Mast engineering stresses

[Yacht Yogi]

To answer the question about heat treatment: Welding the types of alloys commonly used for masts by MIG or TIG processes would require pre-heating of the material around the area to be welded. This provides stress relief to avoid the formation of cracks as the weld cools down. The recommended pre-heat temperature seems to be 350F (177C). Another requirement for a successful weld is to follow "good practice" in design of the joint/repair such as by making it at an angle to the predominant strain direction (so a diamond shaped patch on a mast rather than a square patch for example) and by avoiding big step-changes in thickness (which would cause stress concentrations and out-of-plane bending). On a mast repair I would recommend dye-penetrant inspection of the completed welds to ensure that no cracks are present.

Cheers, Chris

 

[roaringgirl]

I realised that all the rigging attaches either to the aluminium casting at the top or to bolts that go right through the mast so I guess I don't have to worry about aluminium failing through corrosion or fatigue leading to rigging becoming detached.

 

[roaringgirl]

Like this: http://www.kp44.org/images/TangReplacement.jpg

 

[roaringgirl]

No comments? Good idea, bad idea, 'what about ...'?

 

[William_H]

No comments? Good idea, bad idea, 'what about ...'?

I presume this poster is asking about the above photo of mast tangs. This is an excellent way to attach stays to the mast. The through bolt means that pressure on the mast is of the bolt in the bottom of the hole. The wear and pressure tending to cut the hole out deeper at the bottom. This wear mode is reduced by making the hole bigger with the shaped wasters so reduce pressure on the ali. many advocate a tube inserted so thea the bolts can be tightened up on the tube rather than crush the mast. I think that ona smaller boat nylock nuts will be satisfactory to keep every thing tight.
The mast on my little boat (fractional rig) has a wrap around stainless steel plate which incorporates the forestay attachment and has holes for the through bolt and so tangs for cap shrouds. Similarly at the intermediate side stays the bolts go through a wrap around plate which carries and forms the base for the aft swept spreaders. Both these plates are rivetted on so much of the downward load is taken by the bolt in the SS rather than the hole in the ali.
The fittings pictured would be fine for a sand alone shroud attachment but certainly in my case they are not stand alone so those speciall washers are not needed. good luck olewill

 

[roaringgirl]

Thanks for your reply. I guess what I'm asking is, given that this is how my rigging is attached, should I be less worried about mast corrosion - because corrosion isn't going to affect any of the rigging/mast joins

 

[William_H]

Dissimilar metals with no insulation or treatment will still be a worry even though catastrophic rigging failure is unlikely with this through bolted attachment. Worse case is that mast will lose material to corrosion under the fittings ausing weakness. So yes worth dismantling for inspection and treatment with Duralac or similar. good luck olewill