Steel centerboard and fairing

[Clinton B Chase]

I am currently working on a design for a 18 1/2' Maine coast sail and oar dinghy for my family. I am new to steel centerplates. I like the idea of having the weight down low for stability. A good job will make the CB trunk strong enough so that downside to a heavy steel board is not the concern. My concern is being able to still make a nicely shaped foil. My question is: How have people created a foil-shaped centerplate/board to take advantage of the ballast this brings while also acheiving a board that will create decent lift for the boat?

My thinking is to laminate wood on either side of the steel plate or glue the wood to the edges and fair the wood down to the foil shape, but this solution would reduce the weight of the board slightly. Another thought is to make the board a little thinner and build up the foil shape with epoxy fairing compound and do a little grinding at the leading and aft edge. Thoughts are welcome at this stage in the design process. Actual experiments on this stuff would be superb. I need to come up with some ballast solutions for the boat at this point. Thanks.

Cheers,
Clint

[JimD]

...My thinking is to laminate wood on either side of the steel plate or glue the wood to the edges and fair the wood down to the foil shape...


I asked the forum for opinions on this idea a few years ago and was roundly booed. Something about the wood taking up water. I still wonder if it might not work with plywood.

[Banjo]

Angle grinder and lots o sparks! :D

[paladin]

At 18 feet the centerboard ballast will need to be about 250-350 pounds depending on the design.....and steel is a flat surface and fairing it will require a couple inches of fairing.....why not make the board from plywood (dimensionally stable) using several layers.....make the first part out of 3/4 inch ply......layer 1/4 inch on either side....it may be a smaller piece when you figure out the fairing, now you can add lead, either molten in place or lead shot and lead powder in epoxy, then 2 layers of ply either side and faired.....to be really efficient (depending on the board profile) the board needs to be about 3 1/2 to 4 1/2 inches thick, with the thickest par about 33% aft of the leading edge....

[Hwyl]

Every steel centreplate I've seen, has just been a plain plate. I think that there is actually a hydrodynamic anomaly when it comes to thin plates. Something about them giving good lift as is. One of the NA's or Jim Conlin would know.

If I were going to design it, I'd borrow the idea from Uffa Fox's "Fox terrier" where he had a wide chord dagger board, that was hollow and lead could be dropped inside, in smaller units, I seem to remember it was lead shot in 10lb bags.

[Clinton B Chase]

Interesting idea Paladin...I the steel CB would be part of my ballast...some of it in the form of lead pigs easily moved around the boat or out of the boat, but the boat will require 150-250 pounds of permenant ballast, ideally in the board.

I just finished reading something that mentioned the need for the foil to be quite thick to be really efficient. This tells me that there is not a need to go too crazy about shaping the foil. I wonder if a steel plate, trowelled with fairing compound and shaped to a foil (like one does to shape a lead keel on a yacht), the leading edge ground to a round and the rear tapered ....that would be tough to do by grinding...how to remove the steel to make that taper?

You are saying, get the excess weight by putting a heck of a lot more lead in the board than normal? I'de worry about the board getting stressed by all the weight.

Cheers,
Clint

[JimD]

I wonder if a steel plate, trowelled with fairing compound and shaped to a foil (like one does to shape a lead keel on a yacht), the leading edge ground to a round and the rear tapered ....that would be tough to do by grinding...how to remove the steel to make that taper?

Cheers,
Clint

That sounds like a lot of fairing compound. I have a half inch steel plate board, weighs 120#, and more than that again inside our 15 footer. Is yours a high performance design? Can't say I've ever seen a high performance design call for a plate centerboard and I wonder aloud if its worth the extra money and effort to design a foil CB for a boat that may not benefit much from it anyway. I dunno for sure, just curious.

[paladin]

