Shop Tips & Tricks #4: Centerlines - Finding, Marking and Using Them! (Part One)

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Blog entry by GnarlyErik posted 12-06-2012 09:35 PM 10137 reads 8 times favorited 8 comments Add to Favorites Watch
« Part 3: “Declivity” - a trick for dealing with things out of level or plumb Part 4 of Shop Tips & Tricks series Part 5: Centerlines - Finding, Marking and Using Them! (Part Two) »

When working with non-squared and oddly shaped things – like boats, you generally need starting and reference points. The most convenient usually being from the “centers” of something. One general approach is to ‘work from the centers out’, which seems to work well overall too most times. 

For a simple example, suppose you must install a seat in a boat (or, a seat bottom in a chair). You may know the seat width or depth, but ends are not squared to that. Working from centerlines is the handiest and most accurate way to layout your stock, and will best insure your end result is symmetrical. See image #1:

Image 1 – Finding centerline with try square and intersecting arcs

It is usually easy to find the center in a least one direction – simply use your ruler – or divide your dimension graphically as in my post on dividing into equal parts: (

A try square may be used to draw 45 degree angles from the intersections (Image #1). Or, for relatively small dimensions, you may use a compass to swing equal arcs from the endpoints of a common line, which will intersect at your center line. (Image #1).

And here is a little “Gee Whiz” multi-tool you can make yourself in a half hour or so, to find the centers of round stock. You can buy a manufactured tool similar to this called a combination square with a centering head if money is no object, and there are various cheap plastic ones available too. Machinists and lathe using folks use centering heads. (Image #2, #3 & #4).

Image #2 – Layout of Gee Whiz

And, here is how it is used:

Finding center for round stock

Finding center for round stock – mark from two or more locations on end of stock and where lines intersect is the center.

But, this shop made multi-use version can also find and mark the center on long stock like planks (NOT a tapered planks!). In use, this Gee Whiz is slid along the plank with the pins held in contact with either edge of your plank, and a pencil inserted in the centering hole will mark the centerline down the length of the plank, except very near the ends – a straight edge can extend those.

The pictures should explain how this Gee Whiz is made.

Image #5 – Design of plank center finding side of Gee Whiz

If you make one, be very precise to locate your center hole EXACTLY CENTERED between the two pins since any error will be multiplied by two. The pins themselves are screws with the heads cut off and the ends filed for safety, and small bolts are even better. Nails will work too, except they tend to quickly get wobbly in use. You will notice a bit of a countersink at each hole in the pictures. That is so a tapered pencil point can be easily placed in the hole. Just be careful the countersink stops about an 1/8” from the marking side so the hole doesn’t wallow out, and in use, be sure to hold your pencil as nearly vertical as possible when marking. If you paint this ‘bright’ it can be easily found on your workbench or shop wall. Once you try one, you may use it more than you think! See image #6:

Image #5 – Finding & Marking center of length of plank

The overall shape of this tool is unimportant as long as the geometry is followed. Build one to suit your fancy!

You may also notice this Gee Whiz has two extra pencil holes, which I will try to explain the reason and use for, and where they should be placed in another post. Most wooden boatbuilders will already know the reason for those.

(and, here is the reason posted in a follow-up (part two) of this topic:

I hope you find this helpful!

-- "Never let your dogma be run over by your karma!"

8 comments so far

View HillbillyShooter's profile


5811 posts in 3367 days

#1 posted 12-06-2012 11:29 PM

Thanks. Your “Gee Whiz” multi-tool goes to the top of my “to do” list. Another for my favorites.

-- John C. -- "Firearms are second only to the Constitution in importance; they are the peoples' liberty's teeth." George Washington

View shipwright's profile


8711 posts in 3873 days

#2 posted 12-07-2012 12:29 AM

Good post Eric.
Can I make a guess about maybe making a tapered octagon from a tapered square, as in spar making?

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese!

View GnarlyErik's profile


344 posts in 3209 days

#3 posted 12-07-2012 12:45 AM

Yes, I figured boat and spar folks will know the eight-siding layout/tapering scribing twitch. Obviously you are familiar with the technique, and I will post on that in a day or so.

-- "Never let your dogma be run over by your karma!"

View littlecope's profile


3121 posts in 4577 days

#4 posted 12-07-2012 02:07 AM

Thoroughly enjoying your Blog Series Eric.
Please, keep them coming!! And Thank You!!

-- Mike in Concord, NH---Unpleasant tasks are simply worthy challenges to improve skills.

View RaggedKerf's profile


425 posts in 3195 days

#5 posted 12-07-2012 02:31 AM

Okay, mind officially blown. Awesome. I’ll be making a Gee Whiz very soon! Thanks so much for these tips, it’s eye opening to say the least!

-- Steve

View eddie's profile


8565 posts in 3688 days

#6 posted 12-07-2012 03:13 AM

this is plowing pretty deep ,quit interesting

-- Jesus Is Alright with me

View lightweightladylefty's profile


3643 posts in 4787 days

#7 posted 12-07-2012 04:21 AM


It is so nice to come to “school” here and not get charged a fortune! How generous of you to share your expertise free. I’ll need to take time off from work in the shop to make all your wonderful work aids. Ultimately it should speed up my work in the shop, but as slow as I am at making jigs and fixtures, it could be negligible. ;-)

Thanks so much for sharing.


-- “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin -- Jesus is the ONLY reason for ANY season.

View Dave's profile


11435 posts in 3915 days

#8 posted 12-19-2012 04:34 PM

I love it!

-- Superdav "No matter where you go - there you are."

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