"Pumping" headsail.

[rfrye1]

Recently sailing my Hunter 376 beating hard into the wind with a reefed (furled) headsail and main. I noticed my headsail/forestay pumping as I was beating into 16-18kts. Is the pumping an adjustment issue? Rigging issue? Or poor sail trim (pilot error)?
Thanks.
Bob Frye
San Diego

 

[RichH]

Probably a combo of all three that caused an induced harmonic vibration ... probably emanating in your mast (the mast was pumping and since its connected to the forestay it too was vibrating).
All 'solids' in nature have whats called a 'natural frequency' at which they become easily induced to vibrate under the right external conditions ... such as a violin string, adjacent strings on most stringed instruments, cable stayed large antennas, aircraft wings, etc. etc. etc.
Changing rigging tension, changing the angle that wind strikes the sails / mast / rigging, hanging a weight in the mast/rigging, etc. etc. etc. all can help lessen (or increase) the vibrations.

The most common reason for such 'pumping' or vibrating (a natural frequency response) is a mast that isnt set up correctly by special tensions of the rigging. A properly set up and properly tensioned rigging will provide a wee bit of forward 'bow' to the mast which 'stiffens' the mast and is called MAST PREBEND. Go to: http://www.riggingandsails.com/pdf/selden-tuning.pdf look for your type of mast (and in accordance with the exact number of 'spreader sets'), and simply set the correct 'pre-bend' in the mast by properly setting up the rig tension. Note: there is a section in that URL that show how to correctly adjust rig tension without a tension gage - just a 'meter stick'.

Sometimes, the leech of a sail will begin to flutter, and if the speed of fluttering is a harmonic to the natural frequency of the mast or rigging, the vibration will 'harmonize' with and 'excite' the mast, etc. .... If you see such leech flutter, simply tension your leech line or apply more or less jibsheet tension until the leech flutter ceases.

The 'usual' cause of rig vibrations is the mast was not properly set up and doesnt have the proper 'pre-bend' or 'pre-bow' .... typically 3/4" forward 'bowing' on a single spreader rig, or 1/2" prebow for each spreader set on a mast with multiple spreaders.

Your sailmaker when s/he designed your mainsail also expected your mast to have that amount of pre-bend, and if you dont have that prebend the sail will set with a LOT more 'draft' than was designed -- boat now heels over more, is slower than normal, and can become quite 'cranky' especially in heavier winds when the mast develops a 'reverse bend' or a bowing that 'going aft', etc. etc.

hope this helps.
Let us know how this info works for you.
;-)

 

[tkanzler]

The pumping I was able to tune out of the rig on my H340, which also had a B&R rig. The rigging was too loose, and following the tuning instructions made the difference.

The leech flapping I couldn't do much about, as the jib was blown out at that point. Tightening the leech line only cupped the leech, so it's just something I lived with. If I had kept the boat, I would have had a sail loft take a look, or just outright replace it.

 

[ph034495]

check back stay tension...
my 42s7 will pump if to loose (typically loosened for down wind running)
but furled head sails are less than perfect air flow surfaces. Most have ropes added to allow some performance when furled small amounts..

 

[rfrye1]

Tuning B&R rig.

Probably a combo of all three that caused an induced harmonic vibration ... probably emanating in your mast (the mast was pumping and since its connected to the forestay it too was vibrating).
All 'solids' in nature have whats called a 'natural frequency' at which they become easily induced to vibrate under the right external conditions ... such as a violin string, adjacent strings on most stringed instruments, cable stayed large antennas, aircraft wings, etc. etc. etc.
Changing rigging tension, changing the angle that wind strikes the sails / mast / rigging, hanging a weight in the mast/rigging, etc. etc. etc. all can help lessen (or increase) the vibrations.

The most common reason for such 'pumping' or vibrating (a natural frequency response) is a mast that isnt set up correctly by special tensions of the rigging. A properly set up and properly tensioned rigging will provide a wee bit of forward 'bow' to the mast which 'stiffens' the mast and is called MAST PREBEND. Go to: http://www.riggingandsails.com/pdf/selden-tuning.pdf look for your type of mast (and in accordance with the exact number of 'spreader sets'), and simply set the correct 'pre-bend' in the mast by properly setting up the rig tension. Note: there is a section in that URL that show how to correctly adjust rig tension without a tension gage - just a 'meter stick'.

