Junk Rig Masts

(Updated 10-15-2019)
(Minor corrections 10-3-2020)

--Building Minimus II

One of the appeals of a junk rig is that, for the most part, it lends itself to a do-it-yourself approach.  The one exception is coming up with a free-standing mast.  We decided to experiment with using aluminum irrigation pipe for a mast.  Four of them actually, as our 24' (7.2 meter) catamaran sports a 4-masted bi-plane junk ketch rig.  Although we won't be able to test the masts until next spring, we're encouraged by our experience so far. 

Once we had the technique down, we could make a reasonably inexpensive, triple wall mast in a few hours.  By triple wall, I mean triple wall in the lower third of the mast, double wall in the middle third and single wall in the upper third.  The idea is the same as in a tapered mast, which is to maximize strength in the lower part of the mast, while minimizing weight aloft. 

In case anyone else wants to explore this option, here's more detail: 

First, here's what we've learned about aluminum irrigation pipe:  It's readily available in agricultural areas where crops need seasonal irrigation.  It's straight pipe, not tapered, and is available in diameters from 1-1/2" through 10" (38-254mm).  It comes in standard and heavy wall.  The alloy used is either 3004 H22 or 3004 H26.  These have similar mechanical properties to 6063-T6, which is what many of the light poles are made from that are used for junk rig masts.  The structural properties of aluminum alloys can be compared at the makeitfrom.com website.     

In the Willamette Valley of Oregon, where we've been building Minimus II, the closest irrigation supplier is Pacific Ag Systems in Junction City, Oregon.  We were there recently and got a list of diameters, wall thicknesses and prices (as of October, 2019), all for the heavy wall type.  All diameters come in 40' lengths, but they're happy to cut specific lengths.  Presumably, other suppliers around the world have similar policies.

Diameter                           Wall thickness         Price per foot (U.S. $)

--1-1/2"     (3.8cm)            0.042"  (1.1mm)      $2.13
--2"            (5.1cm)            0.050"  (1.3mm)      $2.57               
--3"            (7.6cm)            0.065"  (1.7mm)      $4.06
--4"            (10.2cm)          0.072"  (1.8mm)      $5.72
--5"            (12.7cm)          0.078"  (2.0mm)      $7.49
--6"            (15.2cm)          0.083"  (2.1mm)      $11.11
--8"            (20.3cm)          0.083"  (2.1mm)      $17.30
--10"          (25.4cm)          0.083"  (2.1mm)      $21.96

We made all 4 of our masts from 4" (102mm) pipe with a wall thickness of 0.072" (1.85mm).  That's thinner than most junk masts are made from, but we came up with a reasonably easy way of strengthening them. 

 Our two mizzen masts are 14' (4.3m) long and each supports a 42 square foot (4sq m) sail.  The 4" pipe seems plenty strong for that mast length and sail area.  Just to be on the safe side, we double walled the lower third of both mizzen masts.

The main masts are 18' (5.5m) long and each supports a 91 square foot (8.5sq m) sail.  These longer masts, coupled with larger, heavier main sails caused the masts to flex a bit more than we were comfortable with, so we stiffened them considerably by purchasing 2 more 18' (5.5m) pieces of 4" (102mm) pipe to use as sleeves.  We cut each of them into a 12' (3.7m) and a 6' (1.8m) piece, then cut out a lengthwise strip and inserted them into the original 4" (102mm) pipe.  When we were finished, we had two 18' (5.5m) masts that were triple wall (0.216", 5.5mm) in the bottom third of the mast, double wall (0.144", 3.7mm) in the middle third, and single wall (0.072", 1.8mm) in the top third. 

The main masts are now much stiffer and weigh only about 38 lbs. (17.2kg) each, so stepping by hand is fairly easy.  We're optimistic that the main masts will be strong and stiff enough.  

As an aside, we got another metric on just how strong the pipe is.  When we got home with the 18' (5.5m) pipes that we intended to use for sleeves, I was curious to see how resistant it was to bending, so I put blocks under each end of one of the pipes, then gradually put my weight on the middle of it.  Even with all my 175 lbs. (79kg) on it, it remained dead straight.  So Pearl and I both stood on it, 300lbs. (136kg) or so total.  We even bounced up and down gently, and while it certainly flexed, when we got off we were surprised to see that sighting down the length of it, it was still straight.  

