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Old 24-12-2008, 07:28   #1
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Thoughts on Steel Spars ?

I am converting a motorsailer to a "trawler yacht" and I'm considering making the two mast out of steel. The main mast will be twenty two feet of the deck, have a roller furled steadying sail, crows nest, and the boom will be used to lift a 250lb motorcyle. The mizzen mast is stepped on the house roof and will be eighteen feet. This mast will support a paravane stabilizing rig, roller furled steadying sail, and the radar. Both spars will have tabernacles.
I have been searching the webb for comparisons between different materials for spars but nothing I have found pertains to the kind of rig that I want to build. Although steel will weigh a little more, ( I'm thinking 5" tube with a wall thickness of 1/8" ) Its easy to weld and paint.
Anyone have any experience will steel for spars?
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Old 24-12-2008, 07:33   #2
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Hi Quidam! This is just my opinion, but I think you are adding weight and maintenance problems just where ya dont want them. I recommend finding old used aluminum masts and/or booms...they are everywhere cheap. Look on ebay and the 'net, IMHO, Chris
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Old 24-12-2008, 07:51   #3
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steel spars

Christian, thanks for your input.
Given the short nature of my needed spars and that fact that I can easily taper them in steel, I dont think there will be a big difference in weight. As for maintenence I was thinking that with epoxy based paint it would be pretty low.
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Old 24-12-2008, 09:26   #4
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+1 on aluminum.
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Old 24-12-2008, 09:31   #5
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Aluminum has a higher strength to weight ratio than does steel. A mast is a huge moment arm "trying" to make your boat heel. There is no sense in adding unnecessary weight aloft.

Steel rusts and leaves nasty looking rust bleeds everywhere. Regardless of what coatings you use, eventually there will be rust stains everywhere including your deck around your mast.

Aluminum oxide is an excellent oxygen barrier. Steel does no such thing...its just keeps rusting after it is exposed to oxygen. My guess is that you will eventually get very tired of chasing down rust bleeds on your mast. You may want to get yourself a very comfortable bosn's chair.

I see no good reason to go steel over aluminum for a pleasure boat.
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Old 24-12-2008, 10:20   #6
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steel spars

I understand that there is a good argument for aluminum spars on a "sail boat". However, this is a trawler yacht, 47' in length, 12 1/2' beam, and 4 1/2' of draft. The main mast is only going to be 22' off the deck. According to my calculations, the steel spars will only weigh about 20% more.
As far as rust is concerned, aren't there alot of steel yachts?

The trouble with aluminun spars is that they are made from aircraft aluminum not marine grade, nearly impossible to weld and bare aluminum is ugly and does'nt paint well. If I could afford marine grade aluminum tubing and the cost of welding it, I would be interested but there is still that paint issue.

This boat will have a work boat finish. Semi-gloss enamel, tires for fenders, galvanized rigging, and there is virtually no stainless steel on the boat (too shiny), all the hardware is bronze. A few rust streaks wont bother me.

I'm thinking my choices are steel or wood.
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Old 24-12-2008, 11:13   #7
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I'm thinking my choices are steel or wood.
It sounds like you have made up your mind. Go for it man !

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Old 24-12-2008, 13:40   #8
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The sparmakers all weld their extrusions. Aluminum is more difficult to weld because it doesn't change colors as it's heated. Takes more talent to weld but other, than that, not that big deal to do. You could probably find a mast the size you are talking about for a few hundred dollars. If you are talking heavier tubing, a portion of a taller mast that has bent in the middle would be another cheap source.

Aluminum is more difficult to paint but many paint companies offer systems that work just fine. You just have to follow all the steps. Unpainted aluminum looks fine IMHO, btw.

Steel is way more heavier than aluminum. The steel tubing your are talking about is going to be grossly heavy even if you taper it. That's weight aloft that will have a drastic effect on the boats stability even though it's a short rig.

Sounds like you made your mind up before you posted, why did you waste our time.

