Tom's Rules of Thumb

Tom Whitfield has been my long distance mentor ever since I began building and writing about building boats. He lives as far away as possible, but served his apprenticeship in an old Devon yard, long since gone, a victim of changing ways and the advent of glassfibre. But he forged a new life in Australia, building and designing boats, including some of the country's best loved Tall Ships.

He sends me critiques of my boats, which are always to the point: he spares me nothing. Whereas most people (bless them) go "ooh, aah, how lovely" he'll point out a slight discrepancy in the third land down, near the bow, or a flaw in the timber which he reckons might lead to a problem down the line.

I'll post more of his wisdom as the months go by, but here's his Rules of Thumb for wooden boat building to be getting on with.

RULES OF THUMB.

There are a number of guides to building a boat I have used over the years.

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.

Working oars - the blade is about 1/3 the oars length.

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. 

Tom Whitfield.