Centre plate advice needed

[Frogsider]

Here is the ply template for the Tricorn centreplate.  I was surprised how big it came out, though I think it will end up two or three inches longer when I'm finished doing the fitting.  At the moment the draft with plate down is coming out shorter than the designer intended, according to my calculations, anyway.

The marks at the top are intended to show where metal could be cut out, without losing strength in the important places, in order to keep the centre of gravity low.

That wide bit on the upper right is going to have to be trimmed down, too, so that the whole thing fits inside the case which is closed off by a cover along its entire length.

Patrick  

[hespj]

One thing to bear in mind with metal centerboard is that if you run aground big-time they can bend and become stuck down which can be embarrassing when you want to get back into shallow water, whereas a wooden blade will snap, which can be embarrassing............. Take your choice. Anyway, this one's a hybrid (is that a Prius it's leaning against) made of wood and steel so maybe you'll get the best of both worlds.


John

[Frogsider]

I don't know what it would do in the event of a really hard grounding.  I'm hoping not to find out, but I suspect a well made laminated blade, of the size and meatiness I plan, could rip the centrecase out of the bottom of the boat before either bending or snapping.  Now that would be embarrassing!

I've just cannibalised the entire centreboard from a Miracle dinghy to make a new rudder blade.  The offcut is surprisingly small.  The new blade is 18mm marine ply, so I think it's up to the job.

Yes, it is a Prius.

Patrick

[Frogsider]

I had the steel plate cut, drilled, etc.  This will be the core of my plywood/steel/plywood laminated centreplate.  Although I haven't weighed it, I calculate the steel weighs around 27-28 kg - and it certainly feels about that heavy.  With 6mm epoxied ply, sheathed all over in epoxy/glass the total weight should come to about 30k.  So far so good!

Just a couple of problems

First, I've found to my disappointment that the steel plate is slightly curved.  In the photo here, it is bowing upwards about 4 or 5 mm in the centre - (the centre is higher than the supported ends, I mean, not the other way, with the centre higher than the unsupported sides).

I can sit on the plate and bend it back the other way a little, but it always springs back to its original curve.  I suppose I could just try to take the bend out by jumping up and down to try to bend it the other way, but I'm a bit worried about ending up with a bigger bend in the other direction!

Do you think that lamination under pressure to the two outer layers of 6mm marine ply will be enough to take out the curve?  Presumably, even if it does, the steel inside the sandwich will always be trying to recover its shape, so the ply will always be in compression on one side and tension the other.  It will become a problem if the whole structure eventually takes on the curve and starts to jam in the centrecase, as in some places there might be only 3mm clearance each side.

Must I get it straight before laminating?  If so, what would be the recommended method?

The second problem is a simple matter of budget.  I need to encase this plate inside a sandwich of 6mm ply.  Here in France a sheet of quality 6mm marine ply will cost me an arm and a leg.  If I use the best stuff the total cost of the plate will come to more than if I had had it made from 18mm aluminium plate in the first place.  Do you think I could risk using a cheaper grade of plywood?
   

[Jeremy]

The best way to get the bend out would be to find somebody with some bending rolls and get them to run the plate through a few times until it's flat, but I'm guessing that might be quite hard (although if you can find a local blacksmith you may find he has a set you can use, they look like this: http://www.bulldog-uk.com/sheet-metal-working-machinery/bulldog-bending-rolls.htm).

As a DIY fix then I suggest supporting the plate right at either end and leaving a weight in the centre that's enough to take the plate just beyond flat in the other direction, then leave it for a day or two and see if it's taken the right set.  You may need to do this over a period of time, with some playing around with how far past flat you take it, but there's a good chance you'll take the set out of it this way (the set was probably caused by the locked in stress from rolling when the plate was made, which has unevenly been released by the cutting, drilling etc).

[Chris Waite]

That is the correct scientific version

The backwoods way, straight from the nether end of Arundel is that you put down some (cheap) board on a solid level surface such as concrete, then place the offending steel bowed upwards on top of that and strap it down so that it is already flat against the board.  The quickest strap is you kneeling on another plank over the top of it.  Next find yourself a small baulk of timber/offcut to transfer the force, while preventing damage to the surface, then start to hit it* with a suitably heavy mallet or sledge hammer; this may make your knees quake.  Do this in a pattern working away from the point of greatest bow and keep watching to see how much you've got rid of; if it goes too far, turn it over and work your way over it again, but more gently, from the other side.  *This may or may not be pretty hard.

