The Swing Bulb Keel

Explorer has a 45.4kg (100lbs) bulb at 1.5m (5ft) and two 36.3kg (80lbs) shafts at 0.75m (2½ft) generating 1 356 Nm (1,000 ftlbs) of righting force at 45 degrees of heel.

A hollow long-keel houses the swing bulb keel below the hull. This long keel increases directional stability, prevents hull-wear when taking the ground, and provides lateral resistance when the keel is retracted, for beating in shallow waters.

The keel is retracted manually from the cockpit. The keel-haul is linked to a stainless steel wire guided by solid stainless steel rollers through the transom and under the hull to the bulb. The wire trails behind the bulb when extended, and occasionally creates a slight hum. Protected from fouling by the keel in front of it, this lifting mechanism is preferred for its simplicity, inspectability and lack of complex mechanics.

A nylon shear-pin prevents the keel folding in the event of a capsize. This shears in the event of hitting an underwater obstruction. When the pin is not fitted, a threaded bung takes its place. It is only necessary to fit the pin in strong conditions, as it is extremely difficult to capsize Explorer, even intentionally!

What is the Swing Bulb Keel?

Predicted Stability Curve of Explorer with the Swing Bulb Keel
Larger version

The Swing Bulb Keel is a retractable fin bulb keel. The fin is in two interlocking shafts, one behind the other, each with a pin in the centreboard case and in the lead bulb, forming a parallelogram. The lead bulb therefore remains horizontal at all positions.

What is so amazing about the Swing Bulb Keel?

The Swing Bulb Keel has so many advantages it is difficult to grasp them all in one go. What is interesting is that many of these advantages have a knock-on affect on other aspects of performance, so that when they are all added up, it provides considerably higher performance and stability.

What are the advantages?

• It is faster on the wind
• It is faster off the wind
• It improves self-righting
• It reduces rolling
• It enters shallow water
• It takes the ground or trailer
• It shifts the centre of gravity of the vessel — improving handling on and off the wind

The revolutionary swing bulb keel that presents so many advantages

How does it achieve these advantages?

Faster on the wind

The Swing Bulb Keel is faster on the wind simply because it is practical to have a much deeper draft than a normal fin bulb keel, as one can still get into shallow waters by retracting the keel. Having a bulb weight lower down provides greater righting force, greater power to carry sail, and therefore greater speed.

Faster off the wind

It is faster off the wind because when retracted the keel causes less drag. The bulb weight is then positioned well aft, which prevents the nose diving, and allows you to carry more sail.

Improves self-righting

It improves self-righting in lightweight craft by putting the available ballast very low down and creating a high stability-to-weight ratio.

Reduces rolling

It reduces rolling because rolling is caused by a low ballast weight behaving like a pendulum when off the wind or lying on a mooring. In these circumstances simply retract the keel, and the ballast weight is brought up to the waterline, which stops rolling.

Shallow water

It enters shallow water when retracted. The Explorer houses the retractable keel in a hollow stub keel. This provides sufficient lateral resistance to go to windward in shallow water.

Takes ground or water

It takes the ground or trailer easily with it’s low-draft when retracted.

Centre of gravity shift

It shifts the centre of gravity of the vessel from forward and low when on the wind to aft and at the waterline when off the wind, improving performance, stability and boat handling.

Shifting the Centre of Gravity of the Vessel

An outstanding characteristic of the Swing Bulb Keel is that it moves the centre of gravity of the vessel. This is from forward-and-low when extended, to aft-and-at-the-waterline when retracted. And this is exactly what is required by the dynamics of sailing to windward and downwind respectively!

Sailing to windward

When sailing to windward, the wind hits the front of the rig and pushes the backwards, causing the bow to lift. You need weight forward to keep the bow down. Lowering the Swing Bulb Keel moves the bulb weight down and forward in a quarter-arc, putting the ballast forward as required.

Sailing to windward also causes the greatest heeling forces, so you need ballast as low as possible to create maximum stability. A retractable keel means you can have much greater draft than a permanent keel, and still enter shallow water. The draft of a vertically-lifting bulb keel is restricted by the height of the cabin roof (or interfering with the rig, if it extends above the deck). The Swing Bulb Keel does not encounter these problems, and can have a much greater draft. Explorer’s draught/length ratio of 1:3 is unheard of except in model yachts.

Sailing downwind

When it comes to sailing downwind, the dynamics reverse. The wind hits the rig from behind, pushing it over forwards, and causing the bow to dive. Now one needs weight aft. As the Swing Bulb Keel is retracted through its quarter arc, the heavy bulb weight shifts the whole centre of gravity of the vessel aft. This keeps the bow up, and enables the sailors to carry more canvas. As the keel is retracted it also moves the centre of lateral resistance of the hull aft, which makes the vessel more controllable on this point of sail.

On a broad reach

When on a broad reach, one needs a low centre of gravity to keep the vessel upright, but still needs weight aft. On this point of sail, the keel should be set at 45 degrees. Because the bulb swings up and aft in a quarter arc, this moves the bulb 70% aft and maintains 70% draft. Mathematically speaking this is because Sin 45 = 0.71, but if you sketch out a quarter arc, it is easy to see why this is so.

Reducing Rolling

A problem which low-ballasted craft encounter when off the wind is that their low centre of gravity behaves like a pendulum. This causes them to roll heinously when running or on a mooring. But they need a low centre of gravity to go to windward. The Swing Bulb Keel solves both these problems, by providing a low centre of gravity when extended, yet when retracted bringing the centre of gravity up to waterline level, the ideal position to prevent rolling.

The rolling characteristic of a low ballasted vessel is illustrated by the following quote from Captain J.C.Voss, pertaining to the 30ft (9.4m) canoe he sailed across the Pacific in 1901:

“The Tilicum I had to ballast well down in order to facilitate sailing with a beam wind or when close hauled. The result was that in running, especially with the wind and sea a little on the quarter, she would roll, roll — well, she would roll the teeth out of one’s mouth. But as soon as I had placed my four hundred pounds of shifting ballast on the cabin deck, or still better, tied half of it in two bags to the main-mast about three or four feet above the deck, she would go along as steadily as a lumber-loaded ship. The latter, with their large deck loads, I have found to be the steadiest vessels as far as rolling is concerned.”

The Venturesome Voyages of Captain Voss, Captain J.C.Voss, Travel Book Club ed. 1950