Ocean liner vs Cruise ship

[Isabelle Prondzynski]

Excellent discussion here!

Would there be another difference between ocean liner hulls and cruise ship hulls in the materials from which the hulls are built, and their respective thickness?

[skilly56]

Good morning Ken,

Good question re the doors. It depends a bit on how fine the lines are at the bow, but, generally, the doors will not be effective unless the vessel's normal service speed can exceed 21-24 knots by a healthy margin, and the vessel operates for a large percentage of the time at high speed.

One interisland ferry I occasionally worked on back in the late 1960's had vertically sliding thruster tunnel doors. I recall the ship's max speed was about 23-24 knots, but the hydraulic activation mechanism for closing the doors caused so much grief that they were just left open. However, along would come dry docking & survey time, and the surveyors would want to see the doors fully functional again. Cost a heap of money to fix every time, all for little resistance benefit.

One thing to ponder on regardiing ship speed and big bulbs - warships (even the bigger WWII battleships) only had a bulb similar to QE2, because the warship hulls have fine lines (ie, a very narrow entry at the bow, so it is very effective in 'slicing' a path through the water). The fitting of a bulb is often is often the naval architect's way of reducing the wavemaking resistance on the bow plating of a full-bodied design. So, it often indicates a compromise had to be reached between the design, and the customer's speed requirements.

Isabelle, the QE2's keel was 38mm plating I think, and the plating thickness is gradually reduced as one travels further up the hull sides, and then you get to the aluminium superstructure. Construction also requires that the spacing of the frames inside the hull is reduced at the fwd and aft ends to increase the hull strength in the area where the hull can pound or slam onto the seas.

It would be interesting to know what the scantling thicknesses were for Oasis of The Seas - anybody have any info on this?

The construction requirements of the large 'box shaped' cruise ships will still be similar to QE2, but the thickness of the plating and the depth of the frames would be much smaller. Hence, they tend to show where their inner frames are located on the bow areas a lot sooner than a QE2-type hull. Just look at the bow plating a navy frigate or cruiser - all the 'ribs' can be seen after only a few years and they begin to look like Twiggy!

Anyway, I have to go and pack - off to 'Frisco for New Year, a Caribbean cruise on the Ocean Village, then a lounge on the beaches of Hawaii on the way home again.

Cheers

Skilly  

[Rob Lightbody]

Stephen Payne told me that QE2's hull strength went far beyond what was actually required of her - he said, in fact, it was 95% stronger than necessary.   One reason was to reduce the ballasting requirements and make her more stable than the previous Queens.

[skilly56]

Now how is this for service - in my blurb above about bulbous bow performance I said that a correctly working bulb forms a depression in the water flow just behind the bow... ... etc.

"By fitting a correctly shaped bulb (and they are all different shapes, depending the the beam-to-draft ratio of the particular ship, it's speed, it's beam, etc) the bulb can actually create a 'depression' in the water flow. If the bulb is sized correctly, then that 'depression' in the water flow will coincide with the the forward-facing area of the bow plating where the area of maximum resistance to flow is normally experienced, and will reduce the bow wave to almost nothing."

This afternoon I was standing on Alcatraz Island (and yes, they did let me out again!) looking back toward San Francisco when a container ship sailed past doing about 15-20 knots. A perfect photo to explain my explanation.

Happy New Year Everbody!

Cheers

Skilly

[Twynkle]

Great Service - Great stuff, Skilly56!
A happy New Year too - and good to see that you weren't detained long on the island!
(Those look an interesting pair of watercraft in the background.)

In the early '60s were there many ships already built with similar bows?
As...thinking that QE2 was designed sometime before computer technology had advanced,
do you think that all the testing of models would have been carried out with models, solely in tanks?
Creating all the  different possible combinations of weather, wind, weight, balance etc must have been
really challenging from design and engineering perspectives -
Is seems as if it was pretty amazing they managed to find the almost perfect formula for QE2!!

