Nick_H
Well-Known Member
If you haven't already seen it, look at the video on their web site, its pretty impressive to see the change when the gyro is turned on.
Flopper Stoppers
- Thread starter Nick_H
- Start date
Hurricane
Well-Known Member
Did a quick google
Came across this - adds a little variant.
http://www.magmaproducts.com/Products/Ma...ete_system.html
Came across this - adds a little variant.
http://www.magmaproducts.com/Products/Ma...ete_system.html
Sundays_Child
Well-Known Member
There's an Azimut 85 that rolls like a metronome when a sampan passes, whereas much smaller boats are less affected. I see her owner and family go out on her and actually pity them...
Friend has a Sunseeker on order and was recently recommended a gyro for 50K (not sure US or Sterling, think it was Sterling). He agreed on the phone. The big police boats here make quite a wash. So I suppose the Brit makers can supply gyros.
Friend has a Sunseeker on order and was recently recommended a gyro for 50K (not sure US or Sterling, think it was Sterling). He agreed on the phone. The big police boats here make quite a wash. So I suppose the Brit makers can supply gyros.
Hurricane
Well-Known Member
Just another thought
Why not make the whole thing from plastic.
Then hang a bag of pebbles below to pull it down.
You could then adjust the number of pebbles (weight)
Make it easier to stow
Less sharp pointy bits
No corosion
Lighter to handle
Greener (less aviation fuel used to get them to the boat!!)
Also I think that you should make up a short pole to get the extra leverage - only a short one (6 foot ish) would nearly double the moment. Praps take the load up to the flybridge and fix the pole to the deck cleats.
Why not make the whole thing from plastic.
Then hang a bag of pebbles below to pull it down.
You could then adjust the number of pebbles (weight)
Make it easier to stow
Less sharp pointy bits
No corosion
Lighter to handle
Greener (less aviation fuel used to get them to the boat!!)
Also I think that you should make up a short pole to get the extra leverage - only a short one (6 foot ish) would nearly double the moment. Praps take the load up to the flybridge and fix the pole to the deck cleats.
Nick_H
Well-Known Member
Yeh, but then you have to carry the pebbles around? Maybe I could use a bag instead and fill it with water /forums/images/graemlins/grin.gif
The poles is a maybe if I don't really get results from hanging off the cleats, but it means ropes everywhere, and I didn't really want that.
The poles is a maybe if I don't really get results from hanging off the cleats, but it means ropes everywhere, and I didn't really want that.
Hurricane
Well-Known Member
[ QUOTE ]
Maybe I could use a bag instead and fill it with water
[/ QUOTE ]
The mind boggles - you could always unbolt your anodes and use them /forums/images/graemlins/wink.gif
Maybe I could use a bag instead and fill it with water
[/ QUOTE ]
The mind boggles - you could always unbolt your anodes and use them /forums/images/graemlins/wink.gif
fire999
Active Member
Would a deep bag/sack made from strong fabric (saily type rag) that has a band around the neck to keep it open. maybee weighted at the base to keep it under tension and increase the size depending on the size of boat.....
Worth a try maybee... /forums/images/graemlins/confused.gif
Worth a try maybee... /forums/images/graemlins/confused.gif
Hurricane
Well-Known Member
You mean something like this
http://dngoodchild.com/7729.htm
In the past, I've actually made some sails myself - DIY - so I might give that idea a try sometime in the future. Got a bit too many other things on the go at the moment though.
http://dngoodchild.com/7729.htm
In the past, I've actually made some sails myself - DIY - so I might give that idea a try sometime in the future. Got a bit too many other things on the go at the moment though.
