Centerboard / Daggerboard Shape

[Eric O]

Has anyone tried making their own replacement cb or db and used just a thin sheet of metal? If so, I'm wondering how it worked compared to the stock board. A number of trailerable sailboats use just a flat sheet as a swing keel - the Cat 22 and Aquaris for example.

The advantages of just a thin metal sheet besides being less expensive to replace could be a smaller opening in the bottom of the boat which would translate into less resistance while motoring.

I found the following discussion of shapes that would seem to suggest a thin metal board would be just as good as the thicker fiberglass boards that are standard on a Mac.

As a leeboard, centerboard or daggerboard the difference between the plate and foil would be much less pronounced bcause the boat is usually moving "crabwise" through the water at a much smaller angle than you'd use at the helm when tacking. As long as it doesn't vibrate, a flat plate is probably as effective and a lot less hassle than a shaped foil section. Using aluminum, brass (big in the 1880s and 1890s) or even iron eliminates having to weight a board to keep it down, and allows the plate to be very thin, which is good (3/32 to 1/8-inch, perhaps).

Remember, the smaller your boat the more the drag of a flat plate affects you. (This is why skipjacks don't seem to care that their centerboards are about 4 inches thick and dead straight across the leading and trailing edges, and why keels on big sailing vessels are often flat at the stem and it doesn't matter that much. Loads of sail area = power). So thinner is better.

So on a small boat keep the plate as thin as possible and as light as possible unless you need extra ballast weight. Aluminum, anyone?

Also on the Rhodes 22 sight I notice they promote their "diamond" shape board as being an improvement over the normal foil shape.

Any experience with these or thoughts on the best shape for a board?

[Gerald Gordon]

Eric O
You make a good point that the opening of the CB could be alot narrower. Perhaps a CB and boot could be built as one unit and the boot simply slip into the existing CB opening. This may also stop that infernal CB clanging at anchor.

[adm]

You are right that at relatively slow speeds up to 6 -7 knots the difference between drag of foil and flat plate wil not be that significant.

It may be a good idea to make selfcontaining drop in replacement, especially for 26X. Unit can be made to fill entire CB slot and contain CB support - within peharps weighted side fillers, beefed up raising tackle nad pulleys to facilitate blade raising and fastenig to the hull. Blade itself have to be probably narrower as a drawback of limited space. Sticking out blade will require changes in trailer bunk configuration. I would not go with anything other then bronze due to corrosion potential. But it is not likely DIY project as it will require careful engineering for reinforcement of CB trunk to accomodate sigmificantly heavier board and a lot of precise metal work. It wil not be cheap. but there is nothing to prevent motivated and skilled people to do it.

If I had exact dimensions of slot, CB and laminate layuot data for CB trunk I may try to do some preliminary calculation and design.

[adm]

You are right that at relatively slow speeds up to 6 -7 knots the difference between drag of foil and flat plate wil not be that significant.

It may be a good idea to make selfcontaining drop in replacement, especially for 26X. Unit can be made to fill entire CB slot and contain CB support - within peharps weighted side fillers, beefed up raising tackle nad pulleys to facilitate blade raising and fastenig to the hull. Blade itself have to be probably narrower as a drawback of limited space. Sticking out blade will require changes in trailer bunk configuration. I would not go with anything other then bronze due to corrosion potential. But it is not likely DIY project as it will require careful engineering for reinforcement of CB trunk to accomodate sigmificantly heavier board and a lot of precise metal work. It wil not be cheap. but there is nothing to prevent motivated and skilled people to do it.

If I had exact dimensions of slot,
and CB and laminate layuot data for CB trunk I may try to do some preliminary calculation and design.

[argonaut]

Things I'd be concerned about are:

o Mechanics. Ability of a thinner board to work correctly without tearing out a piece of the trunk. The existing glass board transfers the load to the sides of the centerboard trunk, it's pivot point does not take the full force of the board counteracting. The bracket that supports the board now is scarcely more than a hanger. There has also been discussion that the board is not a simple swinging daggerboard design, but rather cants silghtly in it's trunk slot as it comes under load. Hence you can't raise/lower it underway, you must head into the wind to move it up or down.
o Cost. FRP materials are relatively cheap and incredibly stiff for their weight. Aluminum in thickness and length required will not be cheap or light.
o Maintenance. Galvanic action happens when metal is in salt water. Anything other than aluminum will rust, even aluminum will oxidize and become brittle.

Now on the daggerboard model "M", a metal board avoids the canting issue but still not sure there's a great advantage using metal. Using 0.5" aluminum plate 12" wide x 5' long will weigh about 35 lbs. I venture that's quite a bit more than the glass board. It's easily hefted by those who have damaged theirs. Bringing up maintenance again.
Fiberglass is easier to repair than aluminum. Missing chunks from a bad day are easily patched with FRP. With metal you'd have to file and bondo I guess.

