Alloy centre plate

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jonathan
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Alloy centre plate

Post by jonathan » Thu Feb 14, 2008 9:18 pm

I am bartering with a local engineer to have an alloy plate made for one of my boats. Does anyone know what grade/type of aluminium should be used?

Rupert
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Re: Alloy centre plate

Post by Rupert » Thu Feb 14, 2008 11:10 pm

The rules just state Salt water resistant Aluminium Alloy, that it has to be 8mm +-.3, and that it can be anodised. It must weigh 8kg+-1kg. Main thing is you have to be able to stand on it without it bending...You need diagram 4 in the rules.
Rupert

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jpa_wfsc
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Re: Alloy centre plate

Post by jpa_wfsc » Fri Feb 15, 2008 2:18 pm

AW-7020 heat treated after machining T5 would be nice and stiff, and is also quite common. But totally useless (soft and bendy) if not heat treated! Its the grade used for general welded, fabricated structures.

john./
j./

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Ed
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Re: Alloy centre plate

Post by Ed » Fri Feb 15, 2008 3:29 pm

I did find this out once.....and it makes quite a bit of difference to price....but afraid I can't now remember.

I would go with what Johns says....sounds like it makes sense. Interesting about the heat treatment.....can this be done at any stage?

My Jollyboat has a ali plate but strongly suspect it has not had this treatment as it is painfully bendy. I dare not think what effect this has on performance, but it makes it very hard to bring up from capsize without bending it.

Would I be able to get this heat-treated now...even after years of use? I don't think it has been anodised....but am not totally sure on this.

cheers

eib
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jpa_wfsc
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Re: Alloy centre plate

Post by jpa_wfsc » Fri Feb 15, 2008 7:13 pm

There are basically two types of aluminum alloy - work hardening and heat treated. If you heat treat the work hardening kind it can go soft again (annealed) ... not quite what you would want! And plate aluminium is normally supplied annealed - I E the work hardening that went into getting it flat has been heat-treated out of it.

The heat treatment has to be done after all machining is completed. Your anodizing co. would be able to do it for you. Anodizing would be a very good idea as the plate will always get some wear sliding in and out of the case. The wear rubs of the protecting film of oxide that forms on aluminum which promptly corrodes again - leading to high wear rates. Annodising will stop that. Also looks nice and is probably slippery in the same way that graphite paint was (you can ask for satin finish on the anodizing).

There was some history about all this in the Lark class because early boats had thick, soft boards to make them strong enough whereas later ones had thin, hard, anodized boards. My old lark had a thin plate in a wide case - so was padded to stop it flopping (the class rules allow the board to flop to windward allowing for 'gybing' boards which is fast upwind, but made for an unstable ride on a run!) I just looked at their technical pages but cant find out their material specification.

john./
j./

National 12 "Spider" 2523
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British Moth, 630, early 60's 'Pisces'

!!!! Not CVRDA !!!!
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Pat
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Re: Alloy centre plate

Post by Pat » Fri Feb 15, 2008 9:11 pm

Many Larks are now using the blue boards supplied through Simon Cox which are much better and stronger than the old ones - we've got one on our modern Lark as it was Simon's old boat and it takes the weight well. I quote the advert below but you'll need a lottery win :D

Each centreboard is made from 1/4-inch, high grade aluminium, they are CNC cut by waterjet cutter and shaped to a super smooth finish with the aid of templates. Each board is then anodised and finished with a clear protective finish.

All boards are certified by the Class Measurer to ensure that they are fully Lark Class legal.
These boards are available now for only £375
(Half Cut and What a Lark Removals Ltd)

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Re: Alloy centre plate

Post by jpa_wfsc » Fri Feb 15, 2008 9:32 pm

:!: :!: :!: Same price I sold my Lark for - but that is another thread!

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Ed
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Re: Alloy centre plate

Post by Ed » Fri Feb 15, 2008 11:41 pm

I don't think that is too bad....

If my memory serves...that is the kind of cost I was being quoted for having a plate cut and shaped a good 5-6 years ago.

I may of been unlucky, but I wasn't really able to find anyone who could make up a ali-plate for any less than one of the boat-builders who supply ali plates regularly could.

