Thoughts on Steel Spars ?

[quidam]

Evan,
your data is very interesting. This boat origionally had a 37' mast of solid spruce and it was 7 1/2" in diameter where it went through the deck. It was gaff rigged, had a huge solid teak lookout platform and all the wire was 3/8" in diameter. The solid fir boom was 20' long and the gaff was 16', and of course she carried alot more sail than the two little steadying sails that I am planning on having. This was a heavy rig.
I have added more house than the boat had origionally and consequently added weight on deck, although I have tried to keep that to a minimum by using alot of polethylene honeycomb coring in the house construction. Also, there will be two spars not just the one, so I am very interested in keeping the weight up high to a minimum.
I was thinking of 5" steel tubing. The 11 gauge is actually .125" and weighs 6 1/2 lbs. per foot. The 13 gauge weighs 5 lbs. per foot.
It would be interesting to hear what the comparisons in total weight would be for steel vs aluminum vs wood (hollow construction), for the two short spars that I have in mind. 22' main and 18' mizzen.
Thanks to everyone participating in this discussion.

[Pblais]

It would be interesting to hear what the comparisons in total weight would be for steel vs aluminum vs wood (hollow construction), for the two short spars that I have in mind. 22' main and 18' mizzen.

The last time we hauled we were in a marina with a wood cat boat. It was a 41 ft boat with an 16 ft beam. They were pulling the solid wood mast of 48 ft. It was 1,500 lbs. The 38 ft ft spar was 450 lbs. It took a crane to pull the spar. There was really no way a crew could walk a spar that heavy off the boat.

Here is the link:

Selina II Cat Boat -Sailing the Eastern Shore of Maryland - Boat Charters - Chesapeake Bay

[Brent Swain]

I make the spars airtight, so no oxygen can get in to cause corrosion. No oxygen, no oxidation. What is in there gets used up fast , and doesn't produce enough corrision to be visible.
Once you are more than a few feet above deck level , there is no salt water, and thus corrosion is minimal. Nearer deck level is easily accessible for maintenance. My last masthead fitting was sandblasted and given a couple of coats of cold galvanizing primer. After 10 years I changed it ,for a modified version, and found it almost as good as the day I put it up, zero corosion. Had it been nearer deck level there would have been a lot of corosion. I had done zero maintenance on it.
Brent

[Evan]

Brent,

Never thought of all external halyards, fittings, etc. and making it airtight. Clever idea and I agree if it's airtight that there will be minimal interior corrosion. Ever do an air pressure test on it after it was done, just to make sure? Since it's just a tube, I assume you bolted, riveted or screwed a mast track on it, which might have been hard to keep tight over the years; same for wiring glands which are o.k. when new, but can leak after some years of exposure.

[quidam]

I met a guy in NY who is building steel masts for his ketch and he is also welding them to be airtight, with a twist. He leaves a small hole in the base where he squirts in some creeping oil (used in aircraft construction). Then he welds up the hole and supposedly the oil will creep and eventually coat the entire inside surface.

I have been researching aluminum tubing and was suprised that 5" 6061 tubing with a wall thickness of 1/8" only cost $6.26 per foot and only weighs 2lbs. per foot. Is that thick enough ? They also sell it in 3/16th but the price jumps to $21.79 per foot. I have also seen other mast made out of this tubing where they weld a smaller size tubing at the spreaders.

[mainesail]

I'll add my bit. I strongly discourage considering this spar setup without having someone with serious engineering experience review and calculate loads and moments etc.. I'm have no idea what your hullform and stability numbers are, but I'm fairly confident that all that weight aloft will cause you stability problems. I'm not aware of your motivations for this idea-likely financial-is the potential risk worth it? I also agree about the maintenance issues raised by others.

As a rigger, safety is ALWAYS my highest priority.

Good Luck

[quidam]

Mainsail, I respect your opinion, however judging by your post I dont think your read all the info that I posted on my boat and the kind of rig I am proposing. Even if I built both of the short spars I have proposed out of 1/8" steel, I think they would still weigh less than the origonal rig.
The question I'm asking now is if I built them out of 5" 6061 Aluminum tube how thick should the wall thickness be?

[Evan]

Proper spar engineering shouldn't be done on internet forums. Really.

[James S]

Originally Posted by Evan

Proper spar engineering shouldn't be done on internet forums. Really.

