Is this a good idea? Sable Island is in the middle of nowhere, at the confluence of the Gulf Stream and the Labrador Current, which makes for some truly nasty weather and fog. They don't call it the Graveyard of the Atlantic for nothing. I wasn't aware it was a cruising destination, and thought mariners took great pains to stay the hell away from the place. If you're thinking of trying it, and asking about it in a Learn to Sail colunm, please take some serious advice on the topic first from those who know the area.
Anyway, looks to be a bit under 600 miles due east from Porpoise, with Cape Sable a stopover about halfway. An 'average' 65-fairly solid monohull would probably make 8 knots in a breeze, so a spanking summer souwester could have you there in 3 days' sailing time. A more conservative figure, figuring on the vagaries of wind and current, would be more like 6 knots and 4 days.
The island's described as a very large, moving, sand dune, with some wild horses and a few researchers who travel in by aircraft.
I'd think twice, then twice again, before trying it.