D
Deleted User YDKXO
Guest
Interesting thread, houghn. We experience the same prob in Majorca. In fact I had to leave a v nice cala only 2 days ago when an uncomfortable swell came out of nowhere. Agree with you that outrigger poles are a non starter coz they look naff and I for one would never be arsed to deploy them. Also agree that you don't need much weight in them, just enough so that they sink faster than the boat rolls. Obviously their effectiveness is going to depend on how much drag the device can create. IMHO, the drogue or sea anchor type device as Hurricane has already suggested would be worth a try as they would be easier to deploy than multiple or larger plates. Keep us posted on your experiments. Could be a biz oppurtunity here /forums/images/graemlins/smile.gif
TimAbram
New Member
I understand this is a reply to an old post but as I've only just found it....I also posted this in reply to another thread but no harm in it appearing here too !
I wrote a paper on passive dampers of this type for my dissertation a couple of years ago.
If you are interested I can send you a copy....
The fundamental principle is twofold the resistance of the plate damps the roll through two effects.
1) energy absorbtion .... added resistance to roll
2) Temporary reduction in how stiff the vessel is in roll (the stiffer the vessel or the more stable... the faster the roll).
The simple rules for damper plates are
1) You must size the damper plate properly....(a bit of ply bridled on ropes an weighted eonough to sink .. you may need to weight both edges but one more than the other so the plate flips up and dives as the boat rlls towards it.... is good enough) Too small has little or no effect... too large makes the plate unstable in the flow it is trying to resist and can make the situation worse ! there is a calculation you can do for your boat but it's quite complex and involves an amount of naval architecture or if you drop me the details of the boat I can give you a broad brush guideline as to size (it's smaller than you might think!).
2) the further away from the centreline of the boat and the higher from the water line the suspension point is the better.... the distance from the centrline has a cubed force effect and the higher the suspension point has a greater effect on the fundamental stability.
The other thing to consider is roll harmonics .... every boat has a frequency at which it likes to roll ! so when the wave train frequency matches the boats natural frequency the rolling effect is amplified.
Deploying flopper stoppers on both sides of the vessel just shifts the natural roll frequency... so roll can be just as bad or even worse if the wave trian frequency in the anchorage shifts to match.
Deploying on one side has a broader range of effect as one half of the roll cycle is at a different frequency from the other giving a smoother and broader range of damping but with slightly reduced damping overall ... makes for a more comfortable night as wave frequencies shift regularly.... but if the roll is relly bad you should probably just change anchorage !
Any one interested just let me know and I'll send a copy of the paper (withoout the stats as that's a shed load of data ! but the graphs etc. included)
Regards
Tim Abram
GMRINA; Bsc.(Hons) Yacht manufacturing and surveying
I wrote a paper on passive dampers of this type for my dissertation a couple of years ago.
If you are interested I can send you a copy....
The fundamental principle is twofold the resistance of the plate damps the roll through two effects.
1) energy absorbtion .... added resistance to roll
2) Temporary reduction in how stiff the vessel is in roll (the stiffer the vessel or the more stable... the faster the roll).
The simple rules for damper plates are
1) You must size the damper plate properly....(a bit of ply bridled on ropes an weighted eonough to sink .. you may need to weight both edges but one more than the other so the plate flips up and dives as the boat rlls towards it.... is good enough) Too small has little or no effect... too large makes the plate unstable in the flow it is trying to resist and can make the situation worse ! there is a calculation you can do for your boat but it's quite complex and involves an amount of naval architecture or if you drop me the details of the boat I can give you a broad brush guideline as to size (it's smaller than you might think!).
2) the further away from the centreline of the boat and the higher from the water line the suspension point is the better.... the distance from the centrline has a cubed force effect and the higher the suspension point has a greater effect on the fundamental stability.
The other thing to consider is roll harmonics .... every boat has a frequency at which it likes to roll ! so when the wave train frequency matches the boats natural frequency the rolling effect is amplified.
Deploying flopper stoppers on both sides of the vessel just shifts the natural roll frequency... so roll can be just as bad or even worse if the wave trian frequency in the anchorage shifts to match.
Deploying on one side has a broader range of effect as one half of the roll cycle is at a different frequency from the other giving a smoother and broader range of damping but with slightly reduced damping overall ... makes for a more comfortable night as wave frequencies shift regularly.... but if the roll is relly bad you should probably just change anchorage !
