Access hole in Mast?

Hermit Scott · Jun 3, 2010

If you have to do it, I would do it like you say you intend to; a round hole. I would have a nice thick 1/4" round plate to cover it, with about 6 or 8 1/4"-20 SS allen bolts holding it together. Of course on the flat surface on the side of the mast to make it easy.
I have seen this done where there is a two piece insert that goes on the inside of the hole for reenforcement. Keep the hole as small as possible.

dscribner · Jun 3, 2010

The trick to making holes in structural members is a small hole at the neutral axis. Think of an I-beam (W-shape). If you drill a large hole in the top or bottom flange, you are weakening the beam in the compression web (top) or tension web (bottom). If you drill a hole thru the side (upright part of the eye) you are removing material that really isn't doing much but add weight. Think of the beams in the space ships in those old SciFi movies. So, before you drill a hole, pick the smallest that you can get away with. If you can drill (trepan) it below the pivot of the mast (if you have one) that's good. Otherwise, I think the mast is stressed front-to-rear more than side-to-side so you would probably be better putting it in one side. Smooth the edges well with a file and sandpaper. Sharp edges and drill gouges could propagate into rips and cracks in the aluminum over time.

Hermit Scott · Jun 3, 2010

It's all compression in the base of a mast. Using the neutral axis doesn't seem to me to be a valid way to determine anything on a mast. Most of the bending is happening around the lower shrouds and up. But it bends in all directions not just perpendicular to any one axis.

RichH · Jun 3, 2010

The neutral axis will be on the 'widest face' (side).

Good advice given, repeat/more:
• small as possible hole
• 'rounded' corners, the larger the better - so you dont later on propagate cracks
• hole 'high enough' from the coachroof so it doesnt interfere with any wiring through deck ferrules, etc. Stay as far away from the 'butt end' as possible, leaving sufficient 'meat' for when the butt end eventually starts to corrode.
• no ragged or sharp edges - sand / grind smooth

Ken13569 · Jun 3, 2010

That's how our SanJuan 28 mast it built. I"m guessing the hole is 2 by 3 inches high, smooth oval, and a heavy cover secured with 4 SS #10 screws.

Roger Long · Jun 4, 2010

If you don't have the equipment to work with S.S., you can get a nice cover intended for the purpose from RigRite. It's a bit pricey but comes with four screws and is nicely finished.

I put one on after having to step my mast twice last year due to a flaky connector. The four pin Cole Hersee connectors, when old, will work fine when plugged in to test just before stepping and then break connection when pushed together harder during the tapping. The access plate gives great peace of mine and would have saved me a couple hundred bucks.

I'm skeptical about the idea of drilling it in place though. Good chance of cutting the wires and you will be filling the mast step with aluminum sawdust the will eventually work its way all sorts of places. I would take the mast down.

I using a hole saw to make two holes the desired width and then connected them with a saber saw. This makes for the maximum corner radius Rich mentions.

Electrical Access Plate - Flat: RR-72-99F:

http://www.rigrite.com/Spars/Kenyon_Spars/5072-mast.html

Roger Long · Jun 4, 2010

Bob S said:
I've been toying with the idea of going the season without the radar but I really don't like that idea.

Ask your yard about the cost of lifting the mast just enough to clean and re-tape the radar connector. It's a fraction the time to lay it down. If you do the un-rigging and rigging and let them do it at a slack time they might be reasonable. I used to sail without radar all the time but I could never do it again comfortably now that I've seen how many boats are out there.

If you shorten the drill of a hole saw and take it very slow, you probably can cut the hole without having it plunge in and cut wires or ropes. It went quite easily when I did it. I suggest the slot with two 1 1/2" holes instead of one round hole. It's easier to fish the wires out with a bit more clearance in one direction and the slot has less effect on the mast cross section.

Hermit Scott · Jun 4, 2010

Bob S said:
Thanks for the good advise. I was thinking a 2-1/2" round opening.
Roger, thanks for the link. Not sure why RigRite's cover is rectangle but I think cutting a round opening using a hole saw would be a lot easier. I can fabricate a cover but I was going to make it from 6061-T6 aluminum and use (6) 10-32 screws. I would like to do it with the mast down but I'm trying to avoid the aggravation & expense. My biggest fear is cutting something important inside. I know there is a riveted tube containing all the wiring somewhere in there. There also sheaves at the base of the mast on both sides. I've been toying with the idea of going the season without the radar but I really don't like that idea.

