Islander 36 is a great boat
The Islander 36, designed by Alan Gurney, is a very well proven offshore boat, and I am sure Zac's is not the first one to sail around the world. These boats are in the same league as E-boats in terms of engineering and structure (for the vintage), and IMHO is a great choice for a young person to make this trip in-they are strong, relatively fast and excellent upwind boats-all important features for this type of voyage.
The engine is there for several reasons-to have a reliable power supply to run the instruments-you can have solar chargers, but this guy is going to spend a lot of time and power communicating with sponsors, media and others, so his consumption may be quite high-and hour with a fast charger will get the batteries right up.
Also, since this is not a non-stop voyage, he will want the engine to enter many of the harbors he is likely to encounter-being alone, it will be much safer than trying to sail to a dock or pier. Many of these areas have strong tidal currents, so if entering in light air he may have insufficient control under sail.
Finally, he may want to use it to escape an area of light air (I don't think this trip precludes using the engine for motive power at times). Since he will have good weather routing support, he may be stuck in a big hole and know there is breeze 100 miles away.-Power on and get to the breeze at 6 knots sounds a lot better than drfiting around for days on end...
Cheers,
S