Calculating Amt of Lead to Sink C/B

[sharpie]

Is there a formula for determining the amount of lead needed to provide negative buoyancy to a c/b?

[Ian McColgin]

Figure the volume of the whole board. You'll only need the sink the immersed section but you need some extra to keep it down as your speed increases and with the whole board volume you add between 50% to 100% depending on the board.

Convert from cubic inches to cubic feet and figure salt water at 64# per cubic foot. Subtract the actual board weight and what's left is a good lead amount.

Put the lead near the lowest avaliable corner because that's where stresses are the least and because there removing wood and replacing with lead does the sinking a bit of good.

G'luck

[mmd]

Ian, check your private messages, please.

[Lulworth]

I am always puzzled by those who want to "sink" their blades by adding wieght. On the rudder especially, sticking a hunk of lead as far from the center of the boat as is physically possible is a bad thing. In the case of a C/B, the added lead is just wieght you have to lug around. Instead, use a bungie so the board is spring loaded in the down position and it pops up when the bottom is hit.

Dsvid

[Ian McColgin]

There are many ways to make a centerboard or a rudder.

Especially for outboard rudders, a pull-down can be nice, though this is harder to arrange for an inboard rudder such as on the Chamberlain Gunning dory.

On centerboards, any design pivoting on a low forward pine - simple and common - cannot be held down by any sort of line. You must either ballast or (againd harken to the Chamberlain Gunning Dory) use a rod pendant.

Boards that pivot a bit higher can be controlled by either a pair of lines over the front/top of the board or a locking arm coming out the trunk and attached to a keyed pivot. And we could go on from there.

These boards are of course at the light end. The board of a boat like my old schooner Goblin - drifted oak with just enough lead to bring it down, weighed about 400# and would not be lowerable or usefully held down by bungee cord.

It's a matter of either sticking to the design or, if one is smarter than the designer, doing something else. But it is never a matter of one way solving all problems.

[Thorne]

I'll agree with Ian - there are some board designs that need weight. The standard sealed centerboard case is one, and if ya don't wanna faff about with bungees and cords, many traditional rudder designs are also included.

Here's the quick and dirty method -- just look as some designs with similar sized weighted boards and use the amount of lead shown, possibly adding a bit of weight for the fudge factor.

[bainbridgeisland]

Originally posted by Ian McColgin:

...On centerboards, any design pivoting on a low forward pine - simple and common - cannot be held down by any sort of line. You must either ballast or (againd harken to the Chamberlain Gunning Dory) use a rod pendant...I am confused Ian. Many sailing dinghy centerboards pivot on a low forward pin and are controled by a hold down line or bungee. Having spent years sailing them, I have never had a problem with this setup. I have sailed some large sharpie's with a hold down tackle on the centerboard without problems.

Please explain why a centerboard with a low forward pin "cannot be held down by any sort of line". Am I missing something?

David Mancebo

[Lulworth]

Oh, I think we can agree that there are many ways of doing it and I agree that all bets are off when we are talking about a large boat.

I am thinking of the the very common designs that use a handle emerging from a partially open trunk. The handle can be "bungied" to drop the board upon release of the up haul thusly:

http://www.imagestation.com/picture/sraid153/p8c31ae9dbbad42da370a917520ff7c63/f59af891.jpg

The black and white striped line is the bungie and the light blue is the uphaul.

[Thorne]

Bainbridge -

Sealed centerboard case = "can't be held down with any sort of line".

Most of my daysailors and other boats have had sealed centerboard cases. The one exception has been a hole for a pin to hold the cb in various positions.

[Ian McColgin]

Yeah - I was thinking of the closed top low pivot where even if the pin is an inch back from that lower front corner, hard to see how you'd get enough lever arm on a pull-down line.

Open top gives lots of ways to hang on to the board - can even go no pivot pin at all, though boats like that are often things like the Raven or Thistle with really heavey boards.

[Keith Wilson]

The density of lead is 0.41 lb/in^3
The density of water is 0.036 lb/in^3 (fresh) or 0.037 lb/in^3 (salt)
The density of the centerboard depends on what kind of wood you’re using, and whether there’s any glass sheathing. Representative values are 0.025 lb/in^3 (oak) and 0.018 lb/in^3 (Douglas fir).

You need to figure the immersed volume of the centerboard, and multiply by the density of water to get the weight of water displaced, thus the buoyancy. An example: If the immersed part of the board is 12” x 36” x 1” thick, the volume is 432 in^3, and the buoyancy is about 16 lbs in salt water, or 15.5 lbs in fresh. The board will have to weigh more than that to sink. Note that the part above the water contributes to the weight but only the immersed part is buoyant.

Let’s say that there isn’t much board above the water, and it’s made out of Douglas Fir, so the board without any lead weighs about 8 lbs. If you add enough lead to make it weigh, say, 20 lbs it should sink nicely. That would be 12 lbs of lead; 12/0.41 is about 29.3 in^3 of lead, or a 1” thick disk about 6” in diameter. You lose half a pound of wood when you cut the hole to put in the lead, so making the hole a little bigger would be OK.

A pivoting centerboard or rudder is a little more complicated because lead farther from the pivot will sink the board more effectively than nearer the pivot. To calculate it accurately you need to find the centroid of the immersed area to find the center of buoyancy, and the center of gravity of the board with the weight, and figure moments around the pivot. It’s effectively two dimensional, which makes it a lot easier. OTOH, with a small boat it’s not that critical.

One effective way to weight a board that doesn’t involve melting the lead is to fill a hole with lead shot, then pour in epoxy. The lead makes a good heat sink so it doesn’t exotherm. It’s less dense than solid lead, but not too bad.

Here’s a useful site (http://www.allmeasures.com/Formulae/) that has densities for lots of different materials, and a unit conversion calculator:

[ 01-09-2006, 01:04 PM: Message edited by: Keith Wilson ]

[sharpie]

Thanks for the replies, all. I am grateful for them.

Sharpie

[Robert - Hoopers Island MD]

For what it is worth, I recently built my first approximation of a Hoopers Island Sharpie based on a Bolger's daggerboard equipped WindSprint.

Around Hoopers Island much of the water is is only a foot or two deep much of the time so I added a pivoting lee board following Michilaks general desgn. Initially I held the leeboard down with a bunge cord and the pivoting rudder with friction and the pressure of the bolt that holds the bottom of the rudder to the above water portion.

My assumption was that eventually I would need or want to add weight to both the leeboard and rudder. With out going into details about loss of control of the boat on a windy day etc. I immediately took the boat out of the water and added lead shot and epoxy in 2 and 1/4 inch holes I drilled in the rudder and leeboard. The amount of lead was decided by filling a sock tide to the bottom of the (1) rudder and (2) leeboard with various amounts of lead shot. And putting the same in a large garbage can filled with water. I fairly quickly decided how much lead might be appropriate, and drilled and filled enough holes to add the amount of lead that seemed appropriate.

Now, the boat is a lot easier to handle and generally everything works well with no further intervention on my part. Everything kicks up in shallow water and drops back down when I get back in deeper water.

P.S. I am pretty new at most of this stuff so don't take my word for anything without getting a second opinion from someone that actually knows something.

Robert