Keel bulb mold

 I started construction of the keel bulb mold.  For those who are not familiar with the keel design for the i550, basically it consists of a straight shaft (see below) to which you attach a lead weight at the bottom to help keep the boat upright.  There are some class restrictions on the minimum diameter of the bulb cross section, but other than that the bulb can be any shape that you want it to be.  One of the earliest builders of the i550 had a marine engineer design a bulb with a low center of gravity and hopefully a very hydrodynamic shape.  That design was then shared with the i550 build community.  More recently, Chad from Knot Racing modified that design to include a flat "beaver tail" trailing edge, and went even further in creating a bulb plug to be shared with anyone building an i550.  I was fortunate enough to be the second recipient of the bulb plug, and will send it on to the next builder on the list when I'm done.  

The pictures above and to the left show the the mold being created.  Some builders have created their bulbs in halves vertically which are then bolted together.  I'm going to try to pour a single solid plug.  The top picture shows the bottom half of the mold (really the top of the bulb ) and the picture to the left shows the top of the mold after it was just poured.  


To make the bottom half of the mold I created a wooden box for the bottom part, and then poured in a sand concrete mixture.  I then wrapped the bulb plug in cellophane and then pushed the plug down into the wet concrete.  The bag of play sand shown was used as a weight to keep the plug from floating on top of the concrete.  Once the concrete dried, I removed the plug and took off the cellophane.  I then put the bulb back in the hardened mold and put a sheet of 4mill plastic over it.  The top mold/frame was made with 2x4's and was fastened to the bottom mold box with 1/4 plywood scraps and some nails.  Before pouring concrete for the top half of the mold, I taped some plastic cups to the plug to create holes for pouring the molten lead into the mold cavity.  I then poured the concrete and let it dry.  A small note: I did reinforce the concrete a bit using cut up and bent wire coat hangers.  I really only need the molds to make it through one pour so I thought that this light re-bar approach would be good enough to keep things together until after I'm done.  We'll see.


Once the molds are done, I'm going to have to create a cast "insert" which will basically form the hollow in the lead bulb where the keel shaft will go.  I'm planning on making a mold for the insert using the existing shaft to form the mold.