Flopper Stoppers

[Hurricane]

Firstly, sorry for posting on the other thread - I hadnt seen this one at the time.

This is all fascinating but I picked up from above that you say it is quite surprising how small it can be. Are you referring to the size of the flobber stopper?

Anyway, it would be interesting to have a simple calc that we can all do and it seems that deploying both sides isnt absolutely necessary.

Also some ideas on materials etc. - I believe Nick uses a steel construction - It seems you are suggesting plywood.

Maybe a kind of YBW formula or *rule of thumb" and instructions.

 

[jfm]

By way of debate you are correct to a certain extent.

The analysis is purely analagous to the addition of mass in respect of the righting lever and roll gyradius.

Admittedly geometrically at TDC you are right at at TDC there is no longer any righting lever and the entire system would be in equilibrium but this equilibrium is des not persist due to the mass momentum of the roll and additional excitation by wave action.

However the GM is reduced by an analogous amout to a mass as we are not directly concerned with TDC when calculating the metacentric height. The metacentric height and therby the righting lever is calculated from the force vector of sucessive upthrusts from the vessles buoyancy at small angles of inclination (this is why I referred to the top of the roll what I really mean is as it is approached). The reducion in the righting lever is what actually slows the roll as the applied righting force is effectively reduced.

You're correct when you say the analogy breaks down for mass momentum calculations. There is no mas momentum in this instance as there is no mass, only an applied force equivalent. We can therefore ignore the momentum imparted by the hypothetical mass as it does not exist if we are calculating mass momentum and needs to be treated as a opposing force to the momentum.

The point is that the reduction in roll rate and magnitude is ANALAGOUS to an equivalent increase in roll gyradius where an equivalent compensation on inertial mass takes place. There is no actual increase in gyradius and therfore no mass inertia change other than that observed from the reduced rate of roll (angular velocity). This was borne out by both simulation and experimentation at model scale with full sacle trials (although full scale trials were somewhat limited).

What realy makes the roll uncomfortable (unless relly bad) is not roll magnitude, but rather roll rate. A realy gentle roll of high magnitude does not affect our balance systems as our physiology can naturaly compensale for the heel angles fast enough for it not to bother us greatly. So if we can slow the rate at which the roll occurs this is the key to comfort ....


As I said o active dampers I'm not overly familiar with the forces involved and am happy t be stand corrected in terms of their funcion ..... but I am facinated by this and wil look into it further !


Cheers

Tim

Umm, ok, but you've no longer any argument left to support your contention that increasing the height of the suspension point per se improves the flopper stoppers (ignoring the small tangent effect, as first mentioned by Deleted User). Which is correct - increasing the height per se makes no difference. Changing the angular inertia and momentum would make a difference, and changing the height of added mass would make a difference, but a flopper stopper adds no mass and its suspension height has no impact on angular momentum. Hence, its height is irrelevant

While studying this check out the Feadship water tank flume system that they put in boats >10 years ago, before actives were invented. Viewed now from 2009 the system looks old fashioned and of course terrible space inefficient, but it involved wonderful inventiveness (a Feadship hallmark) and some very clever maths (waves and harmonics in particular) and physics.

 

[jfm]

Firstly, sorry for posting on the other thread - I hadnt seen this one at the time.

This is all fascinating but I picked up from above that you say it is quite surprising how small it can be. Are you referring to the size of the flobber stopper?

Anyway, it would be interesting to have a simple calc that we can all do and it seems that deploying both sides isnt absolutely necessary.

Also some ideas on materials etc. - I believe Nick uses a steel construction - It seems you are suggesting plywood.

Maybe a kind of YBW formula or *rule of thumb" and instructions.

Nick uses steel, and various other design elements, to make sure the flopper moves against the water drag as fast as needed on the downstroke to keep the suspension rope always taught. This makes it more effective on the upstroke. In a similar vein, he also designed the flap valves/vanes so that the stroke of the flopper from fully opened vanes to fully closed was as short as possible, something like 5 inches or so. (He's not just a pretty face that Nick H chap...)

 

[TimAbram]

Hmmm.... if that was the case the height of a derek above a deck would make no difference to the vessels stability which it does.

