Humming topping lift.

[john_morris_uk]

If it isn't the mast that someone else was posting about pumping, its halyards. We get problems with our topping lift.

Basically as its weathered the weave of the line has taken on a slight 'shape'. There are distinct 'edges' that you can see as you sight up the line and the whole thing hums in any sort of breeze. The humming can be translated throughout the boat and gets worse wheh there's any sort of strain on the line. So if we put the boom tent up for shade, its extra weight on the topping lift and exacerbates the problem.

Has anyone else experienced this phenomona? Anyone got any ideas for stopping it? You can't frap the topping lift to any line nearby (and I'm not sure I'd want to as it would be another line to sort out if I needed to make sail in an emergency from an anchorage for instance).

If not its another line I've got to get round to replacing...

 

[Robin]

We have a rigid kicker so the topping lift is really only needed in harbour to counteract the mainsheet & hold the boom in position, but we keep the topping lift anyway as a spare halyard. However in harbour the main halyard is also taken to the boom end and it is this rather than the topping lift that takes the load. The two halyards are in contact and we rarely hear a hum, if we do a slight change of tension will always fix it.

 

[jerryat]

Hi John,

You can cure this quite easily by taking the end of your mainsheet (or any length of reasonable weight line) forming a rolling hitch with it onto the topping lift and sliding it as far up as you can comfortably reach by hand.

Let the other end dangle (helps if there is a small coil to give weight) and bingo! No more noise! /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

Cheers Jerry

 

[Guest]

Have you tried a bungee cord length set into it to keep it lightly taut ... but not tight. Then when you really need it the bungee will extend when topping lift under load ?

I would suggest that the problem is more the TL being taut in a breeze and you may find that the sheeve / system at masthead has some resonance factor that comes into it. That means you have to change some item to alter the factor ... whether it be tautness of TL or service the masthead sheeve, TL attachment to boom etc.

Interesting as I get it with my cap shrouds !!!! but that was cured by a turn on the bottle screw .. altering the apparent resonance ...

 

[kds]

i tie an old towel round it, about one metre up.
Ken

 

[Sheerline]

When refurbishing my mast recently, I changed my topping lift and caused the same problem.

I changed to a thin kevlar topping lift: same breaking strain as the previous double-braid, smaller, lighter...but it hums like mad. Thicker double-braid never did.

I will try Jerry's suggested cure, but I figure I've made a $40 mistake.

(And just to make matters worse, the cleat doesn't like the thin rope either)

 

[MedMan]

It's the tension that causes the problem. Just sit and play with an old guitar for a few minutes. When slack, neither the guitar string nor your topping lift will hum at all! As you increase the tension the frequency of the note increases. The problem comes when you hit a resonant frequency with the structure of your boat. At that point you are sitting inside a sounding box just as if you were inside the guitar.

Changing the tension to avoid a resonant frequency will greatly reduce the volume, but the sound won't go away altogether. The only way to get silence is to slack off the tension completely but, as you say, that isn't necessarily easy. We shift the boom to one side, slack off the topping lift and use a second slackish rope from the boom end to the opposite side of the boat to stop the boom thrashing about.

 

[graham]

depending on the diameter you could try a length of plastic hose pipe split up the side with a stanley knife than pushed over the topping lift and taped on. Probably not need to go all the way up to stop the noise.

I did this on a small boat with beer pipe tubing from the club bar.But that was only about 6 mm.

 

[boatmike]

It's a common problem. The analogy that MedMan has given is correct. It's like a guitar. The mast being the neck, the boom the bridge and the deck the sounding board. The string is the topping lift and the resonant frequency is governed by its length and tension just like a guitar string. The solution is simple and kds and graham have given it to you. Try tying an old towel to the topping lift as far up as you can get it and it will break the frequency and damp the amplitude of the vibration immediately. Try playing a guitar with a sock stuffed in the strings! Grahams solution is amore elegant but possibly less effective alternative to an old towel.....

 

[roger]

The rope shape has little to do with the problem. Round rope does it too. Just for the record the phenomenon is due to alternate shedding of vortices in the wake behind the rope.
Chage the tension, change the resonant frequency by adding wight or spoil the resonance by some form of damping.
The higher the resonant frequency the greater the wind speed needed to get it going. Whatever you do you are liable to get the hum at some wind speed.

 

[peterb]

Take a couple of turns of the main halyard round the topping lift, then fasten it to the end of the boom. The two lines will tend to vibrate at different frequencies, and the friction between them will damp out the vibration. Additionally, the spiralled pair will tend to spoil the Karman vortices that start the vibration.

 

[jerryat]

Might be a risk of chafing though, unless they are tightly wound. /forums/images/graemlins/frown.gif

 

[peterb]

Generally you only get chafe if one or the other is unloaded. If you make sure that the load is shared (unequally, though) then they are unlikely to chafe.

 

[DJE]

Strange, I have never had that problem but the topping lift is 3 strand rope. Maybe the spiral shape acts like those spoilers they put on chimneys to control vortex shedding.

 

[john_morris_uk]

Thanks for the replies chaps.

I was aware that changing the tension changes the notes etc, but had not heard of Karman vortices. As it doesn't happen with some of the boats I sail, I thought it was to do with the profile of the topping lift itself. (Perhaps it is connected as the 'edges' that I mentioned in my opriginal post may exacerbate the vortices).

Anyway.

1. I can't change the load too much. The sail in its stackpack and boom weighs a fair bit, and I can't change to a helium sail. I try not to crank on the mainsheet too much, but when we put the boom tent over the boom, obviously the tension in the topping lift rises and exacerbates the problem.

2. I will try the towel/mainsheet up the topping lift trick. The aft end of the boom when the cockpit tent is rigged is fairly high though and I won't manage to get either old towel or mainsheet very high up - despite me being neary 6'3"!

3. I suspect that the best plan is to bring the main halyard aft and give it a turn or two round the topping lift and disturn the vortices in this way. I have always been relcutant to do this when at anchor for fear of having to make sail in a hurry and not untwisting it properly.

I should have mentioned that I notice the noise most because the vibration is transmitted down the mainsheet to the traveller on the deck above our bunk in the aft cabin!

Thanks again for the ideas and explanations.

John

 

[dralex]

We get the same on our boat- we have a very thin topping lift which thrums. The best and easiest solution we have found is to attach the main halyard to the end of the boom as well and just take a bit of the tension off the topping lift. It also stops the halyard banging against the mast at night. We haven't tried wrapping the ropes around eac other.

 

[Cobra25]

Have heard someone cured this by taking it from 8mm to 10mm