Fin stabilsers at anchor - underwater video

[MapisM]

The main differences between fins at anchor and underway are (i) fin area (bigger is better, at anchor), (ii) hydraulic power (you need higher fin speed at anchor) and (iii) the algorithms (when in "at anchor" mode the systems switch to entirely different algorithms becuase the fins have to "swim" to stabilise the boat whereas underway they act as foils, and these are quite different things)

Yup, all agreed.
But re.the algorithms, my reasoning behind the idea of trying them also at rest was based on the fact that they're actually unaware of the boat speed.
The only input to the system is the gyroscope, which senses the roll (and its speed).
Therefore, my guess was that the system would have moved the fins as widely and as fast as possible, as long as the roll doesn't stop.
And this is what seems to happen. In fact, also the panel gauges show that the fins frequently reach their max angle while operating at rest, whilst this very rarely happens while cruising.
So, I'm not sure that a smarter control system alone could significantly improve the zero speed performance, unless using also larger fins and faster cylinders.
Considering also the pump complications you mention (which I agree with and I didn't think of in my previous post), I suppose that for stabilization at rest I will stick to the proven method I've used so far: looking for quiet anchorages!

 

[jfm]

Yup, all agreed.
But re.the algorithms, my reasoning behind the idea of trying them also at rest was based on the fact that they're actually unaware of the boat speed.
The only input to the system is the gyroscope, which senses the roll (and its speed).

I'm surprised your inputs dont include at least inclination too. To work really well, a system needs to know roll acceleration (gyro), boat speed, fin position and boat inclination. We discussed speed a while ago. I can see how a system can work with just gyro input, but not as well as if it knew other things too. Your system probably knows fin angle

Therefore, my guess was that the system would have moved the fins as widely and as fast as possible, as long as the roll doesn't stop.
And this is what seems to happen. In fact, also the panel gauges show that the fins frequently reach their max angle while operating at rest, whilst this very rarely happens while cruising

Hang on though. "As fast as possible?" I doubt the sytem even can control fin speed. I bet the speed is fixed. The system expects the fins to be foils with water flow over them, and in that mode you do not need to control the fin speed. It is only when the fins swim (at anchor) that you need to control the speed

The fact your fins reach max angle at zero speed actually proves they don't work well at anchor. The boat keeps rolling, so the computer keeps moving the fins to stop it thinking they are foils, but the rolling doesn't stop, so the fin max angle is reached. With an at-anchor system, the computer would deal with this by moving the fin faster to get more antiroll effect than moving it too slowly to the full angle limit

Control of fin speed is crucial to at-anchor performance. In a good system, when the boat is fairly still then a sudden wave comes, the fins will be in the centre position and the electronics will therefore move the fin extra fast, knowing there is only 50% of the available stroke. This helps by stopping the roll before it starts, so reducing the need to play catch up. On the next stroke which will be full angle not 50%, the computer will move the fin more slowly. And so on

proven method I've used so far: looking for quiet anchorages!

I agree that 100%. Stabs of any type should not be a replacement for good skippering!

 

[MapisM]

Yep, definitely the system is aware of fins position, 'cause the panel gauges show it in real time. Besides, the control unit obviously must "know" when the max angle is reached, in either direction.
Re. inclination, if as I understand you mean the boat listing degree at any given moment, I would swear that there isn't an inclinometer in the system, but that the gyroscope takes care also of that somehow, on top of acceleration.
I don't have the manual handy to check that, though.

Re.fins speed, coming to think of it, I agree that in theory there's no need for variable speed actuators, if stabs are only supposed to work under way.
Actually, I always assumed that their speed is variable just by looking at the gauges movement, which definitely gives the impression of slower/faster movements, on top of narrower/wider angles, depending on sea conditions (long swells vs. short wakes, direction, etc.).
Also, when looking at the fins "swimming" in the water, they seemed to be more/less "reactive" at times. This video is the full clip from which the few seconds of the other video were taken, don't you think that also the speed and not just the angle looks variable?
I will check also this on the manual anyway, unfortunately I don't have an electronic version.
In the meantime, a quick search pointed me to this webpage, which shows the same oldish model I have on my boat. According to it, the system controls ...the direction, degree of movement, and speed of ... fins.

