SUNMAID 20 RUDDER

AUTHOR: Alan Burney - NURGLER -102
First published July 200 in Sunlog #86

The original Sunmaid rudder box was fabricated from a marine ply laminated core and marine ply side cheeks. Chromium plated bronze pintle assemblies were secured to the timber by stainless steel screws and a fabricated stainless steel rudder cap served as a tiller mounting. Some rudders had a cast aluminium rod clamp as a blade control device and on others this clamp was fabricated from stainless steel sheet. The rudders were a very serviceable unit and, if the varnish was in good condition, they looked quite attractive. Unfortunately, many of them have not survived the ravages of time, weather exposure and rough handling by helmsmen and have failed completely, or simply split in such a way as to appear that failure is imminent. If the rudder blade is allowed to swing back just a few degrees in heavy weather, such that the effort at the tiller is increased (two hands on the tiller and two feet braced against the other side of the cockpit) then splitting of the cheek plates from the core is almost guaranteed.

There are several ways in which a replacement rudder can be made. One of the easiest methods is to replicate the original using quality marine plies and proper marine glue. For people accustomed to working in timber, this is a reasonable option. At least one of the rudder boxes seen around the launching ramps was completely fabricated from mild steel plate, fully welded and then heavily galvanised. Another rudder box which must not be overlooked is Webster Marine’s cast aluminium open web type rudder box, which is not cheap but is a proven alternative. The rudder box described in the following pages is the preferred option of a couple of members of committee and it has proved to be a solid and reliable replacement for the original. It is not difficult to make.

You will need:

Material: Two pieces of marine grade aluminium plate 6 mm thick, 500 x 300 mm - undamaged and flat. Marine plywood laminated to form the core. The thickness required will have to be determined from the rudder blade. It is about 41mm but take care to measure the blade in several spots around the area which will enter the rudder box cheeks. 41 mm can be fabricated from two x 19 mm and one 3 mm, 43 mm from two x 19 and one 5mm, etc. Use a good marine glue such as Epiglass or Blockey’s Resorcinal. Dimensions are 500 x 300 mm. Hardware - miscellaneous screws, bolts, nuts, washers, dome nuts, etc.

Tools: Bench drill and drill bits, power jig-saw with timber and metal cutting blades, wood plane, flat files, screwdrivers, spanners, clamps and sandpaper.

Step 1.
Mark out (See Fig 1.) and cut to shape the two side plates using the metal cutting blade in the power jig-saw. Clamp the two plates together and dress the edges clean with a file. Break all sharp edges. Note that many of the aluminium suppliers have a cutting service available and will supply the plates ready for drilling.

Step 2.
Cut the laminated marine plywood for the core into the same shape as the side plates (Fig 1.) using the power jig-saw with the timber cutting blade. Clean up the edges with the plane and finish with sandpaper. Use the finished shape of the metal side plates as a guide.

Step 3.
Clamp the plywood core between the aluminium plates, carefully aligning the edges at the top and both sides. Drill three 9/32” diameter holes at positions A, B and C (Fig 3.). Fit three 1/4” diameter hexagon head SS bolts, cut to length and tighten securely, using dome nuts. Remove the clamps.

Step 4.
Position the existing SS rudder cap on the top of the rudder box making sure the front edge is in line with the leading edge of the rudder box. Insert the tiller and check for a neat fit before clamping the rudder cap in position. Using existing holes in the cap as a guide, drill through three holes 7/32” diameter. Drill from both sides to ensure entry and exit holes line up with those in the rudder cap, then open up the holes through the timber section later. Fit 3/16” diameter roundhead or hexagon head screws, cut them to length, and fasten with dome nuts.

