There are a lot of places in the world with big tides and strong tidal currents. We have them all around the UK and especially in the English Channel
. 4 knot
streams are the norm at springs, and in places where the bottom shelfs up or the tidal stream is compressed in some kind of strait, we see 10 knots and more (Alderny Race
, for example).
The consequences of these strong tidal streams are many. Besides a current to fight, there can be vicious overfalls, whirlpools, and incredible (for those who, like I did, grew up in an area with smaller tides) sea conditions when the wind
blows against the tide. There are places like the infamous Portland Race
which can swallow whole container ships, in the right (that is, the wrong) weather
Some people here seem to imagine that you should be ready to motor
against any tide. It don't work that way. As someone else correctly said, hull
speed is hull
speed, and hardly any sailboat can motor
against a 10 knot tidal current. In these waters you more or less strictly avoid sailing against the tide in areas with strong streams -- tidal gates, we call them -- and in general, you would not plan a passage
with much time against even a moderate adverse tide -- it just doesn't make any sense.
So tides affect the power you need maybe not at all. Hull speed is hull speed and naturally you would not choose an engine which could not comfortably get you to hull speed with power in reserve. The power in reserve you need not for tides, but to motor against wind
, when you have to. Here the power required to make any progress at all goes up steeply with rising chop. My 100 horsepower boat with a 46' foot waterline can motor at 10 knots without pushing the engine too hard, in calm weather, but can hardly maintain 4 knots at full revs motoring against a F8 in a short period, steep chop (don't ask me how I know this). And you might really need to motor against an F8 to get into a safe port someday. That's what you need the extra power for.