Hmmm...I going to respectfully disagree...at least in this case...there is a history of the existing engineering on this particular boat, and there are some very qualified people who can answer some pretty fundamental engineering questions.

I think this is exactly the place for this sort of conversation.

[Evan]

I'm qualified, but why take the risk? If I say, sure go ahead, and the mast falls down, might it come back to bite me? Probably not, but since I'm a P.Eng, I take my design responsibilities seriously. I don't do half assed jobs.

And my gut feeling is that local buckling of that thin a wall section is pretty likely (without doing any calcs)

[neelie]

If it rusts on the outside - no problem, you can see it and repair it. When it eventually rusts on the inside, you will have no idea until its too late. We had a steel boat and we lost every battle with rust. But I guess on balance - your intended mast is not that tall and does not support your primary means of propulsion. So any failure wont have catastrophic outcomes.
If you're determined to go steel then consider galvanizing - this may delay the inevitable.

Odd though that, that you consider aluminum to be ugly on a boat that is meant to have a work boat finish. I guess I'm weird, but I quite like the look of bare naked aluminum.

[quidam]

Originally Posted by Evan

I'm qualified, but why take the risk? If I say, sure go ahead, and the mast falls down, might it come back to bite me? Probably not, but since I'm a P.Eng, I take my design responsibilities seriously. I don't do half assed jobs.

And my gut feeling is that local buckling of that thin a wall section is pretty likely (without doing any calcs)

Evan, I am perfectly capable of doing a half assed job on my own but that is not my preference. I want to build the rig I am proposing a little on the heavy side, but since my experience with mast have all been solid wood I dont know what is thick enough. This is not a sleek go fast racing yacht trying to sail as close to the wind as possible so while weight is important I would rather err on the side of caution and build a stronger than necessary rig.

I AM NOT COMMITED TO USING STEEL. I like the idea of using two sizes of aluminum tubing and I thought someone could advise me on what thickness would be appropriate, keeping in mind that a paravane stabilizing rig has to be stout.

If I lived on the coast I would seek out other boats with this type of rig and get a good idea based on what works. Unfortuantely I now reside in the land of houseboats and runabouts.

I liked the idea of steel because I could weld it and it is easy to paint. I like the idea of aluminum tubing because of the weight. It would be great to hear from anyone who could tell me the appropriate thicknesses fo both materials so that I could see what the real difference in weight would be. I know that steel is much heavier but given how short this rig is the actual weight difference might not be that much. For example, If the difference in weight was something less than 100 lbs, would it really make that big of a difference? Keep in mind this boat had a much taller solid spar with a gaff rig.

[Brent Swain]

6 inch steel with 1/8th wall will be considerably lighter than your wooden mast. After welding up their masts , some of my clients have drilled the first hole for the sail track, and heard the hiss of pressure air escaping, after having been left in the sun. Sounds pretty airtight, altho you could thread a pressure guage in , pressurise it with a tire pump, and leave it for a few days .
The bolts for the track are drilled and tapped into the mast, then set in never sieze or epoxy, guaranteeing airtightness. Klingle talks about opening rudders that have been welded airtight for 30 years and finding only black iron inside, not a speck of rust.
When 36 footers with 47 foot masts have trouble telling the difference under sail between a steel mast and an aluminium one, I doubt very much if you could tell the difference with a 20 ft mast on a 47 footer.I can't see the justification for the expense of aluminium on your boat. Yes steel will rust eventually, as will aluminium . If it takes 200 years , then it will become the next owner's problem. One must be constantly on guard against advocates of the "Be reasonable and do it the hard and expensive way" school of thought. That type of elitist thinking has ballooned the cost of boats, and cruising , far beyond what it need be, and beyond the reach of too many low income cruisers, for no benefit.
Steel boats , unlike steel masts, are not welded airtight on the inside. Those that rust from the inside are the ones that nobody bothered to give enough epoxy on the inside.
Brent

[DeepFrz]

Gerr's book "Boat Strength for Builders, Designers and Owners" has a table in App. 2 called "Pipe sizes and properties" that should answer your questions as per strength and weight per foot. Also in the same book he has recommendations on fasteners and stainless threaded inserts.

I suppose you could even use square tubing.

I believe you are right in that the masts you are proposing will be much lighter with a lower center of gravity than the original spars. Solid wood spars are heavy.

[DeepFrz]

Yacht Design Explained

By Steve Killing, Douglas Hunter
has a good discussion on mast design properties and wall thicknesses for steel, aluminum and wood masts.