Starboard would be a poor choice, while it won't rot, it is not a structural product. The chainplates exert forces measured in thousands of pounds and starboard is not up to the task. White oak would work. High quality plywood without voids and knots might also be a choice.Alternatively, you might want to use Starboard or some other plastic wood like Trex to replace the board. That way you would never have to worry about rot again.
I rebedded my chainplates and replaced the wet core under the starboard one this past summer and didn't drop the mast. I supported the mast with both the main and jib halyards clipped to the toerail outboard from the chainplate. I felt confident doing this because my halyards were both less than a year old and I've owned the boat for 7 years and have done a lot of work on it. A new to me boat that I didn't know the history of, or the age of the halyards I would drop the mast to do this repair.I don’t think you have to drop the mast to fix this, but you need to secure the mast somehow (using a halyard maybe) to keep it from falling when the shroud is remove to fix the chain plate. This assumes you do it quickly and don’t leave the mast unsupported by the shroud more than a day while you are there.
They were built similar to the Boston Whalers: Fiberglass shell with a foam core inside. Some say that you could saw the entire boat in half and it would still float! That being said, I'm pretty sure the wood is glassed to the inside of the cabin, reinforced by the bolts going through the shroud plates. If that wood was removed, replaced and re-glassed to its mount, that would probably do the trick. The reason I'm guessing this is because I know a guy who's been trying to sell his Bayliner 20, and it had a similar construction.how Bayliners were built
I think @Caboteur is correct. I did not see that at first but it looks like the forward chainplate is no longer anchored at all. Fix this ASAP before the next big wind storm comes.On the picture the forward chainplate has been pulled up one notch... this is probably for the lower forward stay. that would make it an emergency repair: I wouldn't use the boat until it is repaired. I would lower the mast ASAP: on a 26 foot boat this isn't difficult and you don't need a crane or other mechanical aid; a couple of helping friends suffice!