Although there is no real mass at a derek head (only a force) the mass is effective at that point. The load on a flopper stopper is no different than trying to lift a dead weight from a dockside. The higher the suspension point the greater the change in GM (force x lever).

If there was no effect on gyradius a vessel supporting a container from a derek 1" above the deck would experience an insignifficant change in its roll rate compared to having it on deck but it does. For inertial changes the container is effectively where it was in the first place but in terms of roll frequency and amplitude it's like it sat at the derek head.

The height has an effect on the effective gyradius if it's outside the existing gyradius and on the GM if its above the centre of gravity of the vessel and thereby the stability.

The effect of the tangential change in direction of the force to the gyradius is marginal at a roll of 10 degrees which is a pretty uncomforatble.

Difficult to explain on the forum pop me a PM with your E-mail address and I'll send you some of the reference works listed below and my paper which details the principles applied. :-


Barras B; D R Derrett: 2006 Ship stability for masters and mates: Butterworth Heineman London ISBN-10: 0-7506-6784-2

Ayres Colin: 2002 The study of a roll-damping device (flopper stopper) at zero ship speedata Sorting, Analysis and Results, Physics project 593 . Curtin University of Technology

Klaka Kim: 2000 “Response of a vessel to waves at zero ship speed: preliminary full scale experiments” Report C2000-21 Curtin University of Technology

Klaka Kim and M.R. Renilson: Roll Motion of Yachts at Anchor, 2 March 2002YachtVision02 conference papers Auckland, New Zealand

Klaka K., Penrose J.D., Horsley R.R. and Renilson M.R. Roll Motion of Yachts at Anchor. 17-18 Sept 03 Royal Institution of Naval Architects. Modern Yacht Conference, Southampton UK

Ross. 1980.: “Flopper Stoppers”, Naval Engineers Journal Volume 92: PP45-50 American Society of Naval Engineers, USA.

 

[MapisM]

Away from TDC the height of the flopper suspnsion has an affect as Deleted User pointed out, becuase it changes the effective radius of the flopper's force about the gyradius centre (using tangent trigonometry). That's a mimic of just hanging the flopper from an outrigger. But that tangent effect is much more marginal than you describe, and doesn't arise from the increase in height per se, and indeed works both for and against you depending which side of TDC you are.

Interesting debate.
Re. your last bit which I highlited, why also against?
The higher suspension point of say a port flopper is (marginally - I fully agree on that) more effective when the boat is listed to port, but imho is irrelevant when the boat is listed to stbd, because once the line is lying along the hull, its movement is constrained by the hull itself, hence no radius change applies - unless of course also outriggers are used, but that's another story.
Or am I missing something?

 

[jfm]

Interesting debate.
Re. your last bit which I highlited, why also against?
The higher suspension point of say a port flopper is (marginally - I fully agree on that) more effective when the boat is listed to port, but imho is irrelevant when the boat is listed to stbd, because once the line is lying along the hull, its movement is constrained by the hull itself, hence no radius change applies - unless of course also outriggers are used, but that's another story.
Or am I missing something?

No, you're not missing anything. I was making something of a theoretical point where the flopper could swing freely. On an actual boat you are correct that the hull blocks the free path of the flopper rope, for half the roll, and so the suspension point of the flopper becomes the chine (in effect). That was just an extra niggle I was wishing to avoid in explaining the geometry in a pure sense!

 

[jfm]

Hmmm.... if that was the case the height of a derek above a deck would make no difference to the vessels stability which it does.

Although there is no real mass at a derek head (only a force) the mass is effective at that point. The load on a flopper stopper is no different than trying to lift a dead weight from a dockside. The higher the suspension point the greater the change in GM (force x lever).

If there was no effect on gyradius a vessel supporting a container from a derek 1" above the deck would experience an insignifficant change in its roll rate compared to having it on deck but it does. For inertial changes the container is effectively where it was in the first place but in terms of roll frequency and amplitude it's like it sat at the derek head.

The height has an effect on the effective gyradius if it's outside the existing gyradius and on the GM if its above the centre of gravity of the vessel and thereby the stability.

