[V.Narayan]

Originally Posted by Sutripta

Is the ability to disconnect (neutral for us car people) the powerplant from the propeller a standard feature?

Regards
Sutripta

I believe so for naval warships. Large merchant vessels have a direct drive without a gear box ie the props rpm is the same as the diesels. Jeroen and other sailors will know better.

[Sutripta]

^^^
Diesels can be reversed. So no need for a gearbox. Not so turbines (Which AFAIK power most of the larger naval vessels). These missile boats:- what was their power source. I would think too small for the boiler/ turbine setup.

Regards
Sutripta

[V.Narayan]

^^^^^
Medium speed diesels - 3 of them, one on each shaft

[Sutripta]

^^^
Two Strokes?

Regards
Sutripta

[V.Narayan]

A wee bit of topic from the beautiful Seahawk jet but I cant resist attempt answering your questions

Originally Posted by Sutripta

Diesels can be reversed. So no need for a gearbox. Not so turbines (Which AFAIK power most of the larger naval vessels). These missile boats:- what was their power source. I would think too small for the boiler/ turbine setup.

Originally Posted by V.Narayan

Medium speed diesels - 3 of them, one on each shaft

Originally Posted by Sutripta

Two Strokes?

They were powered by three Zvezda water cooled radial diesels, one on each shaft. These diesels delivered 3942 bhp at 2200 RPM and had 42 cylinders. I suspect they were 4-stroke. The cylinders were arranged in seven banks of 6 cylinders each totaling 143,600 cc.

The Styx Missile of the variant we used in 1971

[Foxbat]

Originally Posted by V.Narayan

The Styx Missile of the variant we used in 1971

There are 1 or 2 P-15 Styx's on display at the Indian Naval Maritime Museum in Kochi, Kerala.

[Jeroen]

Originally Posted by V.Narayan

By keeping engines switched off and letting props free auto rotate

Originally Posted by Sutripta

Is the ability to disconnect (neutral for us car people) the powerplant from the propeller a standard feature?

Originally Posted by V.Narayan

I believe so for naval warships. Large merchant vessels have a direct drive without a gear box ie the props rpm is the same as the diesels. Jeroen and other sailors will know better.

Originally Posted by V.Narayan

These diesels delivered 3942 bhp at 2200 RPM and had 42 cylinders. I suspect they were 4-stroke.

A few add on comments:
There are different ways to go about switching off engines/props.
The obvious one in to have some sort of clutch so you can physically disconnect the engine from the propellor. You are likely to see this sort of arrangement on war ships, but also on ferries, Anchor handling tugs etc.

They often have different size main engines. Say two engine producing 10.000BHP and two engines producing 6.000BHP. By means of clutches you can have different power output on the respective shaft/prop, also ensuring you get the best fuel efficiency. E.g. a 6.000 BHP engine running at 90% is more efficient than a 10.000BHP engine running at 54%.

The other way is of course to have variable pitch propellors. Which are also more effective. Because you would shut down the respective engine, and put the pitch in the trailing position to minimise drag.

You would have special provision on the engine to make sure it doesnt rotate, and in many cases you would let the lub oil pumps run just in case.

On smaller boats, you might not be able to disconnect the engine from the prop shaft. So if you shut it down whilst still motoring on on your other engie/prop, it is likely to be driven from the prop so keep the lub oil pumps on.

I saw an interesting article the other day in a boating magazine. A twin engine trawler tested out running on one engine and with the other one trailing (and spinning). Although that does create drag, running on one engine at a relative low speed, (e.g. 5 knots) was still very fuel efficient. They used this set up to move the boat from one charter to the next over large distances if time wasnt that important, but money was.

They had added a seperate lub oil pump and inspected the engine bearings after their first major crossing on one engine. Seemed to work for them.

The typcial large slow speed marine engines have a a permanent connection to the prop shaft. You reverse by actually reversing the engine. These large slow speed engines are always 2-stroke and it is relatively easy to get it to work in two directions. There are/have been four stroke engines as well that could be started in either directions. It requires more elaborate reversing gears as you will have to deal with both inlet and outlet valve timing in two directions. Two stroke will have no valve or some only outlet valves.

There are some, but not to many, ships with multiple propellors and slow speed engines. E.g. some container ships spring to mind. But the majority of ships with a slow speed engine tend to have one engine / one prop only.

I sailed as Chief engineer on some of the first Supply/anchor handling ships with variable pitch propellors. At the time, there wasn't that much experience with variable pitch, so the ship design also called for four stroke main engines that were reversable. If they variable pitch mechansim would not work, we could put the prop in a fixed pitch manually and still have reverse, by reversing the egine.

In practice, the variable pitch were proven to be extremely reliable. I dont think I have ever encountered any major problems with them. And on these sort of ships they are really put to the test, again and again. Heavy manoevering, idling, continous max output all for prolonged periods.

Most marine medium speed engines tend to be four stroke. At 2200 RPM I'm not sure it is called a medium speed though, but it is only more likely to be a four stroke.

Of course, if you go diesel / electric it all becomes a lot more simple because there isnt a physical connection between engine and prop shaft.

Jeroen

[travancore]

Two Grey Eagles, one too young to be grey!

[Sutripta]

Originally Posted by Jeroen

The other way is of course to have variable pitch propellors. Which are also more effective.
...



The typcial large slow speed marine engines have a a permanent connection to the prop shaft. You reverse by actually reversing the engine.
...

