Bulb Location on Keel for Chad's Bulb Design?

[Warren Nethercote]

Chad (and others who have used this bulb),

I am looking at making a pattern for your bulb design in the 'useful files' section of your build blog. How have you located the bulb nose (or mid-point if you prefer that reference) relative to the keel nose (or mid-chord point if you prefer)? Do you have a centre of gravity location (relative to nose or tail) for that bulb shape?

(As an aside, driveway foundries don't excite me, so I got a quote from a well-known foundry for a bulb: I have only just stopped hyperventilating. Maybe there is a driveway foundry in my life after all ...)

[i550sailor@aol.com]

No worries Warren, a driveway foundry is just a turkey fryer, stainless steel ladle and pot (from Goodwill) and free used tire weights,,,, away from a finished bulb.... Don't forget the Organic respirator.

I found the balance point of my bulb by balancing it on a 1"X1" piece of wood, marking that point, and then centering that on my keel board.

[Dan]

We used Chads files for standard bulb and worked out great, super fun project and very easy to do. We opted for split bulb halves and are going to through bolt to keel.

We chose standard bulb 880mm long and like Warren would be interested to hear about fixing point of keel over original 762mm bulb.

Regards to all

[Warren Nethercote]

Dan,

Your bulb looks somewhat smoother than other 'driveway foundry' bulbs on the site, albeit with evidence of some surface clean-up. Did you do anything different in terms of pattern or mould preparation?

i550sailor aligned bulb CG with fin mid-chord, which seems logical from a structural perspective, although I think I read something from Chad suggesting that the bulb CG be aft of the fin centre of pressure. My concern is having the bulb too far forward which might lead to flutter (although the Aussie boat that experienced that had an overly-long fin). Pragmatics suggests that a bulb CG centred under the foil would reduce the chances of binding in the slot when raising or lowering the keel. FWIW

[Dan]

Hi Warren, It actually came out very smooth, the marks you can see are the start of creating a key for fairing to begin.

Obviously mould will only be as good as the plug. We CNC routered ours out using Chads files (though we did make a few changed to 'smooth' lines between stations). When plug was glued up we faired it with microbaloons until smooth and 100% fair. Two clear coats of epoxy and it was super smooth. Then cut in half and cast in concrete, no plastic. 40 MPa with rebar which was kept wet for a week and then let to cure out for 8 weeks. We melted lead in large cooking pot over HP LPG gas burner and used a torch to keep mould molten between pours with ladle.

We used scrap lead which was a little hassle due to other rubbish in lead, but was cheap at about $75 for the bulb.

If I was to do it again (beaver bulb) I would pre clean lead, and allow concrete to cure for longer.

With smooth concrete release is easy. Use a sheet finishing sander in a plastic bag to vibrate the sides of your mould and a small tamping stick and you will get it as smooth as your plug.

Fun times

[admin]

using a handheld propane torch, to heat the stuff from above, simultaneous with the Turk Fryer, will expedite the melting process significantly.

[ryderp]

Warren,

To address your original question, I lined up the center of gravity of the bulb with the thickest cross-section of my keel foil. No particular reason for doing this, but it had a good look and has a reasonable for/aft balance.

Phil

keel.jpg

[Warren Nethercote]

I broke down and did something about my original question, the old school way. I am not building any time soon (I have other jobs to finish), but decided to progress the bulb and drew up waterlines for Chad's beavertail design, to build a traditional sandwich-construction wooden pattern. I wanted an estimate of CG up front so I could build the recesses for the keel bolt washers and nuts into the pattern. So I dug out a planimeter that was buried in the desk drawer and took areas off of Chad's pdfs of sections. With a bit of Simpson's second rule I get a CG of 14 1/2 inches aft of the nose. I wonder how that compares with reality? (Edit: I may be adrift - my weight estimate is only 143 lbs.)

[Warren Nethercote]

Supplemental to yesterday's post.

I drew up the fin landing on top of the beavertail bulb that I posted yesterday, with balanced leading and railing edge 'overhangs'. I have shown it landing on top of one of my pattern laminations - that would make forming a recess for the fin in the bulb pattern easier (I can cut the recess in the lamination before glue-up, rather than using a router). I show bolt locations in the fin per the plans, and lo and behold, the bulb CG position relative to the aft bolt looks like the same as for the 'stock' bulb (see drawing 60, "Keel Bulb Center of Gravity").

Chad, did you do this on purpose, or was it a happy accident?

[Chad]

There are three bulb designs floating around: the one on the plans, which is a turned, round section almost foil shape; the pin tail "squashed" design by Tokyo's friend Brett (which I used, and have build patterns for on my blog's files section); and the beaver tail bulb, which is a slight modification of the pin tail bulb (same weight, same curve of areas), which I've done a plug for, and which Phil and a few others have used, and is also in the files.

I didn't design the CG of any of them. In fact, I slid my bulb quite a ways aft on my foil, so the CG is about at the 60% chord of the foil, but not for lifting convenience or water flow (jibing keel). I used big threaded rods to mount my bulb to the foil bottom, so I didn't care where the bulb fell in relation to the rods. I did rout a recess in the top of the bulb to slightly capture the foil bottom.