Structural vibrations on the aft

[voyager1964]

This question is primarily for @chengkp75

There are reports on strong vibrations when a former Renaissance ship (all Azamara's + R-class on Oceania) go at full speed, attributing those vibrations to the ship propulsion. With that, based on the posted reviews, there is anecdotal evidence that the vibrations diminish as you go up: the strongest vibrations would be at the Owners Suites on deck 6, while on deck 8 they seem to be less noticeable.

 

I wonder if this can actually be the case from the structural mechanics point of view. Won't the vibration freely propagate throughout the metal construction without ant dampening? 

 

Thanks

[Dr H]

The only time I've experienced vibration has been in the main dining room, and then only when seated in the back. Can't say I've ever experience vibration mid ship or forward. Apparently the vibration does dampen, and significantly. Another advantage to sailing with Azamara...

[Riocca]

7 minutes ago, voyager1964 said:

This question is primarily for @chengkp75

There are reports on strong vibrations when a former Renaissance ship (all Azamara's + R-class on Oceania) go at full speed, attributing those vibrations to the ship propulsion. With that, based on the posted reviews, there is anecdotal evidence that the vibrations diminish as you go up: the strongest vibrations would be at the Owners Suites on deck 6, while on deck 8 they seem to be less noticeable.

 

I wonder if this can actually be the case from the structural mechanics point of view. Won't the vibration freely propagate throughout the metal construction without ant dampening? 

 

Thanks

I can vouch for that, on Pursuits maiden we were in a CW suite on deck 6 when Captain Carl did a couple of high speed runs during the night. The vibration was very bad, so bad in fact that I suffered vision problems after the second run, so much so we spent the morning in Stavanger at the eye hospital being checked for a possible detached retina. Fortunately it turned out to be just a temporary problem, and all was good the next day, Captain Carl was very apologetic.

We have done a few longer cruises in the suites on deck 7 both on Azamara and Princess have never really noticed the vibration other than when the ship is manoeuvring in dock. Have always avoided deck 8 as we’re worried about possible noise from the deck area above.

[MNgardens]

We were in a N2 on deck 8 and had a terrible night due to vibrations on an Australia and NZ cruise.  We heard through the grapevine the next day that the ships had a long distance to go and had been fighting a strong current. The more central staterooms did not have as much shaking; we could feel it getting worse as we walked towards the aft from the elevators. It was really awful and left the three of us feeling very sick and we hardly slept. I think this was on the Quest.

 

The only way I would get another stateroom in that vicinity would be if the cruise does not have any stretches of distance to cover. I don't want to repeat this ever.

[Bloodaxe]

1 minute ago, MNgardens said:

We were in a N2 on deck 8 and had a terrible night due to vibrations on an Australia and NZ cruise.  We heard through the grapevine the next day that the ships had a long distance to go and had been fighting a strong current. The more central staterooms did not have as much shaking; we could feel it getting worse as we walked towards the aft from the elevators. It was really awful and left the three of us feeling very sick and we hardly slept. I think this was on the Quest.

 

The only way I would get another stateroom in that vicinity would be if the cruise does not have any stretches of distance to cover. I don't want to repeat this ever.

 

That can be a problem on all the ship's but only when traveling at full speed for some reason.

It's never been a problem for us at normal cruising speed and we prefer cabins at the stern.

[MNgardens]

To the OP's question:

Based on our experience, I had mentally mapped it to being similar to a see saw.  The further one gets from the fulcrum (engine) the more of an effect.  I have no engineering background and am open to being wrong.

[chengkp75]

The OP asked me to comment here, so here goes.  The question comes in two halves, is the vibration caused by the propulsion at high speed, and can it vary on where in the ship you are.

 

First, yes, it is most likely propulsion caused.  Correct me if I'm wrong, but all Azamara ships are conventional propellers, right?  Propulsion vibration is caused when the tip of the propeller blade passes closest to the hull of the ship (at the top of the propeller swing).  Depending on the clearance between propeller and hull, this vibration will be more or less.  Now, with a given ship, if the frequency of blade passes across the hull (propeller rpm times the number of blades on the propeller) reaches a harmonic frequency of the hull (like a guitar string causing a vibration inside the guitar), the vibration will become much stronger, as the hull resonates in synchronism with the propeller.  So, weather can affect the vibration (if a following sea lifts the stern, there is less water between the propeller and hull), the speed of the propeller can affect the vibration (as little as one or two rpm more or less can take the ship out of harmonics), and also it is complicated by there being a second propeller putting its pressure waves both onto the hull but also into the flow of the other propeller.  Many twin screw ships with have a "synchronizer" to adjust not only the slight difference in rpm the shafts may be turning, but also the relative position of the propeller blades to the other propeller, so that blades from both screws are not passing the hull at the same time.  It will slightly offset the two propellers, and make adjustments to keep them this way, and that can actually double the blade passage frequency, taking is completely out of harmony with the hull.  Okay, sorry for that.

