Ventilators are probably covered to stop saboteurs dropping incendiary devices into the 'tween decks. I know that was a requirement in some ports.

I don't think she was actually trooping with soldiers in the tween decks, it would be about a two day passage from Suez to Port Sudan - probably just carried a few drivers and such on deck.

PS error in my second post - Battle of Gondor was at end of 1941.

And why are the ships ventilators covered ? Must be really hot below decks

Dud link - from the Arnold Hague site -
Dunera
Suez, Dec 27, 1940-Port Sudan, Dec 29, 1940
Port Sudan, Dec 31, 1940-Suez, Jan 10, 1941
Port Sudan, Jan 13, 1941-Suez, Jan 15, 1941
Suez, Jan 25, 1941 - P, Port Sudan Jan 28, 1941
Port Sudan, Jan 28, 1941 -Suez, Jan 31, 1941
Suez, Feb 1, 1941- Bombay, Feb 14, 1941

So that would date the photo as January 1941.

Troopship is Dunera.
She was running Suez / Port Sudan at the same time in early 1941. Several of her dates match pretty closely.
http://www.convoyweb.org.uk/ports/index.html?home.php~armain

Possibly taken while at anchor off Port Sudan - you can see the anchor ball in the other photo.
From the movement card mentioned above.
Silvermaple arrived at Suez on 12th December 1940 from New York and Halifax via the Cape and then went to Alexandria to discharge. She then did 4 trips from Suez to Port Sudan. One side trip was made to India and several of the Port Sudan voyages originated in Haifa.
Leaving Suez on 15/5/41 she sailed to Liverpool via the Far East and Panama.
I would suggest she was moving military equipment to the Sudan to support the British army fighting the Italians in Ethiopia.
That war ended in November 1940 at the Battle of Gondor.

She made one more call at Suez on 4/2/42 on a voyage Glasgow/Suez / New Zealand.

If you go to the National Archives - here https://discovery.nationalarchives.gov.uk/details/r/D8655904 -
and sign up for a free account you can get her movements for the entire war - well - at least until she was sunk.
Ship in the background looks like trooper Devonshire or similar - get her movement card and if you have a bit of free time you can probably put time and place on the photo
Cheers,
Frank