Masts

A Wood Mast Made Easy

by Paul Goodwin - US4061 - March 1990

Many people have not built a wood mast because of the apparent difficulty. There are many ways to build a DN mast from wood, but Jan Gougeon introduced one of the simplest designs yet. This new design uses pieces which can be cut easily on a table saw and requires no jigs, molds, or fixtures.

After discussing this new design with Jan, I went to work developing full size drawings. The simplicity of the design was very apparent and I was able to work out the dimensions of the pieces in a way that also made the setup easy. As soon as I had the basic drawing of the mast complete I started building the first one in my area (Detroit). The mast was assembled within a week, and I was able to sail it 2 weeks after the first discussion with Jan. One week after that, my new mast propelled me to a 7th place finish in the North American Championship. This has got to be the best hollow mast design ever!

My mast was so successful that a building/sailing partner of mine (Chip Cartwright) decided to make one. He arrived from school at 9:30 on a Friday night with a bundle of lumber under his arm, and when he went back to school on Sunday night, his mast was complete (except for the hardware). Plus, he had built a runner plank and a set of insert runners at the same time. He actually glued the two halves of his mast together, complete with aluminum luff tube, within 24 hours of arriving at my house! Understand, Chip is an extraordinary builder, but this still indicates the simplicity of the overall design.

The Basic Design

Now back to the real story--the mast construction. The basic idea is to glue a narrow strip of wood along the edge of a wider strip to produce an "L" shaped section. Then a small triangular strip is glued into the corner of the "L". This basic section is used for each mast half. Two of these sections are glued side by side, with a groove to hold an aluminum luff tube. After some final shaping, and possibly some tuning of the stiffness, an excellent mast can be produced. Fig 1 shows a cross section of the completed mast in the section below the hound.

Start by selecting some wood. The side wall (the large part of the "L") should be made from a wood which is light and not too stiff. Jan Gougeon has used Sitka spruce for most of his masts, but after studying a number of failures, he thinks that Sitka does not have adequate fatigue life in compression. One of my favorite woods for this type of application is redwood. Redwood can be purchased in clear, straight sections at most lumber yards (it is used for building decks), and has the additional benefit of being relatively inexpensive. After building my mast, I found that it was too limber from side to side. This was corrected by applying 2 tows of carbon fiber on each side wall below the hounds. Choosing a wood which is stiffer than redwood for the side walls would probably eliminate the need for any carbon fiber. You will need a 1" x 4" x 16' for each half.

The nose piece (narrow side of the "L") and the triangular corner strips should be made of a hardwood with lots of stiffness. A good choice would be ash, birch, or possibly hickory, depending on what is available locally. If 16' long pieces are hard to find, short pieces can be scarfed together (use 12:1 scarfs on any pieces going into the mast). A 1" x 6" x 16' will be adequate for the nose pieces, corner strips, and the small strips ahead of the luff tube.

Fig 2 shows an exploded view of a mast half with dimensions on all of the pieces. Note that all pieces are 1/2" thick except for the triangular corner blocks. Cut the corner blocks from the ash (3/4" thick), then have all the remaining wood planed to 1/2". It is important to cut the pieces to the exact dimensions shown. These are not rough dimensions. Use a table saw with a sharp blade, and double check your setup before making every cut. Make sure the edges to be glued are square. Cut the small strips (1/4" x 1/2") from the same wood as the nose piece.


 

Building the mast halves

Glue the nose and side wall together as shown in fig 2. Keep the edges aligned while applying clamps (large spring clamps work great here), and remove any glue that squeezes into the comer of the joint. Be sure to keep the pieces perfectly straight while the glue hardens. Repeat the glue-up for the other half.

If there is any glue led in the corner of the "L" from the previous glue-up, remove as much glue as possible and/or put a small chamfer on the comer of the triangular strip. Glue the triangular corner pieces into place. Draw a line 0.43" from the rear edge of the glued-up sections. Glue the small (1/4" x 1/2") strip along this line (see fig 2).

This completes the build-up of the basic mast halves. If you have been careful with the cutting and gluing, then the rest of the work should go easily.

The next step is to machine the face of the halves where they glue together. If you have access to a jointer, this step goes very fast. Simply set the halves face down on the jointer and remove enough stock to just get to the comer of the small strip in the rear, and the corner of the nose block in front (see fig 3).

 

Fig. 3 INITIAL TRIM LINES

 

If you don't have access to a jointer, then use a hand plane to accomplish the same thing. A simple (but effective) trick makes this task much easier. Clamp a block of wood to the side of a hand plane so it is flush with the bottom. This block of wood should ride on one side of the mast while the other side is being cut. Remove a small amount at a time, alternating from side to side, until the desired amount of wood has been removed. The block of wood insures that both edges are true with each other.

