Bert...with you as a seagoing Marine Engineer, I would not ever contemplate to question your comments about ships stability 
, however if you could use lead Shot [from a GunSmith Store] and sprinkled it at the lowest point available within the hull would it not lower the metacentric point of balance?
This then like a lower pendulum, will decrease the frequency of the roll, but increase the extreme angle of roll?
So we see your Alexandra in the bath.......& the rock & roll generated as frightening, so to consider all things being equal in an open water with a little gust breeze, she may do a 180 or capsize 
Another option would be to use whatever lead you currently have as ballast, and cast an external keel bulb ..............but that would look pretty stupid
Derek
Nah... I am not that worried for a capsize or something like that... Maybe when the engine would seize, she could dip her boards, but even then I don't expect her to keel over.
First of all, don't forget that a bathtub is a very unfavourable dynamic environment, any rolling motion of the boat will virtually immediately cause the water to start sloshing and create a feedback on the rolling motion. I expect behaviour on open water to be much more sedate, because any waves made will move away from the boat, not immediately return the energy. She will roll a bit, no doubt, but nothing scary.
Stability is a bit more complicated than just putting weight in the lowest point of the ship.
There's static stability (basically the height of the CoG in relation to the height of Centre of Buoyancy) and there's dynamic stability (a bit harder to explain, but in the bottom line it is the integral of the righting couple graph).
Theoretically, static stability is a torque (force times leverage), the dynamic stability is a ""labour", an amount of energy (torque over an angle of movement).
Inertia around the rolling axis is what absorbs this labour, and the lower that inertia, the faster the roll rate (shorter cycle time of the oscillation).
Most people don't know this, but both in models as well as in real ships, the CoG is actually ABOVE the centre of Buoyancy, and stability is achieved by the shift of Buoyancy due to list angle. The amount of shift per degree of list is determined by the shape of the hull and the draft, and the relation is not linear. When a graph is made of this buoyancy shift against roll angle, the eclosed surface below the graph (the integral of the graph) this represents the righting energy.
And as it is (I tested that) it still takes a considerable amount of force to push one of the boards in the water. Way above what any gust of wind could achieve.
There is a reason that I want to increase inertia and reduce static stability, and that is that it prolonges the roll time.
Look at it like this: the flywheel is a big mass, with a lot of inertia of its own (it basically spins freely around an axis that is near coinciding with the hulls' axis of roll) and due to that inertia it will want to spin a constant RPM.
The sensor that pics up that speed, is fitted to the hull, so any rolling motion will affect the RPM signal (the rolling motion of the boat is added or subtracted to the actual rotational speed. This triggers a correction signal.
If the boat is rolling really violently (low centre of gravity) this can lead to oscillation and increase the amplitude of the roll because any control system has a time lag.
If you think this is theoretical blah blah, I have seen it happen 7 years ago with the first execution of this project. It is real and it does happen.
That hull was too small to add any ballast, that's why that project was stopped.
In fact, it happened in this hull as well, without ballast, and it was so violent I could not video it (needed the one free hand I had to hold the boat instead of the camera
).
Letting the boat loose, the interaction between rolling motion and governor action could clearly be seen, holding the boat reduced governor corrections visibly.
Increasing the draft reduces dynamic stability, but adding weight at the lowest point will increase static stability, while not doing much for inertia (weight added at the lowest point, that weight will have a distance to the rolling axis of only 4 cm). So I added weight on the waterline, in the sides, where that weight won't add to static stability, but have more effect on inertia because now the weight is about 9 cm distance to the rolling axis.
It works, because the boat does not tip over and though there still is SOME interference between governor and rolling motion, it is not much, and it is a bit load dependent: it mainly happens at neutral pitch and is virtually gone when the engine is loaded, despite the constant corrections of the governor still being there.
That it works, is evidnced by the fact that I could actually MAKER the video (I had one hand availlable to hold the camera
).
A big issue is that the governor I use (Futaba GV1) has very poor PID adjustability, and part of it is done in a "hidden menu" that was never intended for the end-user, and there is no proper description of these parameters availlable. Futaba used very non-typical names and I have not been able to either find any additional info online on this, nor was I able to analyze the changes in response in order to figure out what these parameters actually do.
I managed to get some improvement 7 years ago, but that was a matter of trial and error, like a blind man in a maze...
Unless I can find a proper PID controller, that I can tune the way I am used to tune controllers, the only thing I can do is to further dampen the movement hydrodynamically, and right now the only remaining option are bilge keels. I used those in the first attempt, 7 years ago as well and they had noticable positive effect.
There are still a few issues with carburation, making the engine respond less optimal to governor imput, and that is a totally separate issue, but it is illustrative that when I put the boat on the stand with running engine, things quiet down considerably, and in fact, first tests on the stand were actually done without any form of active RPM control, which is remarkable because normally, unloaded engines tend to "runaway", and 7 years ago this was impossible, so I think the adaptation on the valve timing I did a week ago has a positive effect on the engines behaviour.
So there still is also some to be gained by "undiluted" engineering, but that is a separate issue.
Author
Topic: A Krick Alexandra, but.... different... (Read 4487 times)