Thread: The Northstar GIS Build

You should be careful with aluminum flag poles as masts. They are usually 6063 alloy, which is literally half the strength and stiffness of a 6061 alloy, which is the usual preference for a mast. They use this grade aluminum, because they are extruded and need the more malleable alloy. Also try to avoid the "spun" aluminum poles, as these are really weak. Lastly, aluminum flag poles are usually considerably heavier then necessary, once you increase diameter and wall thickness to suitable sizes to compensate for the inherent weaknesses of the build method and alloy.

The last aluminum mast I made carried a 78 sq. ft. (7.24 sq. m) mizzen and a light air mule occasionally. It was two pieces, the upper a birdsmouth section that fit inside the lower aluminum tube. The tube was 2.5" (64 mm) with a .063" (1.6 mm) wall thickness, which is .58 pounds (.26 kg) per linear foot. I used a 12 ' (3.65 m) long tube, which weighed just under 7 pounds (3.15 kg). The upper section was a white spruce birdsmouth, 10' (2.54 m) long and about 14" fit inside the upper end of the tube. It was tapered continuously above this point to the masthead, where it was about 1.5" (38 mm) in diameter. I used a 17% stave thickness, which also tapered along it's length. This portion of the mast weighed about 4 pounds (1.76 kg) including glue, so the total, full up weight for this tapered, aluminum/wooden composite mast, was 11 pounds (4.98 kg), which is pretty impressive for a nearly 21' (6.35 m) stick. Once you include sail track, halyard and topping lift sheaves and exit boxes (everything was internal), the weight did jump up to over 15 pounds (6.8 kg), but this is still impressive.

FWIW . . .

I'm not a naval architect so I can only speak from experience.

The poles on my Gazelle were 6063 as PAR suggests. What I learned at the time was that 6061 had better corrosion resistance. So I painted mine. I never found anything to suggest 6063 was weaker, much less significantly weaker.

The poles were also spun, I assumed. That's how the extruding process works for a round hollow pipe. The poles start off as untapered pipes with a diameter that will match the smallest part of the taper. Wall thickness will be thicker than what is intended for the larger base, by what factor I have no idea. The poles on my Gazelle were .187" at the 8 inch end, probably a little less than .25 at the top which was just under 4 inches IIRC.

I once tried to hold my Gazelle upright while the tide went out by tying off the starboard side to a nearby dock with the port side stabilized a big anchor and long rode; the anchor was manually set in dry ground just above the high water mark. As the tide receded I thought the boat was heeling a little too much toward the anchor on port so I had a dock monkey take in a little bit of the line running to the dock. I said, "pull it in just a little." He pulled it in about four feet and the boat fell over to starboard, landing hard in the mud. When I looked up at the top of the mast where the anchor rode (to port) was I saw that the mast had been flexed about 5 feet! I was shocked it hadn't failed. I immediately set to release the tension by taking the wraps off the cleat. Before I could get the last wrap off the line was yanked out of my hand---ZING! When I checked the top of the mast it was perfectly back in column.

There were a few other incidents that gave me perfect confidence in those 6063 aluminum flagpoles so suffice it to say I had no qualms about using the same sort of pole on my tiny GIS.

Regarding the excessive weight of aluminum: there's no way a tapered aluminum pole is going to carry a weight penalty compared to a solid fir mast. Tom Colvin designed the Gazelle to use untapered, aluminum pipes--6 inches in diameter IIRC, with stays. Those might be heavier than wooden masts but not by much; add the weight of the stays and they were certainly heavier than mine. The tapered poles I used are significantly lighter than adequate solid wood masts.

My GIS mast weighs less than 20 pounds all in, rigged and painted. That's about 33 percent less than the original solid fir mast and maybe 5 percent more than the 4 inch box masts if built of the lightest spruce you can find. I'm perfectly content with that tiny weight penalty knowing that the 3 inch round mast is going to present much less windage than a 3 1/2 inch solid or a 4 inch square and hollow mast.

