The full number as glassed into the transom on my boat is T8579013.
that is a Tanzer 8.5m 1979 year hull
number 13. My sail number is 13. Same thing with the T22 which is number 743. But the T22 were built in the US and Canada
yet they still seem to have sequential numbers. Some class associations like the Santana 525 have a different series of numbers as they are ( some of them ) registered with US Sailing. They have four diget numbers but they did not build that many. What we are concerned with in our local fleets is that boats do not have the same number. This is for racing
. At Swiftsure this year I saw another boat with number 13 on the main. They were in a different class to me so posed no problem. For cruising, who cares, but it does seem logical to use your hull number, unless your local sailing authority has another system.