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Shin'yō Type 1 test run
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Shin'yō Production
by Shigeru Makino, former Navy Technical Captain
Shigeru Makino was Team Leader of the team
responsible for design and production of the shin'yō explosive motorboat.
1. Origin of Maru Yon (Shin'yō)
At the end of March 1944, the Chief of the Naval General Staff issued two
secret military resolutions to the Minister of the Navy. The first resolution
strongly requested the realization of production of nine types of special
(suicide) attack weapons. If these could be accomplished, Japan certainly would
be able to win. These nine weapon types were the following:
Maru Ichi (①) Kanamono - midget submarine to attack conventional submarine
Maru Ni (②) Kanamono - weapon to attack antiaircraft installation
Maru San (③)
Kanamono - kairyū midget submarine
Maru Yon (④) Kanamono - shin'yō
explosive motorboat
Maru Go (⑤) Kanamono - self-propelled depth charge
Maru Roku
(⑥) Kanamono - kaiten human torpedo
Maru Nana (⑦) Kanamono - radio wave weapon
Maru Hachi (⑧) Kanamono - radio wave jammer
Maru Kyū (⑨) Kanamono - shinkai
midget submarine
The nine weapon types were given code names to keep the information secret.
For example, the literal translation of Maru Yon (④) Kanamono is Circle 4 Metal
Fitting, which gives no indication of its true purpose. The other resolution
issued to the Minister of the Navy had 26 or 27 ideas that would be extremely
advantageous to the war situation if they were possible, but many of these were
farfetched notions similar to dreams.
The first resolution was studied carefully and seriously by each department
of the Navy Technical Department, but the only ideas accepted as possible to
realize were items 4, 6, and 9. At the start there were given temporary names of
Maru Yon (④) Boat, Maru Roku (⑥) Kanamono, and Maru Kyū (⑨) Kanamono, and we
started trial manufacture for them. Maru Roku was the later kaiten. As
for Maru Kyū, there was a trial manufacture and was named shinkai, but it did
not reach practical use. The Maru Yon Boat was named shin'yō.
2. Basic Construction of Maru Yon
The Naval General Staff's original concept for the Maru Yon Boat was a vessel
that would carry out a special attack with the objective of sinking a landing
craft. The boat would be an extremely light collapsible type with an outboard
motor attached that would carry out the attack with a small amount of
explosives. However, the production number goal had to be achieved.
When this concept was investigated, the request for something lightweight and
small was reasonable, but there was a large inconsistency between the results of
the loaded explosives and the goal of sinking immediately a ship of several
thousand tons. Also, in addition to the power of domestically-produced engines
being small, the possibility to achieve such production volume was nil.
Because of the goals requested by the Naval General Staff, we thought that
about 300 kilograms of explosives would be needed along with a speed of above 20
knots, and if possible about 30 knots would be desirable. On the other hand, as
for an engine that actually could be produced, we thought that there was nothing
other than an automobile engine, and we estimated that between 70 and 80
horsepower could be achieved if its generating power were increased. We
considered production of the hull and arrived at the conclusion that wooden
construction was most appropriate since the materials and production workers for
this would not compete with other production items. If we went with this
proposal, on the whole a boat of about one ton with a speed of 25 knots was
possible, but the handling of this extremely light boat had a long way to go.
Based on this concept, overall agreement with the Naval General Staff was
obtained as a result of discussions, and trial production started immediately.
3. Design Concept for Maru Yon Boat
As for a domestically-produced automobile engine capable of use as the main
propulsion for a small highspeed boat of one ton, there was only the gasoline
engine produced by Nissan and Toyota for a four-ton truck. Since this engine was
heavy, it was apparent that this would be unfavorable. If this disadvantage were
not compensated for by a lightweight design for the hull, then the boat would
not be realized. In other words, as for the lightweight design, we were
determined that we must proceed based in the belief in the basic design concept
for the Maru Yon Boat. We also strongly demanded that an engine be installed on
the boat. Also, as for demonstration of speed, we could not give up on efforts
to do whatever it took regarding the important point of responsibility to
achieve it. On the other hand, we thought that we would need to remove
everything else in the boat. Because of this, Section 4 (Shipbuilding Section)
leaders felt deeply the necessity of having the Yon Maru Boat designated as a
special attack weapon, and this was able to be realized by going through various
procedures. Based on this, the Yon Maru Boat did not fall under jurisdiction of
general warships, and the aims of the production designers had the possibility
of being realized completely as each department provided cooperation with their
requests. This was a very necessary and reasonable step to realize quickly
performance improvements for the Maru Yon Boat. Through this step it was quickly
possible to have tests for increased power generation for the engine, and also a
drastic reduction of propeller shaft diameter was realized with the goal of
minimizing drag caused by additions. With regards to realization of weight and
speed, this arrangement resulted in much advantageous guidance.
