Monday, March 26, 2012

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.  

Wednesday, March 7, 2012


I decided to build my own spreaders for the mast since I had a partial role of 19.9 oz, 3 inch carbon fiber sleeve (from left over from building my bow-prod and bow prod receiver, and some uni-directional CF cloth left over from the keel.  I really didn't have any idea how strong these should be, but I got some good advice from some of the people on the forum.  

To start, I wanted a nice airfoil design for the spreaders.  The idea of course is to reduce wind resistance, but this shape also has a nice appearance.  In order to get this shape I bought some tapered foam inserts from  These were NACA 0015 shaped.  The standard quick order foils have a maximum length of 36", so rather than ordering a custom shape, I just ordered four 36 inch foils and some shorter fixed chord-length inserts to extend the length of the lower spreaders.  

For the lower spreaders, I wrapped the core with 5.7 oz fiberglass cloth, then covered that with a carbon fiber sleeve, then covered the top and bottom surface with 12 oz unidirectional cloth, and then covered all of that with another 19.9 oz sleeve.  I painted epoxy on the core initially and then on each of the layers that I added.  For the upper spreaders, I did the same thing, but I wrapped the core twice with fiberglass and only used one layer of Carbon Fiber sleeve. 

One of the nice things about the foam cores that you buy from Flying Foam is that they are shipped in the foam that the cores were cut out of.  These "female" shapes are very nice for using as a press after all of the layers of composites are put on and painted.  This avoids having to vacuum bag these things in order to get a very nice finished shape.  So after everything was nice and gooey, I wrapped each of the spreaders in release cloth, then a light plastic sheet and then put the whole thing back inside of the female shapes.  I then put a board on top of the pile and put weights on top of that.  The picture above shows the finished spreaders.  I don't have the exact length that I want them to be yet so they will be cut to size later.  The spreaders actually look quite a bit nicer than in the photo.  The mat look is due to the texture from the release cloth.  I think that I'll varnish these in order to get the nice carbon fiber look.

I was originally planning on mounting the spreaders to the mast with a bracket with a pivot point.  Based on some feedback that I got from Chad on the forum, I think that I might mount them in the shape of a hard V instead of having the pivot points.  

As a final note, these spreaders are extremely stiff.  I can barely get a deflection on the lower spreaders when I lean on them.  The lower spreaders ended up weighing 1lb 4oz each and the uppers weigh 12 oz each.