Easy way to create a lead bulb

error with attached thumbnails.

Hi all,

I saw many ways to build a master bulb : cuting each section of the bulb in plywood and gluing them together, then sanding, etc. Or directly in lead sheets 1mm thickness. It seems to be arduous and to take a long time...

I'd like to describe a simple and faster way to get a master bulb :

First : Use Bulb calculator to define the bulb parameters you need (Naca profile type, weight, ratio lentgh/width, etc.),and print it (easy...)
Then use balsa planks. Thickness dipends of the buld diameter. For a 34mm diameter bulb, I used a section of 10mm for the central plank ( fin thickness being 9mm at the widest ) and 2 section of 15mm for both sides.Total : 40mm; a bit wider than bulb diameter.

Glue the prints on the balsa planks and cut the shapes.

Glue them together with woodglue, then glue another bulb print on the other side and cut the shape.

Trace the center lines of the four sides. Those will be the sanding limits.

Start sanding 45°, then round.

Control the shape all around with the cut-off pieces.

Bulb shape is ready !

I put a G4 coat on it before one or two epoxy resin coats.

Easy and fast ! Next time we’ll see how to pour the mould with synthetic plaster.


This is clearer now ! Sorry, I don’t master all the subtleties of attached thumbails…


That’s how I’m doing it. I make a first cast in one slightly oversize in lead to check or adjust the final weight and shape, then make the final mold. You can always bore out some lead and fill with lightweight putty if you want it a little lighter.

Hi guys,

The box was ready and synthetic plaster was poured. I placed the bulb shape in the center of the box, parallel to it.
Poured the first half of the mold, removed the shape when plaster was dry (12mn), used clingfilm to separate the two halfs, replaced the shape and poured the second part of the mold.
Once dry, I let it totally cool.

Hi Paulin ,
interesting and nice work with balsa, but not fast as you suggested : " I’d like to describe a simple and faster way to get a master bulb"
You started on 22nd of October and today is the 27th and the lead bulb is not yet ready !!
With the lead striping composed of obout 40 layers that I use all the times, the bulb it is done in 2 days max starting from the ‘bulbcalculator’ !! hehehe…and I do not run the risks of breaking the mould or getting molted splashes in the face or breathing lead fumes !!
Don’t mind

Once cooled, I placed the mold back in the wood box and added another plank to prevent the mold to break or explode. I drilled a hole on top to pour the lead.Then put the whole in the hoven to warm it while melting the lead. Even if I was aiming at 1350g and the final weight is pretty good, and the mold didn’t broke or exploded, THAT WASN’T THE SAFER WAY TO POUR LEAD ! The hole chocked twice, there was worrying noises of pressure going out from the mold and the last lead drop gushed out the mold as a geyser… Last, the final bulb looks more like a hand-made cuban cigar than a bulb…
Next time will try to pour half/half…

Conclusion: FORGET ABOUT THIS WAY. TOO DANGEROUS BECAUSE OF PRESSURE (the air/steam/gas is pressed out of the mold from a small hole)


Lucky you !!
BIG ERROR, you did not made any hole to let vapours to escape. You run a very big risk to your health

Was much simpler to pur the lead in the two separated halves and joinning them after trimming with epoxy

OK Claudio, I’m sorry if I offended you, describing your lead striping technic as a slow one. I’ve never said it was a bad one.
Have to know that I haven’t spent five days working on the bulb. Exept the 12 hours waiting for epoxy drying, the rest of work spend few hours.

Anyway, it musn’t be the best way, as I explain in post #7

Best regards,

Once again, you’re totally wright !


Paulin ,
sorry, don’t worry, I’m not at all offended, was just a funny simple remark watching the dates of the messages and comparing with progress of your work !! heheh !!.. .
I’m also happy to see that nothing dangerous happen, but you were very close ! remember !!!


The best way to learn comes generally from one’s own mistakes. But sometimes, mistakes are irreparable… I will remember.
And for my next bulb I may try your method…


Perhaps it is at this point I should interject … “if you sailed a multihull, there is no need for lead - BUT - you have to build at least two hulls!” :stuck_out_tongue:

Just having fun - :sly:

you can still use your moulds but differently and safely as depicted here … some rasp work to flatten the excess of lead and then glass/epoxy bonding.

That’s what I should do first !!! I have to remember I am not a pinoeer to R/C yacht building… but I’m curious minded and so, when I see something interesting me, I try to understand it or/and build it by my self (not only R/C yachts).When I see any method, I try to understand it, and to improve it (not this time…). I noticed that when I understand something, thinking I am the first human being on earth that discovered, thousands people are coming back from this way…
I’m still young (I hope) and hope learning each day of my life.


Hi Dick,

You’re not so far…
I’ve got many projects and no time enough !
My “Open 60” 1:20 I’m working on since january has never been so close to be finished; I still got the Claudio’s “Alinghi SUI 100” 1:20 male mold waiting for me to work on… and since I’ve seen it, I keep in my mind the possibility of building the last “Oracle” multihull, so…


Please keep in mind, we are/were fortunate to have the BMWO (nightime wing babsitter) and also the crew memebr of the winning C Class catamaran champion from Canada contribute to our forum pages. Magnus Clarke (“Blunted” on Sailing Anarchy) has contributed much wing information in the past, and can do so in the future if there is interest. I believe at the moment, he may be relocating to New Zealand to work on the next cup boats…and their wings.

Some discussion already on this forum - do a search on Wings, Wing Masts, Rotationg Masts, etc. for the thread.

I bet these guys wished they had a bit of lead.Link to pic

I don’t get multis, they don’t look like real boats.

I could also point to catastrophic failures of lead keeled boats, but many times they don’t remain floating after their failures, for a photo!

Kites don’t seem like a “real” way to power a watercraft, but they seem to work well …> http://www.youtube.com/watch?v=7iox_-1TAjE

So we don’t get off topic, the problem with plaster type molds, is the water inside the plaster. As te lead heats it, it turns to steam and that pressure has to escape the mold somehow. Plaster will not hold up to multiple pourings, even if it is allowed to dry completely (several weeks). I have made molds from two part silicone that can be poured like the plaster, and can tolerate 600 - 700*. My latest mold is made from refractory mortar. It is specially formulated to be used in high temperature fire brick installations and should hold up to repeated pours. I still “mold in” a pour tube, and a separate vent hole to allow hot air to escape as the lead is being added.

The reason for molds and not the sheet method that Claudio uses, is our need to make many bulbs. When I poured last time, we molded 6 IOM bulbs in an afternoon.

Lead fumes are TOXIC , so use respirator and pour outdoors! Prepare for all mold failures that could result in hot lead spilling out of the mold! Leather gloves and aprons are good too.