The first beam went from corrosion also, the second lasted 7 months and went when I fell off a wave. The cause is an unbraced area of beam between the bolt securing the stainless strap to the beam and the inboard bolt securing the hull to the beam working. The solution is to transfer the stress to the inboard bolt which I have done recently and so far seems to do the job however I'm going to build a new beam incorperating the changes during the build for a even better fix. Until I've proven my fix I wont fit a spinnaker for obvious reasons.
Round corners are good to help prevent cracks. Most sailors are surprised when I point out that their beams are cracked, its particulary prevalent in Boyer boats as they must have built their beams off a jig.
I've noted two different size beams fitted to Mosquitos both of which fit in the tolerance in the building instructions, the smaller size beams which are fitted to the new Cobden built boats are tougher then the larger beams contrary to common sense. The Temper and corner thickness is the issue and the smaller beam is superior on both counts.
I find cracks in Aluminium for a living so maybe I look a bit harder then most.
Only three or four forum users can be accused of badgering, (certainly no Aussie Mosquito sailors who I find to be open and honest about their boats) their endless squaretop main/fit a spinnaker chant along with some of the falsehoods they perpetually bring up such as "its easier to gybe downwind in 25knots with a spinnaker then without" and other classics are amusing to me but newbies to the class (F16 also) actually believe some of that rubbish and get disenchanted pretty fast when it doesn't work for them.
My 2 cents,
Darryn
1704