removing concrete keel in a cp 16/1

[dkclevel]

Just wondering if anyone has removed the concrete in a 1981 cp 16 .. I have a nasty split from side to side and to do the repair right I was advised to remove all the concrete . Also toying with the idea of replacing the concrete with a few hundered pound of lead shot .  any ideas

[crazycarl]

i don't have any advice, but i'm interested in what you find when you get it out.  thought about doing this same thing to our 19.  the extra space the lead would leave could be used for other things.

                                                  carl

[shamblin]

bet keith and johnny scott at www.ipass.net/sailboats will know what to do.

[shamblin]

that must be www.ipass.net/sailboat

[Salty19]

An air hammer ought to break up the concrete easily. 

You want 280.3 lbs of lead, laid low in the keel-within the bottom 3.5", to equal the force of 450lbs of concrete.  Yes I did the math!

[skip1930]

Just what is a nasty split from side to side? Horizontal, vertical, glass only or glass and concrete...
Bare in mind that both concreate and pig iron can be found in the factory mix. How does one know how to distribute the weight?

Hand held air chisel and a 20 gallon, 2hp single phase or 220 compressor. W.W. Granger can set you up. Or you can rent.
Ear muffs and eye protection. Shop vac. Have at it. Reuser the pig iron.

skip.

[MacGyver]

Was pondering doing our cp19 sometime.... but not really a priority.... What is the lead needed for that?
I kind of thought I would weigh what was there, reinforce the glass from the inside, then dump lead and epoxy in.

There was a link of a guy who replaced his, and he went over with the lead, adding more weight.....

I would think that is what I would want to do, go to like 900 pounds........  just a thought.....

Just curious also, how is it that you get by with 280 lbs of lead, for 400 lbs replacement?  just due to tightness of weight and lower the weight becomes?

Mac

[Salty19]

Jason,

Yes.  The keel is a lever of sorts.  Think of it like a teeter-totter.  When equal weight is applied at each end, and in the same position..there is no teeter or totter but rather it will be level.

Move the same weight on one end INWARD, and that end rises as it produces less force. Move the weight outward and the effort increases.  It's a product of the basic lever formula which states that the distance from the fulcrum AND the weight applied at that distance conspire to form the effort on the lever (see website below for details).  The more weight or, the greater the distance from the fulcrum, the more force acts on the lever.

In our case, the fulcrum is the waterline and one end of the teeter totter is weight BELOW this line, the other end is weight ABOVE.  Obviously our boats would look like a fat man sitting on the teeter totter (keel) with a child sitting on the other end (above waterline).   Very imbalanced, which gives plenty of force which can be added above the waterline (wind effect on hull and sails) before the forces are equal.  When they do become equal, you're mast would theoretically touch the water.  If more force is applied, such as even more wind, or a wave above the waterline, technically the boat should turn turtle.  Now, there are other forces at work here--namely the hull shape, windage, keel shape, rudder size/shape and stuff only marine engineers fully understand--of which I don't claim to be.

Since lead is MUCH denser, more weight per cubic area, compared to concrete, the added mass per cube will alter the lever formula accordingly. Thus why less lead weight is needed to equal the same righting force as concrete.

I worked it all all out in Excel, a mathematical program designed to solve formulas which the user can create, modify and alter inputs to form a result. Been using the program for 20 years and have solved all kinds of things with it. 

Anyway  I figured out the total effort the stock concrete keel makes, then determined the same effort using the density of lead.

If you put in 800lbs of lead in your CP19 keel, it will heel much less and be dramatically more stable.  This sounds like a good thing, but you really want to keep the designed forces similar--Bob Johnson and Clark Mills knew what they were doing!  Sailboats are designed to heel!  A little more might be nice, but this is WAY OVERKILL and you'll be sorry you did it.

But this is a CP16 topic and my numbers reflect that boat's 450lb (not 400) stock concrete keel filler.

http://www.engineeringtoolbox.com/levers-d_1304.html

[skip1930]

So if you have say 8 inches of lead that equals 200 lbs and 10 inches of 'air space' with nothing to stiffen the keel's walls, where 400 lb of concreate use to be, then what keeps the keel from falling off the hull when the going get rough? She's flexing above the lead and below the hull. And not a lot of space in there anyway and how you gonna reach anything down in their after it's stowed?

Inquiring minds want to know.

skip.

[millsy]

Mac,

I removed the concrete from the keel cavity of my C23 and replaced this with the equivalent weight in lead pigs (down low- in resin slurry and covered with plywood/FRP) plus AGM batteries for my electric drive system ( and here I always explain that this battery compartment is watertight, vented above the waterline, and I still have the original bilge sump in the aft part of the keel.).  To compensate for the lost stiffness of the concrete, I tripled the shell thickness-tapering off into the turn of the bilge- and added a transverse floor about midway down the length of the keel.    The keel would flex way too much otherwise. 

The lead has probably added some stiffness under sail, but I have not been able to compare this to another C23.  I suspect that carrying all of the weight at the bottom of the keel, as you suggest, might make the boat too stiff.

I don't know the geometry of the C16 keel well enough to say whether or not this would work for you.  From an economic/practical standpoint, removing the concrete makes no sense.  The keel of my boat was heavily damaged when I purchased it, which influenced my decision.  I also had the materials, including the lead, FRP, etc, already on hand.

Regards,
Chris C23

[wes]

I think anyone seriously contemplating this should read Keith Scott's response to the same question (www.ipass.net/sailboat/, click on "answers"). He points out that removing the concrete can lead to keel breakage when trailering, unless you reinforce the fiberglass in the weak areas.

Wes

[Allure2sail]

Curious:
For those of you that have gotten into the cement used for the ballast in the keel. I always thought there was lead mixed in with it. Someone here recently said he found pockets of rust in many voids they found in the cement. Does that mean that the metal mixed in with the cement is not lead or stainless steel. How much of it is cement and what would you need to get the weight ratio needed for the volume used in the keel. Does a cubic foot of rust weigh as much as a cubic foot of cold rolled steel (OF COURSE NOT). Here is just a passing thought to ponder......if a 27 ships with 2,500 pounds of ballast from the factory and in ten years or so when the steel or iron mixed in with the cement rusts away and all that is left is pockets of rust and cement, how much ballast do we have now? Of course it depends on how much scrap metal is mixed in with the cement. Does anyone know the ratio? I guess the engineer side of me is showing LOL.
Bruce
S/V Allure

[fawsr]

wondering which engineering discipline you might be ...

[Tim Gardner]

Bruce,

Being rather simple minded, I tend to think the ballast weight would be the same.  A pound of iron is the same weight as a pound of iron rust.  Rust just occupies more space.  

TG

[fawsr]

A pound of iron in the beginning - after it rusted - would weigh more than a pound due to the oxygen bound up in the iron oxide. If the inference was that the concrete ballast was full of rust - it was indeed heavier than it was before it rusted.