39 knots and the magic that is the weight of destroyers

[christian]

why is it 39 knots is basically impossible without having the destroyer have nothing but its weight dedicated to speed we have several examples in real life of destroyers surpassing that such as Le Terrible 45.03kts which was made in 1936

what interests me is if we look at ships like shimakaze or le fantastique they both used 75000 horsepower power plants yet both made speeds of up to 40 knots or more (le fantastique having the slowest ship in the class be 43 knots) 

also a slight bug with light cruisers
if you set the weight to 5900 you can get 39 knot speed for 14000 horsepower which dramatically reduces the weight needed
the diffrence in weight needed is almost 5000 tons and thus you can have extreme range reliable engines and 39 knots and still have several hundred tons left for armaments

also the math is wonky for example 2500 ton destroyers have -70 tons available with 39 knots but at 2000 tons they have +50 tons ?

it seems there is something wrong with the weight of machinery or the weight of destroyers because trying to build historical ships like for example shimakaze is basically impossible

also having looked a bit at torpedo tubes they seem kinda heavy i can understand it for larger torpedoes (oxygen ones)
but normal torpedoes should weight less for example the american quintuple torpedo launcher weights 19 tons or 42000 pounds while in game they weight close to 90 tons i would be fine with a reduction to 50-40 tons but 90 tons seems a bit much

archive.hnsa.org/doc/destroyer/ddtubes/index.htm#pg1 manual for the launcher

although i was not able to find data for the japanese type 92 torpedo quintuple mounts (even the us technical mission to japan dident state the weight) but since these torpedoes weight almost 2.8 tons per torpedo and the mount is shielded id expect it to be atleast twice that of the american mount

just slight things i noticed while trying to build the shimakaze and other japanese destroyers

Attachments:

[alsadius]

It seems like there's some definite breakpoints in the weight of machinery based on ship tonnage - they aren't continuous functions. My rule is not to worry about it too much - I pick a round number for the size of ship I want, and build it to that spec(my current generation is 1.5kton DDs, 5kton CLs, and 12kton CAs). But a more optimization-focused player might prefer to look for those breakpoints in the formulas and design a 5900-ton ship instead of a 6000-ton ship.

[elouda]

The main issue seems to be that as you approach the 'forbidden' part of the curve (which is 6000t and over for 39 knots), it uses 0 as the value for that instead extending the curve sensibly. Simple fix would be to provide one more set of HP values (so in this case for 6000t) in the SpeedHPTable2 (sadly right now doing this will just extend the allowed tonnage to 6900t instead of 5900t, so it needs a code change).

[evil4zerggin]

A hack to fix it without a code change would be to set the last value to 10x what it would be otherwise.

Anyways, if for some reason you really want to milk this exploit, the possible configurations are:

5900 t @ 39 kts
6900 t @ 38 kts
10900 t @ 37 kts
17900 t @ 36 kts
21900 t @ 35 kts
53900 t @ 34 kts

[rimbecano]

A few things to keep in mind:

1) Wave drag increases the shorter your waterline is, so smaller ships require more power per ton for the same speed. The reason that smaller ships tend to be faster is that:
a) Even though they take more power per ton, with fewer tons, a small fast ship is still cheaper than a big fast ship.
b) Because they're cheaper, they're more expendable, so heavy things like armor are less of a priority.
This is the reason that destroyers tend not to as much shorter than cruisers as their tonnage would suggest, and cruisers are not as much shorter than battleships as their tonnage would suggest: smaller ships are built with a higher length-to-beam ratio. But to keep things simple, RTW only models one hull form per ship type (modified by technology), so destroyers get an effective length-to-beam ratio that's closer to the historical average length-to-beam for a destroyer than the maximum length-to-beam ever built. And because of that constant hull form that's modeled, smaller ships of a given type will be draggier, and thus potentially need more machinery in an absolute sense for very high speeds than a smaller ship would need.

2) Many of the speeds quoted for ships like the Le Fantasque are sea trial or record-attempt speeds, and *not* representative of the ships' actual speeds in service. Sea trial speeds were often the builder's attempt to make the ship look good to the purchasing military, and often involved light loads, locations with fairly shalllow and carefully chosen sea bottom topography, and machinery operating in excess of rated power, and often were maintained for only an hour or so. At some points in history for some navies, it crossed the line from the builders trying to paint the ship in the best light to the builders straight up cheating, and the Royal Navy had to introduce rules for exactly what conditions sea trials were to be performed under when alot of their supposed 30kt destroyers in the period leading up to WWI turned out not to actually be 30kt destroyers.