Center board weight?

[Docpal]

One of the projects I am working on today is shaping my center board. The board, like all of the wood members in "Petunia" is Philippine mahogany. I chose that because from prior experience I know the wood's properties ( light, easy to glue, can look beautiful when finished, etc.), and also I've learned that by using a better grade of clear wood I don't have to take the extra time to cut around knots, checks, or other assorted "boogers". After searching on this Forum with no definitive answer I'm wondering; is there a "desired"weight to the centerboard? As I cut away the profile the board keeps feeling lighter, and lighter and I've only seen a couple of pictures on Graham's CD of other people's boards which all look like they are made of heavier woods.

Obviously I'll gain a little of that weight back when I glass the board, but has anyone ever weighed their board? If I'm on the light side I can always add some lead weight down low to compensate, but I really don't know what the optimum weight IS...TIA for any ideas...

[Gordy Hill]

I think the board can be too light. I used to sail in very shallow water and found that after I let the board up it would stay up and actually float at a setting much higher than I wanted. Then I would have to pull it down till it touched bottom and then up a bit. Then I'd encounter shallower water and have to pull it up again. Then I'd be sailing along with it up long after I was in deeper water.......

I drilled a 3 inch hole through the lower section of the blade, tapped one side and filled it with bird shot and epoxy. I weighted the rudder blade the same way and was really happy with the results. In fact, even more weight would be nice. Even as it was the board would still float, but a few inches lower and it would tend itself when the bottom showed up. Of course this was all in light winds.

Mahogany is a great wood to work with, but it's rather light and I don't think it's as strong as fir. Fir was my choice back when I has no idea what I was doing.

I just got lucky...

[Gordy Hill]

I think you have a point Ray.

However, once the boat lists wouldn't lift or buoyancy of the board contribute to the listing moment?

[JeffM]

It's never occurred to me to weight the board to keep it down.  Mine (doug fir) floats high in the trunk when left to itself.  I have it on a two-part downhaul with a releasing clam cleat, so it pops up automatically when I touch.  Sailing to weather in close quarters I have to jump to the board, pop the cleat back in place and pull the board back down.  It might be nice to have it float lower so I don't need to tend it right away.  On the other hand, I'd have to remember to cleat the uphaul before I winch the boat on the trailer or the heavier board would drag over the frame and rollers...

[wkisting]

Basically, there are two ways I know to do it: downhaul (to hold the board down manually, usually on shock cord or a release cam so the board can still "kick up") or weighted (i.e., gravity).

In the case of a weighted board, I don't know of an "ideal" weight, but I've heard many folks say that you should add just enough weight to give the board "negative buoyancy" (not sure if that's a real term, but I've heard many people use it... sounds like a contradiction to me that I just call "sinking"). Anyway, the idea is that you float the board or calculate its buoyancy, then add just enough weight to negate that buoyancy plus a LITTLE extra to give it an inclination toward sinking.

Beyond that sinking point, any additional weight is just dead weight (and extra strain on the uphaul) UNLESS the centerboard is also intended to help counterbalance the hull (i.e., help with heeling forces). The latter is something to ask the designer, but if no target weight is spec'd in the plans, I suspect the CB isn't intended for counterbalance. To my mind, it would take a VERY long/large CB (and a very light boat) for weight to do anything useful for heeling, and doesn't seem like a practical advantage, but what do I know?

[Edited to clarify the second paragraph.]

[Gordy Hill]

I liked the board to float, but deeper than originally. That way it was still at an angle and would tend itself when the bottom showed up. The same with the rudder. When I was in water too shallow to set the blade down and lock it it would float higher than necessary and steering was terrible. After adding the weight the blade floated about 4-5 inches deeper.

[Peter HK]

Ray said

Bring on the solid steel galvanized centerboards

Actually, I have a galvanized centreboard on my Folding Schooner and there are a few of the good points.

No downhaul needed- but the uphaul's a brute.

No worries about damaging the board when you run aground- my brother calls it the galvanized depth sounder.

It's a really convenient anchor when you get to the beach- just let go the uphaul. 

Peter HK

[PAR]

I think what you are referring to wkisting  is "neutral buoyancy" which is not floating or sinking but the object seems to levitate in the water. It's weight is equal or very nearly so (very slightly less) it's displaced volume. This makes the object, very easy to manipulate in the water.

You can go in each direction with boards (and rudders). I prefer weighted boards, but not overly so. They'll bounce along on the bottom when they encounter it, but don't have the mass to do much damage to themselves, unless some velocity is involved, such as running up on a partly submerged (neutrally buoyant) log, with everything flying, planning along at 10 knots. This will ding a leading edge or mash the trailing edge of a dagger into the case.

My boards will sink with authority, so a down haul isn't necessary. It will also stay at depth when hauling the mail and some serious flow is attempting to shove it back into the case. Leverage on a hoisting lanyard isn't difficult to arrange. On the other hand a bungee cord used to keep a light board down works fine too.

Every gunkholer I've known has used their board as a depth sounder. It seems easier with a weighted board, because the board is always "engaged", but if it's forced down with a bungee, then it'll remain engaged too.

[Ken_Potts]

  I can confirm what par says about holding the board downwith a bungee.  I returned tonight from a wonderful cruise on the Outer Banks of NC and we had plenty of contact between the board and the bottom.  When the board would bump up I loosened the downhaul a bit and the board would stay in the same position due to friction.  Then when we hit shallower water the board would bump up again, etc.  Once we were in deeper water we'd haul it back down again.