Once both halves are planed, the they should fit together tightly for the full length of the mast. Make a drawing of the mast cross section, cut it out, and use this template to check your assembly for accuracy.

The next step is to make a groove to accept the aluminum luff tube. Using an aluminum luff tube is important since it gives the final mast adequate fore and aft stiffness, and prevents shear web failures. The tube that I have been using has an outside diameter of 5/8" and is extruded with a slot the full length. The tube is available from Sailing Specialties in Wisconsin. An alternative is to use a 5/8" diameter aluminum tube and put the slot in it with a table saw.

One way to put in the luff tube groove is with a router. Use a 5/8" core box bit to produce the groove. A router table with a fence is useful for this step. set the fence so that you can run the edge of the mast half along the fence, cutting the groove in the correct location (see fig 4). Make several cuts with the router, increasing the depth of the cut with each pass, to produce a groove which is exactly half the depth of the tube (5/16").

An alternative method is to make the groove on a table saw. This is the way that I make grooves, and is actually very simple. Clamp a fence to the table saw at an angle to the blade. Set the blade depth to half the diameter of the luff tube (5/16"). Run a scrap piece of wood through to check the setup. Change the angle of the fence until the groove is the correct width. This produces a roughly semi-circular cut, and is fast and accurate.

After cutting the luff tube groove, put the halves together to be sure of a good fit. It is acceptable to have a slight amount of clearance around the tube, but excessive slop be avoided.

The next step is to decide if tapering of the mast is desired. Most people building wood masts have been taking advantage of the rule which allows tapering above the hounds. Tapering adds a significant amount of complexity to the construction of the mast, but I think most people should be able to accomplish it without too much difficulty. Start by deciding how much taper is desired. The basic mast is about 2-1/8" thick. A good place to start is with a taper that starts at the hounds, and drops to a thickness of 1-3/4" at the tip. This is done by planing down the edges of the halves before gluing them together.

Place a pencil mark 3/8" away from the front edge of the mast wall at the tip (do this on each half). Put a pencil mark at the location of the hound. Now draw a line which runs from the 3/8" mark at the tip to a point on the edge just above the hound. This wedge shaped section must be removed from the edge. use a hand plane with a block of wood clamped to the side as previously described, and carefully plane the edge down to the pencil line. The small strips in the rear will also have to be planed down (almost to the comer of the side wall) for a good fit on the luff tube.

The next step is to cut a 45° chamfer on the front edges of the mast. Draw a line on the nose and side wall to mark the edges of the chamfer (see fig 3). Also a draw a reference line on the side wall at the point where it is tangent with the final curved surface (see fig 5). This line will help with the final fairing. A table saw can be used to perform the chamber, but a hand plane does the best job of cleaning up down to the line. If the mast is tapered, then the size of this chamfer will be reduced where it is tapered, and you might not be able to use the table saw for the full length of the mast.

 

Final assembly

Blocks of wood should be used to close the base and the tip, and the tip block also contains the halyard. William B. Sarns Co. sells a halyard with a tube which works well. A better alternative is to use a sheave or small exit block at the tip for the halyard, since it makes hoisting the sail easier. Some people glue the blocks into the base and tip and install the halyard at the same time as the halves are glued together. I prefer to leave these out until after the mast has been glued up.

The hounds can either be mounted internally, or can be mounted on the outside of the mast External hounds are recommended for the first time builder, since it simplifies the building procedure.

So far so good? Then glue it together!

Prepare the luff tube for gluing. Here are two methods which seem to work:

1. Use an etching system to prepare the surface of the tube for gluing. Coat with epoxy.

2. Sand the outside of the tube with coarse sandpaper to remove any oxidation and dirt. Brush a coat of epoxy on the outside of the tube, and sand the epoxy into the tube (sort of like wet sanding).

Mix up a batch of epoxy (using slow hardener can give you a few extra minutes for the critical alignment). Brush a coat of epoxy on all glue surfaces. Also coat the inside of the mast with epoxy to seal out moisture. Mix up another batch of epoxy, and thicken with microfibers. Coat all glue surfaces with this mixture.

Place the luff tube in the groove with the slot towards the back of the mast. Place the other mast half on top and put several spring clamps along the mast. put the clamps on the mast with the pressure applied in the middle of the side walls.

Set the mast with the tube up, and make sure that the tube is in the correct position (the slot in the tube must be centered at the back of the mast). Once you are satisfied that the tube is aligned properly, add additional spring clamps to the mast. A clamp every 12" should be adequate.

If enough glue has been used, there will be a bead of excess glue along the entire length of the mast. If it appears that the joints are glue-starved, it would be a good idea to open the mast back up and apply more glue.