The effect of the tangential change in direction of the force to the gyradius is marginal at a roll of 10 degrees which is a pretty uncomforatble.

Difficult to explain on the forum pop me a PM with your E-mail address and I'll send you some of the reference works listed below and my paper which details the principles applied. :-

Tim, I'm sorry but your analysis is quite wrong.

Earlier you said the relevance of the suspension height is that it simulates, in terms of its affect on the roll, a mass outside the gyradius centre. That is simply wrong., There is no mass effect, no change in the gyradius length, nothing.

Now in your last post you've shifted ground to forces and geometry, which is actually the correct approach becuase masses were/are a red herring. But your geometry is wrong. If you put a short stubby derrick on the gunwhale/edge of a (straight, not listing) 20m beam ship, and hang a 1 tonne weight (and i mean weight, not mass, there's a difference) on it, there is turning moment on the ship of 10m x 1 tonne. Now, if you make the derrick 100m tall, vertically, and hang the same weight, the moment is still 10m x 1 tonne. no difference

The lower stability that in practice occurs with the too-tall derrick is caused by the ship listing when the 1 tonne is lifted, and the taller the derrick the greater increase in the lever caused by the list, and so you get a vicious circle where the ship lists more, and the 1 tonne moves further out, and so on till the ship capsizes. This is precisely the tangent effect described above, and is the reason why a too-tall derrick is bad on a ship. But that's a red herring here: it does not at all justify your original assertion that the higher suspension point of a flopper stopper per se makes it more effective. It just doesn't, and you need to reconsider your geometry/engineering.

Some of your statements (like "the mass is effective at that point") show a complete lack of understanding of the geometry of forces and indeed the difference between mass and weight. Mass is mass, namely an inertial resistance to acceleration. Weight is the force of gravity on a mass. Quite different things. Take a car to the moon - it'll need much softer springs becuase it has less weight (the force of moon's gravity is 1/7th x the earth's) but it'll need the same brakes to stop and the same engine to match it's earth 0-60 time becuase its has the same mass on the moon as on earth

Your statement that really astounds me is this one: "If there was no effect on gyradius a vessel supporting a container from a derek 1inch above the deck would experience an insignifficant change in its roll rate compared to having it on deck but it does. For inertial changes the container is effectively where it was in the first place but in terms of roll frequency and amplitude it's like it sat at the derek head." That is just utterly utterly wrong. It is absolutely not correct to say that the roll rate effect is the same as if the container were sitting up at the derrick head

To get back to the point, lots of things can be tweaked on flopper stoppers to make them work better, but hanging them on a long rope from a very tall suspension point on your boat doesn't intrinsically add any benefit

 

[MapisM]

That was just an extra niggle I was wishing to avoid in explaining the geometry in a pure sense!

You should know by now that some anorak would have spot even the most irrelevant niggle...

 

[TimAbram]

The lower stability that in practice occurs with the too-tall derrick is caused by the ship listing when the 1 tonne is lifted, and the taller the derrick the greater increase in the lever caused by the list, and so you get a vicious circle where the ship lists more, and the 1 tonne moves further out, and so on till the ship capsizes. This is precisely the tangent effect described above, and is the reason why a too-tall derrick is bad on a ship. But that's a red herring here: it does not at all justify your original assertion that the higher suspension point of a flopper stopper per se makes it more effective. It just doesn't, and you need to reconsider your geometry/engineering.

Thank you for explaining the geometry ..... but you've just explained the principle nicely.

The EFFECT on the ship's stability is ANALOGOUS to this situation .... I did not say it was the SAME.

The geometry might not work for you but the observed results show that the vessel does indeed react as if a weight is suspended from a derek with the boom slewed out to the side.

The GM of the vessel can be measured to have shifted if you apply a force that opposes the roll.

The vessel then rolls at a frequency as if the GM and roll Gyradius had been altered.

Unfortunalely pure geometry does not in reality satisfy the observed results otherwise the Ross equation would would prove to be accurate which it is not.


