[Public-List] Dynamic loading on the Alberg

[John Birch]

I don't have a lot to add to this as, unlike my good friend Don, I am not a 
true Renaissance Man - wish I were.

However, all racing A-30's carry the mast butt as far forward in the plate 
slots as possible.

The second point is that it is always peak dynamic forces that bring down a 
rig as opposed to static forces. Unless the rig is in terrible shape. Also 
the one need remember that cyclic loading plays a part. everything only has 
so many cyclic loads before fatigue sets in.

It was an unanticipated peak dynamic load that sank that Aussie America's 
Cup boat.

Fortunately for us, Whitby did not push the limits to define the limits and 
there is sufficient reserve strength in the a-30 rig.

However, peak loads under the truly dynamic conditions a boat sails in are 
difficult to calculate as wind wave height and wave period  and wave shape 
can cause radically high peaks in loading.

Often, what brings a boats rig down is the mast getting out of column when 
hitting a wave at the same time a peak compression load takes place.

So what to do ?

Oversize the chain plate bolts; ensure the rig is not too loose; ensure the 
wire is in good shape and robust enough; sight the mast to ensure it is 
remaining in column; replace the wire every 10 -15 years in salt or 15-20 in 
fresh water;  use mechanical terminals like norsemen or stalocs; make sure 
there is a compression tube inside the mast at the spreaders; ensure the 
spreaders are in good shape and not flexing or rotting if wood; and fix the 
laminated wood beams on the older boats - use the time proven Rankin method 
and if you hate aluminum put a wood covering piece at the aft end of the 
beam.

And go sailing.

Best

John      PS It was great to finally meet you Gord Laco (at the Toronto Boat 
Show) - we have to sit down and have a beer some time - maybe two ; )


----- Original Message ----- 
From: <crufone at comcast.net>
To: "Alberg, Public List" <public-list at alberg30.org>
Sent: Saturday, January 16, 2010 10:44 AM
Subject: [Public-List] Dynamic loading on the Alberg


>
>
> Don Campbell wrote,
> Because the mast base plate is not centered over the mast beam, it
> becomes obvious that the torque on the mast beam is more that we think
> at first glance, even though the force is now spread over the area of
> the mast base plate, most of which is behind the mast beam. That too
> will strain the connective joints of the beam to the hull even under
> static loading.
>
> Don and Others,
> I really would like to see this point clarified. On boats that I have 
> observed the mast base plate and the mast butt are centered over the mast 
> beam, both athwartships and fore and aft. The beam is attached to the main 
> bulkhead by it's forward face using bolts. The aft portion of the mast 
> beam is less substantially supported by pilasters attached to the aft face 
> of the bulkhead. I assume that this difference in strength of support of 
> the mast beam by the main bulkhead is what causes the torsion action on 
> the mast bean, not the assumption that the mast butt is not centered over 
> the beam.
>
> On #133 the mast butt is positioned directly over the center of the mast 
> beam below. So on my boat what would cause the mast beam to twist downward 
> on it's aft side if not how it is attached and supported by the bulkhead? 
> I assume that the strength of tabing to the coach roof is inconsequential.
>
> It would be good to have this explained to all Alberg owners.
>
> Michael #133
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