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How to Curing leaking keel bolts

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 Good working knowledge for small boat owners

Welcome to the first of the articles on maintaining / upgrading your Yacht. Over the coming weeks we will be publishing more of these fact sheets.

Curing leaking keel bolts

This text is intended to advise experienced and qualified marine and boating persons and assumes the availability of suitable conditions and equipment for the procedures described. Issued free of charge this is a guide only and the company does not consider itself responsible for the consequences of its use.

Detection a bolt leek

Almost without exception, the keel bolt which is leaking will exhibit signs of deep rust staining. This is not deterioration of the keel stud, nor the nuts and plate washers which are made from 316 stainless steel. Water entering the hull from outside passes between the face of the keel stud and the top of the cast iron keel, and in the process becomes stained by unavoidable rust on the face of the keel itself.

Bolts which are leaking will normally leak all of the time, not just when the craft has been sailed. Should there be some evidence that a stud only leaks when sailing, a closer structural inspection including stiffening, floors, etc should be carried out.

On the rare occasion that a keel bolt may be leaking, but shows no signs of staining, this can be pinpointed by first drying out the area of investigation thoroughly. Using modeller’s plasticine and sticking it to the inside of the hull, make a complete circular wall around the stud which you are checking. If over the period of your check, water becomes trapped thereby, this can probably be counted as a positive leak.

How to cure a keel bolt leek

When a Westerly is built, generous amounts of a highly viscose polysulphide or thiosulphide bedding compound is applied to the top of the keel prior to the process of attaching the keel and bolting it down.

This sealant or bedding sticks tenaciously to the clean new surfaces of the bilge keel stub and the top of the keel in its early life, when it also cures into a stable but flexible synthetic rubber. It is not entirely correct to assume that this bedding provides the waterproof barrier in the interface between stub and keel.

The positive seal against water ingress is made at the keel studs. In the fullness of time, corrosion to the keel from which it cannot be fully proofed, and which is not structurally detrimental, may mean that the positive adhesion and seal between the bedding and the top of the keel may deteriorate.

Leaking bolts are treated one at a time, and from within the hull. To attend to them, suitable sized sockets (in some cases spanners will suffice it the sockets are not available) with extensions and a robust turning bar will be required.

On old yachts built since 1978, the studs used have been metric, usually in the main part of the keel the studs are 24mm diameter, on the leading edge they may be 20mm diameter, and on the trailing edge of the keel 14mm diameter.

In the case of the metric nuts

In the case of the metric nuts, the across flats measurements (A/F) are 36mm, 25mm and 17mm, respectively. As well as suitable sockets, a screwdriver and sharp scraper will be useful for cleaning the general area, as will rags for drying purposes. Materials necessary will be caulking cotton for making up into ringlets as described in the attached diagram below.


Having identified the stud to be treated, clean off the covering gelcoat (this is used for cosmetic purposes and for locking the nuts to the thread), if this is not cleaned away thoroughly there may be a danger when undoing the nuts of unwinding the stud from the heel, and this is to be avoided if possible. Selecting a suitable socket, engage this carefully onto the locking nut only and then unscrew this off entirely. Next remove the fullnut and the heavy plate washer which lies beneath it.

It is better if the leak is being attended to whilst the vessel in question is dried out, but if it is afloat an increase amount of water seepage will at this stage become quite apparent, though will not usually be to such a degree that is will prevent treatment ,see diagram B below.

The plate washer should be cleaned and any sharp edges that might damage the grp when it is replaced should be filed away.

A ringlet of caulking cotton as described in diagram A should be made up to fit over the stud in question tightly and this should be pushed down the stud until it rests against the inside of the hull. Sealant can then be applied to the face of the plate washer that will rest against the hull, its replacement, and tighten down with the fullnut first of all. Proper adhesion can be achieved between the sealant plate washer and the hull in dried out conditions, however, in wet conditions adhesion is less thorough and less likely. The purpose of the caulking cotton is to take advantage of its property for expansion when it becomes wet, which serves to enhance the compression seal created by tightening down the plate washer upon it.

Finally, the locking nut should be placed into the position and tightened down to around 30% of those pressures used on the fullnut. When the bedding has cured, the whole assembly should be over coated preferably with gelcoat, but otherwise with ordinary resin or paint, to both seal and lock the threads.

The keel stub and its surrounding area on your vessel (as with other boat) is a highly structural area, and though the hull lay-up at this point is very thick, the stiffening webs and floors bonded into and around the stub are important reinforcement. When looking at keel studs for whatever purpose, it is wise to carry out an inspection of the grp bonding onto the webs/floors/partition bulkheads, etc, which might be used to support the structure. If the bonding is lifting, or cracking, or maybe in any other way suspect it should be made good as soon as possible.

The keel is damaged. But Now?

the keel bolts

The scantling rules for keel bolt design contain a very large factor of safety. Depending on the material of the bolts (galvanized steel, bronze, stainless steel or monel) and how they have been cared for, the life span of most keel attachments ranges from 15 years to infinity, with galvanized steel being at the lower end of this range.

Spider cracks around the keel flange are not uncommon in boats with deep-keels. While not desirable, these cracks often indicate nothing more than that the boat was run hard aground at some time in the past, flexing the fiberglass around the base of the keel. If the boat has been bounced hard repeatedly on rocks or a hard surface, repairing the damage to the fiberglass may be in order.

The keel bolts can be inspected. The common method of tapping them to "hear them ring" leaves a lot to be desired, however. To be done correctly, the bolts need to be drawn (removed) and tested by X-ray or ultrasound.

If the soundness of the bolts is in doubt and you have them out of the keel, why not just replace them. The cost of the operation is in removing the bolts and replacing them in the keel, new ones probably won’t be much more than the X-ray costs on the old ones.

All that aside, the classic signs of bad keel bolts are:

  • Obvious corrosion, pitting or other problems inside the boat
  • Leaks into the bilge that are untraceable
  • Failure of the caulk line between hull and keel from movement of the keel
  • Rust streaks from the caulk line after hauling
  • Loose nuts on the keel bolts
  • Compression fractures or enlargement of the holes in the hull
  • Ability to physically move the keel from side to side
  • with the boat in the lift slings, you can usually see the keel move up and down as the weight of the boat is put on it and then lifted off.

If you find that the keel bolts are truly defective, you have several choices. Depending on how they were installed, it's usually not necessary to remove the keel from the hull. In some cases, sistering, that is adding one new keel bolt next to each old one without removing the original, is a good option.

One thing you may want to do is to contact the boat manufacturer. Some are pretty good at keeping track of their boat’s repair histories and helping owners of their products. They will also be able to tell you if your particular year or model has problems of which they are aware and who in your area is the most knowledgeable as to possible repairs. They will want the boat’s HIN (boat hull identification number or serial number), so have it handy.

Next: How do you repair a leak in the 14' aluminum rivet construction boat