Aluminium Keel Ballast

Hi everyone

Because of aluminium's low smelting point of 592 degrees C, compared to steel smelting point of 1427 degrees C, is it safe or possible to pour molten lead ballast directly into the aluminium keel box?

 

Yes, it can be done .
Pooring lead in alu keels is done in a very controlled manner. And not only the melting point of the alu is the problem, but also the stresses in the keel caused by the heat, these will deform the keel resulting in problems with the straightness.
Pooring is done with the outer surface being constantly cooled with water. A shower system is prefered over a bath as in a bath air bubles form on the keel surface as water starts to boil and this prevents the surface being cooled.
There are several companies in Holland doing this regularly, but I don't think they'll be willing to disclose their working methodes.

 

You can also make lead ingots of the approximate shape of the keel and set them in resin or other adhesive.

 

Hi Dutch Peter

Thank you for your opinion. What you say rings true, but, Alu has a further disadvantage, it can burn....

Imagine pouring molten lead ballast into you alu keel and, phoof, the goes your labour of love up in smoke if you do not take extreme care. Yip. this is an extreme scenario, but then again, I am born on Friday 13th.

Maar dit is goeie raad wat jy gegee het. Ek sal dit probeer.
Just a sideline people.

Gonzo, ignots can be used and lead shot as well. But I pose the question especially for pouring molten lead into the keel box.
By pouring in molten lead the keel is solidly filled with the heavy stuff, and the CG lower than with ignot or shot. Another advantage is that the lead would be anchored well fast to the keel because of the structural "formers within.

Any other ideas?

 

Wynand,

I have to add to my first post that the biggest part of the keels is normally filled with lead ingots before pooring lead. Otherwise it would not be possible to loose all the heat.
The keels are protected against catching fire in the methods I discribe, but I can imagine this happenening when someone goes about it the wrong way.

Met wat voor boot ben je bezig?

 

Wynand
I would recommend casting separately then lifting into place and bedding in resin or tar. I would paint the aluminium inside , (etch then at least 150 microns of epoxy ) lead and aluminium are (from memory) about half a volt apart on the galvanic scale. If you pour directly into the keel with the metals in contact and get some moisture in there you will have a replating job within 3 months. You cannot treat alloy as you would steel.

 

Mike,

Sorry to say, but you sound a little paranoid. How do you get moisture between the alu and the lead when the lead is poored in at, let's say 500 degrees???
In addition, this methode is used at yards that build vessels with an open account!!!
They don't see that vessel back in 10 years, and when they do, it's for a refit of the interior, and not the keel.

 

Peter

Add aluminium , warm Australian sea water, marine grade alloy and then drop in something more noble (which means just about anything) then sit back and watch it fizz.

I have reported on the result of a 3/4" copper washer that ate through from the bilge to the ocean in just over one month. The mechanic lost the washer so we verified the time scale.
The bilge was damp and unpainted the washer was wedged against a stringer.
Now the owner of that work boat paints his bilge, as we recommend to all Al boat owners.

Even very large commercial craft can have some ridiculous inbuilt corrosion starters, just because commercial boatbuilders do it doesn't mean its ok. I could go on for a while here !

with a bit of flux, lead can be persauded to stick to the surface of steel and the whole lot becomes one solid mass. Aluminium doesn't bond to the lead consequently it works a little and moisture can collect down between the lead and the plate. If it does you have a disaster in short order.

If it is poured then it should be either capped with fully welded plate and or bitumin or resin to fully insulate the bimetal region from any moisture. If you had bare Al and Pb together in a wet bilge the alloy would be lucky to last 3 months before the keel fell off. If you have pinholes in the welds below the waterline then any seawater gaining entry will again cause extensive damage.

Incidentally even with steel keels designers often prescribe either painting or zinc sheeting to line the keel prior to the pour. I have never specified zinc sheeting but I suppose you could use this method with aluminium. The zinc sheet is folded and laid all around the insides of the pour area and the molten lead softens it adheres to it and forms it against the backing metal.

Corrosion inside the hull works quite differently to that immersed outside, you can't work on the ratio of areas to assess likely corrosion rates. The old friend of steel hulls a good alkaline cement doesn't work with aluminium either.

Yep utterly absolutely and totally paranoid when it comes to Aluminium.

