Making the Keel of So Close

Preface this is a blog about making a keel for a model boat. If you haven't read the post about that it won't make a lot of sense - Link to the original So Close post.

So Close needs a high-density low heavy keel to keep it the right way up. Sounds easy to make something heavy, but it also needs to be a streamlined shape and have some means of fixing it to the two bits of sheet metal that it will hang between. Still doesn't sound very hard? Just 3D-print a mould of the shape and fill it with concrete, cast in a few bits of threaded rod and job done?

Not quite so easy, So Close needs a 6kg keel, but the density of concrete is approximately 2.4kg/L, so 2.5 litres of concrete. Not a small mould to print, but not impossible if done in a few bits. The problem is when put under water it will displace 2.5 litres so now the concrete only weighs 3.5kg and it's huge. Now, of course, I could keep adding more and more concrete, not a very practical strategy, or pick a more dense substance to use. Lead is a classic choice. Nice and dense at 13kg/L. Easy to cast and very practical. Can poke a few holes in it with a drill. Slip a bit of threaded rod in, job done. However, preparing a green sand mould is hard work. Melting the lead is a pain. Lead is not cheap. If it turns out too heavy or light, then I will have to start again (this was back in the design stages). 

A quicker and easier keel design was what I needed. Lumps of steel in a 3D-printed fairing is what I needed. The steel is a good compromise. It's about half as dense as the lead at approximately 7kg/L, is economical to purchase and readily available. So I set to work designing such a keel. I found I could purchase steel rods for a good price online. Can even have it all cut into the lengths I need for free (from metals4u - no joke).  My big problem was once the round rod is packed neatly in hexagon shapes, it left gaps between each rod costing more density and making the keel bigger and slower. This is the circle packing problem. 

Luckily I have 100% solved the circle packing problem (a problem plaguing mathematicians for thousands of years). 

The solution. Purchase square bar. They fit together without gaps and are just as cheap as the round bar. 

This worked well. I printed a fairing and stacked all the rods up. By a stroke of luck more than judgment, the fairings fit very tightly on the rods so the complete assembly held itself together. I fitted it on the boat and we were off sailing. It worked well till I got it back out of the water. The tiny gaps between the rods were not all that flat and accumulated water. The water rusted the steel bar and then leaked out all over my car. Less than ideal. And it's still huge. 

The keel needed a rethink.

Lead is still a good choice and now I know how heavy the keel needs to be.  I could have made a single monolithic casting, but to save work in the future I designed a modular keel part. This is a streamlined shape cast from lead that can be stacked up next to each other to reach a target weight. I designed an aluminum mould tool to cast the lead keel sections and cut it out of a solid billet of aluminum at my local maker space. I made 6 castings and drilled holes in them all. Bolted them all together and we were off to the lake again. The new lead keel should give much less drag in the water and is less trouble to handle. It also gives the handy ability to make more lead keels in the future of any required mass (in increments of 820g) with a streamlined shape.