Hi Howard
By, you like opening a can of worms really don't ya
and before anyone says anything - the honest truth about propellers and design, there is no hard and fast rules. Each vessel, or shall we say, each group of vessels is individual. If you are talking about MTBs and fast run abouts there is a set of rules which govern their designs and the amount of propeller shafts and engine horse power plus hull shape, these are all have to be taken into consideration if we are going to discuss this type of vessel. If we are talking cargo ships these have another set of rules as these come under what is known as displacement hulls. Their propellers and engines are like I say governed by their type of rules.
By what you are talking about - the Titanic - you have to take into consideration the time she was built and the Company she was built for. Also the reason she was built - the top of bottom of it is - not being too harsh - was greed. How fast we can get from A to B as cheaply as possible. The centre engine of the Titanic was steam turbine - two outboard engines were 4 cylinder reciprocating engines - the exhaust steam from these 2 wing engines fed the steam turbine. Now, the turbine, I don't think had a reverse gearbox and I think she could only drive forwards. So, when she left port, she would leave on her 2 wing engines (presumably) so she could manoeuvre easily and once she left harbour she could engage her turbine engine to give her maximum speed. That is all good.
We also have to take into consideration why they never put a 3rd reciprocating engine in the Titanic - possibly the reason being - and very strongly it may have been the space require for the engine + the extra fuel which needed to be carried + the extra boilers to maintain it. Now, we will come to the direction of the propellers.
If we stand and look at the back end (stern) of a triple propped ship - we will see the 2 wing propellers are actually in free floating water - in other words the water can pass along the sides of the ships on the bottom - directly onto the blades of the propellers, giving them maximum efficiency, whether they turn inboard or outboard. Your centre propeller is actually starved of free flowing water, compared to the outboard ones. The only way it gets its full efficiency is when the ship is moving, the thrust from the propeller starts to move around the radius of the propeller - in other words from say 7 o'clock to 12 o'clock the thrust will move say from say 7 o'clock to 2 o'clock - so therefore increasing the efficiency. To check this out, stick one of your models in the bath with the motor running - and CAREFULLY - feel the thrust from the propeller and see how it varies. To which way the centre propeller actually rotates doesn't actually matter - because - we are going from A to B as quickly as possible and we can correct our course with a slight alteration on our rudder. This in real life would have been done on the sea trials on the measured mile.
THIS IS A VERY SIMPLIED - VERY SIMPLIFIED explanation - some points may be way off the mark - to those who want to live the life of propeller design
but those like the rest of use who enjoy modelling - its just a quick explanation.
Howard - here is a picture of a bit of a complicated set up and all this was done to correct cavitation
