I found out a document (in french) explaining that N18 was originaly the first part of a hydroplane that should have been completed with a wing.
But the wing manufacturer named "Voisin" never delivered the wing.
"No. 18
No. 18 was to be a hydroplane of
speed consisting of three floats built by
Lachambre joined by a metal frame
carrying engine and sails. The sail, which was to
coming from "Voisin", was never delivered.
After tests on the Seine river without an engine at the end
of the summer, Santos-Dumont installed on No. 18
the 50 hp V8 Antoinette engine then in September the
100 hp V16 engine which comes from No. 17, before
flow with her into the Seine river and escaped from drowning
. On October 17, 1907, he returned the machine to
condition for a photo shoot.
If it can help ...
machines_volantes_santos-dumont.pdf