Centerboard restoration and refit

The restoration began with knocking out the old plastic pivot bushing at the workbench, keeping it and other pivot bolt parts together so measurements could be taken to replace the entire pivot unit assembly.

The rust was removed, mostly with a power wire brush on a 5" grinder and 9" angle grinder with cutting wheel.

One thing that surprised me was the completely square edges on the 12mm thick centerplate. These looked very un-hydrodynamic so i set to it with the 9" grinder to reshape the forward and trailing edges into semblances of a parabolic foil shape to improve sailing performance.

There, that's starting to look better! A final bit of rust converter acid  to neutralise the deeper pits. The hot dip galvanising plant recommended some paint stripper to make sure no remnant paint remained anywhere as cold gal won't stick to paint etc.

A pic of what the center-case slot looks like without the plate, its a hardly-ever seen (let alone maintained) part of a trailer yacht, its either under water or on the trailer near the ground with rollers and steel beams all around it which prevent easy access to it.

The center plate of the 18 foot Teria is a similar size to the daggerboard of the 8 foot long Fatty knees dinghy. The Teria has a shoal draft fixed keel that makes up about 75% of the yachts lateral resistance, the center-plate is about 25% of the lateral resistance. The Fatty knees has only a 1" deep rub-keel and small skeg so it's dagger board does most of the work.

The pivot assembly was re-built from scratch. A stainless 10mm diameter bolt and the remains of the 3 bushes were taken around town to see what could be done. I started at the marine chanders, went to a power boat chandlers and they knew of an engineering firm just down the street - Arndt Engineering.. In luck, they had refitted yacht rudder shaft assemblies. The callipers were taken out, centerplate and pivot bolt bits were left with them, they made the bushes on a lathe with white polyacetate? a tough hard wearing plastic suitable for marine immersion for a very reasonable fee. They drilled out the pivot hole a bit and allowed for some clearance after the cold gal was on. The center bush was slightly larger than the original which made it thicker walled and stronger.

The center plate went to the galvanizing plant, it weighed around 20 kg which was just under the 25 kg minimum lot weight. So paid $55 for the plate's acid bath and hot dip in the molten zinc vat. The going rate for hot dip galvanizing at the Bohle plant is $2 / kg. Due to a long work Que it took a week to get done but it was worth the wait, it sure did look like new.

Back at the boat yard cheek pads were fabricated and fitted. I bought 2 mm black polyethylene? plastic sheet (aka chopping board plastic - is a bit slippery) at Northern Plastics. Cut round circles 270 mm in diameter with a 20mm hole in the middle.

The plate was refitted with the help of a friend. Mike got the pivot bolt in while i lifted the plate up. Some pencil marking and a strong torch helped align it. Afterwards it was apparent that the 4 mm packing was not enough the plate was too loose in the case and wobbled about. So i dropped it out and added a third 2 mm spacer.

This time i got it in solo and allot easier. The plates 20 kg weight was held up by the trolley jack below, when up in the case there's almost no fore and aft play so the pin aligned with the hole far more easily. This time, with  6 mm of plastic packing, the board fit snugly with no sideways movement.

The benefits of a "deco" floor, dug a small trench for centerplate to lower into.

Note: Decomposed granite "Deco" is an economical material often used for car driveways up here. It can be spread out like soil, but after being compacted (by cars wheels) sets hard. However it still remains workable, it can be added to or even dug through (eg to lay a pipe). It's great on the expanding clay soils we have, as it never cracks up like concrete. In the open, grass will grow over it. Under a roof it's resistant to grass and weeds, i just spray the few that do sprout.

The pivot bolt assembly got bedded down with liberal amounts of silicone sealant to prevent leaking.

Pivot bolt nut and washers inside the bilge area. The high compression plastic washers spreads load out while having a little "give" 

To refit the lift wire i bought a 3.2 mm swaging tool. It looks like a pair of bolt cutters but with a different business end that crimps the swage tight around the wire.
This time the eye was not integral with the plates lug, it was attached with a screw pin shackle instead, so future maintenance (possibly by some new owner?) will hopefully not require the purchase of an expensive swaging tool!

Lift wire now shackled to center-plate for easy plate removal.

Everything together - the plate worked well. Rising up into it's case with a haul on the lift rope in the cockpit. Lowering down by letting it out. I marked the lift rope with 2 positions, 1/2 raised and 3/4 raised. The system is simple, no purchase block and tackles required, just a pipe for the lift rope-wire, exit sheave and horn cleat.