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Jim Budde
01-25-2005, 06:57 PM
A thought ... when small boat plans call for a weighted centerboard (plywood with a glob of lead down low, all encapsulated in epoxy), why not do the following. Encapsulate a piece of flat steel of appropriate weight and dimensions just a little short of full centerboard dimensions. Achieves two things: (1) achieves the weight called for and (2) makes centerboard out of something other than wood .. no risk of swelling, warping, etc. Obviouisly, cost of epoxy might be a considration if plans called for 1" thick centerboard and 12 mil (1/16") steel was all one could use for purposes of weight .. what do you think?

paladin
01-25-2005, 07:01 PM
depending on the boat/design...a flate steel plate may introduce unwanted vibration under sail..and some other undesirable characteristics...such as more prone to stall...

JimD
01-25-2005, 07:09 PM
Also, the size of the cb is designed to provide sufficient lateral resistance. If you make it smaller than planned you may end up getting blown to leeward more.

Bruce Hooke
01-25-2005, 08:43 PM
A thick, streamlined board is actually more efficient than a thin, flat plate. In addition you have the vibration problem noted by paladin. So, you would want the board to be roughly as thick as the board called for in the design. Since the lead in most centerboards is just enough to make the board sink you would be facing the situation you mentioned of 1/16" thick steel plate that needs to be, say 1" thick. I can't imagine trying to build up that much thickness with epoxy but I suppose you could sandwich a steel plate between two pieces of plywood. However, if water ever did get to the steel you would have a real mess because steel expands as it rusts. Also, to be most efficient the weight should be concentrated at the bottom of the centerboard -- weight at the other end is just extra mass to drag around.

The only advantage of your proposed approach that I can think of is that it avoids messing with hot lead. Another way around this issue that I've heard people talk about is to simply embedding lead shot (or some other sort of lead that comes in small bits) in epoxy to create your weight.

If warping is your concern I would use plywood coated in epoxy and glass cloth.

ahp
01-25-2005, 09:46 PM
Why not aluminum? If it is thick enough it would have the weight and strength and no rust problem?

The 18 ft "Oslo Jolly" (or Oslo Dingy) did have a steel plate CB.

I would stay with weighted wood, then you can get a good hydrofoil section that is a lot more effective than a flat plate.

Bruce Hooke
01-25-2005, 11:15 PM
Originally posted by ahp:
Why not aluminum? If it is thick enough it would have the weight and strength and no rust problem?
In addition to the problem you mentioned -- the difficulty of shaping a metal plate into a foil -- a plate of aluminum big enough to make a centerboard would not be cheap!



I would stay with weighted wood, then you can get a good hydrofoil section that is a lot more effective than a flat plate.

Buddy
01-26-2005, 09:42 AM
Many , many times I have seen a steel plate embedded in mush between two fiberglass "halves" in a production boat centerboard rust, bulge up and deform the fiberglass surface in the rectangular shape of the insert, split the seams, and really no a number locking the nboard in the centerboard trunk. I wouldn't wish this on anybody. Similar I have seen many fiberglass rudders where the poor quality stainless bars welded to the rudder stock tube did the same thing. Of course stainless isn't corrosion proof under water, mild steel sure isn't.

I can swear by lead shot ballasting using either epoxy resin or polyester casting resin ( doesn't shrink) 27 years under water and still working.

Jim Budde
01-26-2005, 11:02 AM
Thanks, folks.. Looking out window at unheated shop with temp hovering around zero gives me time to "think" as opposed to work on my boat project. Staying with plans and wood it will be.

ssor
01-26-2005, 12:00 PM
Any local gun shop can sell you a bag of lead buck shot. Just glass one side of the board and pour the shot resin mix into the other side and glass that side. Unless you have access to a machine shop you should shy away from trying to use raw metal.Ssor

Dan Cavins
01-26-2005, 10:39 PM
Hi Jim. As always - I am not the expert - but...
My boat called for a flat slab of aluminum cb. Short story is that's what I did, rounded the front edge. I'm doing fine but would not do it that way again. I would do a thicker and foiled wood cb. They are more efficient and not that hard to build. JimD is right, don't downsize. And Bruce was right on about "not cheap" as to aluminum. It's really gone up. And buckshot cost nothin'. Just seems a weighted wood board is a good balance of cost, performance and effort. Just my experience and thoughts. Dan.