Bob, why did you choose 6061 for this? I just built a triple clutch out of 6061, but a machinist friend of mine gave me a reference manual on metals, if you can fine 5005, 5052,5083 or 5086, they are listed to have better corrosion resistance in a marine enviornment. I made a little pedestal for my chart plotter out of a lesser alloy aluminum and it was pretty well corroded after a year. I hope the triple clutch holds up with out me having to anodize it. If it corrodes I will anodize it because it was a lot of work. $10 worth of Aluminum, but 10 hours in the shop. lol I even took the time to install Igus inserts so my cams will stay pretty.
I attached the page that lists the different alloys and their rating. 6061 is a B rating, where the others I listed are an A rating with no other coating or anodization.
The second page is just a better photo of the book, the first didn't come out well.

dscribner · Jun 4, 2010

It seems to me that the mast would be in compression, until the wind blows. Then the 300 + pounds of pressure exerted on the sail will be transfered thru the sail slugs that are in the mast slot to the stays, the shrouds and the tabernacle thereby exerting bending stresses and torsional stresses on the mast, depending on the point of sail.

Probably the easiest way is to see how the big boys are selling them and shamelessly copy it. Also, for material you might try McMater Carr. They're pricey but for a small slab of something, it might make sense.

Hermit Scott · Jun 4, 2010

When you bend something, one side compresses and one side is in tension. The dividing line for these two areas is the neutral axis and is neither in compression or tension. If you have a square stock and bend it the neutral axis is the median lines.
If you draw a free body diagram of a mast sitting at the dock, most of the forces are going straight down into the compression post. The bottom of the mast is almost an extension of the compression post as far as how the forces are distributed.
Masts bend over two neutral axes. They bend forward and backwards and they bend sideways. Mine is very easy to bend sideways with one upper shroud off. I took one upper shroud off and you can push on the other upper shroud and it was shocking how much the mast head bent over. Plus it is twisted, I bet it's a crazy set of equations to resolve all the forces. That's what I think anyway.
Roger are you into any of this, or are there tables you follow to determine the mechanical properties of masts, when you design boats? Or is your primary focus on hull structure?

Roger Long · Jun 5, 2010

Hermit Scott said:
Roger are you into any of this?

Not as much as I used to be but I have done things like the indeterminent computer structural analysis of the rig for the "Master and Commander" ship "Surprise" (ex. "Rose") for her certification as a sailing school vessel. Very cool, I could put the wind loads on and produce a printout of the whole mast bending and see just how flexible those rigs were when the ship was pressed hard.

I assume that this is a deck stepped mast which is why there is a connector. Such a mast typically has way more cross section area than it needs at the base because the extrusion is sized to be stiff enough to keep the column between the stay supports from collapsing. Ever noticed how some radio towers taper to almost a point at the bottom? There isn't much bending at the bottom of a column so a reasonable size hole is unlikely to cause a problem.

Stretching of the stays will let the mast lean over a bit putting all of the considerable compression on one side of the mast. I would favor a rectangular S.S. plate since it's stiffness and the four screws will help stiffen up the mast locally against any flexing created by the local compression at the edge of the step. That's also a reason not to put the hole right down at the base. Neutral Axis and bending aren't relevant issues here though.

Bad Obsession · Jun 5, 2010

Why cut a hole in the mast? You should have two sets of blocks on both sides of the mast on your Catalina 30. Remove one set of the blocks (shieves) and you can work in that area. Mine look like they wouldn't be that hard to remove.

CABixler · Jun 5, 2010

I drilled a 2" hole with simple hole saw in my Catalina 30 tall rig many years ago. About maybe a foot off of the deck, right in the center. Absoloutey NO issues in all weather conditions.

I stuggled with how to make a clever cover. Settled on a very simple solution. I had a couple of very nice Catalina 30 decals! Just stuck one over and it looked great. Of course there is no mechanical support but there really is no need for it as it has been well pointed out.

I never regreted having access to the electrical wires.

Regards,
Craig

Access hole in Mast?

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