[Alberg30] Mastbeam Repair Revisited

[Dan Morley]

Has anyone out there removed the beam.  This was something that was suggested to
me by Whitby Boat Works when I contacted them (they even actually offered to
construct a whole new beam...)  Can the beam be unbolted and removed from the
boat without taking down the bulkheads?
Dan Morley

And Mike:
  - many thanks again for the additional info.  I have a week off in early April
to do several repairs, including the beam.  I'll let you know how it all works
out.
Dan

Mike Lehman wrote:

> 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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