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Old Jul 17, 2015, 09:58 PM
Dick L. is offline
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Question
Keel bulb design for specific weight

are there any tables that show keel bulb design/dimensions for specific weight? I am looking for tables where you can enter desired final bulb weight, and the tables will provide (I hope) the length and diameter for a bulb to hit or come darn close to the design bulb weight,. I thought there was one but haven't been able to find it. Any help or links appreciated.

Thanks in advance. Dick
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Old Jul 17, 2015, 10:22 PM
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http://www.onemetre.net/design/bulbcalc/bulbcalc.htm

All you need. Takes a bit of learning how to drive it though.

found it in http://onemetre.net/index.htm

There is always a tradeoff between length/dia (surface area) and drag. I've seen a chart of surface area vs drag relationships but can't find it.
I seem to remember for lowest drag the diameter is around 8 to 10% of length. Makes delicate ballasts though.
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Old Jul 17, 2015, 10:32 PM
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@KIWINB

Thank you. I believe this was the one I was looking at some time back, and forgot to bookmark the URL. Will give it a go and see what kind of shape & size I wind up with. Appreciate your quick response.

Dick
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Old Jul 18, 2015, 05:44 PM
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Attached is a non-lifting, parallel bulb calculator and lifting profile drag analysis for completeness.
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Old Jul 19, 2015, 11:32 AM
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Thanks for the calculator. I am hoping to take a chunk of cedar and turn it on a lathe in garage. I was thinking that I wanted a diameter and a length that would = desired weight. My initial guess is the first response will provide that - or - a darn close ball park shape to begin.

Want to get away from the # 5 or #6 shape (Image above) and head towards the #1 or #2 shape. I have no issue in finding the best total bulb weight - just visualizing how much lead needed to make a good bulb shape. I prefer to be close on the wooden turned plug, rather than having to try to reduce a lead bulb.
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Old Jul 28, 2015, 10:32 PM
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Bulb Moulds

Has anybody tried making moulds of portland cement.Plaster of paris is just to fragile!Maybe even chopped fiberglass in cement.
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Old Jul 29, 2015, 08:17 AM
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bulb mold

The best material I found for making bulb molds is alumina ceramic. You can use a wood plug and make a small box out of balsa to pour the mixed ceramic into with the wood plug inside. After a few hours it will be ready to go. A bit expensive if you buy the material from McMaster-Carr: $85 for 10 pounds, part #8498K35. You can use a mold made out of this stuff dozens of times with no degradation.
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Old Jul 29, 2015, 08:23 AM
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I have been looking at this stuff. Haven't purchased as yet.
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Old Jul 29, 2015, 09:15 AM
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I actually use silicone rubber for molds. You can mold the bulb inside the rubber, then cut a single slit to get it out. One pc bulbs and since the rubber is soft you can cast in a fin slot.
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Old Jul 29, 2015, 09:28 AM
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RTV silicone is cheap and easy. People have been casting lead soldiers in it for years.
Not as cheap as sand mind you, but cheap enough.
I make a box out of lego bricks to pour the silicone in to around the form.
You can either use this box when casting or it you add enough wall thickness the whole thing is self supporting. LIke Slotracer says you can basically encapsulate a nice bulb form with filler and vent, in silicone and then slit it to remove the form once the silicone has set.
Then close if back up with either the orignal box or a rubber band or two and you can cast in it.

here is an article by me for casting tanks - but it's the same process...

http://www.madponies.net/mold.html
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Old Jul 29, 2015, 10:52 AM
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Dick:

I have made several "cast to weight" bulbs for different boats. I use the Bulb Calc software to get the correct dimensions, factoring in the amount of lead that will be lost for the keel slot that is later milled into the bulb and an additional tiny bit because the lead shrinks as it cools in the mold.

Then I turn a hardwood plug and apply a good waterproof finish and let it dry. The box for the mold is made from fan folded foam (BlueCor) parts stuck together with hot glue.

I have tried several materials for the mold, but the cheapest and simplest is Ultracal 30, a gypsum cement used in the ceramics industry http://www.plaster.com/Ultracal.html for duplicator molds. It is super strong, has virtually no shrinkage, cures quickly and withstands the heat of the pour lead so it can be used multiple times. Petroleum jelly is an easy and cheap mold release.

I get it in 50 lb bags from a local ceramics studio - the type where people buy greenware, apply the glaze and pay to have the shop fire it. You can get small amount from a seller on eBay, but the shipping amounts to more than the product.

About the only cheaper molding process is to use sand molds, but even the best technique still leaves a rough surface. The Ultracal molds require only a light touch up with surface putty before priming and painting.
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Old Jul 29, 2015, 01:34 PM
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Bulb Molds

Thank you all for info.and quick response.Now have to decide what material i'm going to use to make mold for several bulbs.
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