6061 alloy:
tensile strength (PSI) 45,000 (3,164 Kg/s cent)
yield strength (PSI) 40,000 (2,812Kg/s cent)
Brinell hardness 95

6063 alloy:
tensile strength (PSI) 27,000 (1,898 Kg/s cent)
Yield strength 21,000 (1,476 Kg/s cent)
Brinell hardness 60

Simply put, you can have a 6061 alloy mast, with half the weight at the same strength of a 6063 alloy. So whatever you mast weighs, it could be nearly half of this, with the same physical properties. This is the primary reason I spec up my two piece composite, aluminum/birdsmouth mast for most of my designs (I offer a few choices). I can get 6061 in the most common sizes fairly inexpensively, though many can't. I get mine through a local welding shop who orders it along with other stock they need, which saves me shipping. The last stock I got was two 24' (7.3 m) lengths of 2.5" (63.5 mm) tubing. I pulled them directly off the flat bed truck that delivered them, in the parking lot of the local welder (who I've know for years). I even cut them to length, right there in the parking lot, with his chop saw, so I could load them on my 16' trailer and drag them home. Finding sources like this can be difficult, but they're out there.

I'm glad you're pleased with your mast.

Thanks for the specs re: 6061 and 6063.

When I was trying to decide whether or not to buy the two poles I needed for the Gazelle I did some basic calculations based solely on the wind speed ratings for the flagpoles. The poles were rated for a force one hurricane if carrying a flag, and 100-plus wind speeds if bare. Since I had no idea how big the flag was I had to back into the decision based on a bare flagpole in 100-plus mph winds.

I started with a base of a 400sf sail in a moderate 15 knot wind. The stress on the mast was a fraction of what it would experience in the bare-pole-in-the-ground at 100mph.

I then ramped up to calculations based on a reefed sail in 30 knots of wind; the stress was still a fraction of what the pole was rated for.

Remember: a flagpole is set solid in the ground which doesn't give while a mast is fixed (relatively solidly) in a boat which heels under force.

One other factor played a role in my choice of masts. After hurricane Hugo devastated the Charleston SC area I found myself shopping for bottom paint. The supplier I found was just inland from Charleston, the Summerville area IIRC. We found ourselves driving through a disaster area--the steeple of a church was lying on the ground out front, huge southern live oaks were down and blocking the streets, power poles were snapped along the roadside. But as we drew near to the place where we were going to be buying the bottom paint we passed a small industrial warehouse which had five flagpoles standing out front. The poles were standing tall, with no apparent damage. All five of them.

Masts and flag poles don't have much exposed area if bare, though admittedly, I've scooted along at over 16 knots under bare poles, on my big boat in a storm once. Most of the time, while aboard a small sailboat, we'll have sails hoisted at some level. I sailed twice during Hugo, one over by Daytona in a specially equipped 26' sailboat, that was only used in very heavy air and later that evening, we drove up the east coast and windsurfed Hugo as his southwest corner was battering Jacksonville and areas north. I helped in the clean up the following weekend in Georgia. I remember driving up that Friday to see how bad it was, then staying for a week helping clear debris.

The real advantage in using well suited materials in an aluminum mast is weight and of course corrosion. As a young man, I remember climbing up an old cat boats mast, taking a new peak halyard with me (in my teeth). It was a solid grown mast on a fat, hard bilged hull, like all these old cats where. I remember climbing up the forward side of the mast, but had to reach around to feed the new halyard through it's sheave. In doing so, I had to lean to port a fair bit, just so I could look around the mast and see what I was doing. We were at a dock, just before the usual Saturday morning races and I heard a sudden bunch of gasps, by the onlookers that were amazed someone was 22' or so up the mast of one of these puppies. Comments of "she's going over" didn't boost my confidence that this was going to end well for me, but I did get the halyard fed through and back down the stick without incident. I was concentrating on getting the halyard in the sheave, but the folks on the dock could see the boat heel dramatically, as soon as my arm reached around for the sheave. The gasps started when I shifted my weight to port and the boat started to roll. Fortunately, my crew had the throat halyard and where hauling as hard as they could to keep it from rolling over. All I did was shift my body from the mast centerline, several inches to port, so I could look around the mast, but this 150 pound mass, just a few inches off the boat's centerline taught me a valuable lesson. In mathematical terms, I was exerting about 3/4" of a ton on the righting arm of the boat, at that height (3/4's up the mast). No wonder she wanted to capsize.

