Lead casting

[Kiteroa 8]

Casting moulds for keel bulbs, What are they generally made from? I'd like to do a bulb weighing around 550kg, using a machined foam plug. I imagine the mould would be 2 peices (port + stbd) as the bulb is not symetrical.

 

Anyone with some experience?

[wheels 543]

It is not a job for the faint hearted. You are dealing with a lot of very heavy and very hot liquid metal that will run everywhere in the event of a mishap.

However in saying that, it is a job that once achieved, can give you a tremendous amount of satisfaction.

The type of Casting you are looking at is called "Investment Casting" and I suggest having a look on the net, as there is some really good info. The more you read, the more you wil become confident.

A couple of big issues to consider.

Firstly the weight. 550Kg of lead is heavy. Sorry to sound patronising, but you would be surprised at the number of people that don't get this through their heads, especially when the lead is molten. Being a liquid, it is hard to imagine the stuff is so heavy till you go to pour it and one of two things happen. As the crucible tips it can get away on the person and if the cast was not supported to take half a tonne of molten liquid, it can fail and you will have a mess as well as a lot of danger.

Investment casting can be done in several ways.

The first question would be is this a one off design, or if this bulb is common, is there a mold available. A foundry maybe the best place to ask.

If it is a one off, then the easiest in this situation is probably to use Sand as the outer Mold. Firstly you need to start with a Steel framed box, with enough clearance to have say a good 200-300mm of sand between the Investment mold and the Steel box sides and bottom. Across the top of the box you need supports to hang the investment mold from.

So you have your foam plug CNC'd, this is called your investmentand you hang that from the frame on top. You need to provide a filling tube and an exhaust tube for fumes and air to funnel out of, or you will end up with blow backs as the foam burns away and creates the gasses and the air is displaced. You need to use Silica sand as the filler around the mold and you pack this in tightly around the mold and those filling tubes/plugs.

The sand MUST be dry or you will have moisture bubble into the bulb and cause defects. The investment allows the use of dry sand.

The Furnace is a different kettle of fish. For 500+Kg of lead, I would be looking at one that can be tipt by a geared wheel. I did see one of trademe not long ago, in the machinery section. You simply can not tip 500Kg of lead by hand and expect to control it.

To be honest a foundry will not be a lot more expensive if even anymore expensive at all than you doing a one off. The big expense is always the Investment and that is where the term came from. You invest in an expensive mold. The rest of it is the cheap part and is used because it is quickly and easily repeated. The Foundry will have the Sand most likely the frame work to support the sand and weight, the furnace to handle a big load of lead, all you have to do is turn up with the investment Mold. They will also have access to lead if you don't have it already.

And finally, lead is toxic to handle and breath. So you need to ensure you have excellent ventilation and breating protection and gloves. Then you have the mess to clean up afterwards. So a foundry may well be the better option.

[muttley 0]

Ring Mike at Mike Rees castings on the north shore, Send him your plug and have him cast you a bulb not too expensive you can even give him the lead yourself to save costs.

 

Safe way to go, Hot metal is not a great thing to play with.

[wheels 543]

I forgot to add, you will need an additive to mark the Lead harder.Usually Antimony is used. But how much is important. So if you have scrap lead, it's a bit hard to know, unless you know what the lead actually is. Likewas it old flashing or something, where the Lead needed to be soft. But if you have a mixture, you really don't know what to add. A foundry guy should have enoguh experience to determin what is needed to be added. If he is starting with his own Lead base, then he will already know it'smake up and know what to add.

[Kiteroa 8]

Thanks for the info. I have an existing bulb that will do for lead. Sounds like providing a foam plug to a foundry is the way to go. I have a nice bulb solid model with the correct volume and shape for my boat so am pretty keen to use that design rather than an off the shelf existing bulb. That way these is no wasted lead volume with a less than ideal fin socket also.

[Fogg 427]

Hey WT, I presume this is for WT itself? If I recall from before you went away you wanted to stiffen things up by going slightly deeper with the same weight keel?

