Model Boat Mayhem

The Shipyard ( Dry Dock ): Builds & Questions => Working vessels R&D: => Topic started by: NFMike on September 05, 2012, 11:50:05 pm

Title: Spare props
Post by: NFMike on September 05, 2012, 11:50:05 pm
The Aziz kit and prototype carry a pair of spare screws on the superstructure. This has been puzzling me. I can't imagine changing a hefty thing like these is a DIY job, especially with Korts around them, so surely a dry dock visit needed. I also can't really see that these are particularly likely to 'fail' anyway.
I've not noticed other ships carting such things around though spares for small stuff like outboards is not unusual I think.

Can anyone enlighten me as to when/why and indeed how these spares might get used?
Title: Re: Spare props
Post by: deadwood on September 06, 2012, 12:57:05 am
Usually these days the propeller(s) of almost every seagoing ship is particularly designed for the very ship (i.e. hull shape) and engine combination,
for the most common or typical loading situation encountered by this vessel.
This means that props are highly sophisticated and and pretty unique parts that are difficult to obtain as spare parts in any odd port en route
where a dry-dock may be available for the change of props.
I think this is even more true for those highly loaded ducted propellers of AHTS and the like.
Of course, if there isn't a 100% matching prop for the ship some nearly fitting stock propeller
(i.e. with similar particulars such as diameter, (mean) pitch to diameter ratio P/D, ratio of expanded area to disk area Ae/Ao, blade number etc.; see ITTC Wiki ( for definitions),
will do as well, but this isn't quite optimal.
For that reason it is safer to carry ones own spare props if one can afford.
If the props are controlable pitch propellers (CPP) then it usually suffices to carry some spare prop blades.
On the other hand, with fixed pitch propellers (FPP), such as in AZIZ's case, one has to carry the whole spare screw.
For mounting the spare prop of course docking will be required.

Marine propellers might fail more often than their rigid appearance may suggest at first sight.
For once, because of the higher density of the medium they operate in, they are more heavily loaded than for instance air screws of aeroplanes.
Then the detrimental effect of cavitation, which often in some load cases cannot be avoided, can severely erode propeller blades up to total loss of blades.
Another peril are vibrations due to a bad (or rather incorrectly predicted) wake field in the propeller plane.
For vessels that are operating in icy waters, like icebreakers, of course particles of cut or crushed ice may damage the propellers.
Or any other parts  or debris that happen to hit the props.
In that respect I'm not sure whether the standard procedure of anchor towing from the sea bed,
especially when hoisting vertically, doesn't bear the risk of the tow line or even anchor to accidentally hit the props.

Title: Re: Spare props
Post by: Bryan Young on September 06, 2012, 06:07:06 pm
Twice during my sea-going career The ship has suffered prop damage. The first was during some submarine cable-work in shallow water and we bent a blade on a rock or something. This was back in 1962. Drydocked. then found that the spare prop had sometime between 1923 and 1962 had been stolen and replaced with a concrete one. A reasonable copy had to be made and sent to Uruguay. But having twin screws we could still (more or less) operate.
The second was very similar, but much more commonplace.
In parts of the tropical world where hardwood timber used to be chopped down and exported (Malaya, Indonesia and the west coast of Africa) many huge logs either detached themselves from the rafts or fell overboard from a ship. After many years in the water they could have drifted to just about anywhere. Unfortunately, they eventually became waterlogged. They didn't actually sink, but floated vertically with only a few inches visible above the surface. Some of these things were up to 8ft in diameter and 30ft long. They had a name..ending in "----heads" that now escapes me. Just sheer bad luck if you hit one with a prop.
As we did on c/s "Recorder".  Made a mess of the prop, but probably didn't damage the log too much. Another dry docking (Dakar) and one of our 2 spares was fitted (twin screw again). BY.
Title: Re: Spare props
Post by: Colin Bishop on September 06, 2012, 07:16:08 pm
Carrying spares is quite common as the others have said. The following image shows spare blades for the thrusters on Queen Mary 2. As you can see, they are displayed as a decorative feature and are known as the Commodore's Cufflinks!


Title: Re: Spare props
Post by: deadwood on September 06, 2012, 07:34:19 pm
As you can see, they are displayed as a decorative feature and are known as the Commodore's Cufflinks!
Yes, indeed they seem to have made a special exhibition of them on QM2. That's why they put railings around them.
I'm only missing the "Do Not Touch" plaque.
Title: Re: Spare props
Post by: NFMike on September 06, 2012, 09:15:15 pm
Thanks for the info people. I didn't know any of that.

I'm only missing the "Do Not Touch" plaque.
Yes, they look pretty sharp :)