After throwing the boat in the tub and figuring out how much weight she needs to put on to sail pretty, I had to figure out how to form that weight. I already planned an ostrich egg shaped lead bulb, but I needed to figure the size of the thing to get the weight I wanted, and then make a plug of it to sand-cast the lead.

It popped into my head to 3D model and print the form for the bulb, then I could change the size and shape to get the volume to get 25 pounds. I have 50 pounds of reclaimed lead shot laying around just for this purpose, but I don't have the tools and materials for melting and casting. Then I figured, why not make the 3D printed bulb a container for the lead, just like my Constellation uses a 2" ID PVC pipe filled with lead shot for ballast, I'll make a plastic egg shaped version of that.

I started by making a ball in Anim8or (my 3D modeling software) with it's surfaces facing inward. Then I looked up how to calculate the volume and found a nice Ellipsoid Calculator where I could plug in numbers in millimeters (what Anim8or uses) and get an answer in cubic inches (what my brain uses). Lead weighs .41 of a pound per cubic inch, meaning I need a volume of about 61 cubic inches. Playing with the numbers on the site, I settled on a=50mm, b=105mm, and c=45mm, giving me 60.62 cubic inches, or 24.9 pounds. The weight of the metal fin, the resin shell, the epoxy fill, etc should put this really close to 25 pounds.

Adjusting my 3D ball into an ellipsoid of those dimensions gave me the inner surface of my bulb. I copied that, enlarged it, and flipped the surface to face outward, giving me the outer surface of the bulb and making it slightly over 1/8" thick. I closed the gaps between the two surfaces, and added a slot and some structure for the fin to sit on. Even in two halves, it wasn't going to fit in the 3D printer, so I divided it into quarters and then I could print 2 parts, or half the bulb at a time.

Once printed, the pieces were bonded together to form a left and a right half. These were filled with lead shot, and epoxy poured into each half to bond everything together and make it solid. The fin has a hole that will be inside the bulb with a metal pin so when the halves are bonded together, that pin removes any chance of the bulb coming off the fin.

The two halves haven't been bonded together yet as it's been too cold in my unheated shop.

In the warm house, I've been 3D modeling boats for Constellation, and a kind of generic inflatable boat for Pride. When I was on board, Pride had four boats;
Two inflatables; a beat-up old faded red or orange one, and a new gray one meant to replace it.
The "Bequia Boat" that went into storage soon after I reported on board, and came back out just before I left the boat.
A lapstrake boat about 16 foot, that sat inverted on the stern davits my whole time aboard. I was told it would need to soak for a month to swell up, and even then it would leak like a sieve.
Aside from the orange inflatable, I'll only be modeling the stern-boat. I have a 3D model of a boat with about the right shape, I just need to scale it to size and print it.
Ellipsoid Calculator: https://www.omnicalculator.com/math/ellipsoid-volume

Finally the temperatures and my time have coincided enough that I may finally get closer to getting this ostrich egg finished.

It seems the resin egg deformed a little from the weight, so the two halves don't lay flat to each other any more. If I do this again, and for you folks thinking of using this method, I'ill model ribs inside the bulb to stiffen it, and maybe fashion some sort of cradle for it to sit it on that supports it so the bulb doesn't try to flatten out as the lead and epoxy set-up.

There were also dry pockets in the bulb that resin hadn't gotten to and I could hear the shot rattling around inside. If there's a next time, I'll place shot and pour resin in layers to ensure the resin gets everywhere it needs to be. What sounded like the biggest of these pockets was behind the flange I had modeled in for the plate to slide into. I opened the holes here to pour more resin in and hopefully fill this void.

I re-drilled the holes for the pin that will keep the bulb from sliding off the plate, and cut about a 3 inch length of 3/16" brass rod for the pin.

I mixed very fine sawdust from my band-saw with the epoxy to make a paste of it, buttered both sides of the bulb, placed the pin, and plate, and put the other half on.

I used a wood shim at he bottom to hold the seam closed on top, where the plate enters, as close as I could, so most of the gap is at the bottom. The epoxy butter filled this in fairly well, but the seam's still open almost all the way around, which I'll fill with more epoxy butter.

The bulb also needs some clean-up from epoxy runs and cardboard that stuck to it; though I intend to wrap it in some 6oz glass cloth as that'll be stronger than the resin alone if the boat should hit a rock or something.

It got really hot for a while there, and I was waiting for a day not-so-hot to work with glassing the egg, instead, I went and had a stroke on June 22nd.
It wasn't as bad as it could have been, but I'm still working back from it. I was away the whole month of September in the US South-east, visiting family, the beach, and a nice museum in Savannah GA which I get into more in my Constellation thread.
Anyway, now I'm home and I'll try getting back to Pride's egg and maybe getting her sailing by spring, or even a nice winter day if I can.

I mixed up epoxy and sawdust for gluing on Constellation's pin-rails, and made some extra to"glass" Pride's ballast-bulb. Instead of glass cloth, I use a piece of pantyhose. Despite the sawdust, the epoxy was still rather thin, and kept running out of the gap I was trying to fill, while I chased it with a stick trying to keep it in there.

It's going to require some sanding, but it'll have to wait till it decides to quit raining, cause I'm not doing that inside.

It turned out to be a nice day, so after mowing, I got out the disk sander and sanded the lumpy bulb out in the driveway. It was quicker work than I expected, so I mixed up some epoxy (without sawdust) and coated the thing. This is sold as "pouring epoxy" so it's pretty runny, and takes a while to set.
Once it set , it got a sand and another lite coat of epoxy; then it was sanded and primed - twice; then sanded and painted Moss Green - twice. I think that just about does it for the ballast bulb.

The bulb, aside from paint drying, is basically done!
In total, 3 coats of epoxy, 2 coats of primer, and 2 coats of Moss Green paint, to match the hull.
The thin epoxy ran all over the place with me, brush in hand, chasing it, best I could; but there's still drags and runs, but it's encased in epoxy and functional. Now to see it in the boat.

While painting things for Constellation, some things for Pride got paint too

While trying to clean up the shop, I got Pride out of her corner, and put her fin in to get a picture
The holes in it are for depth adjustment, from 15" to 13". It weights 25 pounds, so it the primary ballast for the model, though a little will go inside to adjust how she sits in the water. Obviously it's removable, and without it, the models only weights about 20 pounds, even with the battery in.