I made a board.....started with 3/4 inch ply......board was a few inches shy of 8 feet long and 3 1/4 feet chord. When finished, the board was made by epoxying multiple layers of 1/4 inch marine ply to each side of the board , and addition 3 layers each side. I did use a layer of epoxy/dynel fabric between layers. I laid the board out with the anticipated curves, which were easy, and power planed it to shape. It made a very efficient blade, finished it with 3 layers of dynel/epoxy on the leading and trailing edges, and 2 layers on the sides, the boat sailed for over 6 years, and completely through one circumnavigation, and for several years thereafter. Jim Brown then used it when looking for reefs and other underwater objects for several years, until he finally managed to break it 10-12 years after it was built......the case was also well built...and has never leaked......1969 to present day......I started the job early April 1969 and finished it about one year later....
I have a current project for a nice roomy trailerable cruising boat, adaptable sail or power and will use a ballasted centerboard of similar construction for the sail version.
I used a similarly designed centerboard and centerboard case when retrofitting my Piver AA-31 Trimaran...I also used the ideas from Jim Browns "Manta" tri, by making mainstrength bulkheads to support the ama's of the piver, and used the Norman Cross ideas of canted ama's and also changed the rudder to match the Jim Brown Searunner rudders.....My AA-31 would outperform the similar tri's in California at the time, out pointing and out sailing. I should also like to point out that at the same time I used the recommendations of Rolly Tasker and converted to fully battened mainsails with a good roach in them, the sides and the bulkheads of the floats were raised 9 inches all the way around, then the tips of the bulkhead frames were set up to attach the ama's outward at an 8 degree angle.

[Wiley Baggins]

Angle grinder and lots o sparks! :D

I made one of 5/8-inch plate, and that's the process (I used).

[rbgarr]

Clint-

You might do as well by finding an old Lightning class sailboat centerboard. Many of them have steel centerboards . The stainless one on a boat I raced was shaped very smooth with no need for fairing materials. Her sistership was sailed by a guy in Portland and one day he capsized and turtled her. He climbed up onto the bottom of the boat and having not cleated the board in the down position, it slammed back into the slot and nearly chopped the pinky finger off his hand.

[Oscarvan]

Something about them giving good lift as is. (steel plates) One of the NA's or Jim Conlin would know.

Is there someone with knowledge that can expound on this? Thanks!

[Thorne]

One possible issue is that this is a sail and oar boat, so at least part of the time you'll be rowing it with the CB fully up in the case -- this puts some of that ballast rather high. Presumably you'd drop the CB at least partway during extreme conditions (rowing in a squall), so some of that issue would be negated.

I've always thought there were two reasons why steel CB's were used flat:

1. They are reasonably close to the most efficient shape as is, so it would only matter when racing one-design.

2. Using any material to build the foil shape could create serious problems if you hit anything with the CB. Fairing compound/wooden cheeks/fiberglass pads/whatever --- it might partially detach from the metal and jam in the slot or the case. Also makes it harder to just pull the CB and hammer it back into shape for emergency repairs. And any attached material provides scope for either rot of the material, or rust of the CB.

[Woxbox]

I have a steel centerboard collecting dust in the back of my garage. It's flat, and worked fine in the 16-foot sloop I made it for, but after one season I found two things:
1: I really didn't need the ballast.
2: It was a pain to raise and lower.

So I replaced it with a plywood board reinforced with a layer of glass. I couldn't make the wood board as thick as I'd have liked too because I'd built the centerbaord case for the flat steel plate, and wasn't wide enough for anything else.

The real question is this: what are your intents with the boat? Is maximum performance or maximum convenience the priority? The heavy board adds a lot more than ballast. And if you really need ballast in the board, I agree with Chuck -- make a nice wood board and then add lead down low, where it really counts.

[merlinron]

i can try.....
the lift gained from a steel plate centerboard is more associated with it's relationship with the under water hull shape as the boat heels than any sort of truely hydrodynamic lift from an actual foil shaped centerboard.
as the boat heels, it rolls on it's c/g and and the physical centerline of the hull deviates from the true direction the boat is sailing. the plate ends up slightly pointed up in relation to the course and acts as a foil because flow is disrupted at it's leading edge on the windward side, creating an area of low pressure from the resulting eddies off the windward leading edge. as the eddies straighten out towards the trailing edge of the plate the flow around the plate resembles that of a foil, so a small amount of lift is generated, but, as always, it is at the cost of an awfully high amount of drag from all the disturbed flow over the "mis-aligned" centerboard.

a foiled shaped board is also similarlily "mis-aligned" in relation to course, but it's shape directs the flow around it much more efficiently and drag is greatly reduced yealding much more lift.

[JimConlin]

The appeal of steel plate centerboards is that they're cheap to make, provide ballast whether you want it or not and, if galvanized, aren't hard to maintain. The downside is that they are not very efficient foils and provide weight whether you want it or not.
I expect that very few of us have the patience to grind any meaningful contour into a steel plate, so a little rounding on the leading edge is all anyone's likely to do.
My take is that if you can tolerate the weight in a low-performance boat, they're a reasonable compromise for many of us.

[Banjo]

The two boats myself and a mate are building