Step 5
Place the upper pintle bracket in position (Fig 4) making sure it is fully home against the forward side of the rudder assembly and that the hole in the bracket lines up with the hole in the rudder cap. Check that the pintle is parallel to the face of the rudder box. Note that the thickness of the rudder box is less than the space between the side cheeks of the pintle bracket and packers will be required to make up the gaps on each side. Use aluminium, stainless steel or an engineering plastic such as UHMWP or similar for packers but do not try to squeeze the side cheeks closer together. Clamp the pintle bracket in position and drill two holes 9/32” diameter through the timber (work from both sides as in Step 4) to match the holes in the rudder cap and pintle bracket. Fit two 1/4” diameter roundhead or hexagon head bolts, cut to length and secured with dome nuts.

Step 6
Carefully measure the distance between the top faces of the gudgeons on the boat’s transom. This distance is used to position the lower pintle bracket relative to the top one. Set the lower pintle in position and, as before, use suitable packers to match the thickness of the rudder box to the distance between the bracket’s side plates but do not try to squeeze the side plates in. Make sure the bracket is hard up against the front of the rudder box and that it is square to the front face and the correct distance from the upper bracket. Drill the two holes through the aluminium side plates from both sides and continue the rudder blade pivot bolt hole through the timber core.

NOTE: Check the details of the holes used in the original rudder. 5/16” diameter appears to be the standard size for the rudder’s pivot bolt, but at least one rudder in the fleet uses a 3/8” rudder blade pivot bolt. Also, the smaller holes in the bracket may be tapped to accept 1/4” countersunk screws inserted from within the rudder blade space. This method was used when the side plates were timber. These should now be opened out to 9/32” diameter and countersunk from the outside, and the corresponding holes in the rudder box side plates tapped to receive short CSK screws. The rudder blade pivot bolt (5/16” or 3/8” diameter) is a hexagon head stainless bolt, cut to length, and secured with a stainless steel “Nyloc” nut.

Step 7
Dismantle the rudder box assembly and partly re-assemble it using one side plate, the laminated plywood core and sufficient fasteners to properly locate the two together. Use a long bolt in place of the rudder blade pivot bolt and set the assembly on the bench with the aluminium plate on the bottom and the long bolt standing vertical. Place the rudder blade in position over the temporary pivot bolt and swing it from the up to the down position, marking on the plywood core the blade clearance required. Note when marking out the clearance that the top of the rudder blade is not a true circular arc. Mark also the areas on the core where the rudder blade will be stopped in the both the fully up and fully down positions. Dismantle again and cut out the shape of the plywood core with the jig-saw. Clean up the cut and partly re-assemble to check the blade clearance.

Step 8
Depending on the fittings used on the particular rudder there may be additional holes to be drilled, for the blade control clamp for instance, after which the aluminium side plates should be treated to prevent corrosion. Marine grade anodising is the preferred option. Powder coating is an alternative method but not as durable as anodising. The yellow pages will help you find a supplier in your area.

Step 9
While the aluminium plates are away being anodised, the plywood core should be sandpapered smooth and treated with a good coat of Everdure or Bote Kote and several coats of a marine grade of varnish. There is also an opportunity at this time to send the pintles and gudgeons out for chromium plating, if thought necessary.

Step 10
Assemble the three pieces of the rudder box with a liberal coating of Silastic between the side plates and the timber core. Tighten all bolts, working evenly around the rudder box. Insert the blade and check the clearance as it is swung through the full range of movement. Ideally, the blade should be free to move up and down without any noticeable side clearance. If necessary, plastic packing washers about 6” diameter can be cut from perspex sheet of a suitable thickness and used to make a smooth fit between the blade and the rudder box.

Step 11
The blade control clamp should have been chosen and any holes required for it before anodising. it the clamp and swing the rudder blade through the full range of movement to check the extreme positions. In the fully down position the most forward point of the blade at the bottom should be at least 70 mm forward of a straight line projected downwards from the face of the rudder box. When fully up the blade should be at least horizontal. The dimensions given apply to the rudder box made for Sunmaid No 102. There may be some minor variations for other Sunmaids.

The completed rudder assembly is hardly a thing of rare beauty or outstanding elegance but it is a rugged device which will stand a great deal of abuse. The rudder box is slightly heavier than the original timber one but this adds very little to the total weight of the rudder assembly.