It was not that we could always play our trump card. Afterward, our request
for cooperation from Section 5 (Engine Production Section) was gladly accepted,
and they shared in the work according to their regular jurisdiction. Our
gratitude cannot be expressed enough for their extraordinary efforts made on our
behalf for the boat's realization.
4. Basic Testing
One test was to increase the engine's power generation as previously
mentioned. Section 4 personnel themselves went to the Nissan and Toyota
factories and emphasized the test's necessity to those men directly involved and
requested cooperation. The result was that tests were completed in an extremely
short period. It was concluded that it would be possible to realize about 80
horsepower with the Nissan engine and about 70 horsepower with the Toyota
engine. It cannot be forgotten that Dr. Kazuo Sumibe, Tōkyō University Professor
who was resident as a consultant at the Toyota factory in a suburb of Okazaki
City, gave various relevant suggestions and also achieved success by
recommending them to company leaders.
Another test was concerning the effectiveness of the explosives. In an
experiment with a conventional underwater explosion, there was nothing that took
place on the surface. There was an opinion that in a surface explosion the
pressure produced by the explosion would disperse into the air and would not be
able to cause great damage under the surface. Without delay a test was carried
out at Yokosuka Armory with a box model of a large merchant ship at a reduced
scale of 1/3. This result was a small opening created on the surface and a large
opening produced below the surface. In other words, it was proved that even with
a surface explosion, then the large pressure from the explosion if near the
surface would be transmitted from the surface to under the surface. It was
clarified that the blast opening on the plating below the surface for a large
merchant ship caused by the Maru Yon Boat explosives would be between three and
five meters, and it led to the belief that the boat's power would be sufficient.
With completion of these tests in the middle of April, we looked for
opportunities on ways that progress could be made quickly. Needless to say,
these tests brought about a tremendous effect to heighten motivation of men
involved in the program.
Frame production
5. Basic Design and Trial Production
In April we quickly started on the design, and we set a schedule to carry out
a test run on the auspicious date of May 27, Navy Commemoration Day. We promptly
completed the basic design, and the detailed design was prepared at Yokosuka
Arsenal. We immediately started on production.
For trial production, five wooden boat hulls were ordered from the Nihon
Shipbuilding Tsurumi Factory, and Yokosuka Armory took responsibility for
outfitting the boats. The wooden boats were based on the boat design for a Type
5 Hydroplane Torpedo Boat, but in order to reduce weight the bow was cut and
shortened, which resulted in the sacrifice of lower cruising speed. That is to
say, it was reduced to a total length of five meters from a normal length of
about seven meters. Based on this, the center of gravity when loaded with
explosives was appropriate, the hydroplaning capability was favorable, and the
realization of speed was optimal. The hull, deck, and traverse bulkhead used
waterproof plywood for everything. The framework used Chinese red ash wood, and
a small amount of zelkova (Japanese elm) wood was used for the primary longeron.
It was decided that the assembly would be with urea resin in its entirety, but
there was joint use of rivets where there was compression bonding at adhesive
joints.
There was not sufficient confidence regarding the plywood, so a
countermeasure was put in place for the worst case scenario. It was decided that
there would be a trial production of a boat with very thin steel plate and with
a stepped hull having a length of seven feet. The outside plating on the boat's
bottom was normal plating, but corrugated plating was used for the sides and the
deck with thicknesses of 0.8 cm and 0.5 cm, respectively. The hulls for the two
metal boats were made by the fittings factory of the Yokosuka Armory
Shipbuilding Section, and the factory completed the boat outfitting in parallel
with the main production.
