Drilling holes in masts

[nondikass]

I need to replace the steaming light / deck light fitting located mid mast on a Parker31.
Have been unable to find an exact replacement and so will have to drill 2 new holes ( for 5mm rivets ) for a new unit.
Am concerned about weakening the strength of the mast, with 2 pairs of holes in close proximity, located near to the cable access hole.
Looking for guidance on the minimum distance between rivet holes.
If I leave the original rivets in place, will this help the situation?
Thanks

 

[cmedsailor]

The openings where halyards usually come out of the mast are much bigger than the holes you want to drill. I believe you shouldn't worry.

 

[rob2]

You really have to create a perforated line (tear along dotted...) to weaken a mast profile - don't worry about two rivet holes. Putting a rivet in the redundant holes doesn't affect the mast strength - they are still holes whether occupied or not.

Rob.

 

[merlin3688]

This will be fine on the mast, if original mast its a Z Spar (Z351 section). Just be careful when drilling into the mast that you do not drill into any cables or halyards, put a stop on the drill as when drilling alloy the drill sometimes gets dragged in. Also use Monel rivets when fixings, expensive but the right rivets for masts.

 

[Neeves]

Why drill new holes at all. Attach new light to an aluminium plate (or fibreglass offcut), drill aluminium plate to match existing holes - rivet.

Jonathan

edit - to make it even easier - use the old light fitting as the template for the holes in the intermediary plate that you will rivet to mast. In any event you would surely only need one new hole - as you can use one of the old holes anyway. I prefer the backing plate close edit

 

[rob2]

That's a good point about using original holes, but... Most masthead/deck light fittings have flexible "wings" so as to conform to the mast profile, so I assume the new one doesn't stretch to the original holes? If they did, then just drill the wings to match the hole in the mast. Unfortunately you do have to drill both holes as the light will otherwise be offset.

Rob.

 

[Yngmar]

No worries about two little holes weakening the mast - it's insignificant. You can simply calculate the diameter of the hole in relation to the circumference of the mast (I know that's not exactly how it works, as not all parts of the mast carry the same load and it doesn't account for internal structures and different wall thickness of the extrusion, but close enough for an estimate). If your mast has 620mm circumference, two 5mm holes will be just 1.61% (2*5mm/620mm*100%).

With internal halyards, the most important thing is not to drill into the halyard you're hanging from
Even if you usually don't, this is a situation where you should be attached a second safety line. When I did this (mast up), I used a drill depth stop (the sort that clamps onto the drill bit) to ensure I wouldn't even touch anything inside.

Unless the mast is down, in which case just roll it over so you're drilling from the top and the halyards lay down on the bottom, out of the way.

 

[NormanS]

Good heavens! Imagine paying good money for something like that. I just use a wee bit of plastic tube on the drill.

 

[VicS]

Good heavens! Imagine paying good money for something like that. I just use a wee bit of plastic tube on the drill.

I usually use a bit of insulation tape wrapped round the drill bit but the point of the stops in Yngmar's link is that the drill cannot be pulled in beyond the stop when it breaks though.