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Core Sound 20 Mark 3 Build - Chesapeake, VA


AmosSwogger

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Amos, so sorry to hear about your friend Terry and the other victims. We are on the other side of the world and somewhat protected from that level of violence, but we still hear the news and there are many in Australia who feel for our friends there. It is significant for my family because my sons live in LA and travel a lot within continental US for their work. Thinking of you!

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  • 3 weeks later...

Round one didn't work! :D

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I couldn't get the template out of the concrete. I did wrap the template very well in packing tape (I took the picture after I removed the tape).  I actually screwed a bar to the top of the template and suspened the template with ropes from the kid's swingset, and then hit the concrete with a sledge hammer.  The weight of the concrete was effectively hanging from the template.  That didn't work.

 I thought about burning it out:  drilling holes, pouring oil down the holes, and then lighting it on fire!  That would have been fun, but I decided to break the concrete up with a chisel and save the template.  The packing tape worked well, the concrete easily broke away with the chisel and released the template.  

Although the centerboard tip isn't very big, there is a lot of surface area and the concrete contracts as it dries.

 

 

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I cast in plaster. 

 1. Build a wood box to hold the plaster. Leave about 1" clearance. The box should also be high enough to be 1" over the tip.                                                                                    2. Securely attach the c/b tip to a "base plate". Maybe screws and epoxy.  This is your "pattern", and the bottom of your box.                                                                                   3. paint the pattern. Maybe leave the box sides off until you've painted. Be sure it's smooth. You could use "rattle can" sandable primer, sand it smooth with about 320 grit wet or        dry, spray a coat of paint over it, or just leave the sanded primer.                                                                                                                                                                                4. Wax the pattern. I've heard that you can coat it with oil rather than wax, but I dunno.                                                                                                                                                5. Pour the plaster over the pattern and fill the box. Leave the plaster mold in the box.                                                                                                                                                  6. Let the plaster dry thoroughly. I put it in an oven set at low for a couple of hours.                                                                                                                                                    7. Pour your lead. Be sure the top is flat and level as you can get it.  When cool, the lead will        drop right out----maybe.

It works for me.

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I had issues with casting as well. I did leave the CB tip attached to the rest of the board until after the mold was made. That gives you a better grip on the tip, but you can work around that. For the tip to be 'easily' removed from the mold it needs to have a continuous taper. It is easy to have a flat spot or slight bump or bulge. If you have two inch strips of 6mil plastic wrapping the tip parallel to the long axis of the board you may be able to pull them out after the concrete has hardened.

I did everything I could think of and still could not remove the tip from my mold.  Brother in law suggested cutting a groove with a concrete saw along the outside edge of the concrete but shy of a depth that would contact the perimeter of the board. After that a few taps of a hammer caused the concrete to release the tip. The concrete was still intact though there was a hair line crack visible. Even if the mold split in half replacing it in its wooden box would have made it functional. After letting the concrete cure a week or so I placed an electric element intended to be used to start charcoal in the mold for a few hours. This drove out some remaining moisture even though the concrete appeared to be dry. Dry mold tres important.

Round 2

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Joe, I had read your excellent blog before I poured the concrete.  I remember thinking that I had better wrap the tip well in packing tape so it releases easily!

Chick, good idea concerning the use of plaster.  My tip does have a constant/consistent taper.

I'm just happy I was able to save the tip.  It released so easily from the concrete after I chiseled away a few big chunks that I'm surprised it didn't release after I suspended it.  I think there is also a stiction/suction force that has to be overcome.

At the Messabout Jay mentioned using green sand; I need to look into that.

Alan, how do the pros at B&B do this?

 

 

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Hi Amo, we did pour our center board tip at B&B.  We made a simple plywood box and partially filled it with the green sand (google recipe, but Graham had a box of it).  Cut the tip of the center board from the blank and placed it vertically in the box.  We then filled the box with the green sand and tamped it down with a hammer to get rid of most air inclusions.  Then slowly pull the board tip from the sand, pour molten lead in the mold.  It was a sandy surface but a old block plane smoothed it out with very little work.  Hope the mold picture helps, best part, the sand is reusable.  IMG_0943.JPG

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Thanks for the pictures Jay.  I'm going to go the green sand route.  I picked up some sand from HD, now I need to wander the women's health and beauty isle and find some bentonite clay (used to make beautifying face masks).  Hopefully no one will I know will see me in there.  I'll give the excess clay to my wife; if I'm lucky she won't take it the wrong way!

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Here is how I made the centerboard (with thanks to Steve who posted the tablesaw idea):

Glue up the blank and ensure the grain is alternating.

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Flatten the blank (I used hand planes: it was little to big to fit through a 15" planer).

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Glue a foil shape (provided by B&B) to the end grain of the blank:

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Set the saw blade to the highest height that will be used on the foil template, then draw a line corrosponding to where a saw tooth just disapears beneath the table saw table.  Align that mark to where you want the shaped area to transistion to the flat area of the centerboard . . .

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. . . then clamp a stop on an outfeed table.

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Make progressive passes over the tablesaw blade, adjusting the height of the blade with each pass to match the template (this would be faster with a dado blade; I meant to use one but was too lazy to change out the blade).

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Assign your kid to chisel away the ridges.

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B&B provides an really good template; use an awl to transfer the lines and cut the shape at the bandsaw.

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Complete the shaping/fairing; I used hand planes and a RO sander (I don't have a picture of this).  This took me a long time.  The board tapers in length as well as width.  The template B&B provided had two foil profiles; one for the top of the board and a smaller one corrosponding to the transition to the lead tip.  These two shapes were really helpful in determining the amount of taper along the length of the board.

 

A convex soled spokeshave helps fair the transition to the flat area.

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I cut the tip off at the tablesaw using a sled and shims to keep the centerline of the board at 90 degrees to the blade.  The weight helps keep it stable; you want a good straight cut here so your lead tip has a good mating surface.

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It is rewarding to see a nice foil shape after the cut.

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I did something very similar Amos, and am happy with the result. I kept the cut-off as a memento. The concrete method worked for me with lots of tape and wax to reduce adhesion, but I am sure that the green sand would have been better. I glassed the centreboard and now that the boat is upside down I trial fitted it into the slot (with heart in mouth). Hallelujah, no binding between board and housing.

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I used a router and set of templates to check the work. My board was differently shaped than stock and needed a way to guarantee the sections where the same on both sides. The table saw trick works well on one side, but once flipped it becomes pretty floppy, so I made templates for every 6" along it's length and these were placed on the foil blank, so the router base had something to follow. The foil templates were cut and rested on some rails, positioned fore and aft of the blank. The router was set to plow a flat bottom bit across the foil. Since there was a taper from top to bottom, I thought this more accurate. There's lots of ways to do it, this was just my way. I too ballasted the board, though I buried the weight in the lower leading edge of the board, rather than cut the tip off. The tip idea is better, as it'll take plenty of damage, without much being done to the actual board and dented lead is easy to repair, if not sheathed.

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