Luc,
They work in all winds and waves -- but not in shallow water!  My rule of
thumb is: do not deploy them in less than 20' depth.
I think they make the boat have a slower (more comfortable) motion as well as
reduce the amount of roll.
My girlfriend, Winnie, who had her own trawler a few years back was skeptical
until the day we were running from Marathon to Key West with the wind out of
the South at 15 - 20 Kts.  I had to go out past the reef so as to not worry
about catching the bottom if I stayed in Hawks Channel (she was upset because
this would add a couple hours to the trip -- 5 miles out, then 5 miles back)
and when we turned towards the West there was almost no change in the motion.
That was her first experience with flopper stoppers.  Now she has no problem
with adding a few miles in order to use them.
The biggest problem I have is getting the "fish" back onboard.  This must be
done before entering shallow water and usually the seas are still kicking up.
It could get dangerous -- I'm very careful.  The boat must be stopped and it
is difficult to keep her from coming around beam to the seas.  One has to be
quick.  I manhandle them (about 25 lbs each) aboard and it is easy to have a
fish bump the side of the hull as I lift it out.  This is a situation to
which I have given much thought and plan to add a small winch to each boom
and store the fish at the top on the end of the booms until I get into calm
waters when I can lower them to the deck.
As far as looks ... form follows function on my boat.
Regards,
Wayne Shosie
aboard BLUE MAX, a 1980 Albin 43, equipped with Paravanes.
presently lying Summerland Key, FL

Having just bought a Nordhavn 46 with paravanes only, let me give you all my
reasoning, right or wrong. I am not an expert, merely a boat owner. We elected to
go with paravanes only rather than with active fins, or both, because

1. Active fins are forever; they don't go away, you can only turn them off. Much
   danger of snagging them on something.
2. Active fins cost upwards of $30,000 vs $10,000 or so for paravanes
3. Active fins don't work at low speeds, meaning less than about 5-6 knots
4. Active fins require a lot of hydraulics, are noisy and fill up the engine
   room, already at a premium.
5. Paravanes, as they are done on the Nordhavns, are fairly easy to launch and
   retreive, though not completely trivial.
6. Paravanes can be used in anchorages as flopper-stoppers
7. Active fins can have a problem in figuring out what to do in following seas,
   where the water flow is low speed or even reversed..
8. Paravanes are fairly simple, and their operation can be observed and
   understood.
9. Paravanes can be repaired in most any fishing town with a welder and normal
   rigging supplies.
10. Active fins, if you really want them, can be added later. So, probably, could
    paravanes.
11. Paravanes are the sexiest looking things in the universe.
12. Active fins work in shallow water, but paravanes work only in deep water (at
    least 25-30')

Now having said all that, I personally know two N-46 owners who have both. They
swear that the active fins are great. I believe it would be real nice to flip a
switch. On the other hand, both are currently having minor problems with them.
Leaks are common until you get them sorted out.

from Russ, paravane owner for better or for worse.

|----------------------------------------------------------------------|
|    Russ and Donna Sherwin                                            |
|      "Four Seasons"                                                  |
|    1981 Marine Trader DC44                                          |
|    Sunnyvale, Ca 94087                                              |
|----------------------------------------------------------------------|

Our Defever 49 PH has active fins.  We had Naiad stabilizers
professionally installed in less than a day.  They cost considerably
less than the $30000 I've see bandied about.
On the positive side:
They are trouble free.  They don't leak water or hydraulics.
They work fine at our normal cruising  speed of 6 knots.  (but they were
sized to work at that speed)
The engine room space required a hydraulic pump on one engine and a
small (18in X 23in) panel on one bulkhead and room for the actuators. (
under our engine starting batteries)
They work well in our normal shallow water cruising area, the Florida
Keys and Bahamas.

They do have a disadvantage or two;

In our cruising area, crab pots are everywhere during season.  The float
lines snag on the stabilizers and pull the traps into the props.
Obviously this is in water less than 10 feet which is anywhere 20 miles
offshore in the Gulf of Mexico.

The stabilizers are useless at anchor in a swell.

Our single hydraulic pump was useless when the engine driving it
failed.  My wife now insists on another pump on the other engine.

Tom Little, Defever 49PH