Hi!
Kawanishi K-5 mail seaplane.
Kawanishi began K-5 Mail Seaplane as the first domestic large transport seaplane in November, 1921, and only 1 plane completed in October, 1922.
Two crews were sit side by side and a room with a mail was installed behind of the crews. A room with a mail could also be diverted into a guest room of 4 passengers.
The construction of the main wing and the arrangement on a seat of this aircraft were novel as those days. U.S.A.27 type was adopted as the cross-sectional form of the main wing. However, this aircraft was lacking in output of an engine, and the expected performance couldn't be shown. Tested several times , but take off from the water was difficult and wasn't a gift after all.
Crew : 2, Passengers : 4
Engine : Maybach M4a inline liquid cooling 6 cylinder, take off power 305HP/1500r.p.m
Propeller : fixed pitch wooden 2 blades propeller
Span : 18.7m, Length : 12.635m, Height : 4.6m, Wing area : 68.5 square meter
Empty weight : 1750kg, Gross weight : 2700kg, Wing loading : 39.4kg/square meter, Power loading : 10.4kg/HP,
Maximum speed : 130km/hr/S.L., Landing speed : 90km/h, Service ceiling : 4000m
Source : ENCYCLOPEDIA OF JAPANESE AIRCRAFT VOL.3 KAWANISHI HIROSHO CONTENTS
http://en.valka.cz/topic/view/121595/Kawanisi-K-5
Please try auto translator.
http://www.aero.or.jp/web-koku-to-bunka/2016-03-ishimaru/2016-03.html