Shop Tips & Tricks #2: Dividing a line or space into equal parts

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Blog entry by GnarlyErik posted 12-04-2012 06:43 PM 36201 reads 9 times favorited 19 comments Add to Favorites Watch
« Part 1: Shop Tips Part 2 of Shop Tips & Tricks series Part 3: “Declivity” - a trick for dealing with things out of level or plumb »

I’ve heard people say ‘What good are learning things in school if you don’t use them?’. After my lifetime of careers, I realize you never know what may be useful to you later. One of my most useful high school classes for example, was a one-semester class in typing – a ‘fill-in’ course – way back when they taught ‘Typing’. I guess it is called ‘keyboarding’ if anything similar is even taught today.

Geometry was another, which at the time I studied it I could see no practical use for. But, as it turns out I used geometry in practice a great deal in my boatbuilding career, particularly in layout work. Thank goodness, enough of it stuck over the years to be very useful.

Case in point: Dividing a Line (or space) into equal parts;

Sometimes you need to divide a line, space or length into a number of smaller parts, all equal to each other. For example, suppose you have a space or piece of material 27-7/8” long which you need to divide into five equal parts. You can do this mathematically and end up with each part needing to be exactly 5.575” long. But, now you must convert this into English measurement equivalents (inches and fractions) and then try to measure these divisions with your ruler or scale. This is very hard to do accurately. Oddly, many common dimensions are hard to divide using English measures – for example 24” by 5 or 7 (4.8” & approx. 3.43”).

(And, let me say right here as an aside, that even though I have lived with English measurements all my life, things are much easier using metrics! Among all nations, only Burma, Sierra Leone and the US have never officially adopted the international metric system . . . ! That being said, you CAN buy ‘engineers’ scales which divide English measurements into tenths, but few besides engineers and machinists use those.)

But, back to my example; in three steps, you can quickly and easily divide any length by any number of divisions graphically using simple geometry. For those who are not familiar with this, I present it to you here:

1. Establish (draw) lines at either end of your length or space to be divided, and at right angles (perpendicular) to it. See image #1.

(Image 1 – space to be divided)

2. Place your ruler or scale between the two perpendiculars and angle it until you have a measurement you can easily divide by the number of equal spaces needed. For Instance, in the example above, you can easily use 30” and divide that by 5” increments to get 6 equal divisions. (Or, by 7-1/2” increments for four divisions, by 5” for six divisions, by 1” for thirty divisions and so on.) See image #2 & #3.

(Image 2 – make tic marks at dividing points)

(Image 3 – tic mark at each dividing point)

3. Mark these points (I always use a short line against the scale with a tic mark at the point for accuracy), and lines drawn at right angles from the original line (or space), through your points gives the equal divisions. Saw cuts centered on these lines will make exactly equal parts if you are sufficiently careful. See image #4:

(Image 4 – draw perpendicular from each point back to line or space being divided)

If you need to divide a plank into a number of equal widths, you may use either edge of the plank as your ‘perpendiculars’. Slant your scale and divide across from edge to edge at each end of your plank, and lines drawn with a straight edge through matching points will divide the plank into equal widths. A little thought shows you can even lay out a number of exactly equal tapers by adjusting the length of one end point! See image #5.

(Image 5 – laying scale at an angle between lines to make the divisions equal)

Hopefully, the pictures provided describe this well enough to understand.

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

19 comments so far

View HillbillyShooter's profile


5811 posts in 3378 days

#1 posted 12-04-2012 07:07 PM

Thanks for the reminder on geometry.

You’re 100 % about not knowing what will and what will not be of benefit in school and life. I agree about both typing and geometry. Another “pud” course that I had to take in college, but which has helped immensely in life was “Business Correspondence”. The first thing that course taught was to never start a paragraph with “I” as it showed a totally egotistical bent—it’s amazing how many people start every paragraph with I.

Another contribution for my “Favorites” file.

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

View Grumpymike's profile


2480 posts in 3401 days

#2 posted 12-04-2012 07:08 PM

Hmmmm, need to ponder this for a bit.

-- Grumpy old guy, and lookin' good Doin' it. ... Surprise Az.

View a1Jim's profile


118162 posts in 4663 days

#3 posted 12-04-2012 07:12 PM

This is a trick I learned a long time ago and have been passing it on to my students. Thanks for passing it on here Erik I’m sure many folks have not seen this before.


View hoosier0311's profile


706 posts in 3112 days

#4 posted 12-04-2012 08:21 PM

This musta been one of those lessons I failed to pay attention to. This can be a big time saver, thanks for passing this tip on!

-- atta boy Clarence!

