I agree with you that retractable fins will have less drag than fins that stay out, but am not sure that it would matter too much for the boats most of us are using.
Since that I have the fins I have noticed I need about 100 rpm extra (per engine) to get to the same speed as I had before (without fins). Obviously the fuel economy has gone down a bit, but as an added benefit we basically now have the stabilizers stand by, even when the sea is not that rough. As long as the sea remains calm they won't use up any energy, they will remain in neutral position. At the moment we are in the Aegean which is notorious for bad weather, but this summer we will be in Croatia where the seas are usually calm. Would be nice if we could fold them away. However, I have no idea what that would do for growth of barnicles etc ?
Electric fins do need electricity to keep the boat stable while at anchor, but there is this neat feature on my fins, which is the sensitivity. By reducing the sensitivity the fins will not react to every little disturbance. The higher the sensitivity the more electricity it will use and the lower the sensitivity the less it will use.
In addition to that I have a night mode, which already reduces the electricity usage by 50 %. The way it does that is by moving the fins at a much lower speed, which means the roll dampening effect is still there, but not as fast.
When I then also put the sensitivity to minimum I reduce the electricity usage even further, since is not going to react to every little disturbance. And if it reacts, it reacts at a lower speed. In the end it means that the night time modes use as little as possible, but still provide adequate roll dampening. And if the sea would turn rough all of a sudden the fins will dampen the roll out completely, it just takes a bit longer, but then we are asleep and don't notice it (at least not me
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The reason why a gyro needs more energy when the weather worsens is that the forces on the gyro basically act as friction (or resistance), which would slow the gyro down. In order to prevent that the motor has to increase power to keep the gyro spinning at the desired RPM.
If the weather worsens for us also our system will use more electricity and obviously at anchor it will use more electricity than underway. At anchor the fins are less efficient, therefore need larger deflections, therefore require more electricity.
Underway I usually see about 10 - 12 Amps (24 V) when they operate. Most I have seen was 40 - 45 Amps (24 V).
At anchor the total draw we normally have is about 5 - 10 amps (24 V) for a normal anchorage and 60 - 70 amps (24 V) when e.g. we had the storm plus the ferry in Symi (Greece). Good part there was that we were on shore power, so I could not care less how much they were using
I thought about flopper stoppers and paravanes a lot. I wish I could have installed them on my boat, would have been a hell of a lot cheaper than the electric fins I have now. However, having no mast and being in Europe, where the majority has never heard of flopper stoppers or even paravanes, I knew I was in for a very difficult project. Building them myself was well beyond my capabilities. Adding a mast, booms and all the other equipment to make them work would be quite a challenge, so I decided against that.
Once I figured out that the electric fins would work on anchor and even in port, there was no need anymore for flopper stoppers. As soon as we come into an anchorage or in a port I place the fins in dock mode, put the sensitivity down to minimum and don't even think about them anymore. They will do their job, requiring no attention whatsoever and since I have enough solar and lithium battery capacity I don't worry about the amps they draw.
If I would have chosen hydraulic fins I most likely would have pursued the flopper stopper more seriously. Hydraulic fins on anchor is just a mismatch (at least for us) and then I would have wanted something like the flopper stopper. I think it works perfect for what we need, no electricity usage, not a lot can break down and simple to deploy.
A gyro will also work fine on anchor, will absolutely do the job, but for us the electricity draw was simply too much. I would not be able to run the gyro on solar and our battery pack and that made me decide to keep searching for a different solution.