There is something behind the hull of that sub. I have seen many low res sat images of the Arihant class over the years but this is a unique silhouette. There is something there. A pumpjet seems unlikely but not impossible.Either these kids have next-level eyesight or they see what they want to see.
Submarines are launched with a shroud/cover on their propeller blades. This is done to hide the number of blades & the blade geometry. These details can be used by enemies to estimate the noise pattern of the submarine. To avoid that propeller blades are covered in a shroud (usually made of water-resistant fabric like tarpaulin) during the launch ceremony. The shroud is taken off once the sub is in water as the blades are hidden underwater. This is standard submarine building practice.
Except in this case there is no official launch ceremony. No media was present, there isn't even an official confirmation that the launch even happened. All we have now are some satellite photos. There will be no pics released to the media as was the case with the INS Arihant & the INS Arighat. The Navy is very secretive about the Arihant class.
So what's the point of a shroud here ? Who are you hiding it from ? There is no one there to photograph it anyway. Also the sat pics show the sub in water the shrouds should be taken off once in water. Why is it still on ?
The shroud also appears much much bigger in proportion to the sub than normal. So either the S4 has abnormally large propeller blades or this is a pumpjet. Neither of those 2 conclusions add up. Abnormally large propellers mean larger acoustic signature, why would you what that ?
On the other hand if this is a pumpjet, then where did we get it ? The French have only recently offered us their pumpjet tech, even that is not confirmed. Was DRDO working on one by themselves before the current 35 MWe pumpjet project came up ?
So the DRDO developed a pumpjet (presumably not as powerful as the upcoming 35 MWe model), tested it & put it on one of the most prized assets of the Navy all in absolute secrecy & in the course of just a couple of years ? Our research work into pumpjets started with the Shakti thermal torpedo. That torpedo project was cancelled by the Navy but the R&D work continued. Either they were in a far more advanced state of progress than what I estimated when the 35 MWe pumpjet tender came out or we had some foreign help.
There are other discrepancies with the the silhouette seen here. This is what the Arighat (S3) looks like:
We know the Arihant (S2) & the S3 are exact copies of each other, the S4 & S4* are different. The last 2 are heavier, not much else is known.
Notice in the pic above there is a vertical rudder behind the sub hull. Compare that with the pic in the tweet. No vertical rudder on that sub. Where did it go ? Did we shorten it so much that it is underwater ? Or have we replaced the vertical rudder with a X-rudder configuration ?
The S4 & S4* are clearly more than just stretched Arihant class. They are different in their design & the technologies used. I think I am speculating a lot but we won't have any official details on these subs for years to come. Speculation is the only thing left to do.