[Alberg30] Mastbeam Repair Revisited

[Crufone at aol.com]

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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