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Author Topic: Dynamic stability  (Read 4970 times)

g6swj

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Dynamic stability
« on: June 28, 2016, 09:25:35 AM »

Hello,

I am experimenting with dynamic stability - I have a few boats that are top heavy and lean (roll) quite a lot when turning. A simple stand alone system of measuring the roll and moving a ballast weight dynamically to correct this is what I have in mind. Rather than re-invent the wheel has anybody achieved this / have any thoughts on the concept?

Thank you in advance
Jonathan
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Colin Bishop

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Re: Dynamic stability
« Reply #1 on: June 28, 2016, 09:45:58 AM »

I don't know if it has been done but I think your main problem would be that the counterweight would not react quickly enough to offset the roll. You would need a virtually instantaneous response. In full size vessels this is what stabilisers are used for although they use hydrodynamic forces instead of weights. Maybe stabiliser fins linked to gyroscopes would work, it's an interesting thought.

Colin
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Subculture

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Re: Dynamic stability
« Reply #2 on: June 28, 2016, 10:25:15 AM »

Stabilizers linked to a submarine leveller would probably work. A gyro wouldn't work so well owing to the way they respon.

grasshopper

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Re: Dynamic stability
« Reply #3 on: June 28, 2016, 11:16:36 AM »

Depending on the size of the model, could a gyro just fixed firmly to the hull with its axis vertical slow any heeling or pitching movement without affecting horizontal direction?


It's a system I think I've seen in luxury yacht magazines to prevent motion when a yacht is moored, stops the millionaires getting sea sick. For the size of the yacht the specified device doesn't have to be that large, just spinning at a high rev count.
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g6swj

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Re: Dynamic stability
« Reply #4 on: June 28, 2016, 11:38:49 AM »

Thank you for info/thoughts so far.

My thoughts are along the lines in the image below. As the weight is offset the servo would not need to move much for the weight to move a significant distance. A combination of rudder position, speed of rudder position change, roll angle calculations would determine servo movement.

I think this would be a very responsive system - whether it works in the real world or just makes the boat "wobble" we will see.

Regards
Jonathan

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Colin Bishop

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Re: Dynamic stability
« Reply #5 on: June 28, 2016, 11:48:18 AM »

Model boats bob up and down so the response speed would need to be instantaneous which is where your problem would be I think. On full size ships anti rolling tanks were tried without any real success as they tended to become out of synch with the movement of the vessel. Stabilisers work as only a small movement is needed to generate deflection forces and they therefore react very quickly.

To be honest I think an external keel would do a more effective job by lowering the centre of gravity, even if it is just a bar of metal attached to the bottom of the hull.

Colin
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Subculture

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Re: Dynamic stability
« Reply #6 on: June 28, 2016, 11:49:22 AM »

I think you will find weight shifting will only work when the boat is running at very low speed. At higher speeds the dynamic forces acting on the hull will be too great for weight shifting to act against it.

deadbeat

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Re: Dynamic stability
« Reply #7 on: June 28, 2016, 12:09:16 PM »

Firstly, do your boats have bilge keels, they might help with the roll. Secondly, A weighted keel under the hull might help, similar to what yachts have, you would have to experiment as to the weight and depth of keel, you don't want to make the boats to stiff though as all boats lean and roll.

I had a problem like this with a model and one of the issues was that it was single shaft and I had too much torque from the motor and on take up it would lean alarmingly, the weighted keel did help, but the truth was I had too powerful a motor for the model, hay ho!
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Colin Bishop

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Re: Dynamic stability
« Reply #8 on: June 28, 2016, 12:19:44 PM »

If you suffer from prop torque then one answer is to have a smaller prop running at higher speed.

Colin
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steamboatmodel

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Re: Dynamic stability
« Reply #9 on: June 28, 2016, 02:38:56 PM »

I have seen the moving weight on a servo used in a paddle boat. A friend of mine has a Mt Washington paddle boat model, this is a side-wheeler and has a habit of leaning to a side when a turn is entered and staying leaned after the turn. He had tried a number of fixes, and finally installed a servo on the centerline with a weight on an extended arm. As the model enters a turn he moves the counterweight in the opposite direction. the model stays upright and doesn't lean.
Regards,
Gerald.   
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malcolmfrary

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Re: Dynamic stability
« Reply #10 on: June 28, 2016, 03:26:26 PM »

The gyro mentioned earlier was intended for stationary boats and might not be much help on a moving one.  (No leaning problem when turning, just a problem turning)
Bilge keels will slow the rate of roll, but not stop the roll.
Shifting a weight might help to counteract side winds, but would need to be very fast acting to avoid making the situation worse.  A weight on a long arm might easily overpower a servo and the positional definition of the servo might not be good enough anyway.
Submarine self leveling devices acting on hydroplanes might work, but what about underwater ailerons linked to the rudder?  Correction should be applied as the turn is performed and stop when it isn't.
Best bet is to build superstructures lighter.
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david48

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Re: Dynamic stability
« Reply #11 on: June 28, 2016, 05:04:29 PM »

there was a yacht in the America s cup a New Zealand boat I think that had a canting keel, it used to swing from port to stb .
that would take some thinking
David
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g6swj

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Re: Dynamic stability
« Reply #12 on: June 28, 2016, 05:09:54 PM »

Clearly if this was a new build then it would be better to solve possible instability issues as part of the build however my reason for experimenting is that I have some models that have been built and would need something to be retrofitted. Stabilisers are not really an option I don't think.

