A -completely different method for simming WWII machenery weight and destroyers!

[BraselC5048]

(Note to Mods - feel free to move this to where it should belong, in the likely event it's not here)

I've been using SSb3 for simming WWII era fast battleships/cruisers, and I've figured out a method for properly including the more efficient (more shp/ton) machinery of the era. For cruisers and up, first set the speed slider in SpringSharp to the desired speed, and note required horsepower. Then choose a machinery efficiency in shp/ton, based on historical examples. You most include this!

Then, divide the the shp required by the figure chosen above, to determine what the machinery weight -should- be. Then adjust the speed in SS until the machinery weight equals the resulting figure, and make the design based on that, except top speed will be based on the horsepower noted at the start.

Worked example:
A cruiser (14,953 t standard) was desired to make 35 knots. Setting to 35 knots in SS showed a required 157,553 horsepower. A machinery weight of 33.3 shp/ton was chosen, as it is better than the (on battleships) German ~28 shp/ton using high temperature/pressure boilers/turbines, but less than the ~35 shp/ton of even better US (even higher temperature/pressure) machinery. Dividing out showed a machinery weight needed of 4723 tons. Setting the speed to 31.553 knots resulted in SS showing exactly that machinery weight. The rest of the design was done with the speed/horsepower left there.

For destroyers, even more different. SS grossly overestimates required shp. Making a slightly larger Shimakaze equivalent, Logi's download from here https://www.navalism.org/index.php/topic,6009.15.html was used to determine required shp. It came in about 4000 shp less than the historical figure, and since the ship was ~100 tons heavier, I increased it to about ~84,000 shp (compared to the historical ~79,000 at its 40.9 knots trial). Extensive reasearch and a lot of luck turned up the Shimakaze's machinery weight, which showed ~78.8 shp/ton (destroyers engines produce more power for weight). Adjusting the speed slider to give the machinery weight using the above calculated machinery weight, (don't use the weight from Logi's program, but include the report from it and all decided figures!), the destroyer both worked in SpringSharp, and somewhat surprisingly came up with good seakeeping and a composite strength of 1.0. So, with this method, destroyers have to come in with a 1.0 composite strength.

Certainly useful. :)

[Kaiser Kirk]

Certainly useful. :)

Definitely interesting.
The current system of using different comp hulls is a little odd and could use improvement.
I suppose one could point to the the scantlings  on Destroyers vs. Cruisers vs. battleships as part of the issue, but the engine weights have been recognized as the real one.
If the shp/ton is the problem, that already varies with engine date. So why not establish what the shp/ton per "engine year" is and use the engine date as a variable so the report stats work properly. That way someone wanting to replicate the work can do so.

I would caution against using trial speed for anything. Different countries ran trials in different ways, often with the vessel missing important things one would want for actual service, and tolerated different overloads, so the relationship to a working top speed can be hazy.

[BraselC5048]

Certainly useful. :)

.... but the engine weights have been recognized as the real one.
If the shp/ton is the problem, that already varies with engine date. So why not establish what the shp/ton per "engine year" is and use the engine date as a variable so the report stats work properly. That way someone wanting to replicate the work can do so.

That would work, except SS doesn't do dates past 1950, and WWII-era US drives for battleships at around 35 shp/ton are already much lighter than SS's 1950 engines. Destroyers were 60 shp/ton plus, well past SS's upper limit, and the French somehow got ~60 shp/ton on the Dunkirkie, (sp) and 70+ shp/ton on the Richaleau! (sp.) Really, making the machinery weight slider on ss3b3 actually work would pretty much fix the problem. (For bigger ships, anyway, SS still grossly overstates power needed for destroyers.) This idea would require what Logi posted (or simply the version he had at ~2015 for basically everything hull-related.) Anybody able to get ahold of him for even as-is?)
You certainly would have to post the report Logi's program creates, and your chosen shp/ton, and be prepared to justify it! Frankly "high temp/pressure engines" would have to be a tech, separate from "double reduction gears," and those French boilers likely a second level as well.

I would caution against using trial speed for anything. Different countries ran trials in different ways, often with the vessel missing important things one would want for actual service, and tolerated different overloads, so the relationship to a working top speed can be hazy.

I used her overload power for all calculations, including on my ship. Mine can (presumably) only reach 41 knots on overload.

[Kaiser Kirk]

I used her overload power for all calculations, including on my ship. Mine can (presumably) only reach 41 knots on overload.

Interesting. I knew SS fell short, but  I didn't realize the SHP/Ton ratios got so far out of whack.  There's always been lots of details where things don't line - for example the % vessel given to structure should vary with type of vessel and weaponry, and it doesn't... nor is there an option for STS steel use, or welding etc. 
So I think you're correct, that would seem to be the viable course.

Trials and Overloads were something I only recently started paying attention to. Some nations ran trials with the ships still unarmed. Then overloads in early turbine years could be quite extreme since they really didn't know the proper engineering tolerances - that finally explained the range of speeds I've seen for the German BCs over the years.