The Yon Maru Boat trial production progressed extremely smoothly, and a trial
run was carried out according to schedule on May 27. While the steel boat did
not achieve the desired speed in the beginning, the wooden boat, which was what
was really desired, was confirmed to display capabilities according to plan, and
we decided to mass produce it.
Durability tests and wave endurance tests were carried out promptly by
Yokosuka Armory with the trial production boats. Accidents occurred twice where
both sides of the bow's exterior plating experienced fatigue and broke apart and
then the boats sank. Furthermore, based on drop tests that also were carried
out, it was decided to improve wave endurance capability by shrinking the frame
weld spacing only on the bow, strengthening the outer plating with a two-ply
application, and using a clipper model for the bow tip. Based on these changes,
the boat's overall length was increased to 5.1 meters.
The Yon Maru Boat was essentially a special attack boat, but from the
beginning design there was a mechanism to fix the rudder wheel that was
installed, and a pilot wearing a life jacket could escape out the boat's stern.
Afterward, the Torpedo School training unit carried out this escape, and it was
confirmed to be simple and safe.
6. Hull Construction
The biggest problem regarding the hull was the quality of the beech plywood,
which was the main material. If beech tree raw timber is cut down and left as
is, a rot called fuke will develop. Since plywood that used layers taken
from such raw timber had a considerable percentage that escaped inspection and
were contaminated, it was brought to the attention of production workers that
they should not use such boards.
Next there was technical guidance regarding the framing of the wooden
framework. For close adhesion work, urea resin exhibited an extremely high level
of adhesion strength, but when applied with a certain thickness, a phenomenon of
rapid aging occurred, and the adhesion strength was lost. Therefore, it was a
priority that everyone in the adhesive department perform their work without
leaving any space, but to maintain this during production was exceedingly
difficult. Since the majority of Maru Yon Boat production factories were
handling lightweight wooden highspeed boats for the first time, groups were
formed that made a tour of factories around the country. First, they were made
aware of the importance of this boat, and they were provided guidance to fulfill
manufacturing technical requirements. Also, measures were taken to point out
defects in actual boats.
Boat framework (keel)
7. Cooperation on Training
Tactical training was carried out at the Torpedo School, and the initial
manufactured boats were assigned entirely for training use. Later a torpedo
training facility was established at Kawatana on Ōmura Bay as a Torpedo School
branch school, which was designated solely for training of Maru Yon Boat pilots.
Boat pilots came primarily from the Yokaren (Naval Preparatory Flight Training
Program).
We frequently competed even in training, and we worked out steps to address
problems of boat performance improvement. These steps were thought out based on
training. In tests carried out near the breakwater of Yokosuka Naval Port, there
was research on (1) how to clear different types of navigational barriers if the
boats attempted to enter a harbor and (2) the relationship between the
detonation device structure and the angle limits when setting off a detonation
in a diagonal ramming during a taiatari (body-crashing) attack. Also, it was a
quite late period when a test was carried out where a specially-made balloon was
produced as a training target to be towed by a torpedo boat. Besides these,
there was an experiment to load the boat with a larger fuel tank to extend its
cruising range and experiments to equip the boat with a machine gun and a rocket
launcher, and these were later implemented.
There was a large-scale general staged maneuver carried out at Sasebo
Arsenal. The boats were carried by transport trucks from air-raid shelters on
land to the place on shore where they would be launched. Several dozen boats
maneuvered to the place where they would attack enemy ships. It was an amazing
event.
Those men who had been involved with training were invited to a farewell
party for the Chichijima Special Attack Squadron, which was the first
operational unit. The Torpedo School's somber party was held the night before
departure at Komatsu Restaurant in Yokosuka, and the following morning there was
a huge sendoff when the squadron departed from Yokosuka Harbor. An SB craft
(second-class transport ship) was responsible to carry the boats and men to the
island of Chichijima.