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Old 24-12-2008, 13:48   #9
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An alternative to painting aluminium: Nyalic - polymer resin coating that provides surface protection for years. Ultraviolet protection | corrosion protection | clear surface protect | protective coating
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Old 24-12-2008, 14:18   #10
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Steel spars

I was skeptical when people started using steel spars on my designs. They have worked out extremely well over the last 26 years, and few owners would give up their steel spars. A weld in steel is far more reliable than aluminium which is only 60% the strength of the metal, and extremely easy to screw up. They cost a fraction the cost of aluminium both in materials and fabrication. The spars that spar makers recommended in aluminium were not much lighter than the steel masts we ended up using, and cost ten times the price. That an aluminium spar is drastically lighter is an off the wall assumption, that doesn't bear out in reality.
Corrosion drops off drasticaly more than 15 feet above deck ,where it is only hit by fresh water.
I use 6 inch OD steel tubing with a 11 guage wall thickness for a 36 footer. Recently I started going for scarf joints instead of sleeves, drastically reducing the weight of joints in the 24 ft lengths of tubing, and simplifying the building.
Some have changed to aluminium ,and found the difference in sailing performance was negligible , and barely noticeable. Certainly not worth ten times the price.
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Old 26-12-2008, 09:01   #11
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steel spars

"The spars that spar makers recommended in aluminium were not much lighter than the steel masts we ended up using, and cost ten times the price. That an aluminium spar is drastically lighter is an off the wall assumption, that doesn't bear out in reality."

Brent, when I started this post I had hoped to hear from someone who had real world experience with using steel for spars. Thankyou.
Could you tell me how or if they paint or coat the inside surfaces?

Peter O. FWIW I have not made up my mind and I'm sorry that you feel that I have wasted everyone's time.
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Old 27-12-2008, 20:39   #12
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I checked this assumption. Mast design is governed by the buckling strength, where E x I (modulus of elasticity x moment of inertia) governs the strength of the mast.

I assumed equal EI for a 6" x 11 gauge steel tube, and a 175mm x 6.35mm aluminum tube. The steel one is 29% heavier for equal EI, and a very considerable difference if you are worried about stability. For the original poster, check with the boat's designer if at all possible, about the results of adding the masts. You may be drastically affecting the stability of your boat, perhaps dangerously so.

It's also a false worry about welding aluminum - you don't weld aluminum masts in the middle of the panels, where buckling takes place. Welds are confined to the masthead, and maybe the spreader roots.

Finally, 3mm thick steel doesn't leave you any significant allowance for corrosion. And you will have a hard time coating a steel mast.

Here's the raw data from my spreadsheet:

6" steel x 11 gauge

6.5" aluminum x 9 mm thickness
Outside diameter 152.4 mm
radius 76.2 mm
inside radius 87.7 mm
Thickness 3.18 mm

Moment of Inertia (I) 4151130 mm^4
density 7860 kg/m3
cross sectional area 1491 mm2
weight / metre 11.72 kg/m weight
Modulus of Elasticity (E) 2.00E+11 N/m^2
EI 8.30E+23 N/m^2

6.5" aluminum x 9 mm thickness
Outside diameter 175.4 mm
radius 87.7 mm
Thickness 6.35 mm
inside radius 81.35 mm

Moment of Inertia (I) 12064032 mm^4
density 2700 kg/m3
cross sectional area 3372 mm2
weight / metre 9.11 kg/m
Modulus of Elasticity (E) 6.90E+10 N/m^2
EI 8.32E+23 N/m^2
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Old 27-12-2008, 21:11   #13
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Many things are welded on aluminum masts. Bases for goosnecks, winches and other fittings are all welded on.
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Old 28-12-2008, 21:25   #14
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Sure, but they aren't welded where you need the maximum strength - which is in the middle of a panel (1/2 way between spreader and masthead or spreader and deck). You can certainly weld them where strength requirements are lower.
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Old 29-12-2008, 07:50   #15
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Evan
My aluminum mast is also welded..and not at the spreaders...I think its 56ft from the deck, but stepped on the keel so the total is around 65ft in all.
I'm not sure how they did it and if it was annealed after, but it has lasted over 14,000 sea miles!

Quidam
I would go with aluminum if you can get a good deal on some used stick...if not...I also wouldn’t be afraid to use steel if you can come up with a good way of painting the inside...actually painting isn’t that difficult, surface prep would be my concern.
Don’t forget to keep us posted with an up date.
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