The job of a sailing boat’s keel is to control sideways movement through the water and to provide a counterweight to the sideways force of the wind on the sails, which causes a boat to heel over.
Keels are designed to act as underwater foils that generate lift as the boat moves through the water, counteracting the leeward force of the wind and enabling the boat to sail closer to the wind.
The most common type of keel used for sailing yachts is the bolted-on fin keel. Fin keels are usually made of cast iron, bolted through the undersides of GRP hulls with substantial stainless steel bolts. Other types of keel include bilge keels, shoal keels, encapsulated keels, lifting keels and canting keels, as used on racing yachts.
While iron is the most common type of ballast, lead is also widely used. Cement or concrete is sometimes used in combination with scrap iron as ballast for wooden and steel boats, but this can lead to steel rusting from the inside and is not considered good practice. Some modern racing yachts have water ballast which can be pumped into tanks to help counterbalance heeling.
Clearly, motor boats do not require deep keels to counterbalance the heeling effect and sideways force of a boat under sail, but nonetheless they do have keels designed to keep them stable and provide structural integrity. Displacement hulls are often ballasted to increase stability. As a result they have a low centre of gravity making them less susceptible to the wind and waves than planing hulls, which are designed for speed and performance.
It is very rare for a keel to fall off a boat. When this happens the incident often receives a lot of press attention as the subject is understandably of concern to boat owners, builders and designers. Marine accident investigators are usually called in and reports are published explaining the probable causes of these accidents.
There are more common incidents involving keels that require inspections and repairs to be carried out.
Keel bolts inspection
Keel bolts should be inspected once a year for signs of corrosion. Most keel bolts of modern boats with cast iron keels are studs, threaded into a tapped hole in the keel. Keel bolts can be made from either stainless steel or galvanised steel.
Lead keels often have J shaped bronze bolts cast into the lead. Bronze is only used with lead keels as bronze and cast iron create a galvanic reaction.
An inspection entails looking from the inside and outside of the boat:
- If there is rust appearing along the hull to keel joint then this is a clear warning sign that the studs may be corroding.
- Look for any signs of movement between the hull and keel, such as splits in the seal or cracks in the hull area around the joint.
- Do a visual check of the fastenings in the bilges, making sure you take a look at all of them even if they are hard to access.
- Dry and clean the bilge – the bilges should be kept dry at all times to help prevent corrosion of the fastenings, even if they are stainless steel.
- Some staining and minor rust is quite common but heavier corrosion needs to be checked over carefully.
- Surveyors tap the studs with a hammer and know what sounds to listen for. A ringing sound is good, a dull sound is not good.
- Check for any stress cracks in the bilge area around the keel bolts, which indicate there has been movement. If you see damage like this then it would be wise to have the keel removed for further inspection.
Keel bolt corrosion
Keel bolts can sometimes be withdrawn for inspection to check their condition and for any signs of corrosion. This entails first removing the nuts one by one and inspecting the threads for signs of crevice corrosion.
Quite often, it is the middle part (the waist) of a bolt where the corrosion is taking place, which is where the hull joins the keel. This is caused by the failure of the seal at the hull-to-keel joint, which allows sea water to reach the bolts and corrode them. Under these circumstances, the seal will need to be replaced as well as any corroded keel bolts.
If you have any cause for concern it is wise to ask an expert to take a look and advise whether the keel needs to be removed for further investigation and repair.
Keel bolt and hull-to-keel seal replacement
A typical repair involving keel bolt replacement and new hull-to-keel joint proceeds as follows:
- A frame is prepared to support the keel when it is separated from the hull.
- The boat is supported in the boatyard hoist or crane.
- The keel bolt nuts and washers are removed from inside.
- The boat is hoisted a little off the cradle.
- As separation begins steel blades are inserted into the hull-to-keel joint crack to cut through the seal. Steel wedges may be hammered into the joint to encourage separation. This can take several minutes as seals are sometimes reluctant to give way.
- Once the hull and keel are separated the boat is lifted clear from the keel. The keel now rests in the wooden frame.
- The keel bolt studs are inspected. If any are badly corroded or distorted then they are removed and replaced.
- The old sealant is ground away from the flat surfaces of the hull join and keel top.
- The fibreglass hull around the join area is checked for stress cracks. If these are found then the hull will need to be strengthened with new layers of woven fibreglass mat and epoxy.
- The hull and keel top are prepared thoroughly for priming in order that the new sealant can be applied.
- The boat is slowly lowered back down onto the keel, then the backing plates, washers and nuts are fastened. Excess squeezed out sealant is wiped off.
- The nuts will need to be torque tightened according to the recommended keel bolt diameter torque settings as the seal beds down and checked again after the boat is relaunched for any signs of leakage.