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Weighted centerboards. How much weight?


Tom Lathrop
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I took the CB of Lapwing out to add some lead near the tip.  How much to add?  Paul said 20#, which should certainly do the job but, how much is really needed?  Lapwing's CB weighs 12.1# on the scale.  After fiddling around with estimates, I decided to run a test.

I attached a guestimate 6.7# of lead (including a clamp) to the CB and dropped it in the creek from my dock.  Attached a line to the handle, expecting to measure the weight on the scale.  The CB sunk right to what I think is the approximate waterline when extended under the boat and hung there with neutral buoyancy.  Can't guess any closer than that.

So how much additional lead should I add?  I don't want to pour molten lead into a space on the CB and burn epoxy, wood and paint, So I will cast a plug and epoxy it in.  Turns out that the base of a common one gallon rectangular container used for lots of solvents, etc will do nicely.

These containers have an area of about 3 15/16" X 6 3/8" = 25 sq in

At 1" thick this is 25 x 710/1728 = 25 X 0.41 = 10.29#

(Lead is 710#/cu ft and one Cu ft is 1728 cu in)

A one inch thick plug leaves enough room for fairing on each side.

So there is 16 - 12.1 = 3.9# greater than neutral to sink the CB.  Actual sinking weigh is at least a pound greater because of part of the CB that is above water but was included in the CB weight. Net sinking force is therefore about 5#, which I consider enough to do the job.

Anything greater is OK but I think this is adequate and will make the force needed to pull the board up easier than a 20# plug of lead.  It will also make the force needed to hold the CB up on the trailer much less.  Actual force on the hold up line will be 29# with the suggested weight and about 46# if 20# ballast is used near the tip.

This question has come up so many times that I thought I would look into it more closely.  On larger Core Sound boats with larger centerboards, more ballast will be needed but the above should be adequate for the CS 17 and a pound more for the CS 20 should do OK as an estimate.

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Wouldn't the location of you ballast also effect how much you need?  The portion of the centerboard that remains in the trunk does not have to be sunk.  And if the weight is all the way at the tip of the board, or at least close, then could it not be enough to tilt the board to vertical even without achieving negative buoyancy for the entire board?  This is what intrigued me about Joe's casting of the tip of his board.  But I agree, with all the boats being built, and so many wanting weighted boards, it would be nice to have a working plan to simply follow instead of everyone having to figure it out over and again.

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I went through a similar testing process Tom and the board I employed was different shaped. If you remember, it didn't have an arm sticking up, but had the radius of the pivot point carried around the top of the board, so it would continue to ride on the case stringers. This made the board and case several inches longer, forward of the pivot. I also placed the weight along the leading edge, low in the boat, so it's relative location would just lower, not move forward as much as one concentrated at the tip.

I found it took 16 pounds of lead to make the board neutral buoyant, which worked fine when tested in a pool. I used a screw driver through the pivot hole and she dropped assuredly. My next test was how much pressure on the leading edge did it take to move the board back up. At the neutral buoyant weight, she was very easy to rotate around the screw driver. So I added 2 pounds, which helped tremendously, but she still seemed light with the leading edge pressure, so I added two more pounds, which is where I left it.

The image shows the board with the leading edge reinforced and the first round of structural fillers applied. Note the bearing on my router came off more then once and made a mess of the surface before I could shut it off. The client is holding the board in this image and the hole on the right is the pivot, yet to be epoxy bushed and the left hand side is the lanyard hole, which rides in a groove along the radiused edge. The resulting changes eliminate most of the centerboard tackle and completely encloses the board, within the case (no arm). The hoisting tackle is a whip, attached to the lanyard, trough a hole in the top of the forward king post. This terminates on a cam cleat near the centerline of the thwart, as the line runs between the mast and case. A knot in the lanyard prevent the board from dropping down too far, as it's too big to go into the king post.

The lead was melted down from tire balancing weights and intentionally cast (I suspect much like you did) about an 1/8" short of the total board thickness for fairing.

post-619-12949770715_thumb.jpg

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I approached the weighted centerboard differently. I sail in extremely shallow water. First, when I pre assembled the centerboard and trunk I noticed that with say six inches exposed, the board was showing only a small triangle of surface. I exposed 12 inches, marked a line six inches parallel from the trunk, and cut off the rest. The result was that when I only had six inches between the bottom of the boat and the bottom of the ocean, much more surface of the centerboard was exposed.

When sailing, I noticed the board would touch bottom and I'd release the downhaul. We'd sail along, albeit somewhat to leeward, until I had a chance to search for the bottom. During the Great Race my girlfriend spent a lot of time holding the horn of the centerboard and adjusting to the bottom.

An unweighted centerboard, left to tend itself will float high enough to hide most of itself in the trunk and accomplish essentially no  real work. I wanted only enough weight to make the centerboard want to float about a foot lower. As such, the board easily adjusts to a shallower muddy bottom and automatically resets itself when there's more water. I could sail very well through a shallow area with very little leeway and would reset the downhaul when I got to deeper water.

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The weight of a 1" thick lead pig melted in the base of a 1 gallon solvent can is 10 pounds which will fit very nicely near the tip of Lapwing's CB.  The can must be cut away to release the pig and some minor edge shaping was required.  The pig was epoxied into the opening cut in the CB with the pig centered in the opening.  There was plenty of space left for filling on each side and a bit more lead could have been used.  There was a bit of sag in the center of the pig and a metal plate centered under the can over the heat would have prevented that.  I hammered the pig flat with a 3# hammer.  

The space on each side was filled with epoxy, cab-o-sil and Q cells.

A bit of paint and its finished.

post-257-129497707217_thumb.jpg

post-257-129497707222_thumb.jpg

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