Most seaworthy type of ship?

[wolfracesonic]

Ok, so you find yourself in the middle of an ocean in sea conditions that make the 'perfect storm' look like a millpond. What type of ship would best be able to cope/would you feel safest on? Would bigger be better e.g Nimitz carrier, Iowa class battleship (I know they don't venture out now) or something like an ocean going tug or even one of the self righting life boats the RNLI use?

[wolfracesonic]

I was thinking of surface ships, anyway submarines are boats aren't they?;) The idea of being in a top heavy, unstable ship seems insane to me as well, I think I'd choose the lifeboat.

[wolfracesonic]

MBBlat said:

Its not against physics - its all about the ships response to the sea, which in a storm is anything but flat.

A very stable ship will tend to stay perpendicular to the local sea surface, which means that on the side of the wave it can be at quite an angle to the vertical. A less stable ship (note not unstable) will not follow the sea quite so closely, thus have lower accelerations.. Add a bit of damping such as bilge keels to the equation and you can quite quickly reduce the amplitude as well.

If you still think that's counter-intuitive don't look up anti-roll tanks, http://en.wikipedia.org/wiki/Antiroll_Tanks as this uses the movement of water from side to side to counteract roll.

One example I worked on a while back, unmodified in beam seas the model tests (and reports from the full sized ship in question) showed roll amplitudes of 45degrees. Lowering the GM (ie reducing stability), increasing the size of the bilge tanks and putting in some anti-roll tanks and in the same beam seas the ship was just bobbing up and down with no roll.

That's me and RobDickinson put in our place then!

[wolfracesonic]

jkh112 said:

wolfracesonic said:

MBBlat said:

Its not against physics - its all about the ships response to the sea, which in a storm is anything but flat.

A very stable ship will tend to stay perpendicular to the local sea surface, which means that on the side of the wave it can be at quite an angle to the vertical. A less stable ship (note not unstable) will not follow the sea quite so closely, thus have lower accelerations.. Add a bit of damping such as bilge keels to the equation and you can quite quickly reduce the amplitude as well.

If you still think that's counter-intuitive don't look up anti-roll tanks, http://en.wikipedia.org/wiki/Antiroll_Tanks as this uses the movement of water from side to side to counteract roll.

One example I worked on a while back, unmodified in beam seas the model tests (and reports from the full sized ship in question) showed roll amplitudes of 45degrees. Lowering the GM (ie reducing stability), increasing the size of the bilge tanks and putting in some anti-roll tanks and in the same beam seas the ship was just bobbing up and down with no roll.

That's me and RobDickinson put in our place then!

But how much of the reduced roll was from the effect of the anti-roll tanks rather than the reduction in GM? Normally a reduced metacentric height will cause an increase in roll amplitude, but with a reduction in acceleration. If the roll amplitudes of 45 degrees were reduced by your modifications then I suspect that was primarily due to the anti-roll tanks, the reduction in GM may also have helped if it shifted the roll response frequency of the vessel outside the frequency range of the waves being modelled.

HaHa! Me and RobDickinson were right all along!