Keep watching (like shepherds)

If you're lucky and once you've got it wrong sufficiently often, it'll likely end up flat enough for government work.  Don't wait any more days; using exterior grade ply, welly on the 'cheeks' with loads of thickened epoxy and have some way of holding the whole thing flat while it sets.  Like you in France, I could never afford real marine ply and all my boats are made of whatever WBP board I felt was best out of the selection available at the time.  All apart from 'Polly Wee' that is; she accidentally got some real 9mm. marine ply, saved from thirty or forty years ago, when I thought I was going to renovate an oldie and before I knew better.

None of them have delaminated yet, apart from a small area of deck on a Proa that was left out in tropical sun for several years.  Oh, and 'Tit Willow's centreplate, which was a rudimentary verison of what you are doing; it developed a bubble of water in one cheek after a couple of years, most of which had been spent moored afloat.



So I popped the bubble, washed the salt off it and left it to dry out for a month or so - out doors was good in Oman, but Northern Europe probably calls for somewhere like on the mantelpiece, or behind a radiator or such.  Then I re-epoxied everything and it's been good as gold for well over a decade now.  I also epoxy-sheath everything these days, many know that I use polyester-cotton sheets as being larger, cheaper and lighter than glassfibre.  I would say, "Straight and to the point"

But how about adding the word "approximately"?

Chris W

 

[BrianP]

I bought a Beaulieu Scow some 10 years ago. They have a metal centreplate as a class requirement. I converted her to a Lymington River Scow (same hull), replacing the metal plate with a plywood version.

John Claridge cut me a ply blank and explained how to shape it. As I worked on it, it developed a bend, similar to your plate. John explained various tricks to straighten it, but even though it would work, as soon as my back was turned it would be bent again.

So, having done so much work on the shaping, in the end I laminated the ply with unidirectional glass and fitted it. Never needed a centreboard brake as it was always a tight fit but never too tight.

Odd thing was that this Scow had the best windward performance of all the Scows around. Yes, she was well balanced, but I have always wondered if somehow the slightly curved centreboard was actually improving upwind pointing. She could point 5 to 10 degrees better than any other. It certainly proved that performance will not suffer if there is some curve in the plate.

Brian

[Frogsider]

Thanks Jeremy, Chris and Brian for all your expert advice - even on Christmas Day!

I haven't got access to a big mangle thingy (bending rolls I think you called them, Jeremy), so I went with your more everyman's garage style solution.  Here is a photo of the plate being gently persuaded to flatten out under the weight of two bags of grass seed, a large angle grinder in its case, and 5 litres of anti-moss spray.  What you can't see is an offcut of ply underneath to stop it if it starts to bend too far the other way.

If this doesn't work, I'm definitely going to give it the Arundel blacksmith treatment with a dirty great hammer as recommended by Chris.  In fact, I'm rather hoping it comes to this.  Beating the sh*t out of a stubborn bit of steel sounds like a very satisfying way to build (re-build) a boat.  

Finally, if it all fails to come out perfectly straight, I have the consolation, provided by Brian, of knowing that I might end up with a secret keel design of such hydrodynamic excellence that it makes me faster to windward than any other Tricorn.  Unfortunately I know of only one other Tricorn that might still be sailing, and it's in Wales while I'm in France, so it could be a long tack out into the Atlantic for both of us before we get a chance to compare.

All the best to everyone for the New Year. My resolution is to make it to Morbihan Week in May with my restored Tricorn.

Patrick  

[Chris Waite]

It seems to me Patrick

And I am sure that Jeremy will quite rightly cringe, but to reshape a piece of steel like this requires imparting enough energy to the latticework of atoms to persuade them to shove past each other, just slightly.  They know exactly how much this is and it amuses them to see how closely you can work it out.

I've not tried to do this on quite this scale before, nearly, but not quite.  To see what happens and for practice, take an offcut or some other piece of scrap and just hold it (I use a 'Mole Wrench' / 'Vice Grip') on some flat hard surface as an anvil, then take a hammer to it.  Despite the flat surfaces involved, repeated hammering will tend to make it curl up; or on your plate this will initially take the form of flattening it out.

Excess can be beaten out the other way, but keep sighting down it to avoid particularly high or low spots

And think of the muscles you'll develop

Chris