Galvanised keel bolts

The keel bolts- for the ballast keel, the centre plate and the bilge keels- were all originally manufactured in imperial sizes. Can replacements be readily obtained?

5/8″ keel bolts taken from Mariette.

Choosing the material

  • Stainless steel should not be used. It contains chromium, which forms a passive protective coating – but only in the presence of oxygen. This can be from air or water, but deep in the middle of a piece of iroko keel, there is not enough oxygen for this protection to take place. The bolt is therefore most likely to corrode where it is hardest to inspect, and failure can be the first thing that reveals this problem.

  • Bronze is an option, but is usually used for lead ballast keels since it is less reactive than lead and will therefore not waste through galvanic corrosion (unlike steel which is more reactive than lead).

  • Galvanised steel is a better, and cheaper, alternative. The zinc outer layer protects the steel bolt in three ways: it forms a barrier between the steel and its surroundings, it can act as a sacrificial anode, and the reaction of zinc with its surroundings creates compounds which protect the zinc itself from its environment. However, there are two types of galvanising:

    (1) In hot dipped galvanising, steel bolts are immersed in molten zinc. An iron-zinc alloy builds up underneath the final zinc coating. This alloy is harder than the steel beneath it. The lifetime of bolts prepared in this way should be 20 – 50 years. The coating has a thickness of 0.08 – 0.1 mm; over-sized threads need to be used in the matching nuts.

    Hot dipped galvanised isn’t pretty but has a very long potential life

    (2) Zinc-plated (electroplated, BZP) steel bolts have a more uniform, shiny appearance. However, the layer of zinc plating has a thickness of only 0.01 – 0.02 mm and no alloy layer beneath, so they are less resilient. The anticipated lifetime would be much less than hot-dipped bolts.

Choosing the diameter

The original ballast keel bolts had diameters of 3/4” and 5/8”. These can be substituted for metric sizes, but the keel- especially for the larger size- may well need to be reamed out to take the metric diameter. For reference: 3/4” = 19.05mm (use M20 bolts) and 5/8” = 15.88mm (use M16 bolts).

Metric bolts of these sizes are not easy to find, and imperial bolts are even harder. These are the two sources that I found for imperial, hot dipped galvanised sizes:

RCF Bolt & Nut ( (Ask for Mattia.) These guys are based in Tipton in the West Midlands. They will make small batch bespoke fasteners to order. They quoted two weeks to manufacture the bolts, but this could be longer if they don’t have the oversized nuts in stock.

I asked them to manufacture a small number of 3/4” diameter bolts with Whitworth (BSW) threads (10 tpi, thread angle 55°). The threads were cut to bespoke lengths on BSW blank stock (hex heads with the exotic dimension 1.20” / 30.5mm across flats (AF) as per BS1083) before galvanising. The nuts supplied were 1 1/8” AF.

I ordered 6 bolts ranging from 10.5” to 12” in length to cover the replacement of the inline keel bolts fore and aft (the extra bolt being included to cover uncertainty over the length of the most forward bolt), plus 6 centre board bolts (7” long), with two nuts and washers for each. The total cost was £101.73 inc VAT (February 2022) with no charge for delivery, which works out at £8.48 per bolt- an excellent price for bespoke work.

BC Fasteners & Tools Ltd ( Imperial sizes are readily available in the US, but retailers don’t export across the Atlantic. However, this constraint does not appear to apply to Canada. At BC Fasteners, off-the-shelf products are UNC threaded (10tpi, 60° thread angle) which will have 1 1/8” AF hex heads and nuts. Bolts worked out at about £7.30 each after conversion from Canadian dollars. The delivery time (from British Colombia!) is about one week and doubled the overall cost. I tried this source first, not knowing if the bespoke route would work out, but the bolts were lost in transit (I received an empty box!). BC Fasteners issued a prompt full refund, not waiting for Fedex to complete their enquiries.

Which keel bolts do you need?

If you want the ballast keel bolts before the old ones are removed, these are the measurements that I have. (Let me know if you have any different values). Specify 2.5″ of thread for all the bolts.

NB: Updated November 2022




Length (inch)

Length (mm)

Under engine (in line)





Under keel case sides

5/8” or


2x 16.5,
2 x 16

2 x 420,
2 x 400

Under keel floor cap

5/8” or 16mm


2 x 13.5,
2 x 14.5

2 x 350,
2 x 370

Forward (in line)





Note that differences in thread angle between BSW and UNC mean that nuts are not interchangeable.

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