Fleet Boat Speed Control 1: Fairbanks-Morse 38D 8-1/8 9-cyl

[Rosencrantz]

Last edited: 21th August, 2015 Updated 2/2.1. and added 2.2.

1. Prologue

During the last few days I have been researching speed control in the boats equipped with Fairbanks-Morse 38D 8-1/8 9-cylinder engines. Purpose of this and following topics is threefolded:

1. "Which are the correct speeds for different engine order telegram settings" is one of the most frequent question among submarine simulator users, modders, and other non-professional submarine enthusiasts. Even if there is heap of true information availible today, there is still many questions left, and even more simplified, inaccurate, insufficient or even wrong information availible in the internet, books and other publications. Aim of this and following topics is to provide in the future accurate, reliable and as completed as possible information about the Fleet Boat speed control, based on original US Navy documents.

2. Few years ago I started a project called "Fleet Boat Engine Hour Mod", discussion here:
http://www.subsim.com/radioroom//sho...d.php?t=180989
Project was then hindered and later abandoned due to computer loss and incoming real life tasks. However, I'm now slowly getting back in the saddle. My intention is restart previous project, because some progress was achieved previously.

3. If possible, bring an end to "yes, no, maybe or maybe not" discussions regarding speed control in Fleet Boats. I have found topic somewhat more complicated and diversified than often seems to be understood.


"How to?"

a) All information provided by me in this and following topics will be based on real life US Navy documents, especially Submarine War Patrol Reports 1941 - 1945, not forgetting fleet boat and equipment operation manuals (NavPers), other technical and operative documents, and other possible sources considered both reliable and accurate enough by me.
b) Excerpts short enough will be presented here, otherwise link or reference information will be shared.
c) Original post will be edited by me as more data is found. Date of the last update will be presented on top. As I do this in my freetime, updates won't be frequent. Active and passive periods will vary.
d) Screening boats and war patrol reports is based on type of engine. At the moment I'm researching boats with Fairbanks-Morse 38D 8-1/8 9-cyl. Boats with other engines will follow later.


"What to expect"

Along new knowledge also new question must be expected to be raised during this research. However, I have found speed control question quite interesting and I have already learned something new myself.

Mistakes in grammer and spelling will be present in texts written by me, as I'm not a native speaker. However, I hope data, excerpts and links provided and research done will outweight my personal shortcomings.


2. Narrative

2.1. Even if we are talking about submarines, I want to start from totally different kind of a warship. Different pretty much in all aspects like size, speed, type of missions she was build for and tasks she completed. We'll start from USS Enterprise, CV-6. We'll start from her, because I think she might be able to teach us something new about speed control in US Navy ships during WWII.

From a dedicated site we can find parts of the Ship's Operations Manual (http://www.cv6.org/ship/logs/default.htm). Chapter 9 Ship Control is very interesting regarding our topic as Section 904 describes carrier's speed control. There we can see how for example an order "Standard speed" actually did not mean some particular speed in knots, but quite opposite. This order was situation-dependable to a great extent. While in one situation it could mean as slow propeller rpm as 54 (revolutions for 6 knots) in another situation it could mean rpm 309 (revolutions for 33 knots). Full speed was always 5 knots and flank speed 10 knots faster than current Standard speed. (Probably those orders were not used if stadard speed was sometimes set at 33 knots..?) For additional information about speed control in Enterprise see also Sections 905 - 908.

Previous example raises a question if speed control in fleet boats was organized and handled in a similar manner as in flat tops. Quite often it has been understood that for example "Standard speed" meant always 13 - 14 knots speed in fleet boats. This is actually not true. Standard Submarine Phraseology originally issued by ComSubLant is a booklet describing the voice procedures and phraseology recommended "as standard for all submarine interior communications". I have a copy published by Periscopefilm. On the page 19 "Engine and battery combinations" there is one example of setting stadard speed: "Standard speed will be one eight knots." USS Triton's (SS-201) second War Patrol Report offers another example:

Jan 25, 1942: "Standard speed was set at 13 knots but could not be maintained at all times due to rough head seas..."

2.2. Speed range for one engine on the line

Quote from "The Fleet Type Submarine", NavPers16160: "For surfaced operation... the motors develop power ranging from 20 hp to 2,700 hp per propeller shaft at speeds ranging from approximately 67 rpm to 282 rpm."

Determining the slowest possible speed for a Gato/Balao/Tench and maybe also for a Tambor-class fleet boat is pretty easy. While there is no straight relationship between engine rpm and speed the latter is close tied together with propeller rpm. Knowing the top speed of a Gato-class boat was 21 knots at 282 rpm, 67 rpm equals with speed of 4,99 knots. Tambors were a bit slower, probable slowest speed on the surface being some 4,85 knots in a fair weather and the calm seas. USS Trout (SS-202), Report of the sixth War Patrol: "October 31, 1942: Patrolling on east-west courses... at 4 knots". So far4 knots on the surface has been the slowest speed I have run into while reading the reports.

To find out top speed or speed for 80-90 combination are both much more difficult. Raw calculations taking account the shape of the hull (displacement hull), lenght of the waterline, displacement, and both engine and motor limitations and estimated mechanical efficiency of some 84 % suggests that Tambor-class top speed with one engine on the line was more than 12 knots but less than 13 knots. Simplified example:

1600 hp x 0,844 = 1350 hp (shaft)
3 304 000 lbs/1350 hp = 2447 lbs/hp (shaft)
2447 x 1.19 = 2912 lbs/hp (engine)
2912 = (10.665/x)3 => 0.75
Waterline length 300.85 ft (Tambor-class)
x/square root 300.85 = 0.75 => 17.35x0.75 = 13 knots.

1280 hp x 0,844 = 1080 hp (shaft, 80% load 90 % rpm)
3 304 000 lbs/1080 = 3059 lbs/hp (shaft)
3059 x 1.19 = 3640 lbs/hp (engine)
3640 = (10.665/x)3 => 0.69
x/square root 300,85 = 0.69 => 17.35x0.69 = 12 knots.

Archerfish's first War Patrol Report gives some support to previous calculations:

"Jan 2 - 4, 1944: ...turns for 11.5 knots with one main engine..."

Previous remark covers two days enroute so it is not likely that the crew took all out of the one single engine on the line but engine was running somewhat lower rpm. It is even possible 80-90 combination was used.


To be continued.


-RC-