I always think I can find someone outside the marine trade who can do it for cheaper....but it isn't always the case. Although I think my Tideway plate only cost £40-50, which was cheaper than I found in a marine context....but its tichy and only steel.

cheers

eib
Ed Bremner
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jonathan
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Re: Alloy centre plate

Post by jonathan » Sat Feb 16, 2008 12:29 am

Hello every one and as I have been taking my evening whisky and soda I have read what you have said about ali plates. My boat has a very fine steel plate in an equally narrow slotted case so I think the new one will need to be case hardened after fabrication. I will discuss with my engineer friend and see what he can do.

Oh yes, if any of you run a Sunbeam m/cycle he is your man for bespoke spares/rebuilds/advice. He also makes frisbee golf sets!

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Ed
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Re: Alloy centre plate

Post by Ed » Sat Feb 16, 2008 12:36 am

Nah....Only got a Moto-Guzzi for the road and a Montesa for off....

....OK....if you twist my arm I will admit to also owning a Yam ty175.....but that is a bit like having an Enterbox....so I tend to keep a bit quite about it...

cheers

eib

ps....am looking for someone who makes up clock cables to custom size ....urrrrr cheaply
pps.....also good cheap spokes...

Sure this is the wrong forum for this.
Ed Bremner
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Ed
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Re: Alloy centre plate

Post by Ed » Sat Feb 16, 2008 12:37 am

Oh yes....

....and what on earth is a frisbee golf set?

eib
Ed Bremner
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Firefly F2942
IC GBR314 ex S51 - 1970 Slurp
MR 638 - Please come and take it away
Phelps Scull
Bathurst Whiff - looking for someone to love it

clibb
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Re: Alloy centre plate

Post by clibb » Sat Feb 16, 2008 11:59 am

Just a footnote. Many years ago I was sailing a Lark at Bolton. Getting too near the shore I ran aground and bent the plate. I couldn't straighten it so took it to a steelworks in Bolton and asked them to do it. After some three weeks they called to say they had run it through their rollers many times but couldn't straighten it. I went to collect it, and found it straight. On congratulating them they said, "Oh, it's not straight - it's 10 thou out here and here". I said that didn't matter but they said "You said straight, not nearly straight", and straight to an engineer leaves no room for doubt! I bet it was the truest plate in the country at that time.

Regards

Nick

jonathan
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Re: Alloy centre plate

Post by jonathan » Sat Feb 16, 2008 10:08 pm

Frisbee golf. Sort of university game where frisbees are chucked towards what look like hatstands with chains attached so when the frisbee hits the chains its captured and falls into a basket below. The person basketing his/her frisbee in least number of throws is the winner! Da-daaaaaaa

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Re: Alloy centre plate

Post by Duncan » Sun Feb 17, 2008 4:35 pm

Mmmm!!!!! another entist remark i see Ed :(
DJSpencer-Smith

Michael Brigg
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Re: Alloy centre plate

Post by Michael Brigg » Mon Feb 18, 2008 9:32 pm

Anodizing would be a very good idea as the plate will always get some wear sliding in and out of the case. The wear rubs of the protecting film of oxide that forms on aluminum which promptly corrodes again - leading to high wear rates. Annodising will stop that. Also looks nice and is probably slippery in the same way that graphite paint was (you can ask for satin finish on the anodizing).
Since this thread seems intent upon wandering away from its original subject it is perhaps timely to get back to the subject of aluminium and its various fascinating properties. I note the suggestion that anodising will prevent the plate from wearing too quickly by preventing removal of the surface layer of oxide where it rubs on the plate casing but this explanation is a little misleading as to the exact purpose of anodising, which is to protect the aluminium oxide from the effects of Chloride ions produced by salt (sea) water. In the absence of Seawater or salt such wear and tear would actually have very little effect on the aluminium surface, which has a self-sealing property by reason of its chemical properties.

Aluminium (Al) is an incredibly reactive metal. Anyone who remembers the effects of an Exocet missile on the aluminium superstructure of HMS Sheffield will appreciate this. If you still have any doubt about this then it’s worth having a look at this link to a slightly dodgy recess of the U-tube if only to see what happens to another piece of French engineering!

http://www.youtube.com/watch?v=WrCWLpRc1yM

So, what is it that stands between most members of CVRDA and a swift extraordinary rendition to Guantanamo Bay? (Most of us I believe harbour at least two of the required elements for a Thermite bomb in the close association of our iron and aluminium plates!) For this we need to delve back in time to the A-level Chemistry syllabus.

