In 1915 France and Britain tried to force the Dardanelles. Why did Russia not use its Black Sea fleet to attack the Bosphorus at the same time? Surely a two-pronged assault would have been seen as the most effective way to break Turkish resistance?
They waited until the England & France to break through and expected them to weaken the Turks. Even though Turks were weak, everything the Empire had was hoarded into the Bosphorus and they couldn't get past Bosphorus in a short time and without casualties. It was the best protected place in the Empire. Turkey's founder, Atatürk also requested the reserve troops stationed near Istanbul but the Empire refused which left Gallipoli undermanned.
But in an event of a allied victory in Gallipoli, they were ready to sail, thats for sure.
Ps: As a fun fact, the narrowest place in Bosphorus is 700 meters whilst 1200 meters in Gallipoli. Which makes it even harder than crossing Gallipoli.