Pacific MotorYachts Ltd

When compiling an equipment list for a vessel there are three main considerations we adhere to:

1.  Fit for purpose
2.  Worldwide parts or replacement availability
3.  Price

Note the order of priority. This is important. It is unfortunate, but for the majority of  boats it is in the reverse order. This is not the way to deliver value to the customer.

Building a boat has three costs: labour, typically 50%, materials and equipment; 30% and overhead; if controlled 20%.

In our experience it makes no sense to cut costs by including inferior materials, machinery and equipment. The portion saved is of very little benefit in the overall cost and the cause of problems that invariably follow.

What is interesting about the equipment list for the new W54 – it differs little from the W48 & W60 - is the country of origin for which I have added a separate column. 

The big question is of course what influences the decision for each piece of equipment? The answer is never simple. As an example, due to its importance, we will look at the stern gear. That is the equipment that makes up the drive line from the reduction box to the propeller. For this I will use the W60 example with photos.

Each shaft transmits up to a maximum of 153 BHP at 518rpm. The whole assembly runs smoothly in next to absolute silence. The question for the naval architect is what are the forces generated here and how are they to be contained? What are the components and what is the duty of each?

THE PROPELLER SHAFT

The propeller shaft is 21/2” dia. Centreless ground type 318 (2205) stainless steel. This is a relatively new stainless steel with approx. 22% chromium and approx. 5% nickel content and correspondingly high cost.

It is virtually immune to pitting corrosion which is a problem with 316 steels, particularly in tropical conditions.

ASA 318 has a tensile strength about twice that of the 316 steels. In some ways this is a disadvantage as there is a tendency in boatyards to fit shafts of too small a diameter which are far to slender for the thrust forces that occur. This is one of many features of boat design where common sense should overcome pure mathematics. 

TORSIONAL VIBRATION

One of the forces that must be accounted for is torsional vibration. This is an invisible and silent vibration that is a result of a mismatch of the rotating components of the drive line.

Starting with the propeller, the screw can be a source of these problems or the mass of entrained seawater in a rotating screw itself can be the culprit. For this reason propellers should always be manufactured to ISO Class 1.

The spacing of propeller shaft bearings can be another invisible cause.

At the gearbox coupling there is a large differential of rotating mass. The propeller may rotate at 518 rpm but the crankshaft is running at 2400 rpm.

Engine builders ensure that all their components are arranged to limit the machines inherent vibration. But not being responsible for vibrations caused elsewhere in the driveline they will recommend doing a driveline analysis of the complete system.

SOFT MOUNTING

Today it is common for all engines to be mounted on rubber mounts which are designed to isolate the engine from the hull. The main consideration here is limiting noise and vibration transfer through the hull. From this arises another problem – how does the builder isolate the engine from the rigid propeller shaft.

A common solution is a rigid bolted steel coupling connecting a vibrating engine to a rigid propeller shaft. This is certainly a low cost solution but a method which can only end in trouble. This is not to be recommended except for small low speed cruisers. High hours and power can only lead to eventual destruction of the coupling shaft fit and internal parts of the transmission.

The solution is to install a flexible coupling like the one shown in the photograph. In this case we have used a “Centa™” A.G.M. coupling. This has the ability to absorb shock and vibrations in all directions, thrust fore and aft, torsion, axially. Although the initial setup is always done accurately the out of line can be up to 2 degrees under running conditions. The coupling is torsionally soft, ironing out vibrations both from the engine and propeller.

PROPELLER SHAFT TUBE SEAL

We now pass to the propeller shaft seal. The seal in the photo is a Deep Sea Seals Ltd EY Manebar Seal. This is a flexible face seal with bronze housing and Manetex™ face material. 

The seal is fitted with an inflatable bellows which when pumped up allows maintenance of the seal faces without dry docking the vessel. 

In the photo above, for demonstration purposes, a simple foot pump is hooked up to the bellows air valve. Once the bellow is inflated the seal can be loosened and any adjustments undertaken.

PROPELLER SHAFT BEARINGS

Immediately aft of the seal is the first propeller shaft bearing. This is usually termed the “neck bush”. It provides a support to the propeller shaft seal and serves to centralize this part of the shaft. In this case the bearing is a “Countrose” cutless type. A scavenge sea water main engine supply is provided through both seal and this bearing.