[Kiteroa 8]

Yeah, it's for Wild Thing. Although going down to 2.5m draft is getting a bit excessive for a 28ft boat. So we'll have to think of a solution there... having a deep fin is pretty draggy also. So it's going to be a pretty high tech anorexic foil. Standing under the bow looking aft the keel will look like Victoria Beckham standing on her head.

[wheels 543]

So do you have to go longer rather than heavier?

And how much longer than the original?

[Kiteroa 8]

I'd prefer to keep as much weight out of the boat as possible. As the boat stands now she's high and fast upwind, but suffers aft of the beam. Mainly due to weight and a high volume fin, which is great for lift and generous stall angles at low speed, but not so flash when you want to go quicker and don't need much lift. The current keel is not the original, It's a carbon/wood foil with a modern bulb. Draft is currently 2.0, deeper by 0.1m from the standard plan.

[wheels 543]

Got a Pic of the Keel and Bulb ?

[Kiteroa 8]

Existing keel.

[muttley 0]

Upwind! go fast upwind and add sail area to go downwind and win at both ends

[wheels 543]

And you want a smaller blade again?? Wow.

So how much will the new Bulb differ? or is that top secret?

 

Cord thickness is not the most important aspect because it actually takes little effort to part water and have it flow back in behind. Which is just how a Displacement hull works.

Surface area is the killer due to friction with the water flowing over the surface. But other areas of concern are the trailing edges and the turbulence that comes off those edges.

By making the blade thinner in cord length and overall, longer, do you actually gain anything other than the weight of the Bulb being lower and making the boat stiffer. A similar or maybe even more significant gain could be made with the shape of the Bulb. A "torpedo" is not always the slickest thing through the water. Of course you may already know this and hence the new Bulb you want.

[Kiteroa 8]

Sure, if i'm going to go deeper then I want to keep a similar fin area. So the fore and aft dimansion of the fin will reduce by about 80mm. You can't see it from the picture, but the chord thickness of the foil is about 100mm. That's excessivly thick for the aspect ratio of the fin. The new bulb would be more like a TP52 bulb. With a flat bottom and chines on the aft lower edges, with a thin flat aft edge. It looks really nice so it probably is.

[Fogg 427]

It looks really nice so it probably is.

 

What a great sentiment - and it probably works for foil designs. If only everything else in life was this simple!

[Atom Ant 0]

It looks really nice so it probably is.

 

What a great sentiment - and it probably works for foil designs. If only everything else in life was this simple!

Yeah if only it was that simple with women!

[Fogg 427]

It probably is but we most likely complicate it and look for things that are/aren't there, AA!

[Rhys 0]

Don't you lose volume with investment casting? - ie a plug with the correct volume ends up with a bulb that's a bit lighter than anticipated. My brother used to be a jeweler and they always had to make things larger initially to get the correct end size. Different materials and all, but might be worth checking with someone more knowledgeable.

 

I remember seeing some yacht designer on TV as a kid (can’t remember who) saying that if it looks fast, it probably is.

 

What are you making the fin from WT?

[wheels 543]

You shouldn't do Rhys. You are simply replacing the plug with lead. Originally beeswax was used. That stuff is expensive and hard to get now. Candle wax works OK

Basically any material that melts easily. the only problem with Foam is that the fumes are toxic, Black and thick and full of Carbon and if the vent is not big enough, you can have it block. You also don't want any material that has a slow melt and end up bing trapped in the lead and bubble gasses into it and cause voids and crud to have to clean off later. What ever material used, the Molten lead has to melt it quickly and displace it back out allowing more lead to flood in. All the wax is displaced and the lead fills the entire void. There is shrinkage as the lead cools. If that is critical, it can be compensated for by making the plug slightly bigger than the finished article needs to be. But it's not really quite as simple as that, because the Lead cools unevenly and the final shrinkage tends to be toward the top of the mold. There are various tricks you can do to aid cooling more uniformaly and a Foundry expert will know those and if it is worth the hassle. Fairing maybe the easiest and it can be faired using lead and a real big blow torch.But you can also easily make things worse if you don't know what you are doing.

[Kiteroa 8]

All going well it will be a composite fin. The boat has a socket for the keel fin so changing keels isn't too much of a problem. Just have to stay within the SWL of the keel floors. WT needs quite a bit of work though. Keel is about mid-list.