Place six grape halves into each half of the empty orange skin. Place the skins into cups, a muffin-tray or similar, so that they cannot tip over.
In a bowl, mix together the warm water and gelatine granules and stir until the gelatine has completely dissolved.
Add the orange juice and stir well.
Pour the jelly mixture into each half of orange skin, almost filling the skins. Carefully transfer to the fridge and chill for 2-3 hours, or until the jellies have set.
Fold each mint leaf in half and pierce with a cocktail stick to make sails for the boats. Just before serving, push one sail into each set jelly.