    I rigged the uphaul and downhaul exactly as drawn and it works great.  My board is made of ash and sheathed in fiberglass so it is less bouyant than mahogany but I don't think the density will ba a problem.  If it is you can alway add a bit of weight later.

  It does take a little bit of a pull on the downhaul to bring the board down but the sideways pressure on the board seems to matter more than the bouyancy (easy to haul it down when the boat is at rest or running downwind, hard to haul down when the boat is close hauled).

[Oyster]

On my custom sailboat, I enlayed 10 lb of lead that I melted in a broiler pan on an open burner of my gas outdoor grill which made it a thin and flat piece and then I split it. . I then glassed one side of my board and then flipped it over and routered two sections out and enlayed two seperate pieces favoring the down side of the board. I have no problem one way or the other. It stays down and easy to pull up.

[Cricket]

Hi folks,

As an alternative to melting lead (with the associated fumes and splatter), I've used sheet lead left over from shower pan installations. Cut out enough pieces to equal the thickness you want, hammer them all flat together (no need to glue them), and drop this brick into a mortise in the board. Don't cut the mortise all the way through. Then goosh the brick down into thickened epoxy, and glass and fair as normal. My board on Cricket achieves neutral buoyancy just as it reaches the correct max down position. Pure luck, as I didn't even weigh the brick!

I've also used bungee downhauls on light weight boats, and it works well too.

Jim Luton

[Docpal]

SO, I'm getting the feeling that this is one of those issues which is decided by "feel" more than science?

Thanks to all who have suggested some of the options for adding weight. Instead of firing up a  "forge" in this heat wave I have chosen to take the simple route and I just ordered a 5 lb container of .08"lead shot from McMaster Carr. (Their part #  9030K21). Costs about $32 plus shipping ( can't wait for the UPS guy to ask me "what's IN this little package?" and I tell him- lead...). With such small shot I should be able to pack it in pretty tightly, and get maximum weight for the cavity I decide on...

By the way I glassed the insides of the centerboard trunk yesterday. Used MAS epoxy with the "medium" hardener. One of the few advantages of glassing in 99 degree weather is that I went from pouring the epoxy, to trimming off the "green" edges in about 90 minutes.... ;D

[PAR]

A routing trick I use and is similar to traditional methods of affixing weight to boards, is to cut out the area you want to have the weight in, with straight sides (90 degree cut). Using a chamfer bit in a router, set at 1/2" the thickness or the board, under cut the perimeter of the removed area. It doesn't mater if it back cuts or under cuts the edge, so long as there is a difference in angle from one side of the board to the other. This cut edge will cause whatever is molded to the area to stay put.

Melting lead is traditional and not very hard, but can be a bother, so lead shot (I use bird shot) mixed with slightly thickened epoxy works very well. You'll get about 70% (possibly less depending on shot size and how well packed in you get it) of the mass, compared to using melted lead only, but this is more then enough to sink a board or add some ballast.

I place some clear plastic (painters drop cloth stuff) under the board with a piece of 1/4" lauan as a backer which is tacked lightly in place. Pour in your shot/epoxy mix, mashing things under the back cut edges and generally as tight as you can. If you leave it a little shy of flush, you can fill it later or you can use the plastic sheeting method (like on the bottom) and make a neat job of it in one shot.

To save finishing effort I usually place a piece of 6 ounce cloth inside the cut out first, then a fair amount of thickened resin (without the shot) and let this gel off, then start the shot and goo patch over it. I finish with the same pre cut piece of cloth on the outside, more un-shot, but thickened goo and squeegee a clear plastic sheeting over it for a finish.

[SUKIE]

I like this idea of a weighted centerboard. Does anyone know how much to weight a CS17 board to make it slightly bouyant (I think the term used was neutral bouyancy)? Thanks

[Doug Cameron]

You could test it empirically by putting the board in the pond and adding shot to a dixie cup sitting on it until the board was just submerged. 

[PAR]

Attach a fish scale to the board and jump in a pool with her. Pull on the scale until she's below the surface and you're there.

[Docpal]

Here's what I came up with for the first phase of the experiment;I added 5 lb. of lead shot into a cavity I cut through the centerboard in a way I thought would keep the weight as low as possible most of the time. There is .125"of epoxy resin on both sides of the lead shot, and both sides are "capped" with 8.5 oz. cloth then faired with more resin. Just in case this changes things too much I bumped up the size of the centerboard pin to a .375" diameter.

The perimeter of the board is covered with a Kevlar/Carbon tape.

[Docpal]

I've spent waaaaay too much time on the center board. Got a bunch of coats of graphite/epoxy on her now and after some more wet sanding I'm gonna call it quits. At the moment she weighs in at 23 pounds. Anyone else have a weight  on their boards?

[PAR]

I had to add a fair bit more weight then 5 pounds to my modified CS-17 board to get it to positively sink (over 15 pounds). A board that is in the 20 - 22 pound range will be neutrally buoyant and will not sink, though a down haul will easily pull it into position (though I fail to see the point).

It's pretty simple math, you know what water weighs, so figure out how much cubic volume the immersed portion of the board is (about a third of a cubic foot is a rough guide). This is the weight you need to make it neutrally buoyant. If built to the plans the board will probably be in the 7 to 10 pound range, depending on how much epoxy, wood species, etc.

The best test is to make your board and take it to a pool and drop it in. If held vertical, it'll float about half immersed. Add weight until it's sunk to the pivot pin hole. This will be neutral buoyancy, but as mentioned will still need a down haul. To eliminate the down haul need, you should add another ten pounds to insure it sinks with authority.

[Docpal]

I'll pop in the pivot pin and drive down to the Bay in the next day or so, just to see how she floats when suspended in the water, thanks!