This was a fishing vessel from Sri Lanka. It is best described as a one way proa. The distinctive thing about this craft is that the outrigger is to windward, and when the wind is on the other side, the crew move the outrigger over from one side of the boat to another
The original information from this craft comes from a report written by JAMES HORNELL, Director of Fisheries, Madras Government, in 1920. In other words our best information comes from a report written 91 years ago.
Since that time the South Indian Federation of Fishermen Societies, Trivandrum (wherever that is) have reissued the report on the internet. My understanding is that since the report is 91 years old, copyright issues are not deemed a pressing concern. According to the South Indian Federation of Fisherman Socities, they have undertaken to see if a copyright is still valid on this text, but have been unable to find any claims.
The report is called The Origins and Ethnological Significance Of Indian Boat Designs. It can be read in full HERE for those with an interest in traditional craft of the indian subcontinent.
If I may add a little personal opinion here, why bother to move the outrigger over each time, why not just have two? The reason is of course that an outrigger to windward has lower stresses on it, and can thus be built more lightly and is also less likely to break. Evidence of this can be seen on the very light framework that pacific proas use to support their outriggers (amas) compared to the very heaveer crossbeams used in the Samoan Amatasi. The double canoe of course (now referred to as a catamaran) has extremely strong (and heavy) crossbeams needed to withstand the the forces of the ocean.
My knowledge extends to only one other one way proa that I know exists. That craft was built by Gougeon brothers in an attempt to gain the world sailing speed record. The boat was called Slingshot. The difference between Slingshot and this craft is the means by which the outrigger was moved from one side to another. Slingshot used a sliding mechanism, whereas this historical craft used a swivelling mechanism. More information on slingshot can be found by putting Dave Culp Speed Sailing into a search engine to find his website.
What follows is an extract from James Hornell's report of 1920
Two varities of modified outriggers exits, the one using a single pole to
boom out the outrigger, the other employing the normal two (Fig.6 and Fig.7).
The hull is a simple Malabar dug-out canoe, with usually a narrow wash strake
added vertically – not flared. The rig is a simple squat lugsail of the same form
as is employed in the Tuticorin fishing canoes. No attempt is made to spread
the canoe, so without an outrigger it can be used only in calm weather. It is so
used occasionally but normally the outrigger is fitted. The latter is smaller than
Indian Boat Designs the Ceylon type and boom pole or poles are weaker and each consists
always of a single pole without fascine strengthening.
But the most remarkable divergence is that these outriggers are made to unship instantly by a very
simple device and can be rigged out on the other side of the boat, thus
avoiding the unseamanly custom of the Sinhalese who can never say which
end of canoe is the head without looking at the direction they may happen
to be sailing ! (my personal addition of bold text)
The Kilakarai boats can therefore employ a rudder fixed at the
sharp curved stern by the orthodox pintles and gudgeons. The details of the
stays led to the outrigger are readily seen in the sketches given. The device for
attaching the booms inboard on the one side and to the outrigger on the
other is a form of the Spanish windlass, well known to sailors and shipwrights for
exercising force in bending a plank into position and holding it there till secured;
the principle is that of the surgeon's tourniquet. In the present instance the
loop end of a ring of rope or grommet is passed through a hole either in the
gunwale or in the outrigger float, the boom pole is laid over this and the looped
ends of the grommet are brought up at each side and over the pole; the end
of a short rod or stake is passed through the two loops and then by the simple
device of twisting the two loops round one another by means of the rod, the
two main parts are bound together with the greatest possible tightness. The
free end of the tourniquent rod is then seized to the gunwale or the boom as
the case may be, and if the various parts be sound this lashing will maintain
attachment under any ordinary violence. Its chief advantage lies in the rapidity
with which it can be operated. A couple of seconds suffice to release the
boom and scarcely more are required to reship it.