I have a radar reflector mounted on my backstay in what is a pretty typical installation. My C42 has a single backstay not split near the bottom like some. I first noticed that when it gets windy at my home port the reflector will oscillate harmonically and it can get pretty extreme in strong winds. My solution for that was an easy fix that worked at the dock. I would simply tie a line to my main halyard and twist it around the backstay and then when I got to the point just below the reflector I would tie the line to the stern pulpit and pull it tight. Well, it seems like I need to come up with a solution that works while I am sailing. I was out yesterday sailing close-hauled in 11 knots of apparent wind, which is obviously a pretty light day but the reflector was really rocking. I guess I could connect a small line to the reflector and tie it off to the pulpit like I am doing when I am at my dock but I would like to find a more elegant solution without taking the drastic step of buying a streamlined reflector and mounting it on the mast. I know there is a wealth of wisdom on this site and hope I can tap into it for a solution to this vexing problem. Any ideas? Bob V
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natural frequency oscilations can be changed .....
by increasing tension is the support structure .... or by changing the mass (weight) of the vibrating structure; all of which will change the natural frequency. Take a look next time that you have this 'harmonic' as it may be simply that the reflector is mounted at a 'node' of the oscilation frequency, thus amplifying it. During the oscilation, look carefully at the backstay and see if you can 'see' the vibration amplitude and periods (the wire will have some sections where the vibration is visible and some areas where the vibration seems absent. If so, move the reflector either from a quiet zone to the an active zone or vice versa. Changing the tension of the wire or the 'mounting tension' of the reflector ... will change the resident natural frequency - sometimes making the system 'quiet down', sometimes making it worse. If its only the reflector vibrating take some tape and add some weight to random areas ( tape on some heavy nuts, etc.) Your tools to correct are change of tension and change of mass. ;-)
by increasing tension is the support structure .... or by changing the mass (weight) of the vibrating structure; all of which will change the natural frequency. Take a look next time that you have this 'harmonic' as it may be simply that the reflector is mounted at a 'node' of the oscilation frequency, thus amplifying it. During the oscilation, look carefully at the backstay and see if you can 'see' the vibration amplitude and periods (the wire will have some sections where the vibration is visible and some areas where the vibration seems absent. If so, move the reflector either from a quiet zone to the an active zone or vice versa. Changing the tension of the wire or the 'mounting tension' of the reflector ... will change the resident natural frequency - sometimes making the system 'quiet down', sometimes making it worse. If its only the reflector vibrating take some tape and add some weight to random areas ( tape on some heavy nuts, etc.) Your tools to correct are change of tension and change of mass. ;-)
You have another option. If you add a container half
filled with liquid it will have a natural frequency of oscillation different from the reflector and will provide damping with the liquid being always out of sync with the mechanical vibrations. In short you need a shock absorber or a dashpot.
filled with liquid it will have a natural frequency of oscillation different from the reflector and will provide damping with the liquid being always out of sync with the mechanical vibrations. In short you need a shock absorber or a dashpot.
Thanks for the suggestions
I think I will give the dashpot a try first. It should be easy to mount a plastic bottle in one of the "cups" and tie it on. If that doesn't work, I'll try moving the reflector up the backstay a bit. Bob V
I think I will give the dashpot a try first. It should be easy to mount a plastic bottle in one of the "cups" and tie it on. If that doesn't work, I'll try moving the reflector up the backstay a bit. Bob V
Paulj, This will help how?
The reflector is shedding vortexes as the wind blows across it causing it to lift left and right in harmonic synchrony with the natural frequency of the back stay. If the holes will interrupt the shedding of the vortexes then that will help. But the semi-filled liquid container will damp the vibrations and limit the amplitude of them.
The reflector is shedding vortexes as the wind blows across it causing it to lift left and right in harmonic synchrony with the natural frequency of the back stay. If the holes will interrupt the shedding of the vortexes then that will help. But the semi-filled liquid container will damp the vibrations and limit the amplitude of them.
Try using some "string", that thin line
sold in 50 foot lengths at West Marine. Tie it above the reflector with a rolling hitch and two half hitches and a length good to get to your deck or pulpit. When it gets windy, tie it off down below, just like you do with your halyard. When you're done sailing, yo never have to use the halyard again.
sold in 50 foot lengths at West Marine. Tie it above the reflector with a rolling hitch and two half hitches and a length good to get to your deck or pulpit. When it gets windy, tie it off down below, just like you do with your halyard. When you're done sailing, yo never have to use the halyard again.
Flag halyard?
