I though the reason for larger diameter has much to do with installation of raft inside the submarine and nothing to do with single or double hull
I can understand that any naval structure is cramp by nature with so many equipment and crew facilities competing for space . The other reason is vertical cruise missile tube that require bigger diameter
But I thought China has implemented the raft construction with their latest submarine type 93 III.
The diameter of a submarine is a function of its pressure hull diameter and its whether the submarine is single hulled or double hulled.
A single hull submarine means the entire overall diameter of the submarine is the
same diameter as its pressure hull, see below:
A double hull submarine means that the pressure hull diameter is
smaller than the overall diameter of the submarine, which is the "outer hull". In the below picture, the smaller red circle is the pressure hull while the bigger yellow circle is the larger outer hull.
Now, the
pressure hull of a submarine is the determinant of the amount of machinery, insulation and equipment and living space you can put into the main body of a submarine as a general rule of thumb (though one may argue some exceptions exist like Oscar class SSGN or the entire design of the Typhoon SSBN -- but let's ignore those for now).
In other words, if you want to put more machinery/insulation/etc into a submarine, you need a larger pressure hull.
So, let's say you have two different SSNs, both with a hull diameter of 10 meters.
One of these SSNs is single hulled, the other is double hulled.
The single hull SSN with a 10m diameter has a
10m pressure hull to work with to put all of its machinery and insulation into.
The double hull SSN with a 10m diameter has a pressure hull which is smaller than 10m, because the 10m diameter hull is merely the outer hull and not the pressure hull. For the double hull SSN, its actual pressure hull diameter is almost definitely going to be significantly smaller than 10m, usually up to 80% at most. That means the double hull SSN with a 10m diameter in reality
only has a 8m pressure hull to put its machinery and insulation into.
09III is a double hulled submarine. Public info estimates its overall diameter at 11m -- meaning its pressure hull is likely 8.5-9m.
Everyone agrees that the pressure hull of 09V will be bigger than 09III, the question is not about the specifics of what kind of additional equipment or insulation it may have -- but rather it is important to first talk about what kind of hulled submarine it is.
If 09V is a single hulled or hybrid hulled submarine -- then it can likely retain the same overall diameter of 11m or so, but its actual pressure hull will be larger, perhaps as big as 11m in parts of the submarine.
OTOH, if 09V is double hulled, then if they they want to have a larger pressure hull diameter, that means the overall diameter (outer hull) of the submarine will have to increase as well. That is why the Akula SSN (for example) has such a massive overall diameter of 13.6m, because it has a larger outer hull to accommodate a larger pressure hull compared to previous Soviet/Russian SSNs.
The reason this is relevant is because the new Bohai facility's SSN rail gauge is 7.34m, and the width of each of the slots corresponding to the 7.34m rail gauge is only 22.5m wide.
If you're building a 13m diameter submarine, a rail gauge of 7.34m might be a bit small to support a submarine and cradle of that width (I think 12m might be at the upper limit) --- but more importantly, a 22.5m width slot means building a 13m diameter in that slot will not have enough clearance on either side of the submarine for effective construction to occur.
That is the reason I am skeptical to Chris Carlson's idea that 09V will have an overall diameter that is as big as Akula's, because...
1. He seems to believe that 09V will be double hull, whereas rumours have suggested it will likely be single or at least hybrid hulled.
2. He doesn't acknowledge that the rail gauge of 7.34m and corresponding construction slot of 22.5m width is likely far too small to build a 13+ meter wide submarine.
This is a useful starter article to read WRT single and double hulls (again, from HI Sutton, whose Covert Shores website is where his best work is rather than his articles on Forbes imho), and the source of the above two pictures:
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Thanks for the response
@Bltizo. What do you think of Sutton's assertion that 09-V is not likely to use a pump-jet propulsor?
@by78 posted some scientific studies of various pump jets; given that the work was published in 2016 (and I presume held under a publishing embargo for some time), I feel that China has had enough time and experience with this design to field it on the Type 09-V/VI - do you agree?
Also, do you think the Type 09-V/VI will have full electric propulsion?
I wouldn't put too much stock into the specifics of the depiction -- it is as speculative as we are, and likely less accurate than our rumours.
