Bunker C is about one grade thinner than asphalt. Slight exaggeration
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It needs to be heated, usually with steam pipes in order to get it to flow, especially in ships operating in cold water.
What they could do with it post WW II is not what they could do with it in the 1930s. Yes with good fuel heating and good filters you run diesels on it.
But at what cost (space and maintenance) in the 1930s.
In the history of marine engineering they often used commercial ships for testing. Once they got something to work in the lab/shop/test bed they stuck it in one or more small ships to see how well it actually worked in service. Ferries and coastal passenger ships were popular. If they broke they weren't too far from shore and/or were not far from regular maintenance.
German navy historically thought their machinery builders could do better than they actually could or at least they hoped they could.
With the needed power rising at exponential rates successful commercial use was no guarantee of successful high speed military use in early adaptations.
First commercial ship with steam turbine.
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a little under twice the tonnage of the HMS Viper and 1/3 the power, 1901.
It was used for speed, fuel consumption and other performance evaluations like weight and space of engine rooms compared to other steam plants. It also stayed in service until 1951/52. Within 5 years the Cunard line specified turbines on the
Lusitania and
Mauretania (took about 5 years to complete).
The Steam turbine worked out rather well, but a high powered triple expansion steam engine had some drawbacks, especially for high speed use.
Pushing the boundaries does not always work so well even though the goal does get reached.