Refurbishing / Replacing Centre-Plate on a 22' Trailer Sailer (Mk1 Pandora)

[gregandginas]

In 2009 we picked up an elderly Van De Stadt Pandora.  Ours is a Mk1, similar to the one discussed in http://pandorasailing.com/wp-content/uploads/2009/04/pbo2000.pdf - but we had the drop centre-plate model.

We sailed her for a couple of seasons (see http://pandora-snowgoose.blogspot.co.uk/ ) - but we never did manage to drop the centre-plate.  

After a period in storage, interrupted by failed efforts to get the centre-plate out, we finally had some success: welding a bracket to the plate and applying several tons of pressure with bottle jacks... we got the old plate out



Of course... I now need to either clean it up or (perhaps better still) replace it... and that's where I have questions!

1. Our first issue is that we don't know is what thickness the plate would once have been.  I've had estimates ranging from 10mm to 19mm.  I'm guessing that it was originally 1/2": more than is there now!

2. Our second issue is shaping.  I've access to an oxy-acetylene and an angle grinder... and could perhaps access a milling machine if need be... but what should I be aiming at?  

3. Our third issue is finishing.  I've had one recommendation of International Interprotect.  Is that the way to go?

Fortunately, we've managed to drill out the remains of the bolt that once held the centreplate in place... so if we can get a new plate sorted, we're hopefully only a few headaches away from getting Snowgoose back on her trailer!

[Paul H (admin)]

Greg,

How old is she? At school in 1972-ish our metalwork teacher said the industry had moved to metric production, so around about then onwards the plate could have been 12mm.

Also how much cash do you want to throw at her? Stainless is the way to go if you want your family to inherit the boat. At the very least use a stainless bolt/pin at the pivot.

As for shaping the critical issue is thickness and if you look at typical keel profiles you will find it difficult to get the "perfect shape" starting with 12mm. I gave up going for the perfect shape on my 18mm laminated ply centreboard because the trailing edge would need excessive material removal and also become too thin. By all means chamfer the leading and trailing edges as well as you can with an angle grinder.

Alistair is a bit of an expert on these things (something to do with helicopter blades wizzing around) and he says a sharp trailing edge is the critical factor, consistent with not chipping when it hits the sea bed.

-Paul

[GregHBBR]

Hi Greg,

Paul (admin) wrote

How old is she? At school in 1972-ish our metalwork teacher said the industry had moved to metric production, so around about then onwards the plate could have been 12mm.

Based on my experience with a SeaHawk, I'd have guessed 12mm too, but can't anyone at the Pandora site give a clue on that.

As for shaping the critical issue is thickness and if you look at typical keel profiles you will find it difficult to get the "perfect shape" starting with 12mm.

I didn't think you were talking about cross-section, as Paul initially seemed to assume, because I would be pretty sure the original was just a flat sheet of metal cut to shape. The only shaping would, as he then suggests, just be to remove any sharp edges.

To answer your last question, I'd refer to Graham Richards' work on his SeaHawk keel. He works at a boat yard in St Ives (Cambridgeshire) and has access to the best advice and materials. Incidentally, a SeaHawk keel nominally weighs 70lbs, so it looks as if his lost 4lb to rust in its 30-odd years.

[gregandginas]

Thanks, guys... especially for the Graham Richards / Seahawk link :)

We believe Snowgoose dates to 1971.  Everything appears to be original.  She's perhaps a rarity though: I believe all the Pandoras in the main fleet over near Abersoch are either fin keeled or bilge keeled.  That might limit the usefulness of any advice from that side of the country...

Re. the owner's association: I don't seem to be getting any response :(

The centre-plate bolt appears to have been mild steel and was either 3/4" or 19mm.  It didn't go right through.  The head sheared off when I tried to undo it.  The middle had corroded / worn so badly that the plate just fell out once we'd got the other end free.  At least it was soft enough to drill...

Fortunately, we were able to drill the bolt out so straight that we got a sleeve from one side and even removed some of the thread intact on the other side.  i doubt there's enough left to secure a bolt though :(

As we wouldn't even know where to start looking for a 12" drill bit of the right diameter... and as can't see any way of re-tapping the thread... we were thinking of sticking a butchered bolt in as a pin, and then spot-welding a small plate over the head to stop it coming out.  

Any thoughts on inserting a liberal quantity of the right grade of grease?  Does any grade actually work?

Funds are limited, and certainly don't stretch to stainless, so I'll either clean the plate up or (unless I hear more) go for fresh 12mm mild steel and try and get a sharp trailing edge!

Ps. Looks like Graham Richards used 5 coats International Interconnect epoxy primer & 2 coats Trilux hard anti-fouling.

[Paul H (admin)]

Greg,

There is almost no side load on the pivot pin so it doesn't need a captive nut.

Chris Waite recommends a stainless pin, capping the ends with a plate sealed with silicone and screwed. Installation/maintenance becomes easy; just drift the pin in/out with a tap of a hammer:

http://uk-hbbr-forum.967333.n3.nabble.com/Centreboard-pivot-tp3308074p3317371.html;cid=1373287165930-450

I don't know how that scales up your 22' hull - suitably big and chunky perhaps?

EDIT: Link updated

-Paul

[Jeremy]

Metal sizing is a mish mash, with EU sourced metal tending to be metric and far eastern sourced tending to be imperial, and what little there is from the UK is still roughly 50/50 metric and imperial.  1/2" mild steel plate will often be sold as 12.5mm and is still readily available (and cheap!).

If it were me I'd make a template up and find a profile or laser cutting place nearby.  The older profile cutters (flame or plasma) used a black line follower, so need a full size white card template with the outline drawn on in black felt pen.  They will then automatically cut the plate to size for you.  A laser or CAD/CAM plasma cutter would need a CAD drawing with the outline to be cut as a continuous line.  Some will convert your profile template to the right format for you.  Most will probably have 12mm or 12.5mm steel in stock.  

Once cut, I'd be inclined to just clean up the edges with an angle grinder, finishing with a belt sander and flap discs.  Won't take that long, maybe an hour or so.  

I'd then pop around to your nearest agricultural hurdle/gate/fence manufacturer and ask if they can slip the plate in with their next lot for hot dip galvanising.  Again, this will be pretty cheap (and a lot cheaper than going directly to the galvanisers with a one-off).  If going down this route remember to drill the pivot pin hole slightly over size (maybe by around 1/2mm or so) to allow for the thickness of the galvanising.

A hot dip galvanised plate will last for decades, and won't need regular repainting etc.  Well worth doing in my view.

[simplesimon]

I'm with Jeremy - galvanize it!

Have you tried posting on the Norfolk Broads Forum for Pandora experience? There's a number round here and I suspect a fair few are swing keel.

Simon