Let me try a different tack to show my reasoning:

The inertia of the water accelerated as a consequence of the plate motion is called the added inertia. This inertia must have a location at which it acts. It can not effectively act at the site of the plate as it is not connected to the vessel here but at the fixed point of suspension. With any suspended mass the effctive mass is loacated at the point of suspension and not at the origin of the mass itself. If this mass were placed either at beond the maximum beam, or very high up or very low down, the roll inertia will be larger and the frequency slower; this is moddeled by the effective increase in gyradius.

The loss in GM is not caused by a "list" with a tall crane what you describe is a different situation such as a free surface of water or grain or cargo suspended on one side of the deck your comparrison here is incorrect. We are not concerned with mass movement transversly but it's position vertically. You have confused these transverse shifts in centre of gravity with loss of inherrent stability through a reduced gravitational metacentric height, it is the same as a "lol" not a "list" in extremis.

The instability is caused not by the free surface or mass being off centre or the movement of the weight outboard or inboard but by the vertical increase of the centre of gravity in comparrison to the gravitaional metacentre (a loss of GM - lol) not a movement transversely of the centre of gravity in relation to the transverse centre of bouyancy (list). This loss of GM causes the instability and therefore damps the roll frequency. The slower the vessels is rolling the less momentum. The added inertia continues to have effect as the roll passes TDC and thereby a reduced amplitude.

The Ross equation for the roll energy adsorbed does not take into account this effect or the EFFECTIVE change in roll gyradius.


Hope that helps ? If you are still unclear take a look at http://cmst.curtin.edu.au/publicat/2002-08.pdf

 

[tico]

jfm..... youre wasted in the world of finance!

Should have stayed in Engineering!!

 

[Deleted User YDKXO]

Tim, I'm sorry but your analysis is quite wrong.

Earlier you said the relevance of the suspension height is that it simulates, in terms of its affect on the roll, a mass outside the gyradius centre. That is simply wrong., There is no mass effect, no change in the gyradius length, nothing.

Now in your last post you've shifted ground to forces and geometry, which is actually the correct approach becuase masses were/are a red herring. But your geometry is wrong. If you put a short stubby derrick on the gunwhale/edge of a (straight, not listing) 20m beam ship, and hang a 1 tonne weight (and i mean weight, not mass, there's a difference) on it, there is turning moment on the ship of 10m x 1 tonne. Now, if you make the derrick 100m tall, vertically, and hang the same weight, the moment is still 10m x 1 tonne. no difference

The lower stability that in practice occurs with the too-tall derrick is caused by the ship listing when the 1 tonne is lifted, and the taller the derrick the greater increase in the lever caused by the list, and so you get a vicious circle where the ship lists more, and the 1 tonne moves further out, and so on till the ship capsizes. This is precisely the tangent effect described above, and is the reason why a too-tall derrick is bad on a ship. But that's a red herring here: it does not at all justify your original assertion that the higher suspension point of a flopper stopper per se makes it more effective. It just doesn't, and you need to reconsider your geometry/engineering.

Some of your statements (like "the mass is effective at that point") show a complete lack of understanding of the geometry of forces and indeed the difference between mass and weight. Mass is mass, namely an inertial resistance to acceleration. Weight is the force of gravity on a mass. Quite different things. Take a car to the moon - it'll need much softer springs becuase it has less weight (the force of moon's gravity is 1/7th x the earth's) but it'll need the same brakes to stop and the same engine to match it's earth 0-60 time becuase its has the same mass on the moon as on earth

Your statement that really astounds me is this one: "If there was no effect on gyradius a vessel supporting a container from a derek 1inch above the deck would experience an insignifficant change in its roll rate compared to having it on deck but it does. For inertial changes the container is effectively where it was in the first place but in terms of roll frequency and amplitude it's like it sat at the derek head." That is just utterly utterly wrong. It is absolutely not correct to say that the roll rate effect is the same as if the container were sitting up at the derrick head

To get back to the point, lots of things can be tweaked on flopper stoppers to make them work better, but hanging them on a long rope from a very tall suspension point on your boat doesn't intrinsically add any benefit

