubber. Having nothing to lose, he decided to cut some temporary bearings for the pump from the rubber material, getting the pump back online. He kept an eye on the pump’s operation over the next several weeks while he restocked his hardwood supply.

He was intrigued to find the pump shaft was doing nicely with the rubber bearings, much more so than with lignum vitae bearings. He noticed the shaft was not being grooved or worn by the rubber bearing, even in such an abrasive environment. The rubber bearing had, in fact, “cut less” into the shaft surface than the hardwood bearings. A year or two later, the bearing was patented by Charles Sherwood in conjunction with Lucian Q. Moffit of Akron, Ohio, who coined the term “cutless” bearing.

Mr. Moffit eventually sold out to B.F. Goodrich, where in the 1950s, the bearing division came up with the marketing idea of stamping a sword symbol on its cutless bearing products as a form of branding. The confusion of the cutless bearing with the symbol of the cutlass was born.

Johnson Rubber, also started in the 1920s, in which Duramax Marine was a division, made its own version of bearings and stuffing boxes. Duramax Marine became its own corporate entity in later years, and eventually purchased the bearing division from B.F. Goodrich.

Today Duramax Marine makes all sorts of bearings, cutless bearings included, for hydroelectric turbines, agricultural vertical pumps, and many other applications outside of strictly marine use. The design of the cutless bearing allows it to handle much greater loads than roller bearings, due to the lubrication features of the bearing design.

And to further distinguish the bewilderment between a cutless bearing and the cutlass sword, Mike Schonauer offers that today the proper term for the bearing is a “water lubricated, hydrodynamic standard rubber sleeve bearing.”

You learn something every day!'

~Bill Parlatore, Jan 30 2017, Power, Sale, Marine Systems
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Soft foot is an often overlooked aspect of shaft alignment. Your driveline should always be as close to neutrally loaded as possible.

Learn about the effects and importance of measuring and correcting Soft Foot when performing shaft alignment. ⏬Download white paper: https://ludeca.com/downl...

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Learn the fundamentals of precision machinery alignment. 📝 Learn the basics with us! https://ludeca.com/training/shaft-alignment/

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With over 30 years experience in marine maintenance and laser shaft alignment, I can help you with solving vibration and efficiency in your marine drive and associated systems.

05/09/2025

Your boats engine and propulsion system are the culmination of design through many hundreds of thousands of hours of engineering, experimentation and continuous improvement. Regardless of its age, the mechanical principles that had been selected by the designer of your system would likely have been cutting edge at its time.
Under these principles, all of the mechanical components in your drive train were designed to run loaded in very specific ways, under specific conditions intended to maximise useful life and provide reliable service over many years.
Deviations in these operating conditions, even slight ones, will inevitably lead to poor performance such as increased fuel consumption, increased wear in moving parts, vibration and unusual noise. These changes in your drive train can often be difficult to notice without regular monitoring and adjustment.
Being incremental in nature, this wear can easily go undiscovered resulting in higher running costs or expensive repairs and ultimately catastrophic failure.
We can help you to understand the condition of your drive train and provide recommendations for early corrective action to avoid costly maintenance or mechanical breakdown at sea.
Alignment services are seen to be expensive and time consuming, however you may be surprised at how inexpensive and efficient an inspection is, often around $300 per drive train for visual inspection, vibration checks and laser alignment report.
In addition to these services, we offer a complete suite of preventive and corrective maintenance of mechanical, low voltage electrical and ancillary marine systems.
Leave a message or drop us a line to see how we can help you. [email protected]