NormMessinger
01-27-2005, 09:28 AM
When we say buck shot, do we mean buck shot? I would think the smaller shot sizes would pack more lead, less epoxy, in a given volume. And beware of epoxy exotherm.

I have 100 lbs of lead with your boat's name on it, Jim.

ahp
01-27-2005, 10:26 AM
Home Depot sells lead in sheets of various thicknesses, for roof flashing I believe.

johnw
01-27-2005, 02:18 PM
I have a boat with an aluminum board. It's a 50-year-old Snipe. The original board was 3/8 in. steel, and weighed about 80 lb. The class switched to aluminum about 1970, which lightened the boats by 34 lb. I don't get any flutter from my board, though some do. It has to do with how you shape the trailing edge of the board, not what the board is made from. I shaped my own board, and luckily got it right.

I wish the class could shift to wooden boards, but the daggerboard case is too narrow for a strong enough wood board. I'm glad they shifted from steel, so I don't have to horse around that heavy chunck of steel.

I got my board made by a boat yard and shaped it and made the handle myself. Cost about $400. I could have made a wood board a lot cheaper myself. I had to make a new centerboard case anyway, and I would have loved to make one that would accomodate a wooden board, but class rules wouldn't allow it.

Another advantage of wooden boards is that they float. Capsize a Snipe without a lanyard on the board and your $400 board is at the bottom of the lake.

That said, it's a better deal with centerboards than with daggerboards. The Lightening and the Geary 18 have metal boards. Gearys switched to aluminum, but the Lightening still uses a mild steel board. One advantage is the ballast provided by the board, another is that the opening for the ceterboard case can be narrower and case less drag.

If you keep the boat in the water, rust or corrosion will get to a metal board. Steel ships are often done after 30 years, and I suppose steel boards would last about as long.

Tom Robb
01-27-2005, 02:31 PM
A thick foil has, I'm told, a higher stall angle than a flat plate. Probably a good thing.
On an old glass CS-15 we once owned the galvanized steel plate cb's vibration acted as a sort of speedometer. It also acted as a sonar transducer amplifying the ringing sound of high speed props from clear across the lake. :cool:
IIRC, some here have had a steel cb bend upon a grounding such that retractinjg the thing became impossible - not so cool....

[ 01-27-2005, 02:39 PM: Message edited by: Tom Robb ]

Keith Wilson
01-27-2005, 05:23 PM
Lead shot in epoxy works very well if one doesn't want to deal with melting lead. It's about 2/3 the density of solid lead, so the plug has to be bigger. I install it just like melted lead - drive a couple of bronze nails not all the way into the edges of the hole, clamp the centerboard down on a piece of plywood covered with waxed paper or something else that won't stick to epoxy, fill the hole with shot, pour in epoxy and let it harden. Exotherming isn't a problem because the shot is a pretty good heat sink. Smaller shot is denser.

Norm Bernstein
02-01-2005, 10:09 AM
Here's a solution: obviously not as dense as poured lead, and not as dense as lead shot, but it's free (or cheap), simple, and works well:

http://www.marisystems.com/ellipticat/boatphotos/Dsc01174_websize.jpg

The pocket in the centerboard is filled with used automotive wheel weights, obtained for free from a tire store. They usually recycle these, but you might be able to convince the store owner to either give them to you, or accept a few bucks for them (a 5 gallon pail full of them usually gets the dealer $50 or so, and is way more than enough).

In the project illustrated, I simply packed the wheel weights, as they were, as densely as possible. A bit later, I realized that it would be denser if I simply cut off the lead 'ears' and used them, discarding the steel clips in the center. Once the cavity was packed, I poured epoxy into it, level with the surface. I didn't calculate the relative density, but simply made the pocket larger than if I was using poured lead.