[Alberg30] Mastbeam Repair Revisited

[Mike Lehman]

Another thing to consider is to remove the entire beam from the boat, make 
all of your repairs and reinforcements, relaminations in the shop. Then 
reinstall the beam. I have never done this but I know of owners that have 
and I think they are subscribers to this list. I think that this type of 
repair would require removing the mast.


----Original Message Follows----
From: Crufone at aol.com
Reply-To: public-list at alberg30.org
To: public-list at alberg30.org
Subject: [Alberg30] Mastbeam Repair Revisited
Date: Sat, 23 Feb 2002 15:02:39 EST

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




Mike Lehman
"Gilleleje" #505
(410) 544-9067


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