View FirehouseWoodworking's profile


787 posts in 4359 days

#5 posted 12-04-2012 10:10 PM

Well done tutorial. I learned to do this years ago but no one sat down and gave me that much detail! Thank you!


-- Dave; Lansing, Kansas

View Dennisgrosen's profile


10880 posts in 4201 days

#6 posted 12-04-2012 10:28 PM

one I learned from an old master way back in the stone age :-)
but one I havn´t thought about was the last one with tappered widts
keep let them come :-)


View littlecope's profile


3121 posts in 4588 days

#7 posted 12-05-2012 01:22 AM

Great Trick Eric!!
When I use that method for marking finger joints, or halving a board, I found it to be most accurate if the marks are made parallel to the boards edge from the divisions… :) Tilted Ruler Marking

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

View patron's profile


13720 posts in 4427 days

#8 posted 12-05-2012 01:42 AM

it always amazes me
how many building pro’s
don’t know some of the other trades secrets
and the tools they use

from architecture
boat building
glass cutting
to name a few

that t square they sell for sheetrock
can be up to 1/4” off at 4’
the glass guys have a square that is right on
with one side lower to hook on an edge

well and boat builders
have countless ways to do something
like make bulkheads
with a story board
and a taper or squiggly ‘arm’

keep ‘em coming eric
we can all learn
and thank you

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View shipwright's profile (online now)


8717 posts in 3884 days

#9 posted 12-05-2012 04:49 AM

Good one Eric.
Have you ever used a “Texas rule” to divide topside planking equally as it tapers both ways from max girth?

-- 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 3220 days

#10 posted 12-05-2012 03:30 PM

Patron – Most scratch or one-off boat builders have or build a basic set of lay out tools for ‘lofting’ (full size lay out of the boats ‘lines’). This includes shop-built three-sided squares, long straight edges and several sorts of specially made long ‘lofting battens’ up to 20’ long or more. Some battens will be thicker, rectangular in section and stiffer, others squarer in section, more springy, and be tapered towards the ends where they need to handle tighter curves.

Lofting a boat from plans is an art in itself, requiring a range of skills and patience, not to mention flexibility and a strong back. You haven’t really done lay out work until you try lofting, making patterns for, and then accurately building a curved, strongly raked transom for example! That will exercise the range of all your drafting, geometric, patience, and accuracy skills!

And yes Shipwright, I believe your ‘Texas Rule’ is what I call a ‘planking gauge’, made specifically for each boat size or model. The concept is somewhat like dividing with a ruler, only specific to a particular boat and more specialized.

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

View shipwright's profile (online now)


8717 posts in 3884 days

#11 posted 12-05-2012 04:26 PM

I think we’re talking about the same thing Eric. I guess it got the “Texas” tag because it’s big. .... Just a big ruler marked off in “inches” that are equal to (the number of planks to be divided) X 1”. The last “inch” being divided into eighths.
I’m enjoying your posts, very familiar stuff.


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

View patron's profile


13720 posts in 4427 days

#12 posted 12-05-2012 06:22 PM

right you are eric

i worked the boats for 10 years in ft lauderdale mostly with cuban boatbuilders
what i learned from them is invaluable all the old hands on ways
from replacing ribs and keel sections to planking and garboard rabbet

i worked as the lofting guy for a fiberglass company for about a year too
that made 90’ to 100’ custom cabin cruisers
we had a loft floor 120’ long by 40’ wide covered in formica
the ships were hard chinned and made in panels and winched down into cradles
that bent them to profile where the edges were ground to a taper and stitched together
with copper wire then bonded with wider and wider strips of cloth

i made a lofting batten by ripping 16’ pine into 3/4”x3/4” and a jig to taper the scarfs on the ends
glued them together and had a strip 100’ long
then with the help of 6 guys ran it thru the planer to 5/8” square
and we fed it up to the loft floor where i did the layout lines
and placed various smooth ‘plugs’ for shafts and water intake valves
and masking tape and plastic (visquene) on the edges to the wall of the loft
leaving just the shape of the part exposed
the whole thing was then sprayed with release and them sprayed with gel coat
and and the glassing would begin
with 20 guys and roller mops and a carriage that help the cloth rolling over the floor
layer after layer of different cloth types till it was 1/2” thick
guys rolling out air bubbles till you could see clearly thru it
each layer with a different hardener time so all would kick at the same time later
after it was cured i would go in and lay out balsa core (end grain) where needed
and then the whole process would start again till the final thickness was reached

the interesting thing was a sheet of visquene laid over the whole thing
with some 1/2” pvc tubes laid to create air channels for hard to reach areas
then we would cut holes in the plastic here and there and masking tape vacuum hoses (regular shop vacs)
and turn them on and watch the plastic suck down and all the remaining air
work its way out (air pressure is 14 1/2 LBS square inch at sea level)

like paul
i don’t like fiberglass either but i did make lots of ‘plugs’ for molds to be made for multi parts
for production boat company’s

this is a great blog series
pardon if i rattle on sometimes
but what you are doing is very helpful to many here
we don’t take anything away by sharing
so far nobody has ever taken my job from me
regardless of how much i help them learn

