Making a ballasted centreboard...any views on carbon fibre and molten lead?

[dancrane]

Okay, it's not a week since I planned on crafting a beautiful rudder-blade from a single piece of hardwood...and I haven't got farther than the Robbins price list yet.

But I was thinking about the centreboard too, and how much easier it might be to use the existing board as a 'plug', presumably in some sort of tray filled with clay...

...then use one of those £40 carbon-fibre kits to make a strong hollow half-blade, just a few millimetres thick...(then another £40 to make the other side)...

...then use epoxy to glue both sections of the thing together, maybe with a stiffening network of rebars inside...and then...(here's the fun bit, I hope)...

...stand in the dinghy-park with the gas stove, melting bits of lead into a saucepan (lead melts at 327° celsius), hopefully on a breezy day to shift the toxic fumes...

...and pour molten lead into the hollow carbon-fibre casing. About four litres should do it - nearly 100lb (and £100) of ballast in the bottom 30 inches of the centreboard!

I'm sure the centreboard-pivot-bolt would need some reinforcing, and I'd need a wire from the bottom of the board, through a hole in the rear of the case with a handle on the end, to raise the board.

But is there any staring reason (besides the fact I've never used carbon fibre or molten lead before) why the concept won't work?


(If anyone is baffled by the reason for even wondering about this, I'm thinking of the benefit of permanent ballast well below the waterline in a large dinghy which will need serious weight to right her when knocked flat. Likely to be problematic when singlehanded, as I expect to be very often.

Looking at footage of Ospreys capsizing, I reckon that in the moments when the boat reaches 70° or 80° from vertical, the fact of approximately eight stone being already parked on the end of the centreboard would often go a long way towards correcting/recovering the situation before it even happens. )

 

[FWB]

Lead shot in epoxy resin might be safer.

 

[Kelpie]

Dan,
have you actually done a capsize drill yet? You might be worrying over nothing.

It seems counterintuitive to spend time and money making a board using one of the lightest materials available (CF) and then filling it with one of the heaviest. Why not just get a flat steel plate and cut/grind it to shape. A few Wayfarers have done that IIRC.

And one further point... I thought you were complaining about Wayfarers being too heavy? How much is your ballasted Osprey going to weigh!!?

 

[DownWest]

Epoxy gets a bit soft at well below the temp of molten lead, so the advice to mix shot with epoxy is better is better. It is less dense, cerca 2/3rds, because of the voids.

 

[dancrane]

Dan,
have you actually done a capsize drill yet? You might be worrying over nothing. Good grief, no! It's midwinter sir, I'm idly dreaming.

It seems counterintuitive to spend time and money making a board using one of the lightest materials available (CF) and then filling it with one of the heaviest. Why not just get a flat steel plate and cut/grind it to shape. A few Wayfarers have done that IIRC. That sounds appealingly simple, I'll keep it in mind. But steel is much, much less dense and I'm not sure I'd get a useful weight of ballast from, say, five square ft of 1/2" steel, if half of it is close to the waterline. My thinking was to get all the weight low down, to be most effective.

And one further point... I thought you were complaining about Wayfarers being too heavy? How much is your ballasted Osprey going to weigh!!? Glad you asked that. When the Osprey was designed, it was meant to have a three-man crew...over 200kg. Only after they allowed the trapeze did it come down to two. So, while singlehanding, I reckon an extra eight stone won't be a problem.

I like the steel-plate idea though actually - much easier to do without skill, than carbon fibre. I'd been thinking of CF just because I know lead has little rigidity, so I was supposing a carefully-moulded strong outer would be necessary.

I wonder...10mm steel plate, with layers of lead-flashing attached and hammered into the aerofoil shape around the bottom? I can already picture the centreboard case dropping out of the boat, halfway across Lyme Bay.

Epoxy gets a bit soft at well below the temp of molten lead, so the advice to mix shot with epoxy is better is better. It is less dense, cerca 2/3rds, because of the voids.

Ah. I'd heard that it was easily tough enough to withstand weeks at lead-melting temperatures, that's why I thought of it. As to lead-shot density, the gaps are the reason I wanted to melt the stuff instead.