As Malcolm has mentioned
Quote
linked to the rudder?  Correction should be applied as the turn is performed and stop when it isn't.
it is my intention to make use of what ever "data" I can to make the system as intelligent/preemptive as possible which would include analysing the rudder signal. Appreciate that if there is any mileage in this solution then a more meaty solution than a standard servo may be required due to torque

I will let you know how I get on...

Regards
Jonathan

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g6swj

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Re: Dynamic stability
« Reply #13 on: June 28, 2016, 05:44:53 PM »

Maybe a linear actuator -  fast stepper motor and pulley with a platform to carry the weight running on rails (bit like the print head on an inkjet printer) would be more elegant than an offset weight & servo

Another idea for the melting pot...

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derekwarner

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hama

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Re: Dynamic stability
« Reply #15 on: June 30, 2016, 12:30:21 PM »

Interesting subject, can't wait to see what you come up with.
One advice though if you haven't already thought about it, make sure your boat can handle the weight on the "wrong" side in a turn and perhaps some wind before taking off. All systems can fail and it would a pity to see her overturn.
I have no knowledge of gyros at all, but I have tried some rc helicopters and thinking that they would need as fast respons as a modelboat?
Good luck on your trials!
Hama
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g6swj

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Re: Dynamic stability
« Reply #16 on: July 01, 2016, 12:41:12 PM »

Hama,

Wise words indeed. I have a couple of concepts that are waiting to be evaluated. I am currently most hopeful about a moving platform which I can get to move really quickly in response to the roll - however I do not know yet what "ballast" weight will be required and whether this will strain the movement/braking or just simply overload the mechanics of the concept- and of course whether it will correct the roll....

Regards
Jonathan
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flashtwo

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Re: Dynamic stability
« Reply #17 on: July 01, 2016, 01:07:34 PM »

Hi,
At the recent Sumner Ponds Model Show, at guy named Keith from the Swiss Cottage (Shoreham) Model Boat club described a research boat that he served on whilst in the RAF (long story).

The boat operated off Newhaven and was contracted by a government SONAR research department.

Interestingly, he said it was stabilised, while heaved-to, by a "fluvial" tank that was installed on deck. During normal running it was empty, but on heaving-to it was filled with tonnes of water. He reckoned that it would reduce a normal roll from 30deg, down to 11deg. Before returning to normal running it was emptied.

He is planning to make a model of the boat. Funny end story was that the RAF lost the contract to the Army! (one may ask, where was the Royal Navy? ).

See also, https://en.wikipedia.org/wiki/Antiroll_tanks

Ian.
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Netleyned

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Re: Dynamic stability
« Reply #18 on: July 01, 2016, 05:22:49 PM »

As I think I have said before,
Fishermen hoisted an anchor
or a water barrel up the mast
to damp the roll.
Same as a water tank on deck
I would think.
Not dynamic though.


Ned
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malcolmfrary

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Re: Dynamic stability
« Reply #19 on: July 01, 2016, 09:48:11 PM »

Was it firmly attached to the mast - how many came back?  I can see that a sail, by acting as a sort of air brake, would slow the rate of roll with the vessel riding head to wind, but extra weight up there?


I have seen a few threads on other forums where moving ballast has been applied to IOM size yachts with good results (simulating the crew leaning out), but the caveat with any scale type boat has to be that it needs to survive having the ballast in the wrong place at the wrong time.  It is still common in reports on ship disasters to hear that "the cargo shifted".  A big, slab sided, ships do lean in a strong side wind.
I also remember seeing in one of the model boat mags a lot of years ago the suggestion that appearance would be improved by the addition of baffled tanks to cause the model to rock out of phase with the waves.  That was back in the mid-80s, so it probably didn't catch on.
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Colin Bishop

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Re: Dynamic stability
« Reply #20 on: July 01, 2016, 10:01:20 PM »

Raising the centre of gravity can indeed steady a ship - but at the expense of stability. One reason that the Queen Elizabeth class battleships were rebuilt but the later R class were not was that the latter carried their armour deck one deck higher than the earlier class which made them steadier gun platforms but also meant that they could not take on extra weight to the same extent as the QEs. OK, they were slower too but re engining could have done something about that.

The problem with things like baffle tanks and water ballast is that what works at full size doesn't necessarily scale down well as the water viscosity remains the same so scale water is 'thicker' and more 'treacly' by comparison.

Colin
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g6swj

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Re: Dynamic stability
« Reply #22 on: July 02, 2016, 08:06:51 AM »

Many people who camp-cruise in sailing dinghies report that leaving the mast up at night leads to a slower rate of roll (more inertia in the system) and that this is often (surprisingly?) more effective than keeping the centreboard down.

Andy
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Colin Bishop

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Re: Dynamic stability
« Reply #23 on: July 02, 2016, 08:25:44 AM »

Not really surprising re the centreboard on a dinghy as it is virtually weightless in water and only intended to reduce leeway when the boat is moving - the crew is the ballast. A weighted Yacht keel is different of course.

Colin
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malcolmfrary

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Re: Dynamic stability
« Reply #24 on: July 02, 2016, 09:17:02 AM »

Raising the CoG on a model that is already top heavy is likely to be counter productive.  Just a gut feeling.  Putting the mast on my Lindberg Tuna boat made it necessary to fit ballast in the bottom of the hull - its still a bit tender and only suitable for calm wind conditions.
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