I already discussed this encounter way back in my Many-Finned days, but I just thought of a novel explanation that segues into another topic I've got in the works.
Conversions and hyperlinks are my own.
Heuvelmans (1968) citing Ouest-Eclair inquiry from unnamed Captain (possibly P. Merlees):
I saw, some way ahead, a black mass which at first I took for a capsized boat. On approaching and examining it with binoculars, I found it had a strange shape. This resembled a framework over which sail had been tightly stretched. The ribs were very marked. Seen from the side and from some way off it would certainly look indented, for the ridges were very sharp. I had a three-quarter view which enabled me to make a rough estimate of its size.
The colour was black; the length about 16 feet [4.9 m] and the width about 5 feet [1.5 m]. It was quite motionless.
As I wondered what it could be I kept coming closer and clearly distinguished all its features. When I reached some thirty yards [27 m] away a huge head emerged some 4 or 5 yards [3.6 or 4.6 m] from what I could see, and therefore nearer to me.
Although surprised by this sudden apparition, I could observe it very well, and it was very like the head of a turtle, but longer and certainly 2 feet [0.6 m] wide by 3 feet [0.9 m] long; it had two very bright black and white eyes and large nostrils. It was blackish like the rest. I could not see the jaw, the mouth being shut, but the mouth was clearly marked on the sides of large dimensions.
The head turned to look at the ship, blew noisily without spouting water and at once dived, the rest following and making a big wash.
When I reached where the animal had been I could see nothing but the wash in the water, and that was all.
Given the dimensions of this animal it could not possibly be confused with a turtle. For one thing it certainly had no scales, of that I am sure. The skin was more like old tanned leather, and with my binoculars I could see it very well.
The head appeared for a few seconds only, and I did not look at it with my binoculars, but it was very close, perhaps 25 yards [23 m] away, and what struck me most were the eyes. The turtle has only very small eyes, veiled by a membrane, and not big bright eyes like these. And so far as I know there are no turtles of this size.
From the head to the end of the visible part was a good 30 feet [9 m]; so it was indeed an extraordinary animal.
From the disturbance of the water at the moment when it dived and from the part of the body that I saw, the shape of the body must be: a very long and flexible neck, indicated by the distance of the head from the visible part of the body, and by the head which turned without the middle part moving; then a much broader part in the middle, the part that I saw; and finally a fairly long tail which did not show above the surface, but was clearly marked by the disturbance of the water.
Observation was hindered by the fact that the sun was partly hidden behind small clouds and made a white reflection on the surface which prevented one from seeing to any depth.
The poor light may perhaps explain some of the differences between this and other reports of the Along Bay dragon. The projecting ribs and spine are certainly the most interesting feature here. They remind one of the parallel ridges on the leathery turtle and the basking shark, which had [sic] a stabilizing effect, except those are longitudinal-in any case the beast is too big for a leathery turtle, and the basking shark has no neck. It is, however, hard to believe that the ridges across the spine could be taken for a jagged crest, and one cannot help wondering whether the beast's condition was normal. Sea-mammal are, for obvious reasons, usually fat, protected against the cold by a thick layer of blubber. Might not this specimen be wasted away by age or illness? It is hardly likely. The explanation I shall be putting forward is that this emaciated appearance may be due to transverse bands of armour, as on the armadillos.
Heuvelmans' analysis is bullshit. It is painfully obvious he was guilty of the very 'pigeonholing' and 'mental gymnastics' he had scorned; the Hanoi encounter bears no resemblance to the Halong Bay cryptids and neither of them can be plausibly connected with encounters of 'Many-Finned' cryptids, namely by lacking multiple fins. This encounter appears to be the genesis of the 'armored Many-Finned' concept which, aside from the very surreal Tran Van Con report (or dream?), has gone unmentioned in other accounts. This doesn't indicate that cryptid categorizations are fundamentally flawed, but more critical focus is needed on individual reports is fundamental. Who knows, it may turn up something interesting, albeit non-cryptozoological...
It also needs to be repeated that reported size in an encounter should not be taken as an actual figure. With that in mind, I wondered if the encounter actually was of a Leatherback Seaturtle which was mistakenly interpreted as having transverse ridges and a long neck. It still is a possibility, but there is a more parsimonious explanation out there:
|Florida Softshell Turtle (Apalone ferox) from Flickr user dotpolka.|
|Nile softshell (Trionyx triunguis) from Flickr user xor74.|
Obviously neither of these non-Asian species is a candidate for a sighting in Vietnam, but they demonstrate that trionychids could fulfill most of the observed traits. A head quite some distance from the main body (and inferred long neck), leather-like skin, prominent nostrils, big eyes, 'ribs', and ridge all fit perfectly. The long tail is problematic, although since it wasn't actually observed, it could have been the wake. The reported body width (about 1/3 length) is much more narrow than the carapace width relative to SCL in a softshell (typically closer to 1/1), so explanations could be that the carapace was partially submerged, or the inferred length included a non-existent tail. The illustration shows a domed structure, which could either be a misremembered or exaggerated carapace. As for Vietnamese softshell candidates:
Palea steindachneri is moderately sized for a softshell (max SCL = 44.5 cm) and is apparently restricted to freshwater (Ernst and Lovich 2009). Pelodiscus sinensis is small to moderately sized (max SCL = 33 cm, possibly 40-45 cm formerly), and has not been recorded at sea (Ernst and Lovich 2009) - but is tolerant of brackish water. Amyda cartilaginea is large (max SCL = 83 cm) but only recorded from freshwater. Rafetus swinhoei reaches a max SCL of at least 80 cm - possibly much larger for the Hoan Kiem Lake individual - but is too poorly known to determine if its (former) habitat included oceans. Pelochelys cantorii reportedly can reach gargantuan proportions (max SCL = 200 cm), and has been found at sea according to Turtles of the World. and (1975) report an individual P. bibroni caught in a trawl 5 kilometers off Mandapam (India), which lived an additional 14 days in a saltwater aquarium. It is worth noting their specimen with a 57 cm carapace (39 cm bony portion) had a head that was 13.8 cm long by 8.2 cm wide - similar in proportions to the reported 3 foot long by 2 foot wide head of the Hanoi animal - eyes that were 1 cm wide, and a proboscis that was 0.2 cm long by 0.9 cm wide. Presuming P. cantorii is at all similar in habitat and appearance, which it is, its similarity with the Hanoi cryptid would be very striking indeed.
Aside from the reported length (explainable by the omnipresent human tenancy to exaggerate), the dark coloration (explainable by the lighting), and the shell proportions, the most parsimonious explanation for the encounter is a large softshell, probably Pelochelys cantorii. It is possible the Hanoi encounter documented rare surface behavior in a marine setting for this species - possibly multiple - which is/are now endangered thanks to the ravaging Asian turtle market and habitat loss.
Ernst, C. H., and Lovich, J. E. (2009). Turtles of the United States and Canada. John Hopkins University Press: Baltimore, Maryland. ISBN 13:978-0-8018-9121-2
Heuvelmans, B. (1968). In the Wake of the Sea-Serpents. New York: Hill and Wang.