Fractures on the J80's mast

posting disabled

Moderator: forumadmin

Postby Guest » Fri Feb 28, 2003 8:04 am

At our Match Race Center here in Helsinki, Finland, some problems with the root of two of our masts have occured. On the first one the mast has got two fractures on both sides of the main sail foil. The other mast has a dent at its side, the aluminium has also been torned up a bit.
Please take a look at the pictures via the link below:

http://www.kolumbus.fi/daniel.niemi/J80/

Has anyone else got similar problems? Is this (specially the first case) a result of an unbalanced mast or just gap between the root of the mast and the mast foot? Has anyone got a solution for repairing and preventing these types of damages? All answers are appreciated, thank you for any help.
Guest
 

Postby Guest » Fri Feb 28, 2003 11:37 am

Is this a French built boat? Noticed that the mast was unpainted. Call the manufactor of the mast and step. I would think that the cracks can be welded (heli-arc) with no problem. The fatigue appears very unusual with all the scaring. Could the damage have been a result of an outside source? While raising or lowering? During storage? Hard to understand any movement of the mast butt while the rig is up. Should be no "play" between the step and mast.

Craig White
Guest
 

Postby Guest » Fri Feb 28, 2003 12:00 pm

The appearance of the cracks suggests point loading on the aft edge of the mast butt, where the groove is located. This could be the result of excess aft mast rake or an angle on the bottom of the mast extrusion that is incongruent with the junction of the mast butt and the mast step. I would suggest contacting Sparcraft, the manufacturer, about repairs on the damaged spar. I would also suggest examining your other boats for mast butt - mast base incongruity. If the mast butt point loads on its aft edge, a small amount of filing or grinding of the butt can correct the mismatch. In other types of boats I have seen the mast butts filed to a gentle arc that accomodates differences in rake setup.
Guest
 

Postby Guest » Fri Feb 28, 2003 4:48 pm

I have noticed that my mast does not sit squarely fore and aft, and considered filing.

I haven't done so for fear of it being considered an "alteration" and prohibited by the class rules.

While whacking an inch or two off (I don't know if this is a good or bad idea) is clearly an alteration and should be prohibited, slight filing to ensure a structurally sound and safe connection should not.

Could we get a technical interpretation done on this? That will provide some guidance for both us and future owners.

Thanks
Steve
Ghost Dancing
Guest
 

Postby Guest » Sat Mar 01, 2003 3:39 pm

There is a reason for the cut on the bottom of the mast. All Hall Spars cuts there masts that way, it allows for water and other debris to clear.
The other is the mast tends to have more response to butt and partner postions. I would like to recommend that if there is craking that you add a doubler to that area of the Butt or drill a 1/16" hole at the top of the crack to stop further damage.
Guest
 

Postby Guest » Sun Mar 02, 2003 3:12 pm

Everybody seems to agree that the tears appear to be the result of point loading at the aft edge. I doubt that welding would be a good long term solution. The loads exerted by the mast rake are considerable. After seing these photos, if I had a French boat, I'd probably fabricate a stop conforming to the outside front of the mast, and bolt that to the support beam to take the forward load of the mast.
Guest
 

Postby Guest » Mon Mar 10, 2003 7:23 pm

Not sure, but I seem to recall from semi-monocoque structure analysis (how 'bout that spelling!) that the wrong number of holes drilled in the cross-section causes adverse shear flow, and failure of the section. As such, probably not point compression/tensile stress, but adverse shear flow.
Guest
 

Postby Guest » Mon Mar 10, 2003 7:39 pm

Not sure, but I seem to recall from semi-monocoque structure analysis (how 'bout that spelling!) that the wrong number of holes drilled in the cross-section causes adverse shear flow, and failure of the section. As such, probably not point compression/tensile stress, but adverse shear flow.
Guest
 

Postby Guest » Mon Mar 10, 2003 9:24 pm

I had to go to the dictionary to look up "semi-monocoque" For those as befuddled as I was, I have provided a dictionary definition:

mon·o·coque (m¼n“…-k½kâ€
Guest
 

Postby Guest » Mon Mar 10, 2003 11:58 pm

It looks like at least one of the masts has suffered some corrosion. I wonder if the step or support beam is made of a dissimilar metal.
Guest
 

Postby Guest » Tue Mar 11, 2003 9:36 am

There does appear to be corrosion, but it would be hard to determine if the corrosion caused the cracks or the cracks allowed corrosion to form in the exposed metal
Terry
Guest
 

Postby Guest » Wed Mar 12, 2003 11:24 pm

I'm very curious about the pictures you posted on the mast butts. If you figured out the cause of the damage please let the rest of us know. What kind of rig tension are you running? Can you post a picture of the mast steps they came off of?

Robert
Guest
 


Return to Archived Maintenance Topics

Who is online

Users browsing this forum: No registered users and 4 guests

cron