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  • Pacific Driveline Ltd.

What REALLY Pushes My Boat?

I am sure that the vast majority of mariners understand that the propeller is the item that converts the torque supplied by the “Elephant in the room” – the engine, into thrust.

However the item that transfers the thrust to your vessel is the humble engine mount (unless there is a thrust bearing in place).

After 35 years of developing “Integrated Propulsion Systems” meaning ALL driveline items are connected – ALL items have a purpose! Henley's findings are that VERY FEW surveyors, engine suppliers, mariners and marine engineers recognize the importance of the humble engine mount, until something goes wrong – then the world is full of experts!

Fixed or Floating engine / driveline systems:

In the past engines were solid mounted. They were uncomplicated, slow revving high torque beasts that in general ran smoothly in a wooden or steel vessel. With the engine shimmed or chocked into place it was relatively easy to maintain engine / gearbox output flange alignment to shaft half coupling – the biggest issue was the boat itself moving over time due to construction methods of the day.

In the 70’s and 80’s there was a change to increase applied BHP, higher revving engines, stiffer hull construction and rubber mounted engines. Each supplier vowing to offer the smoothest running “anorexic” (low fuel consumption) engine. The short term, immediate effect was less vibration, faster vessel and maybe less fuel. The longer term effects being driveline wear, shaft seal and engine mount failure, gearbox output seal leakage.

  • Supporting the weight e.g. 250kgs to 2 + tonnes of engine while maintaining the position of the engine: either static or when in a seaway.

  • Restricting the forward movement of the engine: when under thrust conditions – e.g. engine moves forward, back of engine also goes down or up depending on gearbox configuration.

  • Restricting extra forces applied by mass, thrust and rotational forces of engine to ensure that the hard fought battle by your engineer to achieve true flange to flange engine alignment when the vessel is in port is maintained when the vessel is at sea.

Points to consider!

Q: Are the standard mounts supplied with my engine configured to my particular installation? A: NO, in our experience mounts supplied with your engine are generic i.e. they are supplied with a particular brand / model of engine regardless of gearbox reduction (which results in different thrust figures) which in turn results in varying amounts of axial loads / movement. The same brand / model of engine can also be fitted into a variety of different craft that have different requirements and working conditions which is not always considered / calculated from an engine mount perspective.

Q: How important is axial movement? A: For most installations that have down angle, drop centre or vee drive gearboxes the shaft axial movement needs to be 2.5mm maximum. Working within this tolerance will ensure that the true shaft half coupling alignment to the gearbox output flange will be maintained to an acceptable tolerance under maximum applied thrust / sea state.

Q: Are Isoflex mounts generic? A: NO, Isoflex mounts are supplied on a custom basis i.e. the gearbox configuration being either in line, down angle, drop centre or vee drive, placement of the mounts, support of the bell housing, the applied mass, thrust and working conditions are taken into account.

Q: Are all engines four point mounted? A: NO, applied power and configuration may require four, five, six, or eight point mounting to achieve the required axial movement, stability while still reducing the engine vibration levels throughout the rpm range.

Q: What technical mount information should be available that is relative to my particular engine installation in my boat? A: Data sheets that provide Axial Load vs Deflection and Mass vs Deflection, Attenuation (vibration reduction) levels. Mount configuration drawing.

Q: What are Isoflex mounts made from? A: Isoflex mounts are manufactured from an engineering grade heat cured polymer alloy. Isoflex mounts and couplings electrically isolate the engine from the engine bed and drive train.

Q: How important is the actual installation of Isoflex mounts into the vessel? A: Due to their static nature the installation is critical to the life expectancy of the mount. Fabricated engine bracketing can be out of alignment to the engine bed. Soft rubber type mounts are more forgiving in this area - the down side is that they allow too much axial and horizontal deflection. It is IMPERATIVE that the top of the engine bed is parallel to the base of the engine mounting bracket in all planes to avoid any pre-load on the Isoflex mount base, stud and core material.

Q: How should we place the engine mounts on the engine and transmission? A: All of the engine mounts used in an installation should be placed on or as near as possible to the crankshaft centreline and or split to gearbox output flange centreline. If the engine needs to be raised or lowered to the hull, the engine mount to bed base must be adjusted to obtain the necessary relationship with the crankshaft centre line.

Q: Will Isoflex mounts retain my engine should the vessel be in a 360 deg roll. A: YES, the Isoflex mounts are designed to hold your engine in a fully inverted situation however you must ensure that they are attached to the engine bed's in such a manner that allows the mount to stay in place.

Q: Should I periodically check my engine mounts. A: YES, most rubber type mounts should be checked every six months for compression, deflection and degradation (due to oil / grease contamination) which in turn effect's shaft / coupling alignment. Isoflex mounts should be checked every two years for uneven compression of the cores, fastenings to engine beds and fastenings to engine brackets.

Q: Are Isoflex mounts fully resistant to diesel, petrol and normal engine lubricants. A: YES

When purchasing your next Engine ask your supplier three questions (among many others):

  1. Soft engine mounts work great in my car as there is zero thrust onto the engine, no seaway conditions to contend with. How will your mounts work in my boat?

  2. What is the applied thrust / mass of this engine you are about to sell me and are the mounts rated to contain these forces in my vessel?

  3. Will the mount system retain the engine to 2.5mm max movement in all directions and working conditions?

The answer may well enlighten you!


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