# RULES OF THUMB

## Dimension of the mast

"An old general rule for lower mast diameter was 7/8 inch for every foot of beam; another was 7/32 inch for every foot of mast from deck to head of mast."

## Length of boat nails for planking

The penny (size) of a nail used to fasten a plank on a wooden boat is one penny for every eight of an inch of thickness of the plank. For softwoods, add two pennies.

## Boat builders share an old rule of thumb for sailboats.

The maximum speed K in knots is 1.35 times the square root of the length L in feet of the boat's waterline.

## How long before sunset?

With your arm outstretched, hold your thumb sideways. Each thumb width equals 20 minutes of sunlight measured from the horizon. In other words, if the sun is two thumbwidths above the horizon, you've got about 40 minutes until sunset.

## Beveling of planks for caulking

"the outer edge should be beveled 1/16 in 1 inch, beveling from the center of the plank." A 2 inch plank would be beveled 1/8 inch...

## Fitting oars to a boat

Measure boat width between oarlocks in inches.
Take 1/2 that measurement and add 2 inches; then divide by 7.
Multiply that result by 25.
Divide that result by 12 to get lenght in feet -- but round answer to nearest 6 inches.

## Time to steam lumber?

One inch of thickness = one hour in the steam box.
The boatbuilders at St. John's Prep School in Danvers MA came up with this one.

## Placing oar leathers

Leverage ratio= 7:18
Multiply oar length in inches by 7.
Divide that product by 25 for the center of the leather location measured in inches from the end of grip.

## BEVEL ANGLES ON TOOLS

15 to 20.........Paring chisels, skew chisels, low angle planes for softwoods, skew blade planes.
20 to 25.........All of the above tools (except skews) to be used for hardwood or end grain use.
25 to 30.........Chisels used for both paring and light mortising, firmer chisels for soft wood, most plane blades and spokeshave blades.
30 to 35.........Mortice chisels, firmer chisels for hardwood, plane blades for hardwood with pin knots.
35 to 40.........Mortice chisels for heavy use, particularly any with brittle steel.

## Adlard Coles has a blog called The Trouble with Old Boats.

He credits Tom Whitfield with these jewels:
Clinker planking for a 12ft dinghy about 3/8in thick lap is 3/4in. 2 to 1
Plank width no more than 5in plus the 3/4in lap narrower is better.
Nail spacing to be six times the plank thickness. 6 to 1
Scarphs to be six times the plank thickness. 6 to 1
Outer end of scarph to have a butt about 1/16in on 3/8in planking. 6 to 1
Ribs to be at every second or third nail spacing.
Plank joints at least three plank or three frame spaces apart.
Ply scarphs to be slash cut and glued. 8 to 1
Mast or spar scarfs to be about 10 to 1 or better still 12 to 1
Rowing seat height no less than 10in above the floor.
Seat riser 7in down from sheer + 1in for the thwart.
Rowlock to be centered 12 ½in aft of the back edge of the thwart.
Oars need to be 1 ½ or 2 times the beam of the boat.
Racing oars - the blade is about ¼ to 1/6 the oars length.
Beam/length about 3 to one or 4 to 1 is normal.
I have built good boats with a 2 to 1 ratio.
Some dinghy designs are as wide as 1 to 1 [eg 6ft long x 6ft wide. Anything is possible.]
Cornish gigs are about 5 1/3 to 1.
Racing eights are 12 to 1.
Designer Pete Culler's stems in light craft were 2 times the plank thickness plus the fastening [¼in] plus 1/8in.
Rivets to have a rove 1¼ the head size of the nail. Allow a bit less than the square of the nail projecting through the rove to allow riveting.

References:
The Gaff Rig Handbook: History, Design, Techniques, Developments by John Leather

The Art of Rigging by George Biddlecombe

A practical course in wooden boat and ship building, the fundamental principles and practical methods described in detail,
especially written for carpenters and other woodworkers who desire to engage in boat or ship building,
and as a textbook for schools.

by Richard Montgomery Van Gaasbeek

Gavin Atkins. Online: "TheRules. What the authorities say should be guiding principles of boat design"

Boatbuilding: a complete handbook of wooden boat construction By Howard Irving Chapelle