Mast pumping, how harmful is it?
Hi,
I have a sailboat and when it gets windy at the slip, sometimes there is a vibration and the mast and rigging seems to resonate and vibrate. Someone told me this is 'mast pumping'. I was also on another sailboat visiting and it did it also. Nobody seemed concerned. My question is; should I be concerned? We get regular wind here at twenty knots and sometimes more. The pumping seems to last about 3-5 seconds. Then, it stops and resumes a little later. Thanks for any info. |
Mast pumping, how harmful is it?
Mast pumping is "a frequency response' of the mast to energy (wind)
pressure. Think of a violin string that vibrates - same thing. Every structure on earth has a frequency that will excite it to vibrate at a maximum .... called the natural frequency. How to stop a mast, etc. from vibrating: change the natural frequency by tightening or loosening the rigging and/or changing the 'vibrational' shape of the mast by adding a little fore/aft "pre-bend" bend when you tune. Harmful - yes in some cases. The metals of the mast, shrouds, and shroud terminal connections have a service life based on 'load cycles' (in the millions) and if these load cycles are appliied beyond a certain strength limit (called 'endurance limit') will rapidly result in *fatigue* of the metal(s). Stainless steel is especially vulnerable to fatigue. Most of these oscilations will be be under the 'endurance limit' and are harmless .... but you can never tell when the endurance limit is exceeded (due to the dynamics of the oscilations .... and the 'bad' cycles start 'deducting' from the number of maximum cycles before the metal fatigues (service limit). At sea, one uses running backstays, etc. to dampen such oscilations. There are other simple methods / tricks on a sailboat mast that will render a mast less vulnerable to vibrate: Most single spreader masts will GREATLY benefit from establishing a 1" fore/aft "prebend" - which bends the mast in the middle towards the stern of the boat.; and which, **radically** changes the natural frequency ... double spreader rigs usuallly need 2" of pre-bend, etc. To set pre-bend, tune the mast dead straight, etc. then tighten the forward lowers and loosen the aft lowers until the pre-bend is set .... that's it. Simply sight up the mast at the sail track, pull a halyard tight and adjacent to the sail track and measure/estimate a space of 1" forward bow/bend. A good sailmaker usually will include such normal pre-bend into the luff cut of mainsail anyway .... so you probably should take advantage and tune your mast with the pre-bend to start with ... and get a better shaped mainsail as a side benefit. In article , MLapla4120 wrote: Hi, I have a sailboat and when it gets windy at the slip, sometimes there is a vibration and the mast and rigging seems to resonate and vibrate. Someone told me this is 'mast pumping'. I was also on another sailboat visiting and it did it also. Nobody seemed concerned. My question is; should I be concerned? We get regular wind here at twenty knots and sometimes more. The pumping seems to last about 3-5 seconds. Then, it stops and resumes a little later. Thanks for any info. |
Mast pumping, how harmful is it?
I know of one case where mast pumping was blamed for compressing the mast step,
which led to severe pumping and rig failure. While the primary fault in this case was a design flaw in the step, it shows that pumping can stress the system in unusual ways. BTW, I know of a number of boats that don't worry about pumping, though they check their rig more frequently than the above mentioned boat. "MLapla4120" wrote in message ... Hi, I have a sailboat and when it gets windy at the slip, sometimes there is a vibration and the mast and rigging seems to resonate and vibrate. Someone told me this is 'mast pumping'. I was also on another sailboat visiting and it did it also. Nobody seemed concerned. My question is; should I be concerned? We get regular wind here at twenty knots and sometimes more. The pumping seems to last about 3-5 seconds. Then, it stops and resumes a little later. Thanks for any info. |
Mast pumping, how harmful is it?
On Wed, 28 Jul 2004 05:01:54 GMT, Rich Hampel
wrote: Most single spreader masts will GREATLY benefit from establishing a 1" fore/aft "prebend" - which bends the mast in the middle towards the stern of the boat. =============================================== This sounds backwards to me unless we are using different terminology. I think of pre-bend as bowing the middle of the mast in a forward direction. The opposite of that is "inversion", i.e., mast bowed towards the stern. Inversion is generally regarded as a bad thing since it is contrary to normal structural design. Any other opinions out there? |
Mast pumping, how harmful is it?
Pardon the 'brain fart'.
You are correct. Thanks for the 'editing'. FORWARD bow at the middle is the correct practice. A lot of the old time sailing ships (Schooners, bugeyes, etc.) that used hoops to connect the main to the mast did in fact apply such an 'inversion' ... as a means to increase draft. In article , Wayne.B wrote: On Wed, 28 Jul 2004 05:01:54 GMT, Rich Hampel wrote: Most single spreader masts will GREATLY benefit from establishing a 1" fore/aft "prebend" - which bends the mast in the middle towards the stern of the boat. =============================================== This sounds backwards to me unless we are using different terminology. I think of pre-bend as bowing the middle of the mast in a forward direction. The opposite of that is "inversion", i.e., mast bowed towards the stern. Inversion is generally regarded as a bad thing since it is contrary to normal structural design. Any other opinions out there? |
Mast pumping, how harmful is it?
Ever see the film of the old Tacoma Narrows bridge? Same thing. Potentially
damaging? You decide. I'd do some rig tuning, but my methods are a bit unorthodox and difficult to explain. The effect you are trying to achieve is to break the ability for the rig to achieve harmonic resonance aka Mast Pumping in your case. "MLapla4120" wrote in message ... Hi, I have a sailboat and when it gets windy at the slip, sometimes there is a vibration and the mast and rigging seems to resonate and vibrate. Someone told me this is 'mast pumping'. I was also on another sailboat visiting and it did it also. Nobody seemed concerned. My question is; should I be concerned? We get regular wind here at twenty knots and sometimes more. The pumping seems to last about 3-5 seconds. Then, it stops and resumes a little later. Thanks for any info. |
All times are GMT +1. The time now is 12:55 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004 - 2014 BoatBanter.com