Superbly argued as ever, jfm. Remind me to put you in front of my tax inspector. I agree with you on the geometry. Contrary to my first statement, of course there is no geometrical advantage to raising the height of the flopper stopper suspension points as any advantage on one side of TDC is cancelled out by a similar disadvantage on the other side of TDC.
However, I do agree with TimAbram in that raising masses relative to the CoG of the boat (reducing GM) does affect the rolling characteristics of the boat. A reduced GM will reduce the rate of roll of the boat (ie it will roll more slowly and hence more comfortably) but the roll period will increase and ultimately, of course, if you put too much mass too high up, the boat will become unstable and be unable to right itself from even small angles of heel. The corollary of this is that if you reduce the height of masses as much as possible, the GM will increase resulting in a short fast rolling motion which is uncomfortable. An example of this is Nordhavn who advise their owners to carry their tenders on the flybridge rather than lower down on the bathing platform in order to make the rolling motion of their boats more comfortable. I suppose the designer has to choose a compromise between a small GM to allow the boat to roll comfortably and a large GM to give acceptable stability
The question is, though, does raising the suspension height of the flopper stoppers simulate the effect of raising masses on the boat or not and, to be honest, I can't get my head around that

 

[wakeup]

keels on yachts then are in the wrong place for rolling??

Superbly argued as ever, jfm. Remind me to put you in front of my tax inspector. I agree with you on the geometry. Contrary to my first statement, of course there is no geometrical advantage to raising the height of the flopper stopper suspension points as any advantage on one side of TDC is cancelled out by a similar disadvantage on the other side of TDC.
However, I do agree with TimAbram in that raising masses relative to the CoG of the boat (reducing GM) does affect the rolling characteristics of the boat. A reduced GM will reduce the rate of roll of the boat (ie it will roll more slowly and hence more comfortably) but the roll period will increase and ultimately, of course, if you put too much mass too high up, the boat will become unstable and be unable to right itself from even small angles of heel. The corollary of this is that if you reduce the height of masses as much as possible, the GM will increase resulting in a short fast rolling motion which is uncomfortable. An example of this is Nordhavn who advise their owners to carry their tenders on the flybridge rather than lower down on the bathing platform in order to make the rolling motion of their boats more comfortable. I suppose the designer has to choose a compromise between a small GM to allow the boat to roll comfortably and a large GM to give acceptable stability
The question is, though, does raising the suspension height of the flopper stoppers simulate the effect of raising masses on the boat or not and, to be honest, I can't get my head around that

I am not sure I get this but from what Tim is saying that in order to make roll more comfortable you need as much mass at the extremities of the hull sides. If that is the case why don't raggie boats build their lead keels into the sides of the hull rather than protrude out the bottom or is it simply a case of lowering the CoG on a yacht as far as possible?

 

[Deleted User YDKXO]

I am not sure I get this but from what Tim is saying that in order to make roll more comfortable you need as much mass at the extremities of the hull sides. If that is the case why don't raggie boats build their lead keels into the sides of the hull rather than protrude out the bottom or is it simply a case of lowering the CoG on a yacht as far as possible?

I suppose it depends on your definition of 'comfortable'. I think the accepted definition in simple terms is a boat that rolls slowly rather than quickly and, yes, I think that moving mass to the extremities of the hull achieves the same effect as moving it upwards ie the boat will roll more slowly
As for yachts, I guess the designers prime concern is righting capability rather than roll comfort so they position the keel as low as possible to maximise righting capability. But there are yachts with twin keels placed towards the extremities of the hull ie bilge keelers. I believe the primary purpose of bilge keels is to reduce draft and allow the boat to dry out upright but I wonder whether bilge keelers do also roll more slowly than single keel yachts. Anyone know?

 

[TimAbram]

I am not sure I get this but from what Tim is saying that in order to make roll more comfortable you need as much mass at the extremities of the hull sides. If that is the case why don't raggie boats build their lead keels into the sides of the hull rather than protrude out the bottom or is it simply a case of lowering the CoG on a yacht as far as possible?

This is because the lift of the keel provides a force in oposition to the sideways lifting force of the sails to force the boat forward and not sideways. It needs to be heavy enough to stop the boat rotating about the centre of rotation providing a greater righting moment.

