Wooden Boat Building... Steel Deck to a Wooden boat

I think you are overly concerned for this condition, the wood hull is large and will tolerate a lot of changes in size and angle before it will fail.

Changes in size/shape at that attachment is small, the steel will not change shape enough to matter, the hull will have to enlarge/shrink around the steel beam length. However, the largest force at the joint is compression, when the hull goes in the water you will get compression on the deck beams, no matter what the material the hull is made from. OTOH, in heavy seas the deck beam connections will see both tension and compression. So the joint has to be designed for loads in both directions.

Lets walk through every combination: hull on the hard, deck in the sun: deck expands and wood hull dries and shrinks; hull goes in the water and deck in the sun, but cooled by water shrinks slightly and hull expands in the water. The steel changes size in relatively small amount compared to temp/moisture changes in wood, so hull for the most part will expand and contract around the steel deck beams. BTW, an all wood hull design would have the same issues because the hull will expand/contract a lot more with moisture than the deck since presumably the hull will always be in the water and the deck out of it (one would hope anyway). But wood deck beams are more flexible than steel ones.

The most severe part of this load condition is there will be rotation at the beam/hull join that could introduce unaccounted for loads in the connection. so you have to design a connection that will tolerate small amounts of rotation. This can be accomplished two ways, use a single pivot bolted design (L-bracket on hull and single large through-bolt at beam end), or to slot beam-end holes to allow some relative movement between the parts. I would design for slotted mounting holes, preferably in double shear, and use of some kind of soft sealant/caulk to allow movement. This will also allow redundancy in the joints.

I do not think the rotation forces will be that large but it is possible that some of the hull planks my want to pull free of the ribs if there is too much reflex in the ribs at the level where the deck beams attach to the hull. So the slotted holes in the connection should accommodate it, or design for an attachment that allows for rotation.

That is how I would design that attachment if someone brought that problem into my office.

Good luck.