Any one interested just let me know and I'll send a copy of the paper (withoout the stats as that's a shed load of data ! but the graphs etc. included)
Regards
Tim Abram
GMRINA; Bsc.(Hons) Yacht manufacturing and surveying
jfm
Well-Known Member
Thanks Tim. Can you explain the above a bit more please? I don't get your geometry. I would have expected the height makes no difference. Nick, the original poster who used floppers on a princess 57 flybridge, attached his flopperstoppers to his midship mooring cleats, but if he's attached them higher up, say using an extra 2 metres of rope to tie them to his bimini frame, it wouldn't have made any difference, would it?
D
Deleted User YDKXO
Guest
Dont want to steal Tim's thunder but I guess the higher the flopper stoppers are suspended, the further they are from the centerline when the boat is rolled at an angle so the more effective they are. Where did NickH get to with these things anyway?Thanks Tim. Can you explain the above a bit more please? I don't get your geometry. I would have expected the height makes no difference. Nick, the original poster who used floppers on a princess 57 flybridge, attached his flopperstoppers to his midship mooring cleats, but if he's attached them higher up, say using an extra 2 metres of rope to tie them to his bimini frame, it wouldn't have made any difference, would it?
Nick_H
Well-Known Member
yep, can't see why having the suspension point higher would make any difference, in fact you'd think the optimum would be horizontally in line with the centre of roll of the boat, due to the effect Deleted User talks about.
Mike, I have 4 flopper stoppers on my boat, and use them whenever we anchor for more than a short time. The stoppers themselves are easy peasy to deploy and retrieve, but they store under the tender which is always a bit of a faff to take on and off.
I'm really pleased with how well they work. We anchored in Cannes bay for the fireworks last year (very exposed bay, if you remember), and stayed overnight. In the morning all the usual leisure boats, trip boats etc were wizzing past. Mid morning we decided to leave so I took the stoppers in, and within seconds of taking them in I heard a smash of glass from the flybridge. We had left a couple of glasses up there the previous night, and they had been alright all night and morning until I took the stoppers in.
I also fitted them when I took the charter captain out last month to familiarise himself with the boat, and he was blown away by how effective they were.
Mike, I have 4 flopper stoppers on my boat, and use them whenever we anchor for more than a short time. The stoppers themselves are easy peasy to deploy and retrieve, but they store under the tender which is always a bit of a faff to take on and off.
I'm really pleased with how well they work. We anchored in Cannes bay for the fireworks last year (very exposed bay, if you remember), and stayed overnight. In the morning all the usual leisure boats, trip boats etc were wizzing past. Mid morning we decided to leave so I took the stoppers in, and within seconds of taking them in I heard a smash of glass from the flybridge. We had left a couple of glasses up there the previous night, and they had been alright all night and morning until I took the stoppers in.
I also fitted them when I took the charter captain out last month to familiarise himself with the boat, and he was blown away by how effective they were.
jfm
Well-Known Member
Hmm, not sure. When the boat is max rolled over towards the flopper that is about to kick in, the higher suspension point makes the flopper further from roll centre, I agree. But at that point velocity is zero so the radius could be infinite and it would make no difference. Max roll speed, and hence max resistance/effectiveness of the flopper, is at the mid point where the higher suspension point causes no increase in radius at all. The bits either side of that max speed mid point cancel each other out - I mean the extra suspension height causes extra radius for half the cycle and less radius for the other half, if you get my drift.
So I'm kinda expecting another kind of answer...!
TimAbram
New Member
Height of suspension
It's basic principle is predicated on the effect on the fundamental stability of the boat. The key is the height above the centre of gravity at inifinitly sall angles of heel (i.e. when max force is being applied at the mid point of the roll) as well as the distance from it. Let me explain:
What you do when you add a flopper stopper (in terms of stability) is effectively add a mass at the point of suspension. (like lifting something on a derek or davit slewed outboard)
This weight effectively raises the vessel's centre of gravity while the force is being applied.
