Rudder Stock Rebuild on a Parker Seal 26 (mk2)

By Peter Lawson

Feb 2014

The 26/27 fleet is ageing (like their owners!) and any boat with the aluminium stock and stainless gudgeons will by now have suffered some degree of electrolytic wastage of the plate cheeks and the spacer channel.This will tend to be worse in older boats and particularly obvious on boats which for one reason or another lie stern-down with the gudgeon and the bottom of the stock constantly submerged in seawater when moored.This could be because, say, a larger engine, diesel or water tanks have been fitted, but is probably an original issue, judging by the range of vessels of varying ages/engine types on which the lower gudgeon lies part below the marked waterline and wastage has occurred.

A typical reaction of owners is to overpaint the cheeks and gudgeon, perhaps with epoxy paint,and to finish in antifoul. This makes a visual improvement in that the pitted face of the aluminium plate and the channel adjacent the gudgeon tends to get filled,but due to the composite assembly is unlikely to fully isolate the dissimilar metals and exclude seawater. Another response is to belatedly add a sacrificial zinc anode,sometimes bolted to the gudgeon.

I have found that it is very difficult to judge the overall wastage without dismantling the stock. This is probably not carried out very often - the number of seized bolts and the limited access is discouraging. In my case a number of the bolts could be released from the channel flanges only by clamping the hex head really tight in a small loose metalwork vice and applying a crowbar for leverage. Once started it was just a question of patient removal of the locknuts. The photos show the extent of pitting/honeycombing of the plate and the main channel, on which the bottom and crucial lower 75mm. or so had the flanges cracked away from the web, possibly due to the weakness of the porous section but no doubt encouraged by the spreading impact of the heavy rudder hitting it's limit - the nylon buffer shown in the manual having long since disappeared. If anyone can describe the precise form of this and how it was fixed I would be interested.

I opted for replacement of the cheeks. A pair of rectangular "blanks" size 1000x 350 in 9.5 mm. aluminium plate of marine 6082 T651 grade were sourced for "home" shaping and drilling,using the old plates as a pattern.The 2¼ x 1¼ x 3/16th channel proved to be a rare section these days and was not available in cut lengths, only in 4 metres, so was uneconomic for a single rudder. The solution has been to cut off the bottom end of the wasted section, to reverse the section top for bottom,and to make up the section at the top with a lapped length of iroko notched into the section and bolted. This echoes the arrangement in earlier Seals where the spacing is formed not by aluminium but by teak or iroko lengths. The short channel spacer at the back of the assembly doesn't suffer in the same way so was re-used.

My plates were cut while set vertical in a large vice using a B&D scorpion saw and metal blade against a clamped plywood guide, with generous applications of cutting agent from an aerosol. This proved a reasonable method using tools available and without undue "wandering" of the cut, which was finished with a flat file. The arrises were taken off using the file and a carborundum stone. It doesn't seem at all necessary to make a heavy chamfer all round the plate, unless you have suitable machinery, but the arris under the gudgeon does need well rounding so as to clear the internal radius of the fitting. The plates were then holed, by drilling through one of the old plates, kept carefully in register by maintaining some clamps at all times,and using the back straight edge as the setting out baseline throughout.


The whole stock was then re-assembled and fully bolted to check alignment before being the separate components were taken for hard anodising.
The layer of 45µ achieved gives a distinctly grey appearance rather than a bright silver so if you prefer the latter a shorter treatment needs to be specified. The re-used channels were also stripped and re-anodised to provide protection at the new drillings and cut ends.

I did consider simply raising the bottom gudgeon and cutting off the wasted material on the plate and the channel so that all were clear of the water. This idea is illustrated in the sketch below, which for convenience was laid over a photo of a rudder without the issue. I did not pursue it, having decided that this apparently short length of the stock is very important in the force couple that resists the great leverage imposed by the rudder blade. I also have a good use for the spare plate on the boat, so the overall outlay of £210 for such a crucial item was easy to justify. I suspect that you could easily double this figure if you got someone else to provide the completed item.

Despite the protection offered by post-anodising and some degree of protection that can be given by assembling with Duralac it seems that full separation of the metals cannot be guaranteed. A multimeter applied to both the old and the new plates shows that the anodising retains it's electrical isolation value but that the damage caused by the bolt threads, perhaps in assembly or during movement over the years,allows an electrical path. So following a check with MG Duff I will tap a stud into each face so that a zinc anode can be bolted on.

The rudder is vital to a boats safety so you might consider a modest outlay for some crucial maintenance before you order that snazzy re-upholstery?


  • Aluminium Warehouse : 6082 plate - both blanks cut out & incl. delivery £135
  • Star Anodising : Erith, local for Kent. £75
  • Iroko :home stock of suitable offcut.