Sight along the slot in the tube, and make sure that the mast is perfectly straight. A string pulled tight above the slot can help with the alignment, but you can usually get it straight by eye. This step is very critical. A slight curve in the mast will make the mast favor one tack, and can cause the mast to counter-rotate (rotate onto the wrong tack) in gusty winds.

If any excess glue has gotten into the luff tube, remove it before it hardens. An easy way to do this is to tie a piece of rag to a string and pull it down the tube from one end to the other.

 

The final touches

The mast must be shaped to generate the final curved surface. Using a hand plane, remove material from each corner until you get close to the final shape. Look carefully at fig 5 to determine where wood must be removed. Once the corners have been planed down the mast should have a fair shape. Continue to remove material from the high spots and the curve win start to develop. Use a sanding block for the final fairing. It is helpful to make a template of the final shape from the drawing, and use the template to check the mast while you are shaping.

Cut an opening for the halyard exit in the front of the mast. The location for this should be as low as practical in order to avoid a stress concentration in the critical area of the mast (about 5' up from the base). A good location is 2' from the base, but the location is frequently determined by the length of the halyard. The exit hole should be at an angle towards the tip of the mast so the halyard can exit easily.

Cut an opening in the luff tube for the sail entry. I think the easiest entry (both easy to make and easy to get the sail into) is the angled entry shown in fig 6. The opening should be close to the base so the boom jaws are above the opening while sailing. Be sure to file or sand all sharp edges after cutting the tube.

 

Depending on the type of wood used for the mast, it may be desirable to cover the entire mast in fiberglass cloth. In any case, I recommend putting one or two layers of 10oz glass cloth at the hounds, and in the area where the boom jaws hit.

There are many ways of mounting the hounds, installing the halyard, and for putting a socket on the bottom of the mast. I won't go into any detail on these steps, but looking at other wood masts can give you some ideas.

If there are any questions about this mast construction, feel free to call, or write a letter if time permits. While the mast drawing can be generated from the dimensions provided in this article, I can supply full scale drawings of the mast, just send a self-addressed, stamped envelope.

Measuring Mast Stiffness

by Paul Goodwin - US4061 - September 1990
Revised February 2010 - Paul Goodwin

This article will explain how to measure mast stiffness, how to plot the stiffness, and provides the stiffness of several different masts that the author has tested. In addition, a target stiffness is recommended for people that are not sure how to tune their wood mast.

The critical area of stiffness is below the hounds, since most bending occurs midway between the hounds and base. When the mainsheet is first pulled in (on the starting line), the mast has a smooth bend along its full length. This bend is primarily caused by leech tension in the sail. As the boat picks up speed, increasing pressure on the sail causes the tension in the sidestay and forestay to increase. The rigging tension pulls down on the hounds, forcing the mast to bend be- cause of the increased compression. As the mast bend increases, the distance between the tip of the mast and the back of the boom decreases, which reduces the leech tension. As the boat approaches top speed, the tip of the mast actually straightens out, and almost all of the bend is below the hounds. Look carefully at a picture of a DN under sail to see this effect (the May, 1990 Newsletter has some good examples).

Since the mast is bending due to compression, small changes in stiffness can have a large effect on how easily the mast bends in puffs. This is similar to measuring batten stiffness by pushing the batten down on a bathroom scale. Once the batten is out of column, it continues to bend with very little increase in load.

The article by John R. Jombock on plank bend measurements (Demystifying Plank Stiffness - January 1990 Newsletter) suggests using a single weight to determine the stiffness of a plank. While this is technically correct, use at least three weights. This gives four data points, allowing a plot of load vs. deflection. Calculating stiffness from this plot provides greater accuracy than a single point measurement.

You will need three calibrated (accurately measured) weights of approximately 40 lb each, and a dial indicator for measuring deflection. Barbell weights work well, and are available at most sporting goods stores. A dial indicator has the precision required for accurate results, and tool stores sell them for a reasonable price (frequently less than $20). A dial indicator which has a range of 1" can make measurements of over 1" by resetting it after each 1" of travel. many" different styles of base are available for holding the dial indicator.

Make both side to side measurements and fore/aft measurements. Use 11' between supports to find the stiffness in the critical area below the hounds. You should also make measurements with 15' between supports, which will help determine how limber the tip is. This gives four stiffness curves for each mast, and allows for reasonable accuracy when comparing masts.

The supports for the mast must be very strong so there is no movement when adding weights. Sawhorses make good supports, but be sure they do not rock or shift when the weights are added. Putting a roller under one end to eliminate any friction will help in getting consistent, accurate results.

To measure the stiffness below the hounds, set the inside edges of the supports 11 ' apart (see figure 1).

