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  #1  
Old 11-10-2007
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Self tapping screws in the mast.

Okay, so this may seem a little weird. In looking into proper mounting for some mast steps to put near the mast top, the clerk at the local marine supply store (seemingly knowledgeable fellow) said to use self tapping screws to affix to the mast.

This seems a little odd... I expected to have to drill and tap holes to install these. Anyone used/heard of using self tapping screws?

The mast is aluminum.
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It really depends on your mast. If the mast is fairly thin wall construction, then self-tapping screws may be a better choice than machine screws, which really need a minimum thickness for them to be secure. If you are going to use machine screws, make the holes about 75% of the size of the screw, so that you've got enough material for the screws to bite into. For instance, if you're using 1/4" screws, the holes should ideally be 3/16" or so.

A better option, IMHO, is using stainless steel pop rivets. I would not recommend aluminum pop rivets because they're considerably weaker.

Whatever you decide to use for fasteners, you really should use TefGel, Lanocote or some other anti-galvanic corrosion gel on the fasteners to help prevent the mast and fasteners from reacting to each other.
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Okay, his line of reasoning was similar regarding the mast thickness. Thanks for the rule of thumb, I'll check the thickness tomorrow and see if it's right for tapping.

There is another selden rig at the yard (similar size to mine), which has the steps already installed. He does have screws, rather than rivets, holding them into place.

Already bought the tefgel

Cool,

paul
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Old 11-11-2007
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Your store guy probably was thinking sheet metal screws. There are true self tapping screws available with a notch in the end to cut threads, but they're going to be very tough to source in stainless.

Stainless sheet metal screws are strong enough to form threads in sheet metal, but in a mast extrusion I think you'll find you'll have a bear of a time turning them in and will likely torque the heads off screws.

Machine screws are much better. You'll need a proper size tap drill for the screw thread you're using. Use a tap drill size table, e.g., like at http://www.efunda.com/designstandard...s/tapdrill.cfm Hardware stores often have the tap and proper tap drill in the same package. (75% of major diameter is too small for a tap drill.) Put lanocote or other lubricant on the tap as you're cutting threads to get cleaner threads and make the tap last longer.

Coarse threads (e.g., 10-24 vs 10-32, etc.) give more strength in softer metals like aluminum.

I like button head allen screws. You will probably have to get them from an industrial fastener supplier, check your Yellow Pages. The heads are nicely rounded and the allen wrench doesn't want to cam-out when you tighten or loosen them. The fastener supplier will also sell taps and tap drills.



Cheers,

Tim
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Old 11-11-2007
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wouldn't self tappers leave a sharp point inside the mast that the halyards could rub against and get damaged?
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Tim-

If the mast wall is too thin, you can't trust a tapped hole to have any serious strength. The mast on my boat is such that the spar manufacturer said that nothing should be attached using screws, since they won't have a prayer of holding under any load. The winch bases and cleats are either through-bolted to a backing plate inside the mast or screwed to a base that is riveted to the mast.

Jacky-

If you're pre-drilling the holes, you can always file the points off...but yes, that would be a possible issue with self-tapping screws.
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You can get the self-tappers in aluminum.
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Quote:
Originally Posted by sailingdog View Post
Tim-

If the mast wall is too thin, you can't trust a tapped hole to have any serious strength. The mast on my boat is such that the spar manufacturer said that nothing should be attached using screws, since they won't have a prayer of holding under any load. The winch bases and cleats are either through-bolted to a backing plate inside the mast or screwed to a base that is riveted to the mast.
I'm surprised the spar maker would say that. What size of mast is that thin?

Here's part of the article "Installing Hardware on Aluminum Spars" from DIY Boat Owner magazine (1998 issue #1 page 27):

Machine screws are used exclusively by custom mast builders to fasten most hardware. More time consuming to install than rivets, they require drilling an undersize hole, then using a tap drill to make a threaded hole for the screw. Screws are identified by the size and number of threads. A No. 10-24 screw, for example, denotes a screw with a maximum diameter of 10 (.190) and 24 threads per inch. Look for National Fine (NF) screws; the finer the thread, the better the holding power.

In this article they interviewed Danny Klacko of Klacko Spars. His company has built spars for a number of production boatbuilders, including for all the C&C boats before Tartan bought the name. C&C were high performance boats, so I would expect their masts to have been as thin as possible.

My previous statement about using coarse thread screws in aluminum stands corrected, although I'm puzzled by their example because 10-24 is coarse and 10-32 is the fine thread in that size.

I've got Klacko spars on a 1976 34' sloop (and a 1973 34' ketch, but that's another story). The boom was assembled with rivets, and much of the mast hardware with machine screws. When I got this boat most of the rivets on the boom were loose. All of the machine screws are tight with none of them showing any signs of working. The gooseneck was worn out, so this boat did do some sailing in that time. The mast is probably 1/8" thick, which is typical. There are no backing plates.

SD - the OP asked about putting steps up a mast. If a mast is so thin that machine screws would pull out, wouldn't you be concerned about the mast dimpling around each step if an adult climbed the mast? Those dimples would really weaken an already fragile mast.

Cheers,

Tim
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A local rigger taught me a clever trick tht lets you thru-bolt to a mast or boom. It's a lot of trouble, but if you want something to be really secure, it may be the way to go. If I were going to climb the mast, I would want to make sure the step wouldn't give way.

Make the mounting hole slightly oversize so a screw with a string attached can pass through. Dental floss may be a good candidate for the string since it's thin, but still fairly strong. Use a screw that's a good bit longer than needed for the job. Here's the process:

Tie or tape a string to the end of a machine screw and drop it through the mast. Use a piece of wire with a hook on it to fish the string through the mounting hole. Once retreived, pull the end of the screw through the hole in the mast and throught the mounting hole of whatever you're planning to attach to the mast. The head of the machine screw is now on the inside of the mast and you're holding the threaded end. Start the nut onto the machine screw, holding the end and getting the nut as tight as you can by hand. Next, grab the threaded end of the machine screw with a pair of vise grip pliers and finish tighteneing the nut. Once satisfied, cut off the excess length from the machine screw and file it smooth. After this much work, I'd also be inclined to use some threadlock or a nut with a nylon insert to make sure it didn't get loose and fall down the mast.
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The finer thread pitch is necessary if you think about it. In a the fairly thin material of a spar wall, you want as many complete threads as possible, and the finer the thread pitch—the more complete threads you'll have in tapped in the wall.

The real problem with using finer pitch thread screws is that they are far more susceptible to galvanic corrosion issues.

As for machine screws pulling out... the real problem is that many people won't do a good job of tapping the holes. If the holes aren't tapped perfecty, then the strength will be severely compromised. I don't think the steps would dimple the mast under load, since their design probably spread the load out over a fairly wide area. The screws are more of a point load.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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