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How We Made a Bulb Keel

Making a bulb keel for our self-build sailboat Alacazam represented something of a challenge. It had to be of an efficient hydrodynamic design to properly optimise the lift-to-drag ratio whilst getting the ballast down as low as possible.

The designer, Andrew Simpson, rose to the challenge as usual and came up with a design for a medium aspect ratio foil that promised good performance and stability characteristics for a fast cruising boat.

Long enough to provide good tracking ability and deep enough to generate an adequate righting moment; it looked good, particularly with that racy-looking bulb.

Incidentally some torpedo like bulbs project forward of the leading edge of the keel, where they're ideally placed to collect mooring lines and fishing nets. Not a wonderful idea for a cruising boat keel.

So how did we convert that on-screen image into a real life bulb keel? Read on...

Forming the Bulb Keel in Polystyrene

After making due allowance for the thickness if the GRP casing we drew the foil templates on polystyrene sheets and built the keel up by stacking one on top of the other, totally forgetting to take any photographs unfortunately.

GRP casing for sailboat bulb fin keel

This was sanded to shape and the GRP skin laid up in layers of chopped strand mat and woven glass cloth using vinyl ester resin.

Once cured, we dug out as much of the polystyrene as when could by hand, finally have to complete the process by dissolving it with petrol. Nobody smoked...

You can see that the GRP keel only included the top part of the keel bulb - the part that provided the end plate effect. To the lower half, a solid lead casting would be attached - once we'd moved the keel out of the workshop.

Building the Bulb Keel into the Hull

The technique is to poke the keel up through an appropriately shaped slot in the bottom of the hull and bond it to the floors, the beam that carries the mast loads, and the bilge.

cutting slots in GRP buld fin keel before bonding into hull
Letterboxing a bulb fin keel into a cedar strip sailboat hull

Cutting the slot in the hull and the notches in the top of the keel provided a ominous opportunity for a spectacular screw-up. Measuring twice and cutting once was far too casual for me; I must have measured a dozen times - then checked, re-checked and re-re-checked. It fitted perfectly...

Once the hull had been re-levelled horizontally and longitudinally and the keel adjusted for verticality, we filleted all joints with high-strength epoxy filler. All joints were then heavily reinforced with layers of epoxy glass woven rovings.

Adding the Ballast

The ballast took the form of lead ingots. These were arranged in layers and all voids filled with a mixture of lead shot and epoxy resin.

Each layer was covered with this mixture to form a bed for the next layer of ingots and the process repeated. We didn't completely fill the keel; a sump was left to collect and bilge water that would drain into it through limber holes drilled the keel.

The remaining half of the bulb was cast in solid lead using a sandbox mould. Once completely cooled this was sheathed in glass mat to provide a bond for the next part of the process.

boat lift for fitting of bulb keel

The boat had to be lifted by a crane, the lead casting levered into position below it - a process involving crowbars, sweat and a lot of grunting and bad language - before coating the top of the bulb with high-strength epoxy gloop.

Then the boat was lowered onto it, squeezing out any surplus gloop.

Finally the two parts of the keel bulb were bonded together with layers of epoxy glass rovings, and later filled and faired.

And that was it; job done. We now had a bulb keel securely attached to the hull.


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