New Radar bracket - will it compromise the strength of the mast?

[KeelsonGraham]

Hi all,

I'm upgrading my radar from an 18" Furuno to an 18" Raymarine Quantum. My fitting guy has a spare Raymarine mast mount and proposes taking the old Furuno one off and attaching the Raymarine one.

I'm concerned that another entire set of holes in about the same place on the mast is going to make it look like a gruyere cheese and compromise the structural strength of the mast.

Am I worrying unnecessarily?

Graham

[Ron Hill]

Grahm : How far out are the holes between the two different brackets??  Also how big are these hole??

A thought

[KWKloeber]

Any how many fasteners in each bracket? The spacing of each bracket?

Consider this -- unless holes are WAY TOO CRAZY in size, or number, or spacing -- replacing a hole with a tightened fastener pretty much replaces the compression strength of the metal that is lost due to the hole.  Not 100%, but it's not like you have drilled a ton of holes and let it as swiss cheese. 
Doing that doesn't do anything if the section is in tension, but bolting on a bracket would increase the tension strength of the area.

Some details please?

[Bill Shreeves]

just a thought, without the benefit of your spacing or sizes your dealing with, what about attaching the new bracket to adapter plate(s) then attach the plate(s) to the old mount points?

[waughoo]

^----- This -----^

If you can stand the visual appearance of an adapter plate on your existing radar mast mount, it will solve any issues of too many holes in the mast.

[KeelsonGraham]

The existing mount has 8 rivets attaching it. The new mount has 12. I think that making a bespoke adapter plate out of 1/4" aluminium would be a better solution. I'm not at all bothered by appearances but I really am wary of a total of 20 holes all drilled in the same area of the mast. Even if the 8 old holes are re-rivetted to fill them, it just doesn't feel like the right thing to do.

[KWKloeber]

The existing mount has 8 rivets attaching it. The new mount has 12. I think that making a bespoke adapter plate out of 1/4" aluminium would be a better solution. I'm not at all bothered by appearances but I really am wary of a total of 20 holes all drilled in the same area of the mast. Even if the 8 old holes are re-rivetted to fill them, it just doesn't feel like the right thing to do.

It appears you've already decided and looking more for confirm, but without a picture or some template of the two or something of solid info (hole size and spacing), and where it's mounted, there's not much solid structural recommendation can do.   If I had some solid info I could determine that, but alas!

[KeelsonGraham]

Just to close this thread out. I spoke to our local very reputable mast and rigging experts. Their view was that it's generally not a good idea to drill more holes in the same place in the mast. It can be done, provided that the old holes are filled with rivets. But their recommendation is to machine up an adapter plate to fit to the existing support.

[KWKloeber]

Their view was that it's generally not a good idea to drill more holes in the same place in the mast.

Are they making the adapter bracket?

No doubt an adapter is the most conservative way and satisfies your peace of mind, but that wasn't the original question.  There's the most conservative and there's perfectly acceptable and safe.  Facts and specifics are king when assessing any mod. "it depends."

Would the riggers say that drilling one more hole when you have only one hole, is unacceptable?  Are two more unacceptable when you have two?  Where's their red line.

Ponder these:
- No doubt, if asked to install radar in the first place, they would absolutely refuse because "generally" it's better not to drill ANY holes in a mast or any spar. 
- It's "generally" better to not add more weight aloft, but they recommend adding additional weight (an adapter)?
- No doubt, they would refuse to install spreader lights or a flag halyard block because it's "generally" better to leave structural members w/o additional hole. 
- It's "generally" better to have floor joists intact with no holes, but if the engineers didn't already figure out where it's safe to drill them, and how large we wouldn't have electricity and plumbed water in our houses.

[KeelsonGraham]

Hi KW,

I consulted in person with AllSpars who are probably the most respected riggers in the UK. They didn't say you mustn't drill more holes in the mast. In fact they said that they have done this for many customers - always re-riveting the old holes. However, on balance they thought it better to machine up an aluminium adapter, anodise it and fit that instead.

They did offer to discuss it with the mast manufacturer but I generally feel happier with the adapter plate approach.

I'm more familiar with aircraft structures, where drilling large numbers of weight-saving holes in key structural members is the norm. But, unlike aircraft, these are small radius holes which must to some extent concentrate stresses. If we can avoid this why would we not?

[KWKloeber]

If we can avoid this why would we not?

Keelson, I agree; Why it it can be avoided?   

Oh, you meant, "Why drill holes," I thought you meant "Why add cost and additional weight aloft" if it can be avoided?  LOL

We all have to do what we feel comfortable with.  I overkill many things that are unnecssary; it's just the engineer in me -- but so is knowing when I overkill because I want to and when it's because it's marginal.

I understand that the cost of worth the peace of mind, but it's just coincidental? that they're also the ones who get the income for an adapter.

As to size, a truss (which is what an aircraft structure is) is made with as large holes as can be accommodated for reasons of economy -- it's infinitely more expensive and more manufacturing issues to anything with many small holes that one large hole.

Cheers,  post a pic of the install if you can.