As the vessels centre of grivity rises the fundamental stability of the vessel reduces. in technical terms the GM value (the metacentric height) reduces; with a smaller metacentric height the roll period is extended .. i.e the slower the roll
To give an extreme example if you placed a few tonnes of stores on your flybridge (either side of the boat's centre line evenly)your metacentric height would reduce so much that you would loll (it would not list that's something different altogether) off to one side (or the other) untill the vessel had enough additional bouyancy to compensate for the overload where it would be stable again. The any roll would be excrutiatingly (and dangerously) slow.
There is also another effect that occurs with a flopper stopper. The roll period is dictated by both you hull form and what is called the roll gyradius. The roll gyradius is a bit complex to explain effectively but looking at transverse rolling (which we are) if you imagine all of the masses of your boat and equipment formed into a cylinder about the vessels centre of gravity infinitly thin ..... the radius of this circle is the roll gyradius. From the roll gyradius you can predict the roll period just like a pendulum swing.
If you add a weight anywhere (further outboard or higher) outside of this normal gyradius it get's bigger and like a pendulum being made longer the slower the swing (or roll in this case)
If you look at the maths involved, the calculation of flat plate resistive forces simply do not adequately explain the reduction in roll actually achieved. Whilst these effects are secondary to the effective energy absorbed by the resistive force of the plate, taking advantage of them properly can add an additional 20% to the oveall effect (not a further 20% roll reduction I'm sorry to say).
The figure of 25% seems to be about right for the limit of roll reduction if your boat rolls like a bi*** at anchor 25% reduction is pretty good for a bit of plywood and string !
Another thing you can do to take advantage of these effects is, at anchor for a prolonged periods, shift as many of your ships stores as far outboard as you can get them... that'll slow down the roll too.
The only realy effective way known to man at present is to stick a gyroscope on board. A bloody effective way to do it too... reseach from Curtin University in Aukland suggests that roll can be reduced by up to 80% ..... but you do need a big motor and dirty great flywheel to achieve it... but if power supply and money's no object you can install one for a few grand !
It's basic principle is predicated on the effect on the fundamental stability of the boat. The key is the height above the centre of gravity at inifinitly sall angles of heel (i.e. when max force is being applied at the mid point of the roll) as well as the distance from it. Let me explain:
What you do when you add a flopper stopper (in terms of stability) is effectively add a mass at the point of suspension. (like lifting something on a derek or davit slewed outboard)
This weight effectively raises the vessel's centre of gravity while the force is being applied.
As the vessels centre of grivity rises the fundamental stability of the vessel reduces. in technical terms the GM value (the metacentric height) reduces; with a smaller metacentric height the roll period is extended .. i.e the slower the roll
To give an extreme example if you placed a few tonnes of stores on your flybridge (either side of the boat's centre line evenly)your metacentric height would reduce so much that you would loll (it would not list that's something different altogether) off to one side (or the other) untill the vessel had enough additional bouyancy to compensate for the overload where it would be stable again. The any roll would be excrutiatingly (and dangerously) slow.
There is also another effect that occurs with a flopper stopper. The roll period is dictated by both you hull form and what is called the roll gyradius. The roll gyradius is a bit complex to explain effectively but looking at transverse rolling (which we are) if you imagine all of the masses of your boat and equipment formed into a cylinder about the vessels centre of gravity infinitly thin ..... the radius of this circle is the roll gyradius. From the roll gyradius you can predict the roll period just like a pendulum swing.
If you add a weight anywhere (further outboard or higher) outside of this normal gyradius it get's bigger and like a pendulum being made longer the slower the swing (or roll in this case)
If you look at the maths involved, the calculation of flat plate resistive forces simply do not adequately explain the reduction in roll actually achieved. Whilst these effects are secondary to the effective energy absorbed by the resistive force of the plate, taking advantage of them properly can add an additional 20% to the oveall effect (not a further 20% roll reduction I'm sorry to say).
