[Alberg30] Mastbeam Repair Revisited

[John Birch]

Hi Michael,

As someone who has been aboard the Rankin/Lehman/et al type repairs, and being a 37 guy, I perhaps can add a note of objectivity. I found the work done to be first rate and not looking like an add on - in fact most would think they came from the factory that way.

Being a free country, do as you see fit, but for my 2 cents the Aluminium job as done by the Chesapeake gang works great. Taking a another look as to whether it is best is the whole concept of progress. But their method works and looks good.

Cheers,

John
  ----- Original Message ----- 
  From: Crufone at aol.com 
  To: public-list at alberg30.org 
  Sent: Saturday, February 23, 2002 3:02 PM
  Subject: [Alberg30] Mastbeam Repair Revisited


  George D, Michael L, Dan S, et al,

  My original concern with using Aluminum for this repair was that owners were using SS bolts in contact with the Aluminum.  I see now that Al 6061 alloy is being used.  This is pretty good stuff, aircraft wing ribs are made from 6061.  While 6061 is reasonably corrosion resistant, I have seem some very nasty SS / AL dissimilar metal corrosion.  In the presence of water vapor it increases and add the Chloride ion and you have the chemistry for active, runaway corrosion, i.e., a battery.

  I didn't think that Aluminum fasteners would be tuff enough, so I gravitated toward using SS for both the stiffeners and the fasteners.

  I also felt that the SS stiffeners would be thinner and less obtrusive.  Originally I guessed that the Al plates would be 1/4" thick, but I have now seen references to both 3/8" and 1/2" thicknesses.

  I did some measurements on my boat #133 from the center of the beam and from both the port and starboard door jamb down to the sole.  I took measurements before and after tuning the rig and then again before and after the 80 mile maiden voyage down to Lake Erie.  In all cases the mast beam was lowering about twice as much in the center of the beam as at each of the door jambs, only 12" distant.

  My hypothesis was that the boat wasn't getting noticeably wider as the mastbeam collapsed and that the center of the beam was flattening more than the entire beam was falling.

  I then thought that I didn't like the conventional fix by drilling 5/16" holes for bolts through the wood laminations, in some cases severing the laminations in half.

  It came to me that if one could prevent the laminations from slipping relative to one another that would solve the majority of the problem.  The idea then came to install bolts or dowels of some material vertically through the stack of laminations to prevent horizontal slip.  The difficulty would be to install the fasteners without penetrating the cabin roof.

  About this time an Engineer friend of mine pointed out that if the beam was prevented from flattening then it could not collapse.  He suggested that if a stiff enough piece of material were laminated to the bottom side of the lamination stack then the beam would not be able to flatten in the center. This guy mentioned that the "sistered" Al plates would not even be needed! This played into my idea of vertical fasteners but against the known fact that the strength of an I Beam is mainly in the web and not the flanges. 

  So... I wanted to do both and came up with the idea of a SS weldment.  SS would allow the material to be thinner and the weldment would have far more strength due to it's channel cross-section.  Hummm......now to address the fastener issue.

  I have not taken the mast base off from the outside of the cabin roof so I don't know if there is an opportunity to use vertical fasteners through the mastbeam at that location.  Someone mentioned in one article that the counterbores for the mast base attachment went down about half way into the mastbeam.  This seams excessive, but could present a location for through fastening the original wooden laminates and repair channel together. 

  My thought would be to investigate adhesive bonding the channel to the original wooden mastbeam.  Perhaps not as the sole method of attachment but as a way to limit the number of mechanical fasteners.  I would consider the location of the mechanical fasteners carefully, avoiding severing the wooden laminates and staying clear of the center of the beam and the web area of the beam.

  My thoughts on installation would be to use a router to reduce the thickness of the lower wooden laminate to receive the channel without any loss of headroom.  I am also considering the use of recessed fasteners so that a thin lamination of teak could be applied over the SS channel to conceal the metal and give the appearance of the original wooden beam.

  Hope that this clears up my thought process on the mastbeam repair.  I just thought that I would attempt to combine an effective repair with the original beauty of the boat.  To make the boat sound again without altering her original looks.

  Michael #133 
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