Bob don't you have a flag halyard on a spreader you can hang it from? Sure would be a lot easier than drilling holes or mounting water bottles to it. If you don't then try wrapping some rubber weld rigging tape under the mounts to isolate the reflector from the backstay. If you can, keep it simple and avoid damaging the reflector. Mike
Bob don't you have a flag halyard on a spreader you can hang it from? Sure would be a lot easier than drilling holes or mounting water bottles to it. If you don't then try wrapping some rubber weld rigging tape under the mounts to isolate the reflector from the backstay. If you can, keep it simple and avoid damaging the reflector. Mike
Size matters
Just out of curiousity, Bob, what is the amplitude of the oscillations at their worst? How far does the stay move back and forth when it's really going crazy?
Just out of curiousity, Bob, what is the amplitude of the oscillations at their worst? How far does the stay move back and forth when it's really going crazy?
Phil, re: size matters
The backstay is not deflected but remains straight. What happen is a twisting motion that I have seen approach 90 degrees each side of centered. I am concerned that the repetative twisting will damage the twisted wire at some point. That was in very high winds, which are fairly common here in the winter. The day I was out sailing in 11 knots apparent wind it was twisting about 45 degrees each side of center. That's why I don't think moving the reflector up or down the wire or that moving it to the flag halyardwill affect it . I think Ross nailed it when he wrote "The reflector is shedding vortexes as the wind blows across it causing it to lift left and right in harmonic synchrony with the natural frequency of the back stay. If the holes will interrupt the shedding of the vortexes then that will help. But the semi-filled liquid container will damp the vibrations and limit the amplitude of them." Now I need to find the right bottle to attach to the reflector and test his theory. Ross, I bet if I filled it half full of mercury instead of water that would be even better, eh? Thanks again to everyone for the great ideas. Bob V
The backstay is not deflected but remains straight. What happen is a twisting motion that I have seen approach 90 degrees each side of centered. I am concerned that the repetative twisting will damage the twisted wire at some point. That was in very high winds, which are fairly common here in the winter. The day I was out sailing in 11 knots apparent wind it was twisting about 45 degrees each side of center. That's why I don't think moving the reflector up or down the wire or that moving it to the flag halyardwill affect it . I think Ross nailed it when he wrote "The reflector is shedding vortexes as the wind blows across it causing it to lift left and right in harmonic synchrony with the natural frequency of the back stay. If the holes will interrupt the shedding of the vortexes then that will help. But the semi-filled liquid container will damp the vibrations and limit the amplitude of them." Now I need to find the right bottle to attach to the reflector and test his theory. Ross, I bet if I filled it half full of mercury instead of water that would be even better, eh? Thanks again to everyone for the great ideas. Bob V
mass balancing
In land sailing with big wing masts, sometime mast oscillation is controlled by adding a mass out in front of the wing mast. On this web site, you can see what looks like an antenna out in front of the mast - I believe its mass (probably lead) put out in front of the mast as far as possible. http://www.windjet.co.uk/ . If the mast on a craft like this starts to oscillate (and they can...), major big problem! Maybe you can use the same sort of "mass balancing" idea on the reflector. Add some mass on the leading edge?
In land sailing with big wing masts, sometime mast oscillation is controlled by adding a mass out in front of the wing mast. On this web site, you can see what looks like an antenna out in front of the mast - I believe its mass (probably lead) put out in front of the mast as far as possible. http://www.windjet.co.uk/ . If the mast on a craft like this starts to oscillate (and they can...), major big problem! Maybe you can use the same sort of "mass balancing" idea on the reflector. Add some mass on the leading edge?
A different sort of mass
You want to increase rotational inertia, not just mass. That would explain keeping it away from the mast. I suspect the periodic nature of the forcing has to do with the changing angle of attack, in both instances. Interesting problem.
You want to increase rotational inertia, not just mass. That would explain keeping it away from the mast. I suspect the periodic nature of the forcing has to do with the changing angle of attack, in both instances. Interesting problem.
I have seen this type of rotational oscillation
on stop signs in high wind conditions. the cable is behaving as a torsional spring. One problem with a water bottle is subfreezing weather. If it froze it would become part of the problem. Streamers attached to each side might break up the air flow. I have watched the cargo webbing on large trucks vibrate this way. Maybe if the attachment was not so secure and the reflector could just weather vane it would be better. Swinging weights would also interrupt the oscillations.
on stop signs in high wind conditions. the cable is behaving as a torsional spring. One problem with a water bottle is subfreezing weather. If it froze it would become part of the problem. Streamers attached to each side might break up the air flow. I have watched the cargo webbing on large trucks vibrate this way. Maybe if the attachment was not so secure and the reflector could just weather vane it would be better. Swinging weights would also interrupt the oscillations.
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