Set the mast on the supports, about 2" from the base of the mast, clamp the mast to one of the" supports so it does not rotate when the weights are added (see figure 2).

Hang the weights halfway between the supports. Set the dial indicator over the middle of the mast, with the tip close to the point where the weights are applied. Be sure to put any clamps or fixtures used for hanging the weights on the mast before setting the dial indicator.

 

Making the measurements

Set the dial indicator to "0". (This is the first data point to lbs, 0 inches). Add one weight to the mast. Write down the weight and the reading from the dial indicator. Add another weight and write down the new reading. If you run out of travel on the dial indicator, reset it to "0" before adding more weight. Keep making measurements until all of the weights have been added. It should not be necessary to use more than 120 Lbs if your measurements are accurate.

Plot the results on graph paper as shown in figure 3a. If your measurements have been perfect, then a straight line will pass through all of the points. If the points are not on a straight line, then check your measurements. If the points fall above and below a straight line, then check the weights for accuracy, and use extra care when reading the dial indicator. If the points are on a curve, then check the supports for movement or bending when adding the weights.



Figure 3a - Measurements at 11 feet

Once the accuracy of your measurements are satisfactory, repeat in the fore/aft direction. Then move the supports to 15', and run another set of deflection measurements (see figure 3b). Keep two charts, one with the 11' measurements, and the other with the 15' measurements. Make comparisons by adding the plots from different masts.



Figure 3b - Measurements at 15 feet

Figures 3a and 3b show plots of the mast deflection taken on a Kenyon 2040 wing mast, a Norton wing, and on a wood mast. The Norton is one of the softer masts (from 1986). Table 1 shows the spring rate derived from these.curves (as suggested by John Jombock).

The measurements in figures 3a and 3b were made with 20 lb weights. Notice how close all of the measurement points are to a straight line. No mast starts out limber and becomes stiff, or vice versa (except in the case of a mast with a wood stick inside for support). So look for measurement errors if you plot your results and the points do not fall on a straight line.

 

Comparing different masts

When looking at figurea 3a and 3b, the lower a line is on the graph, the stiffer the mast is. This rule shows that the Kenyon mast is stiffer than the Norton on all curves. Also, all of the masts are stiffer fore and aft than side to side.

Looking at the plots more carefully, the side to side curves show the effect of tapering above the hound. In the top graph (measured below the hounds), the increase in stiffness between the wood mast, the Norton, and the Kenyon is almost equal. In the bottom graph (measured from tip to base), the difference between the wood mast and the Norton is much greater than the difference between the Norton and the Kenyon. This is because the tip of the wood mast is very soft.

People building wood masts can use these methods to compare the stiffness of their own masts. The wood mast in figure 3 is a mast which has been very fast, but is quite limber. A mast this flexible can be difficult to sail in light wind, and requires sheeting out in heavy air to prevent over bending.

If you have never sailed a wood mast before, then try using a stiffness close to the Norton wing below the hounds. If you're comfortable with a softer mast, then try a stiffness similar to the wood mast in figures 3a and 3b.

Many wood masts which have been going fast are stiff fore and aft (stiffer than a Norton wing), and limber side to side (5 - 10% more limber than a Norton wing). The stiffness in the tapered section above the hounds has been hard to understand, with masts going fast with either a stiff or flexible tip.

One problem with a soft, tapered mast tip is that it reduces leech tension. This flattens the head of the sail and reduces power, particularly at low speeds- to compensate for this, many people use sails with more luff curve above the hounds . A wood mast could have no taper and the same stiffness as the Norton wing. This mast should sail like a Norton, but not break so easily.

Notice that all of the masts are at least 3 times as stiff fore and aft as they are side to side. If a mast is not stiff enough fore and aft, then it will not rotate very well when tacking. This can be demonstrated by sheeting the sail in (not to tight) and rotating the mast. A good mast will snap back and forth from tack to tack. A mast without enough fore and aft stiffness will not snap across, and will be more likely to rotate to the wrong tack in puffs.

One note of caution when tuning wood masts. Adding carbon fiber tows on the outside of a mast provides a fast, easy, and inexpensive way to increase stiffness. However, the stiffness will increase for several days after applying the carbon fiber. Therefore, add enough carbon fiber to bring the stiffness to a level slightly less than desired. Then make the final stiffness measurements after allowing a week or more for full curing of the epoxy.

This article answers some of the questions that sailors have asked about measuring mast stiffness. It should also encourage more people to make accurate measurements of mast stiffness. The techniques described here may sound too technical and complex, but the results are worth the effort. After completing the initial setup, stiffness can be measured in about 1/2 hour per mast. Transferring the urements onto a graph requires about 15 minutes, and provides a permanent record of mast stiffness.