Ray,
I think this refers to Part 1 registry. O.N. is the Official Number and R.T. is the Registered Tonnage. The ON is issued and the RT is measured as part of the application, both are to be “marked on the main beam or on a readily accessible visible permanent part of the boat”. If I recall you are also supposed to have the vessel name and Port of Choice “marked conspicuously on the stern”.
As mentioned RT is an old concept relating to the internal volume of the vessel and is calculated in units of 100 cubic feet, this is why I think the fraction is expressed in 100’s. For yachts I think you could estimate this from (0.45 x LOA x B x D)/100, All in feet and where D is the height from the deckhead to the inside of the hull at midships (other estimation formula are available), but for registration I believe you have to get a qualified measurer.
Jon
SS26/96
Jon Foreman
Calypso SS26, no.96