Returning to our Member 34south's original question :- " what are the best flags under which to sail in 10 years time??"
The answer to that question - could be ; the flags that will give the least hassle in specific foreign waters in 10 years time.
Therefore rolling the clock forward to 2018, and if the rules haven't changed then register the boat in a country that is regarded as friendly to the countries of planned destination. As it is where the boat is registered that shall determine which national flag shall be flown.
Some extracted rules on Flag Etiquette :-
The National Ensign
The national ensign worn by a vessel must be the flag of her registry—not necessarily that of the owner or operator.
Generally, the national ensign should be displayed at the peak of the gaff, i.e., the outer end of the spar extending aft from the mast of your boat—if you boat has a gaff. If it does not, fly it from the flagstaff at your boat's stern. If your boat has an overhanging boom or an outboard motor, your flagstaff may be offset to starboard (preferably) from your boat's centerline.
Marconi-rigged sailboats may fly the ensign from the leech of the aftermost sail (or from the back stay), approximately 2/3 the distance up its length. This puts it in about the same position it would occupy if the boat were gaff-rigged.
At anchor or made fast, the ensign should be flown from the stern staff of all boats
Yacht Club Burgee
Generally triangular in shape, although sometimes swallow-tailed, the yacht club burgee contains a unique design symbolic of the organization represented. If you boat is a mastless or single-masted yacht, fly your burgee from the bow staff. Boats without a bow staff should wear a burgee at the truck of a single-master yacht. On the other hand, if the truck is occupied with instruments or other conflicting gear, a pigstick can be affixed to a halyard so as to carry a flag above the truck. Alternatively, the burgee may be worn at a spreader halyard. If your boat has two or more masts, fly your burgee at the truck of the forward mast. Do not display more than one burgee at a time. The burgee your boat wears should be that of the group in whose activity you are participating, or whose harbor you are entering, if you are a member of that group. Otherwise, fly the burgee of your home organization. Each yacht club usually has rules that determine when their burgee should be flown.
Owner's Private Signal
This is a personal flag, often called house flag. It is usually swallow-tailed, designed by the individual owner to depict a personal interest, hobby, family tradition, initials, or the like. A private signal should be a unique design and always in good taste. It should not include or be the ensign of a foreign country, nor duplicate a design previously adopted by someone else.
When you visit foreign water, your boat should display a courtesy flag (the civil ensign of the country you are visiting) whenever your national ensign is displayed.
If your vessel has one or more masts, display the "courtesy flag" single-hoisted at the outboard signal halyard of the main starboard spreader. Move any flag normally flown there to the inboard starboard halyard or, if your boat has only one halyard per side, to the port spreader halyard.
The customs observed in various foreign waters differ from one another. Try to learn the correct procedure for the country you are entering. For example, is some countries it is customary to fly the courtesy flag only after the quarantine flag (the yellow 'Q' flag) and the vessel has been granted pratique by the appropriate authorities.
Do not fly a foreign courtesy flag after you have returned to your own Country's waters. It is not to be used as a badge of accomplishment for having cruised to another country.
Foreign Guest Flags
When a foreign guest is aboard, you may display the ensign of the guest's country from the bow staff or outboard port spreader. Should more than one such guest flag be appropriate, wear them on spreader halyards
Size of Flags
Flags are often too small. When your purchase your flags, use the following guidelines, rounding up to the next larger commercially available size when necessary.
The national ensign flown at a flag staff at the stern of your boat should be one inch on the fly for each foot of overall length.
All other flags such as club burgees, officer flags, and private signals for use on sailboats should be approximately 1/2 inch on the fly for each foot above the waterline of the tallest mast on the boat. (That is, if the tope of the mast is 30 feet above the waterline, these other flags should be 15 inches on the fly.) On powerboats, these flags should be 5/8 inch on the fly for each foot of overall length. The shape and proportions of pennants and burgees will be prescribed by the organization to which they relate. A union jack should be the same size as the canton of the national ensign being flown from the flag staff.
Many foreign ensigns—courtesy flags—sold in stores are not manufactured to correct proportions. For instance, the flags of all former British Commonwealth countries, including Canada, Bermuda, the Bahamas, and the British Virgin Islands, are correctly proportioned 1:2, i.e., the fly is twice the length of the hoist. As a matter of interest, the United States flag is correctly proportioned 10:19 (nearly 1:2), not 3:5 as is commonly available.
BRITISH FLAG ETIQUETTE
The Red Ensign … is still the flag of the merchant fleet, and is the flag that should be flown by British owned vessels. Its use on pleasure craft is, only optional in UK waters, but in most foreign waters one would normally be expected to fly it.
There are still regulations in force requiring vessels flying an ensign to fly the Red Ensign. Flying other flags (such as the Union Flag, the Saltire, The Jolly Roger, the Cross of St George or a pair of ladies undies) in the position normally occupied by the Red Ensign can lead to a prosecution and a fine not exceeding, in UK £1000. Fines can be even heavier in foreign waters. Two years ago, the yachting press had several reports of British skippers being heavily fined in French sea-side courts for flying improper flags.
The White Ensign … is the flag flown by Royal Navy warships.
In addition, since 1829 an Admiralty Warrant has been in existence permitting vessels owned by members of the Royal Yacht Squadron (RYS) to fly the White Ensign. The RYS is a private yacht club and is the premier yacht club in Britain. It gained the 'Royal' title in 1820 whilst the Prince Regent (later George IV) was a member.
The Blue Ensign … is the flag flown by Royal Fleet Auxiliary vessels.
In addition, vessels belonging to the members of certain designate yacht clubs may also be issued with a warrant permitting them to fly a defaced Blue Ensign - the defacement being a device often associated with the badge of the club. If flying the Blue Ensign, the vessel is also required to fly the burgee of the relevant yacht club.
P.S. as a member of the Royal Hong Kong Yacht Club - before the takeover of Hong Kong by China - I flew the defaced Blue Ensign plus the club burgee Royal_H_K_YC_Burgee.jpg
. On the 30th June 1997 it was taken down at sundown, furled forever - a toast with a wee dram and a tear in the eye.