Why Luftwaffe and Kreigsmarine diesels but petrol Wehrmacht?

z42 said:

In addition to dieselization, an interesting naval propulsion what-if is drastically more powerful boilers. In particular, in 1933 the RN tried a Velox boiler from the Swiss BBC for a destroyer, promising an order of magnitude improvement in steam generation per floor area. Despite this test, at around the same time the RN decided to standardize on the Admiralty pattern boiler, so nothing came of it. But what if they had gone down that path?

And how did the Velox boiler achieve such power density? Well, BBC had the idea to compress the incoming air (=more heat generation per volume), and increase the velocity of the flue gasses. Ok, so add a compressor to compress the incoming air. And how to drive the compressor? Well, there's quite a lot of energy in those flue gasses, so what about a (drumroll..) turbine wheel? I guess you can see where this is going. Now with the technology of the day there wasn't much excess power available after running the compressor (presumably required a lot of extra air to avoid melting the turbine blades with the available alloys, inefficient blade design etc.), so it was clearly a boiler and not a gas turbine engine.

But if one of the big navies had gone for this system in the early thirties and shoveled R&D money into it, we could have had gas turbines a few years sooner, with interesting ramifications both for WWII naval ships as well as aircraft.

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They were looking at "gas turbines" before WW I. They had the theory. The Problem was they didn't have the compressors figured out so that results never equaled theory.
Brown Bovire had a working gas turbine powering a generator before WW II.

They had to sort out the compressor construction and make things lighter than what stationary power plants would put up with

z42 said:

But if one of the big navies had gone for this system in the early thirties and shoveled R&D money into it, we could have had gas turbines a few years sooner, with interesting ramifications both for WWII naval ships as well as aircraft.

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The first naval turbines were tiny in shaft HP output, as they were based off of aircraft engines.

So would be looking at PT boat sized in WWII era

They didn't get Destroyer sized til the Spruance, and that still used a unit based from aircraft, and it needed four GE LM2500 Turbofans derived unit to get 80,000 shaft HP to get 33 knot top speed from the 9200 ton hull, at a decent economy rating from being a high bypass design.

It's decades to get gas turbines in ships, and not boats.

marathag said:

The first naval turbines were tiny in shaft HP output, as they were based off of aircraft engines.

So would be looking at PT boat sized in WWII era

They didn't get Destroyer sized til the Spruance, and that still used a unit based from aircraft, and it needed four GE LM2500 Turbofans derived unit to get 80,000 shaft HP to get 33 knot top speed from the 9200 ton hull, at a decent economy rating from being a high bypass design.

It's decades to get gas turbines in ships, and not boats.

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To further this. Advanced steam power plants in ships were derived from stationary powerplants, usually electrical generation.
Now in a stationary powerplant......
There is no real constraint on weight, the power plant is not going to sink (unless somebody really screwed up the site preparation)
There is no real constraint on volume, Length and width and more importantly height have a lot few restrictions.
The powerplant is not moving, rolling, heaving, the bases are not flexing (ship hogs or sags as hull goes over waves).

On the other hand, marine powerplants usually have more down time for maintenance/repair/boiling cleaning. Between the wars Destroyers were sometimes scheduled for 20% or more "down time" for boiler cleaning (even with oil fuel). Many times ships cruising at sea ran on some of their boilers while one or more other were being cleaned. Something that would be unacceptable for municipal powerplant for a city. Yes they need maintenance but buying 20-25% more capacity than you need to handle regular-scheduled maintenance is a little wasteful. Invest in heavier, more durable machinery to begin with and get longer life and less maintenance you need less redundancy.

Even steam or diesel electrical generators on ships, much smaller than main engines, need to be durable/long lasting. Having to use torches to cut through decks and use large cranes for replacement is not an easy thing. Only once aircraft gas turbines demonstrated service lives of thousands of hours to go with the smaller size and weight they became attractive for marine/naval use. I don't see it going the other way. Navy investing in powerplants (even for emergency generators) that need replacement/overhaul in just a few hundred hours and do much of anything for aircraft powerplant development seems about nil.

Shortround6 said:

Navy investing in powerplants (even for emergency generators) that need replacement/overhaul in just a few hundred hours

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That may be an in for emergency power source, as would be far lighter than existing diesels, and being emergency use, a short life isn't as a huge drawback, as these are small enough to be easily serviced more often

marathag said:

The first naval turbines were tiny in shaft HP output, as they were based off of aircraft engines.

So would be looking at PT boat sized in WWII era

They didn't get Destroyer sized til the Spruance, and that still used a unit based from aircraft, and it needed four GE LM2500 Turbofans derived unit to get 80,000 shaft HP to get 33 knot top speed from the 9200 ton hull, at a decent economy rating from being a high bypass design.

It's decades to get gas turbines in ships, and not boats.

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Yes, historically the first (successful) marine gas turbines were aero derivatives, like the LM2500 you mention. (And no, for a gas turbine you don't want any bypass.)

The alternative path not taken I described above would be to start from the industrial turbo-based boilers like the Velox boiler I mentioned, as a way to improve the power density of the boilers. And later as that concept evolves, it would have been kind of obvious to take some output from the gas turbine shaft in addition to the steam turbine. Poor efficiency of the gas turbine part in isolation wouldn't be a problem, as you have the steam plant to utilize the heat.

Indeed a pure gas turbine, without the steam plant, would have been something only for smaller craft late in WWII, if at all.

There is a paper here that begins by tracing RN marine gas turbine development from WW2 onwards. The first MGT went to sea in 1947 in HMS MGB 2007. It was a development of the Metrovick F.2 Beryl engine that first flew in Nov 1943 in Meteor prototype DG204/G.The experiments continued through the 1950s and resulted in Combined Steam and Gas Turbine power plants in the Type 81 Tribal class frigates and County class destroyers constructed from 1958 onwards and 100% GT power in RN ships designed from the late-1960s, like the Type 42 destroyers & Type 21 frigates.