To understand wooden boats, one has to know a little about wood. Here are the "wood basics".
One can think of wood structure as a large number of short pieces of small pipes glued together, all in a parallel direction.
The short pieces of pipe are called wood fibers. They are usually hollow and their walls consist of compounds called cellulose and hemicellulose, which are chemicals related to sugar.
In addition to short fiber pipes hardwoods (deciduous trees) also have longer and thicker pipes, that take care of nutrient transfer within a living tree. Because of these longer pipes, hardwoods have more longitudal holes than softwoods (coniferous trees). In softwood nutrient transfer is taken care by the short fiber pipes.
The length of the fiber pipes in hardwoods is about 1 mm / 1/25". In softwoods 3-8 mm / 1/8"-1/3".
The fiber pipes are "glued" to each other by a substance called lignin.
Lignin softens at the temperature of about 170 degrees centigrade / 340 Fahrenheit. Moisture lowers the softening temperature below water boiling point. Steambending of wood is based on these facts. The glue between the wood fiber pipes softens, allowing the pipes to slide relative to each other.
For some reason, that I don't understand yet, the softened lignin allows wood to contract but not to stretch. Because of this, when steambending, the wood on the inside of the bent curve gets compressed, the wood on the outside stays pretty much intact.
Depending on the wood species the ends of the small pieces of fiber pipe may be closed or open. For instance spruce has closed cell ends, pine has them open.
The structure, parallel pipes glued together, dictates the strength behaviour of wood. In the lengthwise direction wood is one of the strongest known construction materials. All strength parameters along the length of wood are at least tenfold compared to the same parameters measured across the grain. It is virtually impossible to break wood by pulling lengthwise.
If a batten, plank or plywood is extended with a scarfed joint, the usual advice is to make the scarf lenght 8, 10 or 12 times the material thickness.
This is based on the "all strength parameters along the length of wood are at least tenfold compare to the same parameters measured across the grain", and on the fact that good glues are slightly stronger than wood across the grain.
Using a scarf like this the area of the glued surface is approximately 10 times the cross sectional area of the glued wood members, and the stregth thus approximately equal.
The trunk of a tree consists of heartwood, in the middle, and sapwood closer to the surface. The cells, pieces of tube, in sapwood are living. They carry and store nutrients for the tree.
Heartwood is dead. Heartwood cells don't contain nutrients but, depending on the species, the wood may fill the heartwood pipes with extracts, that may color the heartwood (darker than sapwood) and/or make it more rot resistant and/or make it less moisture absorbent.
No wood species has rot resistant sapwood, because sapwood carries nutrients. Wood nutrients are also nutrients for fungi.
Planks that have been sawn out of trees felled in winter time are more rot resistant than planks felled in the summer, because the nutrient content in the trunk is lowest in winter time when the tree is at rest.
A pine trunk used as a bridge pole. The difference between sapwood and heartwood is clear. Sapwood is infected with fungi, heartwood at least seems healthy.
And some unchopped pine firewood. Sapwood grows fungi in all colours.
And mind You, these logs have definitely not been fertilized, so this is not a symptom of "modern forestry".
In a book from 1957 the chapter on boatbuilding wood says:
In an old pine tree the portion of heartwood is some 50-60 % of the trunk cross section. So if You want to radially saw planks of 15 cm / 6" width, You need a heartwood of some 40 cm / 16" diameter. And that again gives an outside diameter of some 60 cm / 24".