Anyhow, there's no doubt that better/faster processing of all relevant parameters can and does improve the final result, particularly at zero speed. After all, 15 years of development in electronic data processing is probably comparable to more than a century in hydraulic/mechanic devices...

 

[jfm]

I had to watch the video a few times to be reasonably sure but yes I think the speed varies. It seems to me that most of the time the fins moved at constant speed, ie the maximum, but occasionally they were slower

Anyway, that page you linked to is interesting. The full paragraph is here:

Other stabilizers rely on electronic gyros or gravity dependent devices which have to convert the electronic signal to hydraulic output. The heart of the quiet, fully-modulated NAIAD system is an extremely responsive hydraulic turbine-driven gyroscope - the NAIAD difference. Operating at 10,000-20,000 rpm, this patented unit precisely varies pressure, in proportion to the roll tendency, in two signal lines. These lines are connected to a sensitive servo valve located at each fin actuator. The servos, biased by a continuous feedback, amplify this signal and control the direction, degree of movement, and speed of a pair of hydrodynamically efficient fins located beneath the hull.

It therefore seems Naiad do things differently. They work solely on the gyros, and do not have any computer as such. The gyros mechanically alter the mechanical fluid pressure in hydraulic control lines, and then these control lines operate the valves that move the fins. So it is all mechanical, no electronics or computing at all. Interesting stuff. And the system obviously cannot take any input from inclinometers and boat speed; it works solely on gyros. Is this how Naiad currently do it, or is your link to an old brochure for a previous generation system?

The language Naiad use is a little bit "sales talk" where they suggest other systems are inferior becuase they "have to convert the electronic signal to hydraulic output". Electronic converted to hydraulic is obviously a smarter way to do things (here in 2011) and will be much faster and more reliable than Naiad's mechanical system, and can take account of far more inputs and allows for much easier changing of parameters. It is how planes fly, especially Airbus.

On a really long swell, beam on, a Naiad would not hold the boat flat if the sea was tending to hold the boat at a constant angle, ie no angular acceleration. Sleipner (and I guess TRAC as well) use an inclinometer so they can keep the boat flat in these sort of long-wave conditions. Also, on my sleipners, the gyros measure acceleration along the horizontal plane (as opposed to merely angular accelration around the boat's rolling axis) ie they know when and how sharply the boat is turning. When the boat is turning they use the inclinometer to angle the boat into the turn, by a variable amount depending on the speed/tightness of the turn, like a normal fast boat and like a motorbike. The amount of lean-in is an adjustable paramameter. This makes for a very nice feeling when turning at say 20-25kts, whereas a boat held flat by its stabilisers feels weird. This is the sort of stuff you can only achieve with computers to control the hydraulic valves, not mechanical-servo systems

 

[BartW]

On a really long swell, beam on, a Naiad would not hold the boat flat if the sea was tending to hold the boat at a constant angle, ie no angular acceleration. Sleipner (and I guess TRAC as well) use an inclinometer so they can keep the boat flat in these sort of long-wave conditions. Also, on my sleipners, the gyros measure acceleration along the horizontal plane (as opposed to merely angular accelration around the boat's rolling axis) ie they know when and how sharply the boat is turning. When the boat is turning they use the inclinometer to angle the boat into the turn, by a variable amount depending on the speed/tightness of the turn, like a normal fast boat and like a motorbike. The amount of lean-in is an adjustable paramameter. This makes for a very nice feeling when turning at say 20-25kts, whereas a boat held flat by its stabilisers feels weird. This is the sort of stuff you can only achieve with computers to control the hydraulic valves, not mechanical-servo systems

the algorithm in this computer must be some very sophisticated and clever maths, where one brand can distingues a lot from the other

 

[MapisM]

Is this how Naiad currently do it, or is your link to an old brochure for a previous generation system?

It is. I specifically looked for some info related to my stabs model.
I'm pretty sure that already by the time I bought the boat (4 yrs after she was built) Naiad already had a different control unit, and the zero speed option. It's also easy to guess that they must have further moved on since then, though I'm not aware of the details about how their latest models work.