The figure of 25% seems to be about right for the limit of roll reduction if your boat rolls like a bi*** at anchor 25% reduction is pretty good for a bit of plywood and string !
Another thing you can do to take advantage of these effects is, at anchor for a prolonged periods, shift as many of your ships stores as far outboard as you can get them... that'll slow down the roll too.
The only realy effective way known to man at present is to stick a gyroscope on board. A bloody effective way to do it too... reseach from Curtin University in Aukland suggests that roll can be reduced by up to 80% ..... but you do need a big motor and dirty great flywheel to achieve it... but if power supply and money's no object you can install one for a few grand !
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Nick_H
Well-Known Member
not so, there are also active fin systems which are very effective at anchor, check out the Trac system, but they are even more expensive and difficult to install than gyros.
None the less, an interesting post. I've sent you a pm, which you can read by clicking the "your notifications" link above right.
TimAbram
New Member
Trac System
Your deffintily right the track system is certainly effective but boy is it expensive !
These type of actve stabilizers are indeed effective but consume an auful lot of power. In wattage and computing terms.
It would be interesting to see if any data is available for the system.. (think it's commecially confidential though) the fundamental problem with all mechanical (flopper stoppers) or active fins like this at anchor (despite their somewhat interesting claims) is that flow is required over the stabilising surface. Fine for fins while you are underway, but to work at anchor the vessel has to roll otherwise there is no flow to generate the lift. and additionally fins lose the effect of the reduction in stability generated by having the suspension above the CG ...... I've never realy looked too much at these active type stabilisers for use at anchor....... might be an interesting little project if hydrodynamically complex !....
I'll pop the document on a mail later tonight for all of those who have expressed an interest.....
Cheers Tim
Your deffintily right the track system is certainly effective but boy is it expensive !
These type of actve stabilizers are indeed effective but consume an auful lot of power. In wattage and computing terms.
It would be interesting to see if any data is available for the system.. (think it's commecially confidential though) the fundamental problem with all mechanical (flopper stoppers) or active fins like this at anchor (despite their somewhat interesting claims) is that flow is required over the stabilising surface. Fine for fins while you are underway, but to work at anchor the vessel has to roll otherwise there is no flow to generate the lift. and additionally fins lose the effect of the reduction in stability generated by having the suspension above the CG ...... I've never realy looked too much at these active type stabilisers for use at anchor....... might be an interesting little project if hydrodynamically complex !....
I'll pop the document on a mail later tonight for all of those who have expressed an interest.....
Cheers Tim
jfm
Well-Known Member
Thanks Tim. I can see much of your analysis and was expecting it, but I think (respectfully, in the spirit of debate rahter than argument, and I'm very happy to be corrected) you've made a mistake in taking the analogy with extra mass too far. Remeber, you are not actually adding mass with a flopper: you are merely doing something that simulates extra mass, partly.
At top dead centre of the roll, which is the maximum roll speed/angular velocity, and hence the point at which the flopper is doing most good, the suspension height of the flopper is 100% irrelevant. I think your analysis is flawed when you compare a flopper to the addition of mass outside the gyradius. If you actually do add mass outside the gyradius, you increase the angular inertia of the boat and the angular momentum at any given angular velocity (by a square law), and so you reduce the roll period (just like making a pendulum longer ). But adding a flopper doesn't do that. It merely adds a force in the direction of gravity, which does not make any difference (to roll behaviour) at TDC, so is utterly different from adding mass which does have an impact even at top dead centre. Iincreasing the suspension height of the flopper at top dead centre does not mimic adding mass at an increased distnace from the gyradius centre, not one bit
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.
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jfm
Well-Known Member
Tim that is correct for flopper stoppers (and indeed for the earlier Feadship water tank/waterfall device) but not for actives it isn't right. These active stabs flap up and down on mechanical actuators to generate the flow. That's the whole point. With a perfect system the boat can be completely stationary relative to the planet
TimAbram
New Member
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
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
