The question of semi-displacement v planing is an interesting one, I raised it on crew.org.nz a couple of weeks ago.
It is also discussed a number of times on boatdesign.com.
The subject can get technical and there is some disagreement amongst professionals.
It seems to me that yes some design factors allow semi-displacement hulls to exceed hull speed
. This seems to give 12 knots or so or a bit more cruising speed.
A planing hull might do 30 knots but as fuel consumption
rises with the cube of speed fuel use is heavy even in terms of distance traveled ie miles per litre.
Therefore many limit it to around 20 knots/hr.
17 knots is a planing speed in the usual sizes ie not destroyers or cats. I would think it unlikely in SD and it would involve high consumption
Handling in a sea is likely to be better in a displacement and worse in a planing boat. I also note that conditions may not allow for planing and the planing boat would be the worst in that scenario.
As far as design goes it seems there is some imprecision and one has to look at the actual performance achieved. Weight is also a factor here.
What concerned me was that sometimes the same hull say a 32 is fitted with engines from 120-150 or 250-350 odd and the extent to which the speed and mileage reflect this without much difference in handling for the same speed and conditions. That is the boat does not become SD merely because of a lower speed given the same hull.
Whether one would prefer 6.5 or 7 knots or 12 or 20+ depends somewhat on the use, conditions, and intended distances and the wallet, and personal preference.
Those with planing boats may well opt for lighter conditions.
As for crossing the channel. I guess it depends on the weather window you want to wait for and the time factor. I gather the distance is short so I suspect time is not so important.
On the canals though I imagine there is a speed limit. I don't know but 5 knots would not surprise. Whatever it is you would not be planing at 17 indeed that would seem to be contrary to the point of the exercise namely puttering along enjoying the relaxation, scenery food