[Alberg30] Mastbeam Repair Revisited

[sousa, stephen]

Michael,
 
I also installed the Aluminium plates on #114 with 5/16 stainless bolts and
epoxied both plates against the sanded beam with recommedations from Mike
Lehman. I sailed the entire summer last year without incident. The real
issue with the Alberg is that the laminated beam was glued with strip
material in a horizontal instead of vertical. If the beam was laminated
vertically we wouldn't be having these discussions. I sprayed the plates
with white epoxy paint when the installation was complete and every guest on
board thought the boat came from the factory with those plates. 
 
I can comment on stainless and its cost from a machine shop, the port
retainers on #114 are 1/4" stainless which were laser cut. The stainless
pieces were installed on two small ports forward and four large ports in the
main cabin. This combination with lexan windows are possibly stronger than
the hull. The cost of these ports and new chain plates from 316 stainless
were $1000.00 one of the most expensive parts in restoring Carina Vela. It
can be done but get ready to spend some dollars.
 
Stephen
Carina Vela 
#114  

-----Original Message-----
From: John Birch [mailto:Sunstone at cogeco.ca]
Sent: Saturday, February 23, 2002 3:57 PM
To: public-list at alberg30.org
Subject: Re: [Alberg30] Mastbeam Repair Revisited


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 <mailto:Crufone at aol.com>  
To: public-list at alberg30.org <mailto: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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