Simple Spar Gauge

A spar gauge is a tool used mainly in boat building to mark out square or rectangular stock in order to turn it into an octagon. Once it is an octagon it can easily be made into a cylinder. The beauty of the spar gauge is that you can use it even if the wood is tapered, like the mast of a sailboat, or sections of a canoe paddle.

It is very simple and you only need a few things to make it, just some scrap wood, dowels and a couple of nails.

Don't forget to check out the video above to see it being made and if you have any questions, please feel free to ask in the comments.

Below are links to tools and materials I used in this article. It is either the exact tool/supply or something very close.

- Scrap piece of wood. - I used a piece of cherry ~ 5" long by 1" wide and 1" high

- 1/4 dowel ~ 4"

- 1/4" forstner drill bit

- 2x 2" finishing nails

- Combination square

- Drill press

- Hammer

- CA Glue

Note: The links in this article are affiliate links, meaning, at no additional cost to you, I may earn a commission if you click through and make a purchase.

First I marked a the center line in the scrap wood. I used a combination square, but there are many different methods for marking the center of wood, so pick your favourite!

Then I went over to the drill press and drilled two holes the same size as my dowel (in this case 1/4"). I only drilled about 2/3 of the way through the wood.

The thing that makes a spar gauge work, is the ratio between the edge guides and the marking points. It is in a ratio of 7:10:7*. Because the gauge works based on a ratio, the distance between the holes for the outer edge guides doesn’t need to measured ahead of time.

*Technically the ratio is 7.07:10:7.07, but that makes the math harder in the next steps, so often it is simplified.

I took the measurement between the edge guides, in this case 4 1/2". I then whipped out the trusty phone calculator and divided it 24 (I use 24, because 7+10+7 = 24).

4.5 / 24 = 0.1875 (or 3/16")

Lastly I multiplied the result by 7 and I get the distance that the marking points need to be from the edge guides.

0.1875 * 7 = 1.3125 (or 1 5/16")

I then measured and marked out where the marking nails will be placed.

The formula for this is:

X=(A/24)*7

A= The distance between outer edge guide holes

X= distance from outer edge guides that marker nails need to be placed.

I went back to the drill press and drilled a small pilot hole for the nails. These holes went all the way through the wood.

I then used my hammer to drive in the marking nails. I made sure that they were going in the backside, which is indicated because it does not have any holes for the out edge guides.

If you want to make this a bit fancier, you could make the holes bigger and put pencils in the holes. I always seem to be losing my pencils and they are never sharp, so I though nails would be better in my shop.

I cut the dowel using my bandsaw, but any type of saw would work. I didn't actually measure the dowel, I just made two equal pieces that would be about 3/4" longer than the nails.

I then used CA glue (super glue) to attach the dowel. I sprayed the accelerator on the dowel and dropped some CA glue into the hole. I then inserted the dowel and a few seconds later it was rock solid.

I used CA glue here because it sped up the process. You could also use wood glue, but then you would need to clamp it until the glue cured.

To use the spar gauge you place the gauge on top of your work piece and angle it until the outer edge guides touch the work piece. Then you run the gauge down your work piece, ensuring that it does not lift from either side. You do the same process on all sides of the work piece.

If you have trouble seeing the lines you can use a pencil to darken them. The pencil should easily stay within the mark left by the nails.

I used a hand plane to remove material on the edges of the work piece. (You could also use a spoke shave for this step, but my spoke shave needed to be sharpened) I kept removing material until I reached the lines made by the spar gauge.

In the end I take some square stock and turn it into an octagon. Now that it is an octagon, it is much closer to being completely round. From this point it is very easy to make it round just by eye, or you can leave it an octagon if you want.

I hope you enjoyed this project. I will be using this to make some canoe paddles in the future. If you liked it, you may want to see more from me. Feel free to follow me on other social media.

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If you make a spar gauge I would love to know the project you are going to use it on. Let me know in the comments below.