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post #1 of 6 Old 03-29-2015 Thread Starter
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Pumping the Mast

Noob-like question here...(I hope I ask it right so I don't get shot down for asking a dumb question, or asking it incorrectly)...

I've heard about it (pumping mast) several times on the forum here generally in a negative connotation, but not sure what exactly is meant by "pumping"...

What does it look like, feel like, what are the indications that your mast is "pumping"..?

When does it usually happen, ie: what causes it and conversely, what do you do to stop it/avoid it..?

What can happen if it does pump excessively...?

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post #2 of 6 Old 03-29-2015
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Re: Pumping the Mast

Mast pumping is the middle section of the mast having enough flexibility and lack of support such that it can start to move around esp in a seaway. It's most likely in boats with in-line spreaders and single lowers where the middle of the mast is not 'triangulated' into place by the vectors of the standing rigging. (Swept spreaders/shrouds, double lower shrouds, and babystays generally naturally limit mast pumping)

In another recent thread there was a reference to lack of support when sailing headsail-only.. in that case, without the pull of the trimmed mainsail along the length of the mast, it's really only 'held tightly' at the bottom and the top. If it gets 'plucked' in the middle (by breeze or motion) it can vibrate like a guitar string.

This can cause sometimes severe vibration and the constant movement can stress everything connected, including the skipper

It can even be seen on a boat at the dock or on a mooring on a windy day.. the mast can set up its own sympathetic vibration due to vortex shedding in the lee of the mast section. It usually comes and goes at various windspeeds and angles.

Ron

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Last edited by Faster; 03-29-2015 at 12:47 PM.
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post #3 of 6 Old 03-29-2015
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Re: Pumping the Mast

Mast pumping is caused by the velocity of the wind causing the formation 'vortexes' (rotors) being shed by the geometry of the mast; and, when the frequency (rate) of the vortex shedding is in 'tune' with the natural frequency of the mast, the mast will (can) vibrate in a (destructive) harmonic response to the shedding of these 'rotors' being generated.

This is the vortex phenomenon:
..... and includes a moving diagram/illustration of the event.

This is a 2-part phenomenon - the vibration characteristics of the mast PLUS the action of the wind. To remedy, the easiest way is to change the physical aspects of the mast ... changing its mass/weight, or changing the 'stiffness' of the mast so that one changes the natural frequency of oscillation of the mast, the vibrational characteristics (natural frequency) of the mast is the easiest to control/remedy:

The most common methods or remedy/prevention are:
1. Pre-bowing the mast or adding a forward bend or bow by rigging tension - usually 3/4" forward bow for a single spreader set, or 1/2" forward bow per each spreader set for multiple spreader masts. Such pre-bowing radically changes the apparent 'stiffness' of the mast, which changes its 'natural' frequency.
FWIW - all sailmakers expect that all masts are set up with this pre-bow when they design mainsails as this 'expected' bowing is always incorporated in mainsail design.)
For setting up proper mast 'pre-bow' or 'pre-bend' go to: http://www.riggingandsails.com/pdf/selden-tuning.pdf

2. Changing or adding mass (weight) to the mast (a temporary solution) -
The raising of any additional large mass/weight up to about mid-span of the mast ... can be any 'heavy' object that stays 'attached' to the mast.

3. adding a 'spiral' of rope/line about the mast - changes the regular pattern of vortices being shed from the mast. Good for when at a dock, slip, etc. ... a temporary solution.


Importance of remediation - such oscillations 'can' mathematically become severe when in complete 'harmony', and can begin by 'impact considerations' exceed the metallurgical limits of especially the rigging components causing or adding to the fatigue (eventual failure) of the rigging/mast 'system'. This 'harm' would be by the creation of instantaneous 'strength' events that exceed the 'yield strength' of the metals or more commonly events that exceed whats known as the 'fatigue endurance limit'. With fatigue, all these events 'add up' over time and with stainless steels the component will usually catastrophically fail at 1 million load cycles that occur beyond or exceed the fatigue endurance limit ... about 1/3rd the ultimate tensile strength for 300 series stainless (or about 30,000 psi).

Rx:
• The hands down BEST way to avoid 'mast pumping' is to PRE-BEND or PRE-BOW the mast - changes the natural frequency of the mast.
• The hands down BEST way to reduce mast pumping when sailing (heavy wind conditions) is to change the tension in the rigging (up or down) ..... until the pumping stops. For 'distance' sailors this is usually accomplished by applying/changing rig/mast tension by the running backstays ... OR at a minimum by changing the backstay tension .... or in extreme mast pumping situations to reduce/increase sail area ... OR sailing direction.


Mast pumping is a potential failure mode of the rigging/mast system.

Most folks simply never add prebow to their masts ... and without pre-bow/pre-bend the pumping can become 'exceptionally violent' under the convergence of the 'right' conditions.

Here's an equivalent of this 'vortices shedding' phenomenon, can happen with sailboat masts, too: https://www.youtube.com/watch?v=nFzu6CNtqec


hope this helps.
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Re: Pumping the Mast

Thank you for the comprehensive and detailed explanation(s)...

From the description, I don't think I have ever experienced a mast pumping situation on my boat...good prebend, swept spreaders(?)...

At least now I know what to look for...and have a working understanding of what others are describing and experiencing when they mention it.

I remember seeing that bridge video in grade school... It was old then...lol

"Might as well take 'er out...If anything is gonna happen...It's gonna happen out there..."
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post #5 of 6 Old 03-29-2015
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Re: Pumping the Mast

Also lower aft shrouds, checkmates, running backstays, inner forestay and baby stays may be used to eliminate pumping. I am skeptical that adding pre-bend necessarily eliminates pumping. Prebend does imply much greater pre-load on the mast and therefore raises the natural frequency of the pumping - often up to a frequency that might better be called a vibration or hum.

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post #6 of 6 Old 03-29-2015
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Re: Pumping the Mast

Any change of rig tension will change the natural frequency response of a mast. When the frequency of the mast (and rigging, as a 'system') is equal to the frequency of the shedding vortices, the mast will pump (vibrate).
Since the mast usually has the lowest natural frequency of all components of the 'system', pre-bowing produces the greatest effect on raising/lowering its natural frequency because pre-bowing 'stiffens' the mast by changing its geometric 'moment of inertia'.

Pre-bend or Pre-bow changes the point/frequency at which vibration (pumping) occurs ... with the goal of being out-of-phase with the frequency of the shedding vortices. Its when the frequency of the vortices MATCH the natural frequency of the mast+rigging is when one encounters 'pumping'. Change any of the two and the mast will not pump in response.

Last edited by RichH; 03-29-2015 at 02:55 PM.
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