Pine 2x4 (Or Doug Fir if available would be better). Cut to fit and treat with woodlife copper coat or equivalent. Paint with latex house paint or rustoleum enamal.
How do you figure pine or Douglas Fir would be better than Black Locust? Do you know about black locust?
en.wikipedia.org
The
wood is extremely hard, being one of the hardest woods in Northern America with a
Janka hardness test of 1,700 lbf (7,560 N).
[26] It is very resistant to rot, and durable, making it prized for furniture, flooring, paneling, fence posts, and
small watercraft. Black Locust is a highly durable, organic wood product, that does not require chemical treatment to preserve its beauty for 50+ years.
[27] Wet, newly cut planks have an offensive odor which disappears with seasoning. Black locust is still in use in some
rustic handrail systems. In the Netherlands and some other parts of Europe, black locust is one of the most rot-resistant local trees, and projects have started to limit the use of tropical wood by promoting this tree and creating plantations.
Flavonoids in the heartwood allow the wood to last over 100 years in soil.
[28]
Yes it may cost about $8/foot, but for the mast step on a boat it is well worth that cost compared to the cost of pine followed by the cost of the preservatives needed, then the need to replace in any case. Use black locust on your boat and it will last longer than you.