Smooth Runnings: Keeping Vibration to a Minimum

While you may understand the basic working functions of the diesel engine in your boat, have you ever taken the time to appreciate how all that power gets into the water? The thrust is smoothly and somewhat vibration-free as it is transferred to the propellers. Much focus goes into reducing and containing noise and vibration within a vessel’s engine room so those same fundamentals should be applied to what’s around the engine. Reducing vibration comes in a variety of ways, with the most common being engine mounts and flexible driveshaft couplings.

It’s rare that there’s much space available a hull or engine room to configure a bulky driveshaft. Any extra length or height will encroach on available cabin or service area space. Engines are often positioned to balance out the weight of other equipment so in some instances a longer series shaft may be needed with support bearings fitted at the midpoint.

Dealing with Thrust

By combining thrust bearings with a marine Cardan or CV shaft, you get the solution for conventional shaft-driven vessels, reducing noise and vibration and simplifying engine installation and shaft alignment. Thrust vibration is absorbed through rubber pads mounted parallel to the propeller shaft. Rubber performs better under compression. By incorporating these units to absorb all the thrust in the design, softer engine-mount material can be used to further enhance the reduction of noise and vibration. Without the absorption of thrust at the propeller end of the drive line, you risk the transfer of thrust and shock to the gearbox that can be costly to repair if damage occurs.

The ability of universal and CV joints to operate at a considerably higher angle than other coupling designs compensates for hull and mount movement over time. This reduces vibration and load on the propeller shaft seal. Without an independent thrust bearing, thrust is transferred through the gearbox to the engine mounts. 

There are various flexible coupling solutions that sit between the shaft and the gearbox that offer a torsional dampening function though it does not manage misalignment like flexible shaft options. Such a flexible coupling combined with a correctly selected mounting system dampens the torsional vibration coming from the engine and gearbox to the hull.

Vibration Solutions

Turret Driveline offers driveshafts and cv shafts along with its own Turret thrust bearings to ensure that power is transferred quietly with minimal vibration. The drivetrain is isolated from the engine with thrust bearings and universal joints taking the load off the engine mounts and transmitting the thrust directly to the hull.

Python-Drive is another option that lets the propulsion system push the boat, not the engine. This separates the propeller noise and vibrations from the engine while eliminating the need for perfect shaft alignment. Python-Drive Systems are available for engines from 10 hp to more than 1,000 hp. Maintaining perfect alignment of the gearbox to the propeller shaft is no longer necessary with the use of a Python-Drive. Even with unequal angles, the constant-velocity driveshaft guarantees a constant speed of the propeller shaft and reduces wear of other parts of the drive line. An internal taper clamp shaft connection makes fitting the shaft to the thrust bearing a simple task, plus the thrust bearings and CV shafts can be used as standalone units to fit a given application.

Sigma Drive is manufactured by Bruintons Propeller in the United Kingdom. It is another thrust bearing and CV driveshaft system available for applications up to 1,300 hp. The Sigma Drive CV can also be used independently as a thrust coupling but combined in pairs with the Sigma thrust bearing offers vibration and noise reduction.

Aquadrive is one of the world›s most recognized manufacturers of anti-vibration systems, offering engine mounts, CV shaft and thrust bearing systems. The variable-length CV shaft includes two true plunging CV joints that work independently at any angle. This eliminates the need for accurate engine alignment during initial installation or subsequent use. The rolling action of the balls within the CV joints absorbs all axial and radial loads, permitting the use of soft engine mounts and reducing wear in connected bearings. A range of pre- machined adapter kits allows coupling to almost any marine gearbox.

Craftsman Vector Drive is available for shaft torques up to 1,100 Nm and thrust forces up to 11.5 kn. It features a Constant velocity (CV) shaft available in different lengths, permissible angles up to 8° per side of the CV shaft. It has an easily accessible clamping hub that is self-centering and self-adjusting.

Vetus Drive has a combination of a self-aligning thrust bearing and a double-acting CV joint. Propeller thrust is absorbed by the integrated thrust bearing allowing the engine to be set up on softer mountings, resulting in lower vibration and transmitted noise. The VDR is made of stainless, black passivated steel and high-performance rubber and is suitable for maximum thrust up to 24 000 N.

Poseidon Orion flexible couplings are developed to operate with or without propeller thrust and can be installed as a single or twin. The Heracles thrust blocks are Poseidon’s latest innovation and come complete with flexible mounts. Orion thrust blocks are equipped with a twin spherical bearing set and offer a regular and a short building version. Orion thrust blocks can be installed either rigid or flexible.

Flexible Couplings

Poly Flex flexible disc transmission couplings provide a damper between the gearbox and propeller shaft to isolate torsional vibration from the engine and gearbox. It also reduces the shock of forward and reverse gear changes. The couplings work with the engine mounts by providing the softest element in the shaft assembly to improve performance. All metal components are plated in SA5 Cobalt Zinc for maximum corrosion resistance and durability.