 

[Kelpie]

I mentioned all-up weight not because of sailing concerns, but simply the difficulty of hauling out afterwards. I remember something you posted earlier about not being able to get a car to the slip?

 

[macd]

I'd definitely be worried about heating epoxy resin to lead-melting temperatures. Many are significantly degraded at half that temperature.

If you want max density for volume, why not make a plug for the lead, from which you can make a mould (cement works fine), then fettle and fair the cooled lead and sheath in epoxy cloth? Lead is easy to work -- an electric plane carves it like butter.

 

[dancrane]

I mentioned all-up weight not because of sailing concerns, but simply the difficulty of hauling out afterwards. I remember something you posted earlier about not being able to get a car to the slip?

Ah. Hmm...I...err...I did say this was an idle thought, didn't I? The hauling-out weight is something that I idly hadn't thought of.

Actually I concluded that my earlier hauling-out concerns can be dealt with, using a hook at the top of the slipway and a line running from it to the trolley (with a jockey-wheel), then round a nice superfluous ball-bearing ratchet block and back up to a smooth 12" timber bar for hauling on...so the customary hauling-weight would be approximately halved. I was nearly able to haul her up the 1-in-6 just by pulling on the trolley. I'll try it out in the gloomy months to come. But added total displacement needn't destroy the lead keel idea.

...why not make a plug for the lead, from which you can make a cement mould, then fettle and fair the cooled lead and sheath in epoxy cloth? Lead is easy to work -- an electric plane carves it like butter.

Nice idea, thanks Mac. Sounds foolproof, always a benefit for me.

 

[Avocet]

Depending on the geometry of the slot for the centreboard, it might be better to use thinner steel and then you can get a small "bulb" welded on the bottom. Having the weight above the waterline is counter-productive. Having it "on" (or just below) the waterline will add weight without much benefit in righting moment. In fact, as the boat heels further and further, there will come a point where it becomes counter-productive! Also, if the unthinkable does happen and you get the mast in the water, you'll need to be careful that the centreboard is well restrained otherwise it might swing up under its own weight - somewhat violently.

 

[dancrane]

Depending on the geometry of the slot for the centreboard, it might be better to use thinner steel and then you can get a small "bulb" welded on the bottom. Having the weight above the waterline is counter-productive. Having it "on" (or just below) the waterline will add weight without much benefit in righting moment. In fact, as the boat heels further and further, there will come a point where it becomes counter-productive! Also, if the unthinkable does happen and you get the mast in the water, you'll need to be careful that the centreboard is well restrained otherwise it might swing up under its own weight - somewhat violently.

Good reasoning, thank you Avocet. The guillotine potential of a centreboard returning to its slot, as hands are grappling for the upturned centreboard case, isn't a pleasant thought.

I'd be fairly set on using maximum density of metal, as low as possible. Particularly because the Osprey's centreboard comes to quite a narrow point, so simple steel won't have a very powerful righting effect - the majority of the added weight would only be a foot below the boat. Maybe as I thought earlier, very slender steel plate, bulked-out with lead flashing at the bottom and planed to the aerofoil curve? How rigid is 8mm steel? How thick is rigid enough for a man to stand on one end without the plate losing shape?

 

[ningcompoop]

You could make the centreboard from laminated ply, drill or route out a suitable sized cavity somewhere near the bottom end of the board, pour molten lead into the cavity - you could dampen the wood if your worried, it may scorch but shouldn't O) catch fire, then cover with another laminate of ply and/or epoxy and glass. You can then produce a nice a board with a good foil section, weight at the lower end, and a good width fitting for your centreboard slot, so it doesn't rattle whilst you sail!

 

[dancrane]

Interesting idea!

 

[misterg]

you could dampen the wood if your worried

Seriously !?

Anything you use for melting / casting needs to be bone dry or you risk a steam explosion spraying molten lead at you - or at the very best, porosity in the casting.

 

[lw395]

It will take quite a lot of lead to have any influence. 10kg at the end of the board is going to be a marginal difference.
You would be better off spending time sorting out righting lines that work well, to make better use of your 80kg or whatever body mass.
We looked into this to make a dinghy more stable when left on a mooring, and did the sums for capsize as well.
Don't make the mistake of only thinking about flat water, that's easy, the difficult capsize recovery is when wind and waves are trying to invert the boat.

 

[Seajet]

Dan,

I sailed Bosuns with steel centreplates, raising the plate was no problem but it took an army to get the things up the beach ( not just due to the plate ).

Unless you're thinking of keeping her on a mooring or anchor I'd suggest going the other way and having masthead buoyancy, on my Contender - which proved to be a sod to right - I filled the upper mast with foam and fitted an external halliard, preferring survival to the last smidge of performance.

I'd think something neat looking like the ' Visiball ' radar reflectors ( not the fender-like Firdell Blipper ) might do ?

http://allgadgets.co.uk/marine/pc/viewPrd.asp?idproduct=187

 

[Kelpie]

We looked into this to make a dinghy more stable when left on a mooring,

I guess you have thought through all this, but you'd need some serious weight added to the centreboard to make it worth putting it down whilst moored. Usual advice is to keep the board up when moored so that the boat just skids sideways instead of 'tripping' over the board.

 

[Searush]

Don't you think that designers may already have considered some of these issues before designing the boat the way they did? If you want a balasted keel then an old Flying 15 would make a lovely day boat, or any or the 80's lifting keel micro-cruisers designed for one design racing & weekends away.

 

[dancrane]

Don't you think that designers may already have considered some of these issues before designing the boat the way they did? In principle, yes...but I doubt Ian Proctor will have spent much time pondering whether to add a hundredweight of lead to the centreboard of his new racing dinghy! It's very much my own project, for my own singular purposes. Eccentric, yes; but worthy of consideration I hope.

If you want a balasted keel then an old Flying 15 would make a lovely day boat, or any or the 80's lifting keel micro-cruisers designed for one design racing & weekends away. Yes, I love the smaller keelboats, but the fixed keel limits their versatility. And I love micro-cruisers, but not their price (or performance). I'm only hoping to blur the divisions a bit.

My maths doesn't extend very far, but...

...is it the case that (ignoring the improbability of the numbers), assuming the boat is exactly on her side, 50kg of weight at the bottom of a keel 3' deep, would balance with 5kg, at the top of a mast ten times the height?

In the dinghy park this summer we had the boat on her side, and it didn't seem to take a lot of lift to keep the mast level - far less I'm sure, than would have been needed to keep a lead centreboard level, sticking 3'6" out of the other side! So...notwithstanding the inclination of a centreboarder to 'trip' over a lowered board, isn't it likely that having heeled as far as 90°, the keel would still right the boat? I guess not, since light crew members standing on the board routinely seem unable to right a big dinghy.

As far as masthead buoyancy goes, this summer I tied two x 2 litre lemonade bottles together, threaded the line through the head of the sail, to another two x 2 litre bottles on the other side. I reckon 80 Newtons of buoyancy, right at the masthead, would prevent inversion...though of course I want to believe that.

 

[Avocet]

How rigid is 8mm steel? How thick is rigid enough for a man to stand on one end without the plate losing shape?

I don't know how long the board would be - or how deep (fore-aft direction) but I'm assuming something like 3' of unsupported metal poking out from the bottom of the boat, and maybe a foot deep? If so, I'd have thought a bit of 8mm mild steel would be fine for even quite a heavy crew standing on it! After all, the biggest bending moment will be whatever it takes to right the boat, rather than the weight of the person (whichever is least).

 

[Twister_Ken]

Actually I concluded that my earlier hauling-out concerns can be dealt with, using a hook at the top of the slipway and a line running from it to the trolley (with a jockey-wheel), then round a nice superfluous ball-bearing ratchet block and back up to a smooth 12" timber bar for hauling on...so the customary hauling-weight would be approximately halved. I was nearly able to haul her up the 1-in-6 just by pulling on the trolley.

Well, in that case flog the Ozzie and buy a Wayfarer (or better still, a Wanderer). Less of a wife-frightener.