W54 Project

THE W54 PROJECT 2013

We will start with the first important decision:

 
TWIN OR SINGLE SCREW which is the best option for the W54?
 
We have always offered our range of motor yachts with a choice of propulsion installations and for the purpose of this article set them out as follows:
 
Case 1 Single screw, no auxiliary engine
Case 2Single screw, plus wing engine with feathering propeller.
Case 3 Twin screw, no auxiliary
 
The choice of installation is always left to the prospective owner. We do however recommend that some means of “get home” is really essential for long range coastal cruising and ocean crossing.
 
About half our owners have chosen single screw with auxiliary engine and the other half, twin screw installation.
 
None have chosen Case 1, single screw without “get home” capability.
 
When designing the main propulsion system for both single and twin screw vessels we make every endeavour to maximize the propulsive efficiency – because herein lays the biggest running cost savings throughout the life of the vessel.

To this end the hull in the way of the propellers is shaped to permit the largest diameter propellers with at least a 15% tip clearances to the hull and keel skeg.
 
Something to keep in mind when reading this, and also in comparing vessels, is that the larger the propeller, the lower the speed it is turning at; the greater the propulsive efficiency
 
The opposite is also true, the smaller the propeller, the faster the speed it is turning at, the greater the propulsive efficiency.
 
High propulsive efficiency lies at the two extremes, never in the middle.
 
Of course high speed is unsuitable for a displacement motor yacht. An example of high speed would be Sir Donald Campbell’s speed boat “Bluebird K7” in which he set a World speed record on water. Bluebird’s propeller was two blades turning at 8555 RPM for an efficiency of 72% which is phenomenally high.
 
For the examples below we are working with a 4.9:1 reduction gear. This is about the greatest reduction that we are able to obtain in a standard manufactured transmission. Ideally we would like 6:1 reduction; however the cost of a special order would be excessively high.
 
The following gives a detailed description of each mode of get home capability. 
 
Case 2, Single Screw plus wing auxiliary engine.
 
The first thing to remember is that to be of any use an auxiliary engine must have at least one third the horsepower as the main engine. From this power you may be able to obtain a propulsive effort of about one quarter of that provided by the main engine.
 
The cause of this low efficiency is that feathering propellers have a rather low performance in this regard and the shaft rpm is always high which again contributes to low efficiency. All these disadvantages could be cured if the feathering propeller diameter is increased and the shaft rpm decreased. The trouble is then that the shaft assembly also increases and would approach the cost of a Case 3 twin screw installation.
 
Another disadvantage of the get home auxiliary engine is that it is seldom suitable for any other service. We usually fit a 230v alternator to this engine to be used as a generator set, our GenGetHome™.
 
The other drive consideration for a wing engine is the ships hydraulic system required for stabilizers and bow and stern thrusters. Once again the stabilizer hydraulics requires a continuous supply which means the auxiliary engine must run continuously. We prefer to arrange the main engine to perform this function.

Alternatives
 
Devices are now available which enable the auxiliary engine to rotate the main propeller shaft. One such device is manufactured by WESMAR. This probably eliminates two thirds of the likely problems but the system still relies on one shaft and propeller. We always have in our mind problems we have seen with propulsion systems such as broken shaft, trawl net wrapped around the propeller and damaged propeller.
 
Case 3, Twin Screw
 
The following is a comparison of a single screw propeller (Case2) and twin screw (Case 3) proposed for our Watson 54 vessel.

We have calculated the power coefficient in each case. The LOWER the coefficient the HIGHER the propulsion efficiency.
 
Single screw Case 1 & 2.
 
Power 224 SHP @ 2100 rpm
Propeller RPM (N) = 453 rpm
Speed of advance(Va) = 8.85 knots
Power Coefficient (Bp) =√ power x N
         Va 2.5
 
Bp = √ 224 x 453
8.85 2.5
 
Bp = 29.096
 
This power coefficient (Bp) yields an efficiency of 56.5% on a propeller diameter of 1250mm (49”)
 
Twin screw Case 3
 
Power 112 SHP @ 2500 rpm
Propeller RPM (N) = 505 rpm
Speed of advance (Va) = 9.00 knots
Power Coefficient(Bp) =√ power x N
       Va 2.5
Bp =√ 112 x 505
9 2.5
 
Bp = 21.993
 
This power coefficient yields an efficiency of 60.5% on a propeller diameter of 977mm (38”).

In both these cases the propulsive efficiency is very high. If the efficiency was less than 50% we would need to adjust the inputs to correct the deficiency.
 
These calculations show that a twin screw vessel has (or can have) the same or higher propulsive efficiency as a single screw vessel.
 
The above calculations reveal “true economy”. Forget miles to the gallon or gallons to the mile. These only serve to tell the cost in dollars and disguise the real performance of a vessel. When making comparisons of different models the propulsive efficiency is a key number.
 
So what other benefits are there to a twin screw vessel?
 
1. A symmetrical engine room layout.
 
2. Commonality of parts. 
 
3. Allows for a centerline engine room access.
 
What are the disadvantages?
 
1.Slightly more expensive than Case 2.
 
Based on the above the Watson 54 is offered with a twin screw engine installation.
 
 
© 2013 T.C. Watson & Sons Ltd

Last Updated (Thursday, 07 February 2013 20:13)