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View GnarlyErik's profile


344 posts in 3220 days

#13 posted 12-05-2012 08:01 PM

Right you are Patron. Sounds like you may have been around the Rybovich operation, or Bertram’s? I am old enough to have known Dick Bertram when he was just a yacht bum in the ‘fifties, and once had an employee who had worked for Rybovich for a spell – and well qualified he was too!

My boatbuilding began in my father’s shop in Maine, and from the time I was first able to swing a hammer or wield a broom (around 12 years old). Later, I had two boatbuilding shops of my own – couldn’t be a shop flunky for a fussy dad forever – but he sure knew how to do things right. I had a shop in Midcoast Maine, then in eastern NC, and ended up with a yard in Alaska for years. I’ve done only one-offs throughout my career. When I finally got too old and creaky I got in the brokerage end of things for about ten years and finally sold that and retired about four years ago. Now, I just piddle around in my shop in a warmer climate when and as I feel like it! But, I miss full-scale boatbuilding for sure.

In Maine, the local high school was kind enough to allow my shop to use their gymnasium floor once to loft a 92 footer – at night only! We indexed and lofted on plywood panels which we took up and stored during the day. We build a 92 footer in a 70 foot shop by adding on a 40 foot ‘temporary’ extension, which stood up through three winters!

I never got into fiberglass beyond some repair work (thank God!), but have done some work finishing out steel yachts and workboats. I have done some cold-moulding and vacuum forming too, but with wood laminates only. I’ve used vacuum bagging to build cold-moulded rudders, center boards and dagger boards primarily. My older Maine workers called fiberglass ‘frozen snot’ and epoxy used in cold moulding ‘smegg’. Pretty much the same thing on the West Coast too for that matter.

The old skills are being gradually lost, which is one of the reasons I have begun posting a few things, and writing things up before I get too old to remember them. Case in point, when I had a yard in Maine, I would sometimes lose shipwrights to points south, primarily to MA and RI because they paid at least double the going rates in Maine. But, I also had the so-called ‘builders’ come to me for building their plugs which they no longer had the skills to loft and build anymore – they had gotten 100% into FG and their old-timers had gradually died off or left. Maine and the Pacific NW are still the two main bastions for traditional boatbuilding today, and even there the skills are slowly eroding and disappearing. Maine for yachts, and the Pacific NW for working vessels. Ah well, nothing is forever they say.

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

View patron's profile


13720 posts in 4427 days

#14 posted 12-05-2012 09:37 PM

well as you know
sailors are a tight bunch
so sail was not to often or rewarding

i worked the stink pot mostly
as that is where the rich guys hang

knew some guys at rybovich
and got farmed out to bertram from time to time

but worked the lift yards for repairs
and ran some glass offshore places too
doing the interiors and trims
(don’t you know they all wanted ‘secret compartments’)
then the moron captains would take a 1 million $ boat
and go to the bars at night to pick up chicks
and get busted with coke
and lose the boats before they even used most of them

we moved one shop to a bigger place one time
and two days later the cartels sent some colombians to the old place
and machine gunned it to smithereens
glad i wasn’t there lol

the cubans were great to learn from
(i speak fluent spanish)
they taught me the old ways
and i taught them modern tools
back in cuba all they had was table saws and band saws

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View shipwright's profile (online now)


8717 posts in 3884 days

#15 posted 12-05-2012 10:48 PM

Wood boat building, for commercial boats is all but gone now on the west coast of Canada. There are lots of boats left around and lots of work in the repair side, but many of those doing the repairs have never built one from scratch and don’t have the foundation of knowledge required to do the repairs “right”...................They have no problem charging top rate though. Maybe it’s just sour grapes but there don’t seem to be many people left who can get it done and done well. There are a few schools but they seem to be oriented toward yachts and small boats.
I was lucky enough to get in on the last of the big commercial boat building in wood in the late sixties, seiners and trollers mostly around my end of the world. I learned from some wonderful people who were some of the most intelligent I’ve ever met although their educations mostly ended before high school.
This is a great blog.
Keep on writing.

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

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