Now the Rhodes 22 uses a different keel design. That boat's stub keel is a metal ballast and resin filled casting, with a thin molded slot intended to work with their "diamond board". The board is not a high aspect ratio design, and it is FRP construction, not metal.
These days high perf sailboats use high aspect ratio foils.
I am aware of at least one R22 where that board fouled, stuck in the up position when left in the water due to tightness of fit. Any foil in the water needs anti-fouling. The Mac trunk is wide enough I can get anti-foul paint up in there, and the CB is pretty easy to drop on the trailer.

Ultimately I doubt you'd see a significant speed difference if any even if it worked, since folks here say they sail and drag the motor around and only pick up a half knot by pivoting it out of the water. The Mac 26 is just not a J-24 and a metal board IMHO would make it more of a Catalina 22, the most undesirable feature of which is it's metal centerboard. Their Catalina 250 uses an FRP board, likewise the Hunter 26 footer.
If you'd like to see how much speed improvement is possible just pull your centerboard up next time you're on a dead run & let us know how much faster you go when it's in the trunk.

[Eric O]

Thanks for your replies. My CB is damaged and may need to be replaced. So I was thinking about alternatives. I'm just exploring ideas. Thanks for all of your input. May not be worth pursueing.

adm, why would you only go with bronze and not aluminum?

argonaut, I agree the idea probably doesn't make sense for a daggerboard replacement like the M or D models. I was thinking more about it as alterrnative for a CB. Supposedly one of the slight speed advantages of the M over the X is the difference in drag produced by the bigger, more open cb cavity of the X affecting proformance both under sail and motor. I was thinking of filling in the extra space at least at the bottom of the cb cavity to reduce drag from water turbulence. The Cat 22 uses a heavy steel CB for ballast. I was thinking more of aluminum which should be quite a bit lighter. Do you think the canting of the standard board is a positive feature in terms of how the water moves around the board or would it be more effiecent if it was held truer fore and aft?

[adm]

Eric,

Surface of aluminum rapidly oxidizes and thin layer of aluminum oxide server as excellent protection from corrosion under normal circumstances above and below water. It is very hard, insulating chemical and aluminum will last almost indefinitely Problem starts when surface gets scratched. And in case of center board you can surely expect repetitive scraching around pivot point. That will cause very rapid corrosion in presence of water and small oxygen content there. It can be vary fast and it will go trough pivot hole and its vinicity.

A s for marine bronze - it is the best underwater alloy, period. It has been proven for over couple thousand years and I am old fashioned.

[DLT]

I'd be wary of using metal. Where do you want the failure?

I'd much rather my daggerboard fail, upon grounding than the trunk. Therefore, I want to make sure my dagger board is weaker than my truck, from any angle. From the side, metal would likely bend, but you'r more likely to take a hit head on. This, of course, isn't as big a deal on the X, as the centerboard rotates. But, on a M...




It would be one hull of a grounding plate for a lighting strike, but that's a whole other discussion...

[baldbaby2000]

I was suprised to read in a book I have, "Sailing Theory and Practice" by C.A Marchaj, that a streamlined foil is more efficient than a flat plate. The flat plate would be better if the fluid flow was exactly parallel to it but with a sailboat this is not the case. The flat plate actually has more drag than the foil due to the hydrodynamic forces. The flow becomes separated from the plate immediately aft of the leading edge causing turbulence and thus more drag. These results were verified in actual tests. I think the same holds true for the rudders. Having said that, one of the fastest boats I owned was a 28 ft E-scow which had flat aluminum lee boards so it may not be a huge difference.

BB

[DLT]

That non-laminar flow around the flat plate also degrades any lift created.

In airplanes, that's called a stall, where the wing actually stalls and stop producing lift.

Water is a lot thicker than air, so the problem is less in water...

A foil is just more efficient. But, that level ofm efficeincy may not be required in this application.

[Eric O]

DLT & balbbaby, if you read the quote in the initial post you will find that my research showed that for the speeds involved under sail and for a CB or DB that the foil shape is not that critical.

But while on the subject of lift created by a foil shape, isn't lift created only when the shape is not symetrical causing the flow to be faster on one side than the other.

adm & DLT, perhaps the problem with scratching and corrison around the pivot point of an aluminum plate could be handled by using a hard plastic, teflon, or similar insert at the pivot point.

[Catigale]

But while on the subject of lift created by a foil shape, isn't lift created only when the shape is not symetrical causing the flow to be faster on one side than the other.

We had a thread on this a year back or so. Most of the textbook descriptions of how a plane flies (specifically the origin of lift) are not correct. An airfoil with a symmetric shape can induce lift by other means it turns out.

[Hamin' X]

Good paper on lift Here

[Eric O]

Thanks Catigale & Hamin' for the clarification of how lift is created. Is there anyone who can explain in reasonably simple terms in which direction lift is created on a CB while sailing? Is it better to have the board cocked a bit of an angle, if so in which direction, or is it better to have it pointed straight ahead?

[RandyMoon]

If the dagger board on the M were 1" thick, 12" wide and 60" long, you would have 760 cubic inches of material. If stainless steel were .28 pounds per cubic inch, you would have a 201 pound dagger board. You could sharpen the front edge a tad.

What would that do for stiffening the Mac in the wind?