 

Now, as to vibrations around the ship.  The first thing to understand is that even in the 21st century, shipbuilding is still more art than science.  Two "identical" ships, built in the same shipyard, but at different times, can have totally different harmonic characteristics.  This can be caused by the slight variations in steel thickness and weight, just in the manufacturing tolerance, and the time of day, or time of the year when the welds were done to put the ship together.  So, what causes one sister ship to vibrate madly, may not affect her sister ship at all.  Now, you have to understand that the ship is not just one big steel box, but hundreds of steel boxes (and I'm talking the structural steel, not the cabin walls and ceilings), and each of these steel boxes will have their own unique harmonic frequency.  So, one space on a deck, say a restaurant, may vibrate, while the space next to it, say the galley, does not.  Also, the interjoining boxes and frames used to build the ship will tend to dissipate vibrational energy as it passes through the space, so the further you are from the source, the less the vibration energy, and in most cases the vibration, will be.

 

So, to end a long winded spiel, the vibrations are caused by the propellers, not the engines (they are on resilient mounts), which are aft and underwater.  So, the worst vibrations will be lower decks, aft, and the further away you get, either going forward or up, the less the vibration will be, unless you find the one cabin whose harmonic frequency is just right for that ship speed and weather condition.

 

Throw in azipods, and you have another set of problems with vibrations.

[chengkp75]

And, just a personal pet peeve.  "Aft" is not a noun, so "the aft" is not correct.  Aft is an adjective (the aft stairwell), or an adverb (I'm heading aft).  It would be proper to say "vibrations on the stern", or on the "after part of the ship".  Sorry.

[Host Jazzbeau]

Thanks for helping out those of us who don't know our 'aft' from a hole in the ground! 😉

[Ithikan]

Thank you for your posts, chengkp75!  I very much appreciate your clear explanation of vibration, and all the reasons why one Azamara ship may differ from another in this regard.

[voyager1964]

Thanks @chengkp75 for your professional in-depth explanations, and also to all other posters. From my own experience, the worst vibrations were on NCL Epic in the upper-bow area ("Haven") during just one night.

[Fairgarth]

On 9/5/2021 at 9:56 AM, chengkp75 said:

Many twin screw ships with have a "synchronizer" to adjust not only the slight difference in rpm the shafts may be turning, but also the relative position of the propeller blades to the other propeller, so that blades from both screws are not passing the hull at the same time.  It will slightly offset the two propellers, and make adjustments to keep them this way, and that can actually double the blade passage frequency, taking is completely out of harmony with the hull.  Okay, sorry for that.

 

 

 

Many thanks, chengpk75, for that splendid explanation.  Enquiring minds want to know:  do twin screw ships have contra-rotating props?  How about azipods?

[chengkp75]

26 minutes ago, Fairgarth said:

 

Many thanks, chengpk75, for that splendid explanation.  Enquiring minds want to know:  do twin screw ships have contra-rotating props?  How about azipods?

They do not have "contra-rotating" props, as this describes two propellers on one shaft where each propeller turns opposite to the other.  Fixed pitch propellers on multi-screw ships will usually be "outboard turning", meaning the port screw turns counter-clockwise, and the starboard screw turns clockwise.  This gives the best overall lateral stability (the ability to maintain a straight course) whether backing or going ahead.  Controllable pitch propellers (including azipods) tend to be "inboard turning" (just the opposite of above), since the propeller turns the same direction whether going ahead or astern, so side thrust is not increased as when a fixed prop is reversed, and it also gives the best speed performance between inboard and outboard turning.

[TeaBag]

On 9/5/2021 at 11:10 AM, Host Jazzbeau said:

Thanks for helping out those of us who don't know our 'aft' from a hole in the ground! 😉

Thanks to you, I now have a pain in my stomach from laughing.🤣🤣

[KevintheIrishDJ]

On 9/5/2021 at 1:53 PM, chengkp75 said:

And, just a personal pet peeve.  "Aft" is not a noun, so "the aft" is not correct.  Aft is an adjective (the aft stairwell), or an adverb (I'm heading aft).  It would be proper to say "vibrations on the stern", or on the "after part of the ship".  Sorry.

I assume you know who John Phillip Holland was.

 

[chengkp75]

7 minutes ago, KevintheIrishDJ said:

I assume you know who John Phillip Holland was.

 

Yes.