A sailboat needs a fin under the hull, a centerboard or leeboard extending 40-60 cm below it, to resist the sideways push of the wind (leeway) so that the sail’s force becomes forward motion, and a rudder and tiller at the stern to steer. Without the board the boat slides sideways instead of sailing upwind; without a rudder it cannot be held on course, and the two must be placed so the boat has a slight tendency to turn into the wind, not away from it.
Part of Small Sailboat Construction
The centerboard resists sideways drift and the rudder steers the boat; together they turn the force of the sail into controlled movement.
A Sail Pushes Sideways First And Forward Second
When a boat sails across or into the wind, most of the sail force is directed sideways. A rowing hull has almost no resistance to sideways motion, so it slides downwind as it moves forward. The board adds a deep, narrow surface that grips the water: sideways motion produces a force on it that opposes the slide, and a small angle between the board and the water’s flow (leeway) lets it generate that force. Without a board the angle is large and the boat barely moves toward the wind.
The rudder acts at the other end and gives the helmsman a force to change course and hold it. The two are linked: the sail pushes at one point and the board resists at another, and where those two points lie relative to each other decides whether the boat tends to turn toward the wind or away from it. Moving the board or the mast forward or aft changes that balance.
What You Need
- A board: a plank or plate of dense wood, shaped like a streamlined fin, with a handle or line for lifting
- A slot (trunk) or leeboard fittings (see Centerboard or Leeboard)
- A rudder, a tiller and fittings (see Rudder & Tiller)
- Sealant for the trunk, because a slot in the keel is a hole in the hull (see Boat Sealing)
Method: Fit And Balance
Step 1: Choose the board type. A centreboard slides through a slot in the bottom; a leeboard hangs over the side. The carrier’s size is about a third to a half of the boat’s length and extends 40-60 cm below the hull.
Step 2: Locate the board near the middle of the hull for the first trial, and plan to move it in small steps after testing.
Step 3: Fit the rudder at the stern so that the blade is below the water at all heel angles.
Step 4: Test the balance. Sail close to the wind with the tiller released. A slight tendency for the boat to turn toward the wind is desirable, because a boat that rounds up when the helmsman lets go spills the wind; a boat that bears away does the opposite.
Step 5: Adjust. Shift the board or the mast step by small amounts and test again.
Worked Example: How Big Is The Board?
The carrier’s wording gives a length of a third to a half of the boat. For a 4.5 m boat that reads as 4.5 ÷ 3 = 1.5 m to 4.5 ÷ 2 = 2.25 m. With a depth of 0.4-0.6 m, the board’s side area is 1.5 × 0.4 = 0.6 m² to 2.25 × 0.6 = 1.35 m². Set against the 5.6 m² sail used elsewhere in this series, that is 0.6 ÷ 5.6 = 11% to 1.35 ÷ 5.6 = 24%. The carrier does not say whether “length” refers to the chord along the hull or to the part that sticks out; treat the range as a starting size, make it removable, and shorten it if the boat steers heavily.
Common Mistakes
- Cutting a slot without reinforcing the keel — the keel is the strongest timber in the hull; keep the slot short and fit a stout trunk.
- A board that cannot be raised — in shallows it hits the bottom and the boat stops violently; use a line and a pivot.
- A rudder that lifts out when the boat heels — the blade must stay deep.
Safety
A slot in the hull is a leak waiting to open
A centreboard trunk opens the bottom of the boat. Seal it carefully, keep the top of the trunk above the waterline, and check it before every outing, because a leaking trunk will swamp the boat from inside.
What’s Next
- Learn next: Centerboard or Leeboard — choosing and building the board
- Combine with: Rudder & Tiller — the steering end