The molded components are manufactured from Polénite a special range of engineering heat-cured polymer alloys. Unlike most mounts, the polymer is not bonded to the steel, so worn Poly Flex mounting systems can be rebuilt. All the parts are designed, engineered and manufactured by Poly Flex in Brisbane, Australia.

Isoflex also offer a gearbox coupling similar to the Poly Flex unit. Designed as a “sacrificial link” to fail before gearbox damage occurs, they are intended to cover a large percentage of Twin Disc and ZF gearboxes, plus many other units on the market.

Vetus Marine Combiflex coupling has been designed to ensure optimal damping of torsional vibrations created by cycle irregularities, especially at low rpm. The flexible rubber element ensures low-noise and vibration-free transmission without backlash between the engine and the propeller shaft. The Combiflex coupling is secured against shearing axially and radially, thus ensuring safe transmission under all circumstances.

R&D Marine offers a wide range of flexible shaft couplings that consist of a contoured flexible disc molded in resilient polyurethane. The contoured disc gives clearance for the bolt heads. It can flex freely to take up any temporary misalignment of the engine and shaft due to flexing of the boat structure or the engine moving on its rubber vibration isolation mountings. In the unlikely event of a disc failure, the steel straps make the coupling fail safe and ensure that drive is maintained in forward and reverse.

Resilient Mounts

Let’s move forward to the engine end of things. Resilient mountings are necessary to reduce transmission vibration. Engine mounts play a crucial role in preventing vibration from passing to the boat. Just about every engine already comes with the factory approved mounts, but there are several aftermarket brands available that are explicitly designed to reduce engine vibration.

If your engine mounts are more than five or six years old or have been contaminated with seawater, engine oil or coolant, change them. Even if the mounts look perfect, the tough elastomer (rubber-like) material in the engine mount hardens over time and ceases to isolate the boat from the engine’s vibrations. Seawater rusts the metal parts of the mount and can cause delamination of the elastomer and metal parts. Rust also weakens the mounting bolts and can cause bolt failure. Engine oil and coolant deteriorates the elastomer and hastens delamination. Poorly aligned engine mounts can also cause bolt failure. If you have noticed screws backing out on your boat or if your oil-pressure sensor has failed recently, the root cause may be increased vibration due to bad engine mounts or a misaligned shaft.

It is also important that the engine mounts be properly loaded so they are equally carrying the engine’s weight. If one mount is not carrying the same load as its mate then there will be vibration problems.

Poly Flex mounting systems are designed as true marine propulsion engine mounts with sufficient vertical deflection to obtain proven vibration isolation. They have controlled minimum thrust and lateral deflection under propulsion load and inertia due to sea conditions. They are designed with a failsafe that in the event of a capsize keeps the engine from breaking away from its bearers.

Poly Flex has more than 200 models suitable from 10 hp to 2,500 hp and is continually expanding its range. All couplings as well as mounts are available with DNV Type Approval and are molded from heat-cured polymer alloys that are oil and fuel resistant.

Isoflex mounts are for marine applications and contain no rubber components that can suffer from degradation due to oils, fuels and water. Isoflex’s engineering polymer materials make for long-lasting use and durability. They are available as a fixed bolt down or adjustable mounting system, the latter being ideal for retrofitting. Aquadrive’s proven engine mounts are softer than almost any other and come with a steel hood to prevent diesel damage. They are fully captive so that the engine cannot leave its frame even if the vessel is  turned over.

R&D Marine’s engine mounts feature a fail-safe system, slotted holes in the casting to assist alignment, height adjustment to simplify alignment and all parts are zinc plated and passivated.

Vetus Marine has a range of flexible mounts that contain a special rubber compound with excellent vibration damping properties. They range from the K40 that has a loading of 40 kg (88.18 lb.), through to the LMX500 suitable for marine diesel engines in the power range between 70 and 350 kW (95 – 480 HP). It has a maximum load of 500 kg (1,102.31 lbs.).

Craftsman Marine offer a range of flexible mounts with 45 kg to 150 kg (99.21 lb. to 330.69 lb.) load capacity. The CM type 9 and 10 are extra flexible to absorb more vibrations in light boats that would otherwise be transferred to the hull.

Strut & Shaft Science

The placement of the strut itself can have a significant bearing on things too. If it is located farther away from the stern tube bearing in front of it, then a distance equal to four times the diameter of the shaft. That shaft is likely to begin to behave like a skipping rope being swung between two vigorous athletes. Similarly, allow more than one shaft diameter between the rear of your strut bearing and the front of your propeller boss and the prop becomes prone to wobble about.

This may all seem like a bit of a juggling act, but if you don’t get this end of the business right to start with, no matter how light your hull, how fair its bottom or how hairy-legged the horsepower, you will be disappointed with its performance. The actual size of the prop shaft will depend on how much horsepower it is being asked to transmit and at what revs. The prop man is the best guy to establish this because there are materials of different tensile strengths to take into account. However, you first need to make sure you have your engine setup right. Using flexible driveshaft couplings and suitable engine mounts will go a long way to making sure what goes down the drive shaft is just power.