Some keel and bulb progress.
Here's the bulb with a shallow recess routed into it, about a 1/16" oversize all around. I balanced the bulb and marked its center of gravity, seen as the lateral mark in the pic. I need the bulb's CG to be aft of the blade's 60% chord, so the bulb is a bit further aft than most other builders would choose.
A jig for drilling straight holes into the bottom of the keel blade, using an extra long drill bit.
Counterboring for the washers and nuts. I did this after drilling the through holes, which was a little sketchy but with the drill press it worked alright. I found it easier to block the bulb up and slide the little drill press around under it to position the bit.
The weight check! Tipping the scale at 180#, five under the class weight limit. I'll lose 1 or 2 pounds since not quite all of the all-thread rod gets used, but I'll gain maybe another pound in paint. Hmm, might have to fill those counterbores with lead instead of just filler...
The rods are glued into the bottom of the keel blade, and while their epoxy is still somewhat soft I glued the blade into the bulb recess. That way I had maximum flexibility to make sure the rods wouldn't bind going into the bulb holes.
All bolted up. Next, the rods need to get cut off flush to the nuts, some lead or other filler added to the holes for fairing, and a coat of primer applied to the whole assembly so I can see what still needs additional fairing.
Then it all gets stored in some corner of the shop for several months....
.jpg)
Chad,
ReplyDeleteSorry for being somewhat of a skeptic here but I question the decision to connect the bulb using bolts that go into the end-grain... The quality of your build is outstanding - the pictures on your blog show it beautifully. But in this case I think you are taking a risk with the possible end result of an early keel failure... Just my 2 cents.
Tom (#257)
Hi Tom,
ReplyDeleteAs I put it together, the same thoughts went through my head.
Here's how I'm rationalizing it for now:
There's three basic modes of failure that I can see- one is the bolts simply pull out and the keel falls to the bottom. I'm using 5/8" diameter bolts, deeply buried and epoxy-bonded into the fir core, so I'm not worried about that.
The other modes are that the bulb could basically lever itself off the tip of the foil- either by snapping the tip (shear failure), or by prying the upper end of the bolts out of the foil. If shear is the mode of failure, then it won't matter if the foil is buried 1/2" or 4", it will simply break at the insertion point due to insufficient section size. Assuming it won't break in shear (yes, a bit of an assumption since mine is a bit thinner than others have used), there is a fulcrum point at the bulb/foil intersection that sets up bending moments. To transfer as much of the bending moments into the foil skin as possible, I used bolts that are almost the fill width of the core so that their outer edges are nearly in contact with/bonded to the outer carbon skins of the foil. I also used really big bolts so that they wouldn't bend and progressively load the foil tip (like the zipper effect). By burying the tip into the lead a little (about a 1/2"), and bonding the tip with high density bog, I'm hoping to eliminate the possibility of any displacement there so that the zipper can't get started from the bottom. Also, the bonded portion of the bury helps to transfer the bending moments from the bulb into the foil's skins.
And yes, this is all eyeball engineering, and if I knew exactly what I was doing I could probably do a much better job. I'm going to work out a way to test it a bit before it ever gets wet so I can have some empirical evidence that it works, since I don't know how to run the numbers as an engineer would, nor what safety factor to employ.
Thank you for your comments. I'm doing a lot of odd-ball things on the boat just because to me it represents the opportunity to somewhat cheaply try stuff that rattles through my head, with not a huge consequence if I'm wrong. So I certainly welcome criticism or advice because while I'm trying to wrap my head around all this stuff, I know I'll get a few things wrong. Then commences the refinement and tinkering, which I enjoy as well.
Best,
Chad