Flush vs Broken Decks

[Bjoring]

Can someone knowledgeable on the subject explain the advantages/disadvantages of flush vs broken deck hull designs and what goes into deciding what kind of deck a ship design will use?

[SergeantHop]

Ships with a forecastle (the raised bow of a ship), often have better seakeeping qualities and are more stable in rough seas, though larger ships often don't have this issue due to the fact that their decks are more often farther out of the water. 

[MrDeaf]

keeps the front guns from getting wet, while also not drastically raising center of gravity on the ship as a whole

[RedBear87]

Can someone knowledgeable on the subject explain the advantages/disadvantages of flush vs broken deck hull designs and what goes into deciding what kind of deck a ship design will use?

The discontinuity of the decks create a structural weakness in the ship when compared to a flush deck design; the advantages of a forecastle were already pointed out above. In the old Japanese navy an alternative compromise between the need for high freeboard forward and weight reduction elsewhere was found in Hiraga's undulating flush upper deck, which was continuous but of decreased height from stem to stern. A similar solution was retained in the stern of the first JMSDF destroyers, it was nicknamed "Dutch slope", after the steep streets of Nagasaki.

 

EDIT: The final decision between the two depends on many variables, including the expected theatres of operations and other requirements. Tight designs for example tend to opt for the flush deck in order to save weight, for instance it was the case of Zara and Algérie vis a vis the previous generation of Italian and French heavy cruisers.

[tremor3258]

 

The discontinuity of the decks create a structural weakness in the ship when compared to a flush deck design;

 

Basically - the 'end' or 'break' of the forecastle to the main deck can be (if not careful) a place where the stresses on the hull accumulate to an unusual degree.  These problems weren't always evident before you had computers to do the stress calculations, or necessarily in normal service until doing damage analysis, observing metal fatigue at end of life, etc.  

[Sad_But_Drew]

Also depends on how high the main deck was.

 

USS DDs of the Wartime classes were flush-decked with a high freeboard the whole way back.  Which gave a lot of strength (especially since the hull frame was longitudinal as well (ran the length of the ship, not "ribs"), as well keeping things pretty dry.  But they didn't have tonnage restrictions by then. 

 

Just raising the forecastle was buying seakeeping at the expense of tonnage and topweight (and having a stress point in the hull, often where the ship would break if a torpedo hit).