Minimum size for round the world

Minimum size for round the world without refueling ?
Your ideas, monohull, catamarans ,prao other !?
Your proposition for best fuel efficiency .
In 1990 a collegue propose this currious 20 m boat to us :




IN 1993/4 an architecte propose this more conventional boat :




And you ,your ideas ?

 

Can you use any fuel? How about sail or other forms of energy gathering?

 

In fine we thinking sail boat is more expensive than motor boat.
But il will be difficult for me to explain in English !
For example the cost began at the conception of the sailing boat : pillars,chain plates,'strongest framing in the mast area, strongest framing for the keel, keel ,support for fit winches and all deck equipment,mast,rigging,sails,sheet, halyard etc it cost a lot !
Another energy ? if you are thinking solar ...until now Solar Planet is not a very good example !! Cost a lot for building and go slower than if she have a very small mast with small sail ...
A Japonaise try with "wawe energy",but wawes, sun or wind.May be if one day the solar pannels catch the entire spectre assocaited with light an perform batteries .
Alcyone was fitted with Magnus cylinders but also the result was not so good .
No when I ask for some ideas it is with classic engines ,but why not new ideas on the hull .
Because with 100 feet hull it is 'easy' to perform but for one under 20 m it still possible (for example the hull of our actual boat could be charged with 12 or 13 m3 off diesel ,for a total of 21 m3)
But it could be interesting to thinking a better hull shape for optimize the concept .
I don't know if I am clear ?