as US citizens here are the most likely options:
US documentation--
cheap, no tonnage
survey needed, can take months to get your
registration unless you show you are traveling. Annual renewal
cheap. Public
record on
ownership
US state- Delaware is not expensive, $20 per year, can be for 3 years, need a US address to get the
documentation. they are picky on paperwork like needing a notarized bill of
sale and ORIGINAL registration and/or title
Red Ensign (UK, Jersey, Guernsey,
BVI, Cayman) pretty timely but you need a tonnage
survey from their list of organizations and for a boat your size the survey will run $1000 or so. might have problems finding
surveyor off in
Japan. But if you pay their travel costs, surveyors will fly to you. You have to hold through a local corporation (except in Cayman, which accepts US corps as owners), which can run $300 to $2000 to set up and maintain every year--UK is fastest and cheapest,
BVI you likely get gouged. Jersey registration good for 10 years. Usually Red Ensign countries do quick turnaround.
I last used Jersey, cost about 1200 for the survey, forming a UK corp. and
fees.
Liberia-not really geared to private yachts, you need proof of
insurance and I think you would find the process a hassle.
Panama-also mostly geared to
commercial ships, I believe they are efficient but you must go through a Panamanian lawyer. It will cost you some cash.
Antigua and Barbuda do not require a survey, but, you have to form an IBC . this will cost you about $1500 a year to set up, and about the same to maintain. Or you could form a local corp. for less etc but you have to use a lawyer, No cheap way to do it.
Dominica is $3500 for initial registration.