where the halyards go through turning blocks at the deck, beside the mast, they exert a lot of load upwards. This can easily deform the deck upwards, which starts a chain reaction of loads all through the boat, included a 'pinching' effect that tries to make the whole boat narrower where the shrouds are. The deck stay, in our case usually just a turnbuckle, locks the mast and the deck together to counteract halyard loads.

Very old Catalinas don't have this, because the halyards terminated on winches on the side of the mast. But any boat with a keel-stepped rig and halyards that lead back to the cockpit needs something to tie the two structures together. Some boats use a turnbuckle between the mast and the deck, others use a stay from the mast step to the deck. Either system works fine.

Thanks for that explanation, I too have been scratching my head over why that is there. It never occurred to me that the loads from the running rigging would be that strong. Learn something new every day.

To give you an idea, on the 40'er my parents owned, we broke the deck stay during a rough crossing of Georgia Strait. Once we all recovered from the loud bang, there was about 1-2" of up and down movement happening with every wave where the mast went through the deck. We took up as much load as we could with a rope purchase, and still by the end of the trip the tabbing that held the main bulkhead in to the hull was starting to delaminate. This was more of a race boat, so the loads were proportionally higher, but still it is definitely important.

The purpose of the turnbuckle is to prevent the loads from the main and lower shrouds from distorting the deck upward. As you tension the mains and lowers, the sides of the hull and deck want to pull inwards. This causes the center of the deck to bow upwards. In order to prevent this, Catalina retrofitted the turnbuckle to older ships and made it basic configuration for newer ships. The best way that I have found to tension this turnbuckle is to do it when the standing rig is loose so you put an adaquate downward load on the deck to prevent the deformation when you tension the shrouds.

Tom has it right.