D&R Marine has new Centerboards for sale,
www.drmarine.com Rudy Nickerson at D&R was the Part's Dept Manager at O'DAY from 1973 to 1989 and now runs a little shop making and selling parts for our O'DAYs. He should also have the specs on the original pendant setup (lifting line for CB) so that you can restore the ability to Raise/lower the CB again. I would not want to not have that ability, if for no other reason than to prevent exactly what happened to your boat.
I think the 1975 models used what was to become the "standard" for later O'DAY K/CB designs as far as pivot pin mounting. See drawing below. Older models of the 20 had a different setup using a plastic pin inserted through one side of the keel.
I'd either buy a new CB ($$$ unfortunately) or build a new one based on the original, using epoxy and fiberglass covered marine plywood. Structural foam coring (Airex, Dyvinacel, etc) could be used instead of plywood and would be less prone to rot. foam will require a little more ballast weight to allow the CB to sink as it is lowered, but should still weigh about same as original CB.
Now for a few Definitions for all of us:
SWING KEEL: basically a swing centerboard that contains all or most of the total ballast of the boat. Will require a strong winch to raise and lower, and really should be locked down while sailing. Raising the Swing-keel will often cause the boat to trim down by the stern as the heavy weight of the keel is pivoted aft as it raises. Boat is usually tremendously less stable with keel raised. as the center of gravity has also been raised closer to the center of buoyancy. This is a great way to have a deep-fin-keel boat, yet easily trailer the boat by simply raising the keel. However, they are not really designed to be sailed with the keel in any position except fully down and locked.
CENTERBOARD: A lightweight (O'DAY 20 CB is around 30-40#) adjustable fin that can be retracted into the hull or a shallow keel to reduce draft. When lowered it improves ability to steer the boat and to prevent leeway while sailing to windward or across the wind. Adusting it's position up and down (on pivot) allow one to fine-tune the balance of the helm by reducing weather-helm (tendency to turn into the wind) by raising the CB slightly, or reducing Lee Helm (tendency to turn away from the wind) by lowering the CB. When sailing off the wind or directly down wind, the CB can often be raised to reduce drag. Position of the CB has minimal effect on stability of the boat since it contains very little ballast.
O'DAY designed most of their boats as either Centerboard or Keel/Centerboard types. My DS II is a CEnterboard boat and the CB retracts completely into the hull for trailering as does the CB on the Widgeon, Javelin, Rhodes 19, Mariner, O'DAY 19, and a few older designs. The Mariner and O'DAY 19 were sort-of hybrids, more correctly called "ballasted Centerboard" boats. The Mariner had 200# of lead in the bilge and a 165# centerboard, still most of the ballast was in the boat, not the Cb so she is still a CENTERBOARD boat. The O'DAY 19 has 300# of lead In a shallow-skeg-keel (not really a true k/cb, but I would not argue with someone who called it a K/CB)
The O'DAY trailerable cruisers were mostly Keel/Centerboard designs. There was a shallow keel that contained all of the ballast in the boat ,and then a fiberglass centerboard was housed inside the keel to allow more resistance to leeway and better directional stability (ability to steer) when the CB is lowered, yet still have shallow draft with it raised and some directional control due to the keel. The 192, O'DAY 20, 22 (1979-83), 222, 23-1, 23-2, 25, and 26 were all K/CB models. All but the 23-2 had lead ballast contained within the fiberglass molded keel, the 23-1 had a cast-iron keel with a fiberglass CB.
O'DAY did build a few larger boats with the option of a K/CB, the 28, 30, and 32 had that option during at least part of their production runs. These boats may have winches to raise the CB due to the large size of the CB, but the Centerboard is still a centerboard and not a swing-keel due to it still being a very small part of the overall ballast weight. Again, stability would not change much between CB up and CB down.
