I'm a warship aficionado, and even though they never show entire or close-up shots of the Blucher, I could tell it had telltale signs of an early 20th century battlecruiser (like pivoting single gun turrets below the bridge). Does anyone know which ship they used to shoot this?
I recall visiting the Mediterranean Film Facilities in Malta when I was a kid and seeing one of the models built for the movie. This one was a few metres long. Apparently three had been built in different sizes and one of them had been blown up for the final scene.
It's probably a model and a set, the same as the Black Pearl in Pirates of the Caribbean. The ship does seem to be based on the WW1 era German large armored cruiser Blucher, which historically was nowhere near East Africa - it was sunk off Dogger Bank in 1915 and its sinking is one of the iconic photo images of WW1.
This is where "making of" extras would really help. Unfortunately missing on the new Blu-Ray as well as earlier 120-min DVD.
Remember, this was the 70's - you couldn't just CGI the Blucher, a 1912 battle cruiser! If it's a model, it's extremely well done. Especially the ramming scenes....but hey - movies are magic, right?
*Everything happens to me! Now Im shot by a child! (T.Chaney)
Found it! Thanks, Internet! 36 feet is a pretty fair-sized model.... Although to match the original proportions of the 1909 armored cruiser Blucher, the model would've been stretched notably longer. Lots of WW I images a click away...
This link may help you sailors with your other film questions, too.