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Am about to install a new double-pole mast fitting (photos) on my 505's mast. The old single Spiro will have to come off.  I cannot re-use the same rivet holes and worry that all the holes being in the same section of the mast will weaken it. Can/should I have a welder fill in the old holes or are there other fixes?  Or is my concern unwarranted?   

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I  terms of yrouble thr thing to look out for is hole intrtaction. If two holes are in line vertically there is littlr effect. If they are abreast each other then they can together act like a larger hole.

Well spaced abreast less isssue. Then two widely spaced holes.

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So the new holes will be in line vertically.  Would filling the old ones with epoxy/fiber mix be useful or pointless?

 

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34 minutes ago, PaulK said:

So the new holes will be in line vertically.  Would filling the old ones with epoxy/fiber mix be useful or pointless?

 

Yes. No point. The mast is in compression and bending. It isn't in a high tensile unit stress range there (or shouldn't be).

Also filling holes doesn't automatically make a situation better anyway. And further, the modulus of elasticity o aluminum is about 10 million PSI, whereas epoxy is about 250 thousand psi and with fibers mixed in it like a putty, maybe 750,000. So it does diddly squat in terms of elastic load sharing anyway.

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Is there no way to reuse the old holes?Say by riveting on a plate using the old holes and fastening the new fitting to that?

 

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The existing fitting is an aluminum single spino casting.  The holes are to one side of the mast - not symmetrical - and nowhere near where the new ones need to be.  Would certainly use them if it were possible.  

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Hi,

I had to deal with this more than once, just go above or below the existing holes an inch or two.  The fitting has come down over the years.  I am at 41"  above the deck band, remains of other mast pieces I have are 36-42".  These were all built by/for top teams here on the west coast.

 

 

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Would be nice to figure out how to make it adjustable up/down with some sort of levered clamps that would reach around & hook in the luff groove without blocking it.  Or maybe a sleeve that could slip over the mast, leaving the luff groove open, and get held in place with thumbscrews tightened against the mast. Not something that needs changing every six minutes. 

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I used the waterat double pole fitting. It's bonded to the mast using 5200 and has 2 rivits but is much taller than the old Spiro. Holes are inline vertical. No problems so far in 6 years

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16 hours ago, fastyacht said:

Yes. No point. The mast is in compression and bending. It isn't in a high tensile unit stress range there (or shouldn't be).

Also filling holes doesn't automatically make a situation better anyway. And further, the modulus of elasticity o aluminum is about 10 million PSI, whereas epoxy is about 250 thousand psi and with fibers mixed in it like a putty, maybe 750,000. So it does diddly squat in terms of elastic load sharing anyway.

Any point in putting something semi-solid like a drive rivet in abandoned holes?

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  • 2 weeks later...

Our mast is 46 years old and has lots of holes in it from spi pole blocks, On a 5o5 it should not weaken the mast much especially if you have tweakers on your mast then you will not snap it, just make sure your rigging is all ways looking good and don't turtle in shallow waters.

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Two extra holes are nothing. Keep in mind that mainsail tacks are fastened at regular intervals up a mast. My dad was an engineer. He taught me to keep them 2 x diameter apart. Theoretically, you can go one, so you go two for a safety factor. For example you can put 1/4-20 threads with 1/2" material between them assuming the thickness is 1/4". In regards to your mast 4 holes in a row is of no concern. If you put 20 in a row it might tear like a zipper in a blow. 

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