The reason for the use of wood was the Govt requirement to not call on 'strategic' materials such as Dural and other specialist light alloys allocated to military production.
Some incorrect reports say that Hughes decided to use wood being an easily worked but strong material ...
It has never been cleared up as to why he actually took of from the water for the few hundred feet when in fact he was only cleared for taxi trials. There are reports that he was surprised by it happening having possibly miscalculated lift off speed being the plane was empty and without full fuel load.
We will never know the truth.
The fact is he walked away from the H4 after that day to never try her again saying he'd done his job.
Nigel