Since, I've done countless calculations (since) and saving weight in the rig equates to real speed gains, assuming the boat's potential can take advantage of it, which in this case (your GIS) it can. For example, I just built a mast with a birdsmouth 15% wall thickness. It's well stayed and saves about 10 pounds off the usual 20% wall. This is the equivalent of a 100 pound person, hanging onto the mast about 3/4's of the way up, in savings!

I have no doubt your mast will be strong enough, but I just wonder how much more area you could have up, in building winds with a lighter, but equally strong mast or how much less heel you'd experience in the same wind strengths.

I was confident that my Gazelle poles were substantially lighter than what Tom Colvin specified (straight thick-walled pipe with stays) so the boat's stability was never a concern.

Since my Goat mast is significantly lighter than MIK's original solid fir mast I had the same confidence all would be well, stability wise, for the same reason.

If 6061 alloy had been available I might have opted for it, for the corrosion resistance. The lower weight would have been a bonus. But they don't seem to make flagpoles out of that alloy and I did not want stays to hold up a more conventional (and expensive) 6061 mast, one that could fail by the failure of just one fitting out of the dozens needed to keep a stayed mast up.

I've been a fan of unstayed rigs due in large part to my experience with my old Freedom 33. It was an early model, before they switched to carbon fiber masts, and its aluminum masts looked just like the flagpoles I used on the Gazelle.

I have been busy getting out parts and am about to glue a lot of stuff together. I want to do that before I leave for South Carolina for the month of January. That will let things cure while I am gone so I can paint and varnish when I get back. I can't leave soon enough. It was -26 C here when I woke up two days ago with a wind chill of -37 C. Glad to be working inside on the boat.

You're photos are scaring me… I don't see an exit door!

Starboard side of the bow. Not shown but available in case of capsize.

We keep getting closer. Should be done in plenty of time for this spring. Here is the latest update.

Well, here I am in the sunny south and it is sunny, but colder than a frogs behind. 30F and suppose to be worse Tuesday. Can't complain though, it is -15 F at home. The goat is "resting" while I am away and I hate that as I need it here. Great place to sail around here which is the reason I decided to build it in the first place. I get up every morning and watch the sun rise from Africa. Beautiful. I have the plans with me so I can study while away and think up creative ways to keep from screwing up the project when I get back. I'm going to need some paint advice soon. I want white for the bottom and want the most durable paint I can get. Any suggestions?

Paint is a whole subject in itself. The cheapest paint in the long run for the bottom is a 2-pack paint such as International as it is the toughest finish. It lasts for years. Single pot marine paint is fine though, but you will have to touch up more often. International's Toplac is very good. I used it for my Scow Moth. Both types flow on very nicely for a superb finish. However, if you want a "racing bottom" gloss type of finish that is second to none, the 2 pack is the way to go.


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There are literally hundreds (probably thousands) of previous threads exploring all aspects of painting. The two part LPU's are the toughest, but also the costliest, by a considerable margin. The single part polyurethanes are next on the list, having nearly the same toughness and durability, at a considerably lower cost. This is the usual route for the home builder. Modified alkyds are next on the list, but not nearly as tough or as durable. Traditional alkyds and acrylics are the lowest cost of the paints and their price reflects their durability and toughness.

The LPU's can be difficult to apply well, being sensitive to temperature, solvent type, hardener suitability for the environmental conditions when painting, etc. This is less true with the single part polyurethanes, though some issues can arise, if you're painting conditions are poor. The alkyds and acrylics are pretty easy to "lay down" for the back yard painter.

The key with paint is surface prep. Painting is 80% prep and 20% brush in hand time. The more you put into the prep, the better (usually) the paint job can be. A recent paint job I did had 50 hours of prep with 5 - 6 hours of actual applying paint time in it. This is a low percentage, mostly because I sprayed the paint, saving some time, but a good idea of the difference.

Lastly, do some practice runs on scrap stock, before committing paint to boat. You can learn a lot about cutting, application, etc. with a few square feet of practice on a hunk of scrap. Also look up the "roll and tip" method as you research painting.

Great advice and thank you kindly. I can control environmental conditions pretty well so that should not be an issue. I will have to research roll and tip so I am exactly sure what you mean.