Copied and pasted from the PB1 rules on the RYA website
http://www.rya.org.uk/SiteCollection...2012.08.09.pdf
G13. RACE BOAT NUMBERS
(a) Numbers shall be painted in waterproof black paint on a yellow or white
background or be made of black coloured self-adhesive materials of adequate
strength. No number may begin with a zero. All numbers shall be plain and
upright.
(b) The numbers shall be painted so as to be clearly visible from both sides and
from above. Those on the beam shall be placed on the forward half of the
boat.
(c) Those numbers displayed on the foredeck and on the underside of a tunnel
shall read correctly from the transom and both class letter and number shall
be underlined by a black bar. If displayed on the afterdeck, the number shall
read correctly from the transom and must be underlined by a black bar.
(d) The individual numbers shall conform to the following minimum dimensions,
the only exception being where the size of the boat does not allow the
minimum size to be carried.
Height: 30cm Width: 23cm
Thickness: 5cm Spacing: 13cm
(e) Each numeral shall be contained within the imaginary square made up of the
height and width and the spacing taken between the imaginary vertical lines of
the square. The numeral 1 (one) is governed only by thickness and height
and the spacing is measured from the edge. The background shall extend at
least 23cm in front of and beyond the end numerals and at least 7.5cm above
and below the numerals. All numerals shall be plain and upright.
(f) The class letter shall precede the numbers and letters must be the same
minimum dimensions as the numbers.
The letter and numbers shall be separated by a dash.
Class letters are: Class 2 litre C
Class 1.3 litre B
(g) On all boats the numbers shall be positioned as far forward as possible whilst
adhering to the requirements of rules G13 (c) and (d).
On catamaran boats the numbers must be placed on:
(1) The foredeck within front third of boat
(2) The tunnel underside within front third of boat
(3) The outside of both sponsons within front third of boat
On all other boats the numbers must be placed on:
(1) The foredeck within front third of boat.
(2) Both sides of the hull within front third of boat.
(h) The beam race numbers may also be painted or glued on slightly curved non-
vertical surfaces provided that the whole number is within 30 degrees from the
vertical position. The height must be 0.30m measured vertically.