By the way - is it possible to change anything about the shape and dimensions of the bow after completion?  

[highlander0108]

Did you know that QE2 does not have here original bulbous bow?  The whole nose was replaced after it was damaged from hitting something.  Commodore Warwick showed pictures of this operation in his lexture onboard, but I do not remember the specifics.  They did not change the shape, but cut off the damaged portion and welded on a new replacement....a wee bit of a nose job!

[Twynkle]

This gets more and more interesting - Thank you!
So much so, that on seeing a bit of Titanic's double-bottomed hull
on a fragment of a TV programme,
it wasn't difficult to think about her hull in relation to this thread and QE2.
A double bottom must make quite a big difference - both in terms of weight and strength
and I wonder about other benefits v non-benefits to performance etc...?

(hope this isn't duplicated elsewhere on the Forum!)
 

[Peter Mugridge]

A double hull is a legal requirement anyway isn't it?  So they'll all have to have one regardless of what type of vessel they are?

[skilly56]

Good Morning Rosie,

Has the snow melted up your way yet?

A few posts back you mentioned the interesting looking vessels in the San Francisco Maritime Museum (they were in the background of the container ship/bulbous bow photo). The sailing ship is the 'Balclutha', built in Glasgow in 1886,(the history board beside the ship doesn't even state where and when it was built, and said that she only ever traded between the UK and US). However, the marvellous internet produced the following info - see links below.

At 124 years old she is the oldest vessel afloat in San Francisco, and is believed to be only the 4th steel-hulled sailing ship ever built. If you Google it there is a mountain of information and photos available of her.

http://canterburyheritage.blogspot.com/2008/04/general-cargo-vessel-balclutha.html

http://en.wikipedia.org/wiki/Balclutha_(1886)

A bit closer to me than I realised - I was born in Christchurch (As a kid I used to hop on the Lyttelton train and spend my weekends fishing off the wharves there) and I also have family ties to Balclutha in Southland and stayed there numerous times.

Cheers
Skilly

[Twynkle]

Good Morning Rosie,
Has the snow melted up your way yet?
A few posts back you mentioned the interesting looking vessels .....
At 124 years old...
A bit closer to me than I realised - I was born in Christchurch (As a kid I used to hop on the Lyttelton train and spend my weekends fishing off the wharves there) and I also have family ties to Balclutha in Southland and stayed there numerous times.
Cheers
Skilly

Kia Ora! Skilly  - and Good Tomorrow to you, from the still snow-smothered SE of UK!

Thank you for all that interesting info about the lovely old Balclutha - what a coincidence!!
If I go on here about Ch Ch, the Mods might chuck me overboard 
so had better get back to hulls!

Thinking about the strips of 'rolled' steel - and the likely-hood of the iron content rusting...
were hulls ever made from proper stainless steel?

Then about propellers - is  cavitation 'damage' always caused by movement etc and / or can it be avoided by the use of certain materials?
Or - probably much more likely - have I missed the point?!!

btw -I daren't ask questions over on the speed topic - partly because I'm still very insure as to the definition of dead water!

Cheers

Rosie

[Peter Mugridge]

Cavitation is a water pressure ( or lack of it ) thing caused more by the rotation of the propellor regardless of what materials are used; reduction is a matter of propellor speed ( obviously closely related to diameter ) and shape.


Rosie - you still have snow?!  It never reached us here in Epsom although I did see a decent quantity in Ashford on Thursday when I was photographing trains...

[skilly56]

Rosie,

Your question regarding stainless steels - SS is a mild steel alloyed to a minimum of 13% chromium, and often has a % of carbon content as well. This makes it a real pain to weld, especially in thick hull-plating sizes, and therefore it can be difficult to find shipyards with welders who are qualified to repair this material. Unless SS is welded properly, the weld heat can produce extensive cracking, which then leads to early failure of the welds. It is also more expensive than mild steel or toughened steel plate.