8. Engine Problems
When rigorous training began with boats produced for actual use, engine
problems appeared one after another. Causes of the problems included no gasket
countermeasures had been implemented to take into account that cool seawater was
used directly with the engine that was originally designed for cool fresh water,
seawater vulnerability of electrical parts such as the starter motor and dynamo,
no appropriate measure for exertion of continuous power output at an extremely
high level, and inexperienced handling by pilots. Based on enthusiastic
investigations by General Staff Section 5, Yokosuka Armory, and Sasebo Armory to
address these problems, at a later stage appropriate items went into limited
operational use. The strenuous efforts of those involved with this were
extraordinary. Although we can say that from the beginning we were resolutely
thorough in producing a marine boat that would not have accidents, since it
would not be used for a long duration and since there was an urgent situation
with regards to material and production, actually we made certain decisions so
long as they were not unreasonable.
The shaft bracket was a highspeed boat single support. The installation unit
cut it and did a highspeed wave endurance test while it hung down from the
shaft. From consideration of the fact that the occurrence of even the slightest
vibration was not noticed and that a shaft accident was not seen even once, it
seemed that the Yon Maru Boat's shaft had more than enough strength for the
short time that the boat would be used.
9. Production Results
With regards to Maru Yon Boat production results, 75 boats were produced in
July 1944, and afterwards in many months the production volume rose to between
500 and 700 boats. By the end of the war, 6,200 boats had been manufactured.
Just by looking at the 20% to 30% allotted to Maru Yon Boats of the Navy's
small number of truck engines at that time, it can be seen that the Navy had
high expectations for them. The Navy Technical Department responded well to
these expectations and produced actual results up to 85% of demand. These were
remarkably good results not seen with other engine types.
Excellent production results could be achieved so long as there was a
sufficient allotment of engines. Also, there was little competition from other
programs for other materials and manpower, and the boat was a simple structure
to assemble with few departments that were involved. Probably the primary reason
for these favorable results was that production went forward until the end
without any significant changes in the original design.
Shin'yō Type 1 assembly work
at Yokohama Yacht Chōshi Factory
10. Technology Level of Maru Yon
At the initial training by the implementation unit at the Torpedo School,
various deficiencies and improvement points were pointed out. However, with
regards to the basic design, it was thought that there was no room whatsoever
for these to be incorporated. It was because we believed strongly that from a
technological standpoint the best design had been produced and it was the
highest-level product technologically at that point in time. That belief was
shaken, and I heard at some point that some wanted to challenge the men in
charge, but there was no compromise in principles. Based on continued testing,
finally its real value was understood. Later this led to obtaining cooperation
for improvements in performance and production.
Looking technologically, the men involved with the Maru Yon Boat considered
it their pride as a lightweight high-performance boat.
11. Persons Responsible for Design
The Section 4 men involved in the basic design at that time were Section 4
Director Vice Admiral Iwakichi Ezaki, Planning Chief Rear Admiral Yūki Katayama,
Design Chief Rear Admiral Masatsune Yagasaki, Team Leader Captain Shigeru
Makino, and the following design personnel: Engineer Shō [1] Koyama, Technical
Lieutenant Eiichi Watanabe, Engineer Torao Ōno, Technical Lieutenant Junior
Grade Ryō Hayase, Technical Lieutenant Junior Grade Keitarō Takagi, and others.
Design skills based on Engineer Koyama's deep knowledge and proficiency
concerning highspeed boats produced excellent results without equal in the basic
plan for the Maru Yon Boat. Afterward, he established detailed technical
guidelines for manufacturing operations. His contributions to production of the
Maru Yon Boat were tremendous. I as the Personnel Leader, other than carry out
my duties to communicate the technical knowledge of my competent staff members,
did nothing more than only provide encouragement. When I look back, it is
embarrassing.
Note
1. The given name of Shō is uncertain since his
given name of Japanese (捷) has several possible readings, and the correct
reading could not be confirmed. Shō is one of the more likely readings.
Source
This article comes from pages 22 to 27 of Volume 1 of the following book:
Shin'yō Association (Shin'yōkai), ed. 1990. Ningen heiki: Shin'yō
tokubetsu kōgekitai (Human weapon: Shin'yō Special Attack Corps). Shirō
Arai, general editor. Two volumes. Tōkyō: Kokushokankōkai.
Shin'yō Type 1 Boats
Translated by Bill Gordon
July 2025
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