But as per another post you must remeber shifting the weight outboard will slow the roll but will make it bigger, it's the frequency of the roll that makes it unbearable !

a bilege keeler of the same design will roll slower than the same boat with a fin keel for lots of reasons but fundamentaly they are less stable and therefore can not poit as high to the wind as they do not resist the sail lift as well. I'm sure that someone will dispute this but having raced my old moody 27 fin keeler against and cruised with a moody 27 bilge keeler personal experience says this is the case.

 

[jfm]

Superbly argued as ever, jfm. Remind me to put you in front of my tax inspector. I agree with you on the geometry. Contrary to my first statement, of course there is no geometrical advantage to raising the height of the flopper stopper suspension points as any advantage on one side of TDC is cancelled out by a similar disadvantage on the other side of TDC.
However, I do agree with TimAbram in that raising masses relative to the CoG of the boat (reducing GM) does affect the rolling characteristics of the boat. A reduced GM will reduce the rate of roll of the boat (ie it will roll more slowly and hence more comfortably) but the roll period will increase and ultimately, of course, if you put too much mass too high up, the boat will become unstable and be unable to right itself from even small angles of heel. The corollary of this is that if you reduce the height of masses as much as possible, the GM will increase resulting in a short fast rolling motion which is uncomfortable. An example of this is Nordhavn who advise their owners to carry their tenders on the flybridge rather than lower down on the bathing platform in order to make the rolling motion of their boats more comfortable. I suppose the designer has to choose a compromise between a small GM to allow the boat to roll comfortably and a large GM to give acceptable stability
The question is, though, does raising the suspension height of the flopper stoppers simulate the effect of raising masses on the boat or not and, to be honest, I can't get my head around that

All correct mike. The reason you can't get your head around the conceopt that higher suspension point mimics higher location of a mass
is that its tosh and not get head get roundable

 

[jfm]

Thank you for explaining the geometry ..... but you've just explained the principle nicely.

The EFFECT on the ship's stability is ANALOGOUS to this situation .... I did not say it was the SAME.

The geometry might not work for you but the observed results show that the vessel does indeed react as if a weight is suspended from a derek with the boom slewed out to the side.

The GM of the vessel can be measured to have shifted if you apply a force that opposes the roll.

The vessel then rolls at a frequency as if the GM and roll Gyradius had been altered.

Unfortunalely pure geometry does not in reality satisfy the observed results otherwise the Ross equation would would prove to be accurate which it is not.


Let me try a different tack to show my reasoning:

The inertia of the water accelerated as a consequence of the plate motion is called the added inertia. This inertia must have a location at which it acts. It can not effectively act at the site of the plate as it is not connected to the vessel here but at the fixed point of suspension. With any suspended mass the effctive mass is loacated at the point of suspension and not at the origin of the mass itself. If this mass were placed either at beond the maximum beam, or very high up or very low down, the roll inertia will be larger and the frequency slower; this is moddeled by the effective increase in gyradius.

The loss in GM is not caused by a "list" with a tall crane what you describe is a different situation such as a free surface of water or grain or cargo suspended on one side of the deck your comparrison here is incorrect. We are not concerned with mass movement transversly but it's position vertically. You have confused these transverse shifts in centre of gravity with loss of inherrent stability through a reduced gravitational metacentric height, it is the same as a "lol" not a "list" in extremis.

The instability is caused not by the free surface or mass being off centre or the movement of the weight outboard or inboard but by the vertical increase of the centre of gravity in comparrison to the gravitaional metacentre (a loss of GM - lol) not a movement transversely of the centre of gravity in relation to the transverse centre of bouyancy (list). This loss of GM causes the instability and therefore damps the roll frequency. The slower the vessels is rolling the less momentum. The added inertia continues to have effect as the roll passes TDC and thereby a reduced amplitude.

The Ross equation for the roll energy adsorbed does not take into account this effect or the EFFECTIVE change in roll gyradius.


Hope that helps ? If you are still unclear take a look at http://cmst.curtin.edu.au/publicat/2002-08.pdf

Tim, I think best to leave it there. I can't argue with you when you keep introducing new rew herrings like free surface effect. If I reply in full I'll just write loads trying to explain why you're miles away from correct. As for your last sentence I was never unclear and don't need to follow weblinks for stuff this simple. Let's agree to disagree!