Here's how we went about strengthening the main masts:

The local irrigation supply yard.	One of our finished main masts, showing triple wall thickness at mast step. The following photos show how we did it. (The black ABS pipe is just an exterior sleeve to help distribute lateral loads at the mast step.)
A line is scribed the length of the piece to be used as an internal sleeve. The weld gives a straight reference line.  Penciling over the weld line makes it easier to see.	A straight edge is used to connect the pencil lines.
A sewing tape is used to measure where a second line will be drawn, parallel to the first one.  The formula we used to calculate where the second line will go is: circumference of sleeve =  [(outside diameter of pipe minus 2 times the wall thickness) times pi] minus 6mm   For example, here's the calculation for our sleeve circumference: [(102mm OD pipe minus 2 x 1.83mm) x 3.14] minus 6mm = [(102 minus 3.7) x 3.14] minus 6 = [98.3 x 3.14] minus 6 = 309 minus 6 =303mm The 6mm allowance insures that the sleeve will not be too large to fit inside the mast.  Without that margin, the sleeve could jam inside the mast. If making a triple wall section, where a second sleeve will be pressed inside the first one, adjust the formula accordingly.  The second sleeve goes in after the first one is all the way in.	A circular saw makes quick work of cutting along the lines.  A piece of 2" x 4" board screwed to the bottom of the saw base helps to guide the cuts.   Eye and ear protection are a must.  An ordinary wood cutting blade works well, but not a plywood blade, as the spaces between the small teeth tend to fill with aluminum.
A right angle grinder with a metal grinding wheel turned upside down works well for removing burrs.  Caution is advised with the blade guard off.  The inside and outside edges of the cuts should be free of burrs.  A final smoothing with a hand file is recommended.	A toilet brush is taped to a long pole and covered with grease to coat the inside of the mast.
3 hose clamps are used to compress the sleeve.  The compressed sleeve isn't perfectly circular, so a bar clamp is used to make it circular long enough to insert the end of the sleeve into the mast.	The bar clamp is then removed and the hose clamps slid back so a portion of the sleeve can be coated with grease.  The hose clamps are then put back on, as in the previous photo.
Gentle taps with a sledge hammer easily drives the sleeve into the mast for the first few feet (1m) or so.  Note the board to protect the end of the sleeve while tapping. Each time the sleeve is driven in until a hose clamp is reached, the 3 clamps are moved back 6" (150mm) or so along the sleeve and a new section is greased. After the first few feet are tapped in, it's time to switch to a hydraulic jack.	The mast is then positioned between two immovable objects.  In our case, that was two walls of the shop.  A hydraulic jack then drives the sleeve the rest of the way into the mast.  As the sleeve goes in, the hose clamp and greasing routine is repeated each time the jack reaches maximum extension.  More and more blocks are added behind the jack as the sleeve goes in.  It's important to make sure the mast, sleeve, jack and blocks all remain inline with each other.      If we hadn't had two walls at the right distance apart, our plan was to park two vehicles parallel to each other and the appropriate distance apart, then brace the jack, mast and sleeve against the wheels. A shipwright friend of ours suggested the hydraulic jack idea--thanks Drew!  He also suggested that one might use a couple thick boards connected by two long pieces of chain as braces for the jacking operation.
All 4 masts finished except for masthead fittings. The black collars are ABS pipe pressed over the mast to help distribute lateral loads at the mast step and deck collar.  The green objects inside the masts are pool noodles to give buoyancy in case a mast ever goes overboard.  We'll be reporting on how they do during sea trials next spring.	Our mast head fittings are made from a 4" (102mm) PVC cap.  One of the dyneema lines is for the halyard block and the other is to anchor the topping lifts and mast lift.  The lines are free to rotate around the mast as the sail angle changes.  We used dyneema because the masthead gets lots of sun exposure and dyneema is highly UV resistant.
Masthead fitting on mast.  A piece of 4" (102mm) Schedule 40 PVC or ABS pipe several inches long is pressed over the end of the 4" aluminum irrigation pipe mast, then the 4" PVC pipe cap is pressed over the plastic pipe.    This makes a secure and weather tight fit.	A close-up of the parts making up the top of the mast head fitting.  We got all the parts at the local hardware store.  Here's the parts list in case anyone wants to try it. --Schedule 40 PVC slip cap, 4" (Spears brand is a nice, rounded shape) --stainless machine bolt, 1/4" x 1-3/4" --stainless fender washer,1/4" x 1-1/2" --2 nylon bushings, 5/8" OD x 1/4" ID x 3/8" long --aluminum tube, 13/16" OD x 5/8" ID x 7/8" long --1/4" x 1" stainless fender washer 1/4" stainless lock nut I filed down the raised lettering on top of the cap so it wouldn't abrade the dyneema lines.
All four masts stepped.