Sometimes, the leech of a sail will begin to flutter, and if the speed of fluttering is a harmonic to the natural frequency of the mast or rigging, the vibration will 'harmonize' with and 'excite' the mast, etc. .... If you see such leech flutter, simply tension your leech line or apply more or less jibsheet tension until the leech flutter ceases.

The 'usual' cause of rig vibrations is the mast was not properly set up and doesnt have the proper 'pre-bend' or 'pre-bow' .... typically 3/4" forward 'bowing' on a single spreader rig, or 1/2" prebow for each spreader set on a mast with multiple spreaders.

Your sailmaker when s/he designed your mainsail also expected your mast to have that amount of pre-bend, and if you dont have that prebend the sail will set with a LOT more 'draft' than was designed -- boat now heels over more, is slower than normal, and can become quite 'cranky' especially in heavier winds when the mast develops a 'reverse bend' or a bowing that 'going aft', etc. etc.

hope this helps.
Let us know how this info works for you.
;-)

Really good info. But with my Hunter and my B&R rig, I assume the "tuning manual" is a little different. It isnt a really big issue for me or the pumping isnt really bad. Next time I will experiment more on trim and watch the mast. I have a decent amount of "bend" in my mast now. I'm due for bottom paint this year, maybe a good time to have the rigging checked/adjusted.

 

[RichH]

B&R rig tuning is included in the URL listed.

 

[Don12364]

My guess is that you have a slack forestay. It gives the boat a horrible feel when going upwind in a blow.
I set my H376 up with 6" pre bend in the mast and 6" rake on the mast.
Do this on a non breezy day as follows:-
Tie end of main halyard to gooseneck and pull halyard tight as you can by hand. Then sight up mast and look for about 6" between halyard and mast.
Then check mast rake. Hang something heavy e.g. your toolbox on the main halyard and lift it about 1" off the deck. Now note where it touches the boom. It should be about 6" back from the mast.

Also I use 1 ton (approx 2,200lbs in the forestay).
Whilst setting the rake I use about 2000lb in each main shroud, all the while sighting up the mast to eliminate any sideways bend. Then I tighten the lower uppers fairly tight and re-check the rake and pre-bend.
Then I go sailing. If the lee shrouds go really slack when on the wind then they are not tight enough so count the turns as you take some of the slack out. Then tack and put the same number of turns on the other shrouds and lowers.
If you've got it right the slack in the lee rigging should have gone.
Now recheck the shroud tensions with the boat upright i.e. non sailing.
The caution is to not go much over one fifth of the breaking strain if the wire.

Wire BS is available everywhere. The forestay is 5/16" diameter, main shroud (lower upper) is 5/16" and lower Intermediate is 9/32" diameter.

How to measure rig tension is dealt with elsewhere but I use a spring balance hooked onto the wire. I put a marker of tape 79" above the lower toggle and extend a measuring stick or a steel tape measure from it to the mast. Then, using the spring balance, I pull the shroud either towards or away from the mast by one inch exactly. If the gauge reads 40 lbs there is 2,000 lbs tension in the wire.
It is all worked out for a H376 and all the distances and loads scale within reason.
I have it on an Excel spreadsheet if you would like it.

 

[Warren Milberg]

While you never can go wrong by ensuring your rig is properly tuned, I've sometimes found that mast pumping (and other problems) occur when a headsail is reefed, but the sheeting cars have not been moved forward to create the optimum sheeting angle. Give that a try and see what happens.

 

[shemandr]

refrye1,
Is your boat equipped with the struts? Is the main in-mast furling? The fact that Selden's publication on the mast says that you don't need the struts on the furling mast because the sections are larger, suggests that you do on the non-furling. Just a thought.
We have the in-mast furling and struts and I've never felt the mast was pumping or in any kind of harmonic vibration.