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Annoying Genny!!

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Feb 15, 2005
14
Chrysler 22 Sheepshead Bay Brooklyn, NY
Hi Don, It's certainly a pleasure to "meet" you. I have a 22ft. Chrysler sloop, mast head rig, with a 150 genoa bent onto a roller furling extrusion that fits over the forestay. I have a standard main. In air over 10 knots and close hauled, my forestay starts to "pulse". I have really cranked down on both stays, but it doesn't seem to help. The boat moves very well and points beautifully, but this "thumping" is annoying and only increased with wind speed. Any ideas?
 
M

Mike

Do you have a halyard?

You didn't state what manufacturer roller furler you have. Some like a CDI have an internal halyard, and merely sit on the drum. If that is what you have, maybe the foil has too much play at the top, and a piece of plastic pipe can eliminate the play there. Others, like a Harken, use the halyard to put tension on the luff. Maybe your halyard is too loose.
 
Feb 15, 2005
14
Chrysler 22 Sheepshead Bay Brooklyn, NY
Information I didn't relay the first time.

Thank you for the quick reply. Yes, I have a CDI and I have tried to tighten the internal halyard as much as possible. However, the pulsing in not up and down the forestay. It's more of a flexing that causes the "thumping" vibration that can be felt all the way back to the tiller. As far as the extrusion, it is attached to the barrel so it really has nowhere to go. I hope I gave you enough info. I have a habit of keeping my thoughts in my head assuming the listener can read my mind. Sorry.
 
May 17, 2004
2,023
Other Catalina 30 Tucson, AZ
Annoying Genny

Windchaser22: Yes, I do have an idea. If it were me, I'd contact a local rigger and have him take a look at your situation. Tightening everything down is generally not a good idea as there are specific tensions for for boat. The rigger has probably run into your problem before and when he fixes the problem he could re tune your rig for you.
 

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Harmonic Vibrations

Two very different things can cause such vibrations: Sail set/trim and improper rigging tension: Next time out, set the sails as usual and with an assistant at the helm go forward and look closely at the sails especially at the upper leach of the sails. Look carefully for any cyclical movement anywhere on the sail surface. Leech problems of fluttering can usuallly be solved by changing the fairlead position (forward or aft) or by tensioning the leech cord, if you have one. Sometimes because of age and permanent stretch, a sail 'just wont be quiet' in a particular area - some times the remedy is installation of a small/temporary batten across the affected area. Sometimes the rigging/mast system will vibrate at what is called a 'natural frequency vibration'. Such vibrations if very severe can damage or break the rigging by a phenomenon known as fatigue. Again with an assistant at the helm, when the 'vibration' is evident go forward and with your eye close to the mast look upwards to the mast top. If you see cyclical flexural movements in the mast ---- something has to change! Usually a change in rigging tension (more slack or more taught) will change the 'natural frequency' of the mast rigging .... just 'tune-out' this frequency by adjusting rigging tension to another value. :)
 
B

Bob

Remote possibility

Is there a chance the vibration isn't actually coming from the forestay, but from the lifting cable for the swing keel? It is not uncommon to get vibration at a given speed from them.
 
W

windchser22

Definately not.....

....This is not a vibration. It is a pulsing of the forestay as if someone was plucking it to a set beat. The keel cable is my trusty speedometer and I use it to my advantage. Thank you for your thoughts.
 
W

windchaser22

Thank you much!!!!......

Although I have done some rigging tension changes, I never thought of eyeballing the mast. I will most definately give it a try! Just an afterthought...wouldn't a mast oscillation also affect the main? since my main has always been rock solid, I always looked to the foresail rigging???
 

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
.... more on harmonic vibrations.

No, not necessarily ...... if this case of possible mast vibration is the case, anything that has the same (or fractional) vibrational characteristics on the boat will all vibrate at the same time (some more, some less). Its like the strings on a guitar or other stringed instrument .... if you strike a high A note, the lower A (on another string) will also vibrate because its a harmonic and 'resonates' at the same or fractional frequencies. If you strike an A on one guitar, the A on a close-by guitar will also start vibrating - harmonic. Youve experienced this in automobiles and machinery .... all of a sudden one particualr part or section starts to magically vibrate (natural frequency) all by itself and if you change the 'excitation frequency' (changing speed, rpm, tension, mass of the part ... the natural frequency will change and the part will no longer vibrate. On a boat, every mast will oscilate at a certain frequency (sometimes very noticeablly - called mast pumping); usually it is encountered during high wind situations but if the 'tuning' matches the frequency, it can occur at quite low wind velocities (rare). Changing the tension on the rigging will usually shift the natural frequency and stop the vibration ..... only to have the vibration appear at different situation that matches the 'next' excitation frequency. Mast pumping at low wind velocities although rare is a fact; but, not as rare as a section of a sail that is oscilating to a certain frequency and causing an adjacent structural member (such a a jibstay, etc.) to vibrate as a 'harmonic'. Next time out, take a good look at the leech of the genoa and see if the vibrations (even very teeny vibrations) are coming from the genoa leech .... probably the most common cause/source of such vibrations. Another cause of 'mysterious' vibrations on some boats is caused by thin flat centerboards. At certains angles to the flow of water going past them they start to vibrate (or acutally 'hum') and if this hum/frequency is at the same natural frequency (or fraction of) of any other part or section of the boat that part will vibrate (harmonic resonance) also. Induced harmonic oscilations are a nuisance and can cause rapid rigging failure (fatigue); the solution is usually to change the frequency by changing tension or mass.
 
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