
Location of serial number (HIN) on a 1978 O'Day boat.
Key to Month of Model Year
| AUG |
A |
|
FEB |
G |
| SEP |
B |
|
MAR |
H |
| OCT |
C |
|
APR |
I |
| NOV |
D |
|
MAY |
J |
| DEC |
E |
|
JUN |
K |
| JAN |
F |
|
JUL |
L |
Example #1: XDYF26040373
In this example XDY is the designation given to the O'Day company and F means it's a Mariner. 2604 is the sail number, the 0373 means the boat was built in March 1973.

Here is an actual molded in serial from a 1978 boat.
Code XDY (O'Day), F (Mariner), 3627 (Sail Number), M (Spacer), 78 (Year) D (Nov. built)
Example #2: XDYF2547M72E
In this example XDY is the designation given to the O'Day company and F means it's a Mariner. 2547 is the sail number, the M was used as a spacer and the 72E shows that the boat was built in December 1972.
Starting August 1, 1984 the letter designation for the month built next to the year built starts with A=Jan, B=Feb. etc.
Boats manufactured or imported on or after August 1, 1984, also have a duplicate secondary HIN affixed somewhere on an unexposed location inside the boat or beneath a fitting or item of hardware. The purpose is to help authorities identify your boat if a thief or vandals remove or damage the primary HIN on the transom. It is illegal for anyone (manufacturer, dealer, distributor, or owner) to alter or remove a HIN without the express written authorization of the Commandant, U.S. Coast Guard.
Mariner Style Chronology
1963 - O'Day fixed keelboats No. 1-92. Some of the initial centerboard boats were also built. We know of centerboard boat No. 20 still sailing @ Surf City YC
1963 - 1968 - O'Day centerboard design with the open cabin and hatch forward to approx. Hull No. 1414.
1969 - O'Day 2 Plus 2 design that has sleeping for 4, self bailing cockpit, full cockpit to the transom, closed in cabin and a forward hatch. Approximately hull No. 1385 to approx. 2372
End of 1971 - O'Day forward hatch removed, aluminum interior cabin mast support added and modified the cabin companionway.
1973 -1979 - O'Day built sail numbers 2372 to 3771. Sail number 3771 was the last O'Day Mariner built.
1980 -1984 - Rebel Industries Spindrift Design built hull No. 4000 to 4078
1986 - to present Stuart Marine Corp. has built hull No. 4079 to present. (No. 4193 built by late 2005)