[Public-List] mast beam repair

[Bill Wallace]

I'm not totally sure about the stress on the mast beam, but I think it would be sheer - a force applied on the deck on top of a couple of the beams would push them down more than the other beams, thus applying a sheer force - regardless, long term applied sheer or tension is hard on non-reinforced plastics.  On the dinghy, there are a few places where screws are used to keep the joint under compression - this is for a joint without fibreglass.  Fibreglass changes the mix as long as the stress is along the fibres - that is why you can pull the tabbing apart as it is a straight glue joint, but once you adding the slots, the fibreglass takes the stress.  I haven't done my deck beam, and can't even see it so this is all based on other epoxy/fibreglassing I've done.

Bill


----- Original Message ----
From: Rachel <penokee at cheqnet.net>
To: Alberg 30 Public List -- open to all <public-list at lists.alberg30.org>
Sent: Thu, January 7, 2010 10:05:58 AM
Subject: Re: [Public-List] mast beam repair


On Jan 7, 2010, at 9:21 AM, Bill Wallace wrote:

> Even Epoxy glue joints fail for things like tabbing and gluing things in sheer.  For tab joints, the recommended solution is to use slots cut through the material, and strips of fibreglass.  Don Casey's book This Old Boat (2cd version) has a picture in it of how to do this.  I would guess that just wrapping the entire beam in 3 places with 3" fibreglass would work well.

Wouldn't the beam laminations themselves be in tension though, not sheer?

I can agree that there is a "peeling" type force on the tabbing, but if the beam does not sag or move, then I don't see why that peeling force would be any more pronounced than that which is on all of the other tabbing in a boat (bulkheads, etc.).   Most of the force in this case wouldn't be a peel force either, to my mind.  Unless I'm missing something (?)  (I'm not a physicist.)

Rachel
ex #221
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