Centerboard

Graham has redesigned the centerboard for EC22. He wanted to remove an annoying touch of lee helm. which became worse when the board was partially raised as when sailing close hauled in shallow water.

The new centerboard has a lead tip sized to make the board sink and stay on the bottom but kick up readily if an obstruction is encountered.

First task is to laminate a dozen or so boards to make a 61 inch by 16 inch blank. Laminating from a lot of small sticks is time consuming but allows you to select for good straight wood and control things like grain orientation. Hopefully this results in a centerboard that has little inclination to twist, bow, warp, crack or mess up.

There needs to be enough lead on the board to make it sink and stay down even when the boat is at speed.

 Graham suggested about 15 pounds of lead. Someone else used about 18 pounds for a similar board.

 I decided to try to calculate the weight of lead needed and the amount of the board that should be lead.

That is should the bottom 2 inches of the board be lead or bottom 4 inches or 6 inches?

I could figure this out empirically by taking the completed board out into the water and piling weights on it until it sank. Then add a few pounds for good measure.

Another way involves calculating the volume of water displaced by the board and using the density of the board and the density of water to arrive at the amount of lead needed.

I carefully measured and weighed my centerboard blank to determine a density or 0.017 lbs/cubic inch.

The density of water is 0.036lbs/cubic inch

The density of lead is 0.409lbs/cubic inch

The density of my Douglas Fir centerboard is about half of the density of water.

After the blank was glued up and smoothed I cut it to shape in two dimensions. Then I marked the edges in preparation for shaping the board in the third dimension.

Did I mention that some centerboards are flat, however a board with a air foil like shape is more efficient.

 

I used a plane and a draw knife to bring the board to shape.

I was able to transfer the profile of the board from the plans to some scrap plywood. These helped guide my progress.

Here the board is nearing its final shape.


The weight of the blank was 22 pounds.

The weight of the shaped board was 15 pounds.

15 pounds divided by the density of the board 0.017 lbs/cubic inch =  882 cubic inches.

Not all of the board is submerged when it is installed in the boat and lowered only about 700 cubic inches.
700 cubic inches of water weigh 700 x the density of water 0.036 lbs/cubic inch = 25.2 pounds.
700 cubic inches of board weigh 700 x density of board 0.017 lbs/cubic inch = 11.9 pounds

25.2 pounds- 11.9 pounds = 13.3 pounds of lead would just make the board neutral.

I want to have the board sink with authority so I will add an extra 3.7 pounds for a total of 17 pounds of lead.

How much of the board will be taken up by 17 pounds of lead?

Archimedes anyone!

I put some marks on the board.

Up until this point you may have thought I was pretty clever.

Several things could have been done to make the removal of the board from the concrete easier. I do not know where I was but alas none of them were done.                  
I hung the board from the ceiling and used progressively larger hammers to pull the board from the stone.        Fortunately before doing any serious damage I moved to plan B.                                                                                                                                                                           

                           

  Plan B was to use a concrete saw to cut a shallow kerf around the block and than strike the block in order to split it open. I heard a change in the sound of the hammer and slowly I was able to work the board free,   though I was not able to find a crack in the block.                                                                                                                                                                                                                                                                                                  This is when I said                                                                             



                                                          Eureka !

Next we need to cast some lead.                                                                                                                 

I wanted to insure that the concrete was absolutely dry before pouring molten lead. Instant vaporization of the moisture can lead to a small explosion spraying molten lead everywhere.

After a brief consultation with Sally I decided that the 60 pound block of concrete was not going into the oven.

Fortunately I remembered seeing an electric element intended for lighting charcoal on the shelves in the cellar.

 After the element had been on for several minutes I noticed steam rising from the form. Looking inside I could see drops of moisture in the mold. They were being drawn out of the apparently dry concrete.

I added lead from an old centerboard and wheel weights to an old iron pot heating over a backpacking stove.

  After about an hour the lead was molten. I then skimmed the dross from the surface of the molten lead.


I tried to stay upwind.

 Finally the big moment. Pretty incredible watching the molten lead flow bright and silver thin as water.

I had wanted to make the pour in one shot but I ended up a little short and had to melt some more lead.

I think I should have kept the surface of the lead in the form molten by working it with a propane torch, but I did not think of this until later.

The result is a bit of a cold joint, but I do not think it really matters for my purposes.

   I thought the lead looked very good even the surface seemed free of impurities.


I let the pour sit overnight.

I removed the box and after a few wacks with a hammer.

I think it looks beautiful. Not too shabby for my first casting.

The target weight was 17 pounds.

 I will lose a little when I dress the casting, but I am sure it will end up over 16 pounds.

More later.


Lead is fairly soft. I used a rasp and a block plane to shape it.

     

I considered placing some pins to help align and attach  the tip to the board. Graham assured me that a simple butt joint reinforced with fibergass would be strong enough. A pin located near the center of the axis would provide little additional strength. Intuitively it just seemed a little dicey. The lead is sooo heavy.

I drilled a hole in the lead tip and jammed an epoxy soaked rope into it. The rope lays in a groove that runs down the leading edge of the board. I placed the rope under about forty pounds of tension until the epoxy was set.

    

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I draped two layer of 6 oz cloth around the leading edge and about two thirds of the way down the board.

Then I flipped the board so the trailing edge was up. I draped three layers of 2 oz cloth around the trailing edge and to the middle of the board. The lighter cloth was able to make the tight turn at the trailing edge

 Here I am using a roller to apply epoxy to the 2 oz cloth.

The fiberglass cloth has been applied and wet out. now I am filling the weave with epoxy to which a little carbon has been added. The carbon epoxy mixture results in a low friction and abrasion resistant finish.
So hopefully the board will rotate up into and down out of its case smoothly.


Speaking of which the case is barely larger than the board in length, depth, or width.




Here the nearly completed board is laying on one half of the case.

 I decided to test empirically the buoyancy of the completed board.

The board when completely submerged has a slightly less than one pound tendency to sink.

About four or five pounds of force is required to hold the board at the design waterline.

The black and white pattern on the board is caused by my shadow

The lead tip was sized about right

I am very pleased. The next test will come when the board is installed in the boat.