Monday, October 12, 2009

Bring on the heavies

So this week and weekend I did some gross tuning on the keel bulb.

I started out by drilling the holes for the cross bolts. I first drilled the recess for the bolt head using a 7/8" standard spade bit. Slow and steady is the name of the game when drilling lead. I know that other builders used loads of cutting oil, but I opted for water as I wanted to avoid extensive cleaning before trying to apply fairing filler. I also set the clutch on the drill to about half to avoid shearing off a drill bit. This worked great. After the countersink was done, I drilled a 7/32" pilot hole through the first bulb half. I then pulled out a 12" long 5/16" diameter brad point bit. With the drill on the low setting, small "bites" into the lead worked awesome. Pulling the bit out to clear it after 50 or so revolutions seemed to be the key. Once I made it through the first half, I laid the bulb over the keel foil to drill that hole, and then laid both on the second bulb half and drilled through. Finally, I drilled the counter sink on the other side.

If I had to do it again, I would have bought a long bit to drill a 1/4" pilot hole all the way through. With a 5/16" hole on the second half, it was difficult to drill the counter sink without the bit wobbling and I had to lug the 80-lb bulb half to my office to use the drill press. Incidentally, I tried a 1" Forstner bit on the second set of countersinks, and when using low RPM, it worked a treat.

OK, so the bolts are drilled and I am good to go, right? But, it turned out that the cast bulb halves contracted quite a bit as they cooled in the mold. This left the a 1/2" - 5/8" gap between the halves when I put then together. A preliminary weighing told me than I was also about 10 lbs below max weight. So I came up with a decent solution to fill the gap and add as much weight as possible. I started by folding a piece of aluminum foil into ~1" strips (probably 8-10 layers thick) and layed the Al strips onto the tacky side of regular old duct tape. I then used my homemade foil tape to create a dam to seal up the gap so that I could do a second lead pour.

My scheme worked great and the duct tape only melted a little. Best of all: no leaks! The second pour probably added about 8 lbs to the bulb weight and I am darn close to max class weight (185lbs for the bulb and keel strut).

I now have a coat of fairing filler on the bulb and fairing continues on the rudder, but it is quite close.

Stay tuned!

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