In order to reduce the wetted surface yet retain stability, Istria's hull shape consisted of a long forward overhang and a wide beam at the waterline. Yachting Monthly described her as "Short on the waterline, with large displacement, a fine tail and a useful snout, she would not have been a pretty vessel had she not proved clever. But as I have always insisted that efficiency is beauty, the blue 'fifteen' was admired". Her midsection also featured a distinctive tumblehome, a feature that Nicholson would repeat in subsequent boats designed to the IYRU International rule.[2]
Nicholson devised a topmast that slotted atop the mainmast, and fitted with a track to fasten the topsail luff. This design eliminated the need for a topsail yard, thus permitting a lighter and loftier rig as well as easier hoisting and dowsing. Nicholson produced similar rigs for Pamela and Paula III and the rig was dubbed "Marconi mast" because the elaborate standing rigging reminisced a radio mast.
For the sake of safety in case a yacht should flounder, the rules at the time stated that a dinghy should be on the deck of the yacht during racing, and was typically stored over the main skylight. In order to reduce the windage, Nicholson created an open dinghy with a gunwale overhang which could be recessed into an aperture of the deck of the yacht, the open dinghy cockpit thus serving as the yacht's cockpit. When the Istria attended her first race, all the opposition protested against the arrangement. However, after successfully proving that the dinghy could be launched in under 10 seconds, the protests subsided.
| Length on deck | 23.94 m |
| Load waterline length | 14.7 8m |
| Beam | 4.16 m |
| Draught | 2.82 m |
| Displacement | 40 tons |