Actually, “plastic” is the wrong term here. Plastic infers that the flexing causes permanent deformation (plastic deformation). Elastic is the better term. Elastic deformation is when the material returns to its original position/dimensions after flexing due to force. Sorry, I am an engineer AND a grammar Nazi. . .
And one also has to consider the critical/barred speed from a slow speed diesel engine.
That’s O̶K̶ acceptable, you’re e̶x̶c̶u̶s̶e̶d̶, forgiven. There is no e̶x̶c̶u̶s̶e̶ reason for inaccuracy. I am chastened 
DOF is Degree Of Freedom
Slow speed Diesel engines are usually selected as the main engine in large cargo ships for reasons of power and economy. However, they usually have significant imbalances at several frequencies, and as they have to be bolted directly on to ship structure due to their invariably large size and mass, vibration excitations can be fully transmitted into the ship. Keeping the total lengths of the propeller shaft, intermediate shaft and crankshaft relatively short can result in less probability of resonance because of higher natural frequencies of axial and torsional vibrations of shafting.
The low-speed diesel main engine and the propeller are the principal vibration exciting sources. The hull structure responds as a both ends free beam when subjected to dynamic loads.
Excitation forces from the propeller are transmitted into the ship via the shaft line and also in the form of pressure pulses acting on the ship hull surface above the propeller.
The vibrations generated by the propellers, RPM times number of blades, cannot be altered. Well, in theory the RPM can be controlled but that isn’t the solution we are looking for. Viscous-spring dampers can be used to control the torsional vibration of the shaft system.
there are enough similarities between this subject and how ice behaves on water when trucks drive on it that perhaps some comparison could yield a benefit.
Capndirk !! a grammar nazi … I may be one also but supress it … also cuz I ain’t all that good compared to my family of authors. (there may be something in err here)
RE: ship bending … always going on but when you can see it happening (a lot) it’ll give you the heebie jeebies.
Not necessarily, on the contrary. It is in fact a good thing because if it doesn’t bend it will break…
I understand the bend don’t break concept, still gives me the creeps.
Between two years at Maritime School I did a trip on a small ship that was built extra “strong” to carry Titanium Ore from Rekefjord, Norway to Rotterdam.
Titanium Or has a Specific gravity, 4.70–4.79, so a full load for that ship was only small heaps in two holds.
The problem was that the ship did NOT flex much, thus “behaved” differently to other ships of the same size and had to be handled accordingly, even when carrying different cargoes.
PS> Titanium Ore is also susceptible to liquefaction, so water content was another thing to watch carefully.
Dutchie, I did not say bending was bad, just that if you’ve ever seen ships as big as Ro Ro’s or tankers bending in heavy sea it still catches your attention, especially below deck where it is much more obvious.
Apart from flexing such ships have a very low G, large GM. What they call cruel ships. I understand that with bad weather they could hardly walk.
Since bad weather has something to do with it, you’re not talking about prop walk. You lost me, what are you referring to?
The GM would depend on the cargo. With titanium ore it would certainly be a VERY uncomfortable ride.
The ship was also carrying bagged fertilizer from Glomfjord in Northern Norway to Oslo and special fertilizer for vineyards from Rotterdam to Portugal and Spain.
Return trip was with Potash in bulk from Barcelona back to Glomfjord.
PS> As well as a lot of 96% spirituous and other “trading goods” that would be distributed along the coastal passage from Koppervik to Glomfjord. (From there the rest of Northern Norway was supplied by other means)
Here is the ship in her original configuration:
She ended her days in the Caribbean as Colombian owned, doing much the same, but the “trading goods” had change to suite the market.
You are losing me and now I am losing you. What is ‘prop walk’?
Just semantics obviously as I don’t doubt you know about the phenomenon: I’m referring to the effect that prop rotation has on a vessel with no way on, a clockwise rotating prop pulls the stern to starboard in forward and to port in reverse.
And what has Prop walk got to do with a ship under way in bad weather?
I think it is fairly clear that he was referring to “hard to walk” for the people on board due to the rapid and violent motion a VERY stiff ship makes.
With very heavy cargo in the bottom a ship becomes VERY stiff, as you know.
’
PS> That particular ship had high wing tanks, but that eat into the cargo capacity.
Hey I asked him first. There was no mention of people having difficulty walking on deck, only this:
I trust that Dutchie can answer questions directed to him on his own and I didn’t see him request assistance from a Swede or Norwegian or whatever the hell it is you claim to be.
I get it. What I meant to say was that I heard from somebody I knew who was sailing on the Halcyon Line ore trade ships between Norway and Rotterdam that they had difficulty walking on the deck because of the unexpected and sometimes violent heeling moments. They were almost thrown of their feet, very uncomfortable.
Yeah, thanks man. I made the connection when Bug put his 2 cents in but I had to give him a hard time anyway, you know, just because.
Back in the day of conventional cargo ships we worked very hard to achieve the Goldilocks moment when things were right and any planner worth his salt does his best planning a box boat. The worst part in sailing on say a bulk carrier carrying a very dense ore or in ballast is you can do nothing about it. A bulk carrier an typically roll up to 45 degrees in ballast which makes walking anywhere a challenge.
Working in the oil patch has made me more familiar with American idioms and while I generally leave measurements in metrics I sometimes change words. For instance prop walk is known a paddle wheel effect which is descriptive particularly in the case of the Milano Bridge where the propeller was not fully immersed. In text books the term transverse thrust is used.
It is also called ‘wheeling effect’. That comes close to what the phenomenon is called in the Dutch language where this is known as ‘wieleffect’. Yes, we glue all words together… A striking example is Hottentottententententoonstelling: Hottentotten (African tribe) Tent Exhibition.