Super yachts frequently have SS anchor boxes so less damage is incurred when dropping/raising the anchors, but superyacht owners tend to have more money than shipping companies! We used to have a lot of trouble here in Auckland trying to find qualified people who could weld THICK SS plating correctly.

Also, strangely, some SS's do not like environments that have NO oxygen - it tends to fail faster in these conditions than does mild steel.

Around 2004 a new superyacht (which, fortunately, I was no longer involved with) was launched here in Auckland, and the yard crew decided to pressurise the 316L Stainless Steel fire mains to test the pumps and manifolds. The hoses, etc were all run out and tested, but, the crew DID NOT purge the SS pipework systems with fresh water after the tests. Many months later, they pressurised the system again and a number of guest suites (all that beautiful joinery, and full of electronics!) and other spaces got wet when the pipework welds began to fail because of crevice cracking. Unfortunately, because the firemains had been installed first, all the remaining systems pipework and cable trays/wiring were now covering the pipework - it was a major.

Regarding propeller cavitation, this is generally caused by imploding air bubbles on the thrust face of the propeller blades, and is dependent on shaft speed, blade tip clearance to the hull, prop blade pitch, diameter (the bigger the diameter, the faster the circumferential speed), propeller immersion, propeller material, water flow into the blades, etc.

One ship I was on with a controllable pitch propeller was found to have a bad cavitation condition when the blade pitch was between Dead Slow Ahead-Zero Pitch-Dead Slow Astern positions, but also had a considerable problem near Full Ahead. The blade design was not good, and after a major failure at sea, then continual blade cracking & ongoing replacement, it was decided to retrofit a totally new propeller and shafting system, with the blade design being of the skewed (ie, the blade profile is 'swept back') type. All the cavitation & vibration disappeared, you could safely leave a cup of coffee on your table without it vibrating over the edge, and life was a lot more pleasant. Although the blade areas of both props were identical, the ship also had a small speed increase resulting from the more efficient design, which had more of the blade area located toward the outside of the propeller circumference and thus developed more thrust for the same rpm. Fortunately, the ship also had sufficient installed horsepower to deliver this thrust. Refer to Before & After photo attached.

Many different solutions have been tried in order to reduce the cavitation effects - these days, large propellers sometimes do not exceed 75 rpm at full speed, this giving a more efficient propeller, with cavitation effectively eliminated.

Peter,

With reference to your mention of double hulls, all TANKERS are required to have double hulls around the cargo tanks from this year, but this requirement does not apply to vessels that are not tankers...extract from IMO Rules...

   A speeded up programme for the gradual phasing-out of single-hull oil tankers - notwithstanding a number of limited exemptions, single-hull oil tankers will not be allowed to continue operation beyond 2010. The exemptions foreseen by the IMO concern a limited number of single-hull tankers which will be subject to new and more severe regular technical inspections.

Cheers
Skilly

[Cruisemarsh]

Ocean liners have a much more rounded hull than the modern day cruise ships which have a flat bottom. Cruise ships have stubby looking bows whilst ocean liners have long sweeping streamlined bows that are designed to eat into any wave across the Atlantic or in any heavy seas. Ocean liners have a thicker skin than that of cruise ships due to the rigourous nature of the North Atlantic. Also, Ocean liners have a deeper draft in order to maintain a better balance in rough seas whilst modern day cruise ships don't have as deep a draft due to the flat bottom.

[Jakester]

Simply excellent discussion here. Thank you, Skilly for your outstanding Hydrodynamics course!

[Mauretania1907]

i am amazed there are still tankers with single hulls around. The scrappers of Alang have some (dangerous and toxic) work ahead. I realiase the shape of the propeller would be a factor in a ship's performance.
The designers of Mauretania `1907, and Swan hunter (shipyard) did many hundreds of trials to determine her hull-form. She, of course had a very slim length to beam ratio, which made her faster. A set of new propellers helped her along as well. Of course, no bulbous bow, thrusters or stabilizers and no computers to help her designers work out their sums.
A question, what if the length to beam ratio called, is it block co-efficiant?