The answer lies in two words: Oxide, and Stoichiometry. (OK there’s also activation potential to take into account but that’s just another word for a detonator and most of us don’t turn the heating up to 2000’c either.)

A mild steel plate as we all know will suffer from rust. As soon as it is exposed to oxygen the surface molecules of the Iron will form Fe2O3, which is a “Non-stoichiometric” substance. This does not have a regular crystal lattice and so it leaves gaps on the surface, which exposes the underlying iron to further oxidation and so on. This means that the stable oxide is steadily flaked of as rust. The absence of oxygen at extreme depth or when encased in mud explains the relatively good preservation of Iron artefacts from wrecks or for that matter the hull of wrecks such as Titanic. Prevention of rust is only possible by sealing the iron under non-permeable preferably chemically protective paint immediately after treatment such as shot blasting or chemical treatment of the surface to produce a stable coating. Galvanising on the other hand uses Zinc, relatively close to Iron on the electrochemical series to act as a Sacrificial coating, being more reactive than Iron the Zinc slowly dissolves away and effectively acts as a weak battery in which oxidation takes place at the positively charged Zinc and hands over its electrons to the Iron thereby protecting it from the attack of Oxygen. Once the Zinc is consumed the Iron will rust (beware if you have galvanised rigging!)

As noted Al is very reactive. It is the most abundant metal element on earth but is almost entirely in the Al2O3 oxide (AKA corundum, Igneous rock and also Rubies, sapphires and Emeralds.) It rapidly oxidises (ie: Immediately) when its surface is exposed to Oxygen. However the oxide is both exceptionally stable and extremely hard. Furthermore it is also Stoichiometric, forming a uniform Crystal lattice on the surface, which is impenetrable to the Oxygen and protects the surface immediately from any further oxidation. The thermite bomb works because it is hot enough (through the Exothermic reaction) to melt this protective oxide layer so that unoxidised aluminium is continuously available to sustain the reaction, and the energy is suffcient to steal oxygen away from the Iron Oxide, giving sufficient Oxygen to the process to sustain a chain reaction.

However in seawater there is the problem of Salt. This dissociates into Sodium (Na+) and Chloride (Cl-) ions. The Chloride is able to displace the Oxygen in the Aluminium Oxide to form one of the only soluble aluminium salts, (Al Cl3) and so thus breaches the protective Oxide layer and slowly dissolves away the underlying Aluminium. The process is sloww because there is not much Oxygen available in the water that is required for this process, being as it is a rather insoluoble gas.

The anodising process uses the Aluminium product as an Anode, which in turn causes the surface oxide layer to turn its Negatively charged bottom to the surface. Like an army of Kilted Braveheart Scotsman this repels other negatively charged Chloride ions thereby preventing the mechanism of electrochemical transfer that would otherwise leech away the aluminium plate. Perhaps those surface electrons also account for the surface sliperryness, as noted, like graphite

This moves onto the subject of Corrosion, which is all to do with Batteries and the different positions of metals on an “Electrochemical series.” By my book it is aluminium's achilles heel on account of its very high position in the series meaning that practicaly anty other metal in contact with aluminium will set up a corrosion circuit. Starting with Lithium at the top and moving generally up through atomic weight via Sodium & Magnesium to Aluminium and onward through Titanium,Chromium, Zinc, Iron,Cobalt, Nickel,Tin, Lead, Copper, Silver, Platinum and Gold..Unlike oxidation, which is kept in check by properties such as activation energy, electrochemical reactions occur with the effortless ease of an electrical current. This flows from the higher potential to the lower. The Electrons are supplied by an Oxidation reaction at the top of the chain flow from one reactive metal into a metal of lower potential. This causes the aluminium underneath that protective Oxide layer to crave the company of oxygen so much that it will effectively steal oxygen from the surface oxide and thereby allows the oxidation process to penetrate its skin. It lacks the drama of a Thermit , being the chemical equivalent of a John Le Carre novel. Quiet but destructive.

Well I do hope you've enjoyed all that! All in all I wouldn't touch anything aluminium if its not anodised and even then only with stainless steel or other aluminium products.
Michael Brigg

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