Crevice Corrosion

Between the 1960s and 1980s, keelbolts, rudder shafts and chainplates were nearly all fabricated from 316 stainless. Most types of stainless steel require a free flow of oxygen to remain corrosion free.

When stainless steel, especially type 316, is trapped inside a wet environment and deprived of oxygen, an acid is created which corrodes the stainless. It can be a quick process – I’ve seen 25mm diameter 316 keelbolts corroded away to virtually nothing in under 10 years. There have also been several recent mast failures on 1970s and 1980s yachts due to failed 316 stainless chainplates and/or bolts.

Any original 316 stainless component that’s been encapsulated in damp timber, for example chainplates, chainplate bolts, keelbolts and rudder shafts, is at risk. Any sign of rust from these components should ring alarm bells and, even if it means difficult dismantling, such components should be inspected as a matter of priority.

ASSEMBLY

When assembling metal rudder shafts into a timber blade, it’s essential to keep seawater from getting past the shaft and into the timber. Gluing stainless-to-timber is a risky business at best. More often than not the glue will crack when the rudder blade flexes under pressure.

A better method of sealing the shaft is to create a rebate at the top of the blade around the shaft, which is later filled with a quality, flexible sealant. Alternatively, the rebate could house an O ring or two.

Creating a rebate is easy enough on a spade rudder, but because they have a lower rudder gudgeon, Townson rudder shafts require a second rebate at the bottom. There, the only access to the shaft is via the narrow horizontal slot for the skeg gudgeon. Creating a rebate, then trying to fill it with sealant through this narrow slot would be highly problematical.

I decided a more effective, long-term method of sealing the shaft from blade was to sheath the shaft and its pins in fibreglass/epoxy. Unlike timber, glass/epoxy won’t split, crack or swell if exposed to seawater. This meant any slight seawater seepage that might occur between shaft and fibreglass can’t get to the timber to cause damage.

Another major advantage of this approach would be easier gluing: the only gluing required would be timber-to-fibreglass, a proven marriage and there’d be no need of any tricky timber-tostainless gluing. Note: this approach requires a 2205 stainless shaft as 316 won’t tolerate being buried in a damp, salty environment.

Glassing the shaft was simple. With the pins out and the shaft de-greased with acetone, it was set into a simple jig and, while rotating it slowly by hand, wrapped in three layers of 450-gram double-bias (DB) 75mm tape and epoxy.

The next day, holes were cut through the cured DB glass to enable the three pins to be pressed home. These were then glassed onto the shaft with more DB and epoxy. When cured, the DB glass/epoxy encapsulated shaft and pins had essentially become one solid unit.

Meantime the timber rudder blade had been cleaned, flushed of salt and slowly dried. After truing up the saw cuts along the rear

edge of the blade, a piece of kauri was machined with a hollow to match the shaft and drilled for the three pins.

After careful measuring, the kauri and the glass-sheathed shaft/pin assembly was glued back into the original blade with thickened epoxy. The smaller pins gave additional clearance inside the old holes making aligning the shaft and its blade easy. As this was a critical operation slow epoxy hardener was used so there was no rush.

Next, six kauri strips were machined and glued around the front of the blade onto the DB glass/epoxy. Kauri strips were used in preference to one block for the leading edge to avoid having weak cross-gained timber around the front of the shaft. Also, it saved some kauri.

After the glue had cured, the kauri strips were faired back to the original rudder profile minus two millimetres to allow for sheathing the whole blade in 450-gram DB glass/epoxy, doubled around the leading edge.

Following a suggestion from Grant Beck of Adhesive Technologies, six millimetres was planed off the rudder blade’s trailing edge and during sheathing the DB glass was brought past the edge either side, then glued together with thickened epoxy. This gave the rudder a solid glass/epoxy trailing edge, considerably stronger than the thin vulnerable timber edge of the original.

After the rudder was faired and sanded, it was painted with multiple coats of Altex #3 epoxy primer. To ensure a good bond, the first coat of antifouling was applied to the last coat of epoxy primer while the latter was still partially cured, i.e. after four hours.

With Talent’s rudder finished, it was time to look at her remaining underwater components, the rudder housing, rudder bearings, skeg and keelbolts. B