I did some searching on this and was surprised that the use of coated canvas was used on upper decks starting the early 19th century.  For me, that is not a pretty thing. Some thoughts regarding trennals........ they were not generally used on decks.  Wrought iron spikes or dumps were used then covered with a diamond shaped or round wooden disk. For our purposes, a wooden trennal does give the appearance of the wooden disk cover.   When the wood of the decks was holystoned, the wooden disks were subtly visible, if at all.   On scales smaller than 1:48 leaving out the wooden covers or trennals is often a good idea.   
Allan