All your observations are correct, but, again, it depends.
The reason boat have big ICE engines has absolutely nohting, zero, to do
with designers.
It has to do with customers.
When customers want big engines, and they do, and they don´t care about
efficiency, and they don´t, engines get absurdly big.
Designers know perfectly well the engines are too big.
However, customer perceptions are far more important than technical reason.
Most customers believe that they want the "extra power" for when
conditions are "hard" or tough.
Now, this is desirable for example on a commercial freighter or fishing
vessel.
When you are going against 20 ft waves, with 40 metric tons of cargo
(say crab and water), you might need a full 1200 hp on a 30 m vessel.
And you are in a hurry, to sell the cargo.
And the wind is 50-60 knots.
And you are still going onwards, rather than seaking shelter.
None of the above five applies to a recreational vessel.
And, 99% of owners, have nevr been, and will never be, in such conditions.
And of the 1% who get into such conditions, over 2/3 or 3/4 abandon ship
before they ever need the "full power".
That why Nordhavns, for example in the 62 ft range, have 400-600 Hp engines.
80 Hp would be enough.
120 would be plenty.
Fuel costs on Nardhavns are about nr 5-6-7 on list of yearly and overall
lifecycle costs for a big trawler yacht like a Nordie.
If the fuel cost was 1/10 of normal, all the other costs would still be
the same.
Thus, because resale value is directly related to how desirable a boat
is, and the next customer will want big engines, everyone gets big
engines for their boat.
So, and owner who buys a boat with a big engine is not "wrong". Often
they are "right" in that they lose less money with the big engine.
As an example, 200-400 ton commercial ships used to have 100-150 Hp
engines, early 1900s.
On commercial cargo craft, used day in - day out.
The situation is different for someone who ios going to live on the
vessel, make money with it, or travel extensively (over 10,000 hours)
with it.
Here, operational costs become the nr1 driver.
But again, this applies to maybe 0.2% of owners.
--
The reason for the upraised flaps at the end of the wings is to keep
from air slipping out alongside the wing.
For the same reason there is a small vertical fin on the wing, about 6
inches high.
Wings are also twisted a little, so that they lose liftoff near the
body, rather than the tips, where most of the control comes from.
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