In This Section:

Building notes and additions to this section are now on a monthly schedule, so watch our "news" pages, or check our ever changing menu panel as more building pages come on line:


one minute guide
building overview
• materials
• tools
marking out
hull assembly
hull assembly 2
hull completion
foils
spars
rigging
finishing
sails
signs and graphics
kits and supplies

 

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Spars001
BUILDING THE SPARS

The plans for the Oz PDRacer illustrate a very simple low-tech method of a building lightweight hollow timber mast which is designed to flex to allow an automatic depowering of the sail in gusts. Building masts using this method is very simple, and uses a little over half of the timber of a similarly dimensioned solid spar, which would not have the same performance characteristics, or built in safety.

In this picture we are using a rather unsophisticated method for testing the amount of flex in one of our first spars. As you can see, it's going to take a lot of load before it actually breaks. Once we've measured the deflection, we'll calculate the amount of curve needed in the sail luff.

Spars002
LAYING OUT THE TIMBER

While building masts from long lengths of timber is desirable, there's no need to panic if you don't have bits that'll go the whole distance. We don't recommend quite this number of scarf joints, but it is possible.

Here enough 12mm thick bits are laid out to build two masts, and we're going to have to join them.

Spars003
TIMBER QUALITY

If you are going to use dodgy free timber, as we did for our first mast, expect this sort of result!

Here you can see the sort of thing to watch out for, and be careful to eliminate from the timber used in the mast. A properly constructed scarf joint will be as strong as a good length of timber, so don't be afraid to cut out all the dodgy bits!

Note 1: The knot extends not only the full depth of the timber, but across the width of it as well, leaving no strength at all where you need it!

Note 2: You can clearly see in the photo (click on it for a bigger view) that the grain in the timber takes a right hand turn around the knot, creating a shake which is akin to sawing mostly through the mast. We actually have a cross-grain happening which will have no strength at all when forces are applied.

This is a "snap line"!

Spars004
SCARFING BY HAND

Note: The angle shown here is too shallow for structural work, you need at least a 6:1 ratio for mast timber, if you can do 8:1 that would be better still.

It's not a difficult thing to do though, simply mark the line and cut. If you have enough space and a good vice, you can hold the two pieces you are going to join one above the other, so that they are in line and overlapping where the joint will be, and cut through both. Even if your cut is a little off square, they will both be in line, and you'll get a near perfect join.

I'm not sure if that description makes sense, if not, ignore it and do it your way!

Spars005
SCARFING BY BANDSAW

Note: The angle shown here is too shallow for structural work, you need at least a 6:1 ratio for mast timber, if you can do 8:1 that would be better still.

It's really easy to make a simple bandsaw sled to hold your pieces at the correct angle. Use a bit of scrap MDF or ply, and glue or screw any old piece of timber so that it provides a face at the correct angle (1).

Glue or screw a hardwood runner under the MDF, and insert a screw at the trailing edge to act as a stop, so that the sled won't go further than the depth of cut requires. This may save your fingers from needing a scarf joint, and therefore indefinite delays in completing your boat.

Spars006
GLUING THE SCARF JOINT

Our sprits were made out of old construction timber, but they were too short on one end, so the trusty scarf joint fixed all that. Here you see a nice amount of glue squeezed out indicating a nice properly glued joint. We'll leave the clamps overnight and clean up the spars further down the page!

Spars007
MAST BITS

Here our mast bits are joined to length, and we'll shortly be tapering the narrower pieces as described in the plans. This is a very easy process. Setout the taper in accordance with the drawings, and using your weapon of choice, cut down to the line! I don't mind a bit of work with a hand plane, but some may choose to saw much of the waste off before planing down to line, or even trimming with a belt sander.

Spars008
MAST LADDER FRAME

The plans set out a number of stations along the masts where square spacers are used to ensure the accuracy of the taper during glue-up. Of course the location corresponds with the marks you've already made while setting out the taper, so it's a pretty easy process. To save a bit of time if you coat the inside face of the timber with epoxy and glue up while it's still wet or tacky.

Sticky tape makes a good clamp, but staples or panels pins that can be removed later will hold the slippery little suckers in place till the clamping pressure can be applied.

Spars009
MAST SPACER

Here's a spacer in place with two sides of the mast finished. We've cleaned up the joint with a belt sander, and we're ready for the last two sides.

Spars010
MAST GLUE-UP

We've already spread the glue onto the mast frame using a snap-lock bag to spread a nice even bead of thickened epoxy glue, and we're coating what will be the inside face of the mast before final assembly.

Spars013
COATING THE SPACERS

Here the underside piece is in position, and the spacers are getting a dollop of epoxy, just in case some time in the future the mast springs a leak, and we end up with water in it. No chance of rot now! The top piece will be in place in a minute!

Spars011
MAST READY FOR CLAMPING.

The only trick at this point is to ensure that the ladder frame doesn't have a bow in it before gluing the final two pieces. It is quite flexible so it's worth sighting along it to ensure that it remains straight, unless of course you want a camber in your mast!

Spars012
CLAMPING

There's no need for a million clamps, even though that's probably what you are aspiring to. A couple of clamps about where the spacers are located is all that's needed to hold everything in place, then wrap packaging tape tightly round the bundle to secure it firmly.

Spars014
SHAPING

Once the glue is set, the wider staves of the mast need to be tapered to match the original ladder bit. This is easy to do with a router fitted with a bearing bit.

If you don't have a router, that's not a problem, most of the taper can be cut off with a simple hand saw or jig saw, and the whole lot sanded to a nice finish. A belt sander is good for quickly removing material, but be careful that you don't get too aggressive with it. Finally, run a round router bit over the corners of the mast to give it a nice masty look!

Spars015
SHAPING THE SPRIT

Remember our old structural timber above? It's amazing what a few laps with a hand plane will do.

Here the taper is marked and the pieces are being planed down to the line of the taper. They will be rounded with the router on completion as well, and the lot sanded.

Spars016
GLASS THE ENDS

Wrapping the ends of the spars in glass tape is not necessary, but it is a great way to ensure durability, and prevent splitting, particularly as this is the point where lots of load is transferred into the timber. Simply roll a resin soaked roller over the glass to gently wet it out in position.

Spars017
SAND THE ENDS

Don't worry if the glass looks like a complete mess.

Once the epoxy has set, sand all the sharp bits, and lumps and bumps, and once the thing is varnished the glass will be barely noticeable.

Spars018
TWO MASTS

Here we have two masts in their final shape, epoxy coated ready for a final sand and varnishing.

Spars019
TWO SPARS

Again, a final sanding is needed, then half a dozen coats of varnish, and we'll have a pretty nice looking set of spars.

These have a few gouges in them, and a few old rusty black nail holes, but once shiny and on the boat, most people won't notice any of that.

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