Used to sail around there in a 20' yacht with 8hp 2 stroke Yamaha on the back. Displacement about a ton, so similar to the Norman. Other boats would head off into the distance but was possible to make headway and get round Asland lamp. Engine was working hard and a displacement hull vs. the planing hull of the Norman so not direct comparison. Would probably want more hp, in the 20hp+ range, on a 20' motorcruiser on the estuary/link but a healthy and reliable 8hp should allow progress, albeit slowly. (disclaimer: if you end up going backwards, these are only my thoughts, so don't blame me!) A good anchor and rode is essential, plus plenty of fuel, as the outboard will drink it like a wino in a free bar when pushing against the tide. Without sails, in an ideal world, I'd also want an auxiliary engine. There's no chance of drifting to the side and dragging down a towpath if the engine fails.