I sailed as Chief engineer on some of the first Supply/anchor handling ships with variable pitch propellors. At the time, there wasn't that much experience with variable pitch, so the ship design also called for four stroke main engines that were reversable.
In practice, the variable pitch were proven to be extremely reliable.

Wasn't variable pitch propellers the norm in planes?

Any thoughts on the propeller design of bow thrusters?

One thing not mentioned is that one might need a gearbox (reducer) to match speeds of prime mover and propeller.

Of course, if you go diesel / electric it all becomes a lot more simple because there isnt a physical connection between engine and prop shaft.

Not just that. The trend seems to be to put the motor/ prop in a pivotable/ steerable pod.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Wasn't variable pitch propellers the norm in planes?

It certainly is these days on most planes. Smaller planes still might have fixed propellors. Actually, part of your pilot training is to get a so called endorsement for what is known as a complex plane. A complex plane is a plane with a retractable gear and or variable pitch propellors.

Not all variable pitch propellors can produce reverse thrust, but some can.

Here a little video I took some time ago:

https://www.youtube.com/watch?v=8pyWMv2pWiM

Originally Posted by Sutripta

Any thoughts on the propeller design of bow thrusters?

Haven’t given that much thought. Most of the bow thruster I have sailed with were electrically powered. On some of the larger AHT we had dedicated dieselgenerators that would provide electrical power to the bow (and or stern) thruster. On smaller ships (yachts) you might also see them hydraulically powered.

Originally Posted by Sutripta

One thing not mentioned is that one might need a gearbox (reducer) to match speeds of prime mover and propeller.

Yes, except for the low speed engines, everything else requires a gearbox.

Originally Posted by Sutripta

Not just that. The trend seems to be to put the motor/ prop in a pivotable/ steerable pod.

True. And these pods are showing up on increasingly larger vessels too. Some of the largest cruise vessels have these pods now. I have never sailed with them other than on a motor yacht. Very easy to manoeuvre.

Just for members who are not familiear with the concept:

https://www.youtube.com/watch?v=dv4C_LdoDaE

Jeroen

[Sutripta]

^^^
My point was that variable pitch propellers was established technology, albeit in planes. Why then the fear over the reliability when first used on ships.

We are seriously polluting this thread. Maybe the mods can remove the pollutants to a new thread.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

^^^
My point was that variable pitch propellers was established technology, albeit in planes. Why then the fear over the reliability when first used on ships.

This was really well before my time. By the time I was sailing on these ships they were 20-25 years old. But I did hear some of the stories from the guys who were around then.

I would say the variable propellor for ships really came about in the early sixties with the oil exploration on the North Sea. Before there had been supply and anchor handling vessels, mainly in the Gulf of Mexico. Pretty shallow and calm water, especially compared to the Northies.

Everything was relatively new and untried, from the way they build drilling rigs to platforms to supplyships and AHT. Till then most oceangoing tugs had fixed propellors too.

So whereas the concept of variable pitch propellors was available and used in aviation there was simply very little experience with it in maritime environment.

Especially supply and AHTs operate under extreme conditions, sometimes for weeks and reliability is key. You just can’t afford to have anything critical break down. At the time, just about all maritime propulsion engine came standard equipped with reversing gear. So effectively at no additional cost, you got redundancy, just in case.

I really don’t know how much know how is/was transferred from aviation to the maritime sector. Today, very different players. I don’t know offhand of any company providing variable pitch propellor to the aviation as well as the maritime industry.

There is a fundamental difference between air and water, density. Whereas in aviation the variable pitch is (partly) used to accommodate the density of air, this is not the case for water application. Density varies very little. So although the design I assume follows certain common approached there are specifics to air and water specific design.

I don’t think/know if maritime variable propellors get designed as constant speed, Even though they might be operated at constant speed in practice.

Jeroen

[Sutripta]

Originally Posted by Jeroen

There is a fundamental difference between air and water, density.

That certainly affects the propeller design. Which is separate from the pitch control mechanism - which I would assume the cause of the worry.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

. Which is separate from the pitch control mechanism - which I would assume the cause of the worry.

Yes, also getting things caught in the propellors happens on a fairly regular basis with supply ships and AHTs.

I remember the following incident which happened on my very last trip on the SmitLLoyed 32 working of the cost of Gabon late 1985. I was on duty in the engine room and we were going to pick up some cargo from an oil platform.

Reasonable weather, so at about 100 meters of the platform we would drop one or two anchors and then reverse back towards the platform paying out anchor chain. With the stern near the platform the crane would lower a mooring rope, one for starboard and one for port. Whilst manoeuvring near to the platform something went wrong and the loose end of the mooring rope, went into the water where it immediately got caught in the port propellor. The captain immediately hit the emergency stop, but the rope was already being wound around the propellor. With the port engine now stopped he slowly went forward on the starboard engine and anchors. The other end of the mooring rope was securely fastened on the platform. By moving forward the mooring rope was pulled off the propellor in the opposite direction as it had been spun on. Which also meant the engine was forced to turn in the opposite direction. Quite the racket in the engine room and the engine spewed out black soot and smoke through the inlet filters!

Once we we were at a safe distance of the platform, we turned the port engine over a couple of times, checked the seals on the propellor shaft. Checked the variable pitch mechanism etc. Then we started the port engine and everything still worked as before. No harm done whatsoever.

You can imagine the sort of forces exerted on the propellor blades, shaft etc. This mooring ropes are 8-10 cm in diameter!

These variable pitch propellor proved to be extremely reliable and robust!

Jeroen

[travancore]

Looks like another photo from Malta, during the ferry flight.