 

Mike,

Don't see this as me disregarding your experience, but the point I'm making is that this methode is used on yachts of €50,000,000.=. I'm sure the yard knows what their doing. That you have bad experiences with the combination doesn't mean it can go right. You mention several factors that can be eliminated.

 

Lead Ballast in Alu

There is another, more simple method and used by well known shipyards: no, ingots, no molten lead ( although the way Dutch Peter describes it can do no harm ) Wynand, aluminium can burn but not by smelted lead, it burns only at a very high incendiary. In a mass of fire. Not by just coming in contact with a hot item.
No, the simple method is using hail, like hail in a shotgun cartridge.
It is supplied in bags of 25 and 50 kgs and poured bij way of a pipe in the holde of the keel. After that, and the boat have been trimmed again, the ballast is sealed.
As you say, fair winds, oftewel goeie vaart!

 

Peter

Proff yards are not manned and run by engineers, our involvement is low in the actual building of the vessel, we do the design and the specifications, then you find the yard knows better, and although within the rules the vessel gets approved the corners that are cut shorten the vessels lifespan, all for a few hundred dollars.

The big problem I find is the yard only has to guarantee its work for 12 months then its the owners problem.

I would be very suprised if there is not some sort of basic sealing, even just epoxy paint used in the yards in Holland. If they dont in wet bilge areas than I suggest they will start to do so soon as alloy boats become more common.

D'Artois
Its called lead shot. Problem is the cost and availabiity, in many parts of the world lead shot is around 5 times more expensive than scrap lead. Some builders use lead shot poured in layers and they fuse it together with a gas torch to increase its density.

I have always thought it a good way of handling the ballast since you can shovel it around, shape it and then pour resin over the top. It does need restraining for inversion with capping plates unless its all mixed with resin and runs through the frames.

As an aside, even lloyds gets things a bit skew at times, they approve some dissimilar metals that should not be used.

 

No sealing applied, but to clarify, these keels are generally not connected to the bilges! The area wit lead and alu is dry.

Have to agree on that point, only argument I can put against this, is that corrosion is a very difficult subject. (and sure all the other class societies make the same mistakes!!! For my re-assurance!!)

 

I have to place around 250 kg of lead ballast in the keel of a 28' semi displacement alloy hull that I launched last year. ( I was being clever with long thin hull forms and slightly overdid it)The keel is welded to the outside of the hull plating and is 80mm wide and about 6 mtrs long. ( See attached photo) Height varies from 100mm forward to 550mm aft where the stern tube penetrates the hull. There are formers every 500 mm in the keel, which is nominally sealed shut ( I have no confidence that my welding is pinhole free, having pressure tested too many alloy fuel tanks. The 3 coats of epoxy paint on the outside of the hull will cover a multitude of welding sins and hopefully seal a few leaks ). I really need to drill a hole in the keel base to find out how dry it is.

I am planning on chopping a couple of oval holes into two adjacent keel spaces about 75 x 50 mm in the upper part of the keel from the outside of the hull and pouring in leadshot mixed with epoxy resin. The resin is supposed to achieve two objectives: first it is supposed to seal the lead shot and minimise the galvanic effects of contact between the lead and the alloy. Secondly, it will stop the lead shot rolling around in the bottom of the keel space and migrating along the keel base through the rat holes in the bottom corners of the keel formers.

I do not underestimate the potential for a galvanic reaction between the two metals but effectively encapsulating the lead shot in resin should moderate this. If not I may need to graft on a new keel eventually.

My original plan had been to cast up some lead bricks just narrow enough to drop in the keel and cut bigger access holes in the keel side, but the lead shot will be much quicker. I will weld patches back into the keel side when I have finished.

Lead shot will cost me around AUD$4.00 per kg, when I could buy scrap lead around AUD$1 to AUD$1.50 kg.

If all turns to custard I will let you know.

Cheers

"May your keel never fall off" ( Old Western Australian proverb )

 

My designer has suggested to fill up the space between lead and aluminium with oil, wait for a week to get it to every cavity and then preassure test it. If it does not leak, leave as it is. Might that be a solution to the galvanic corrosion problem? Perhaps even some transformer oil with enhanced corrosion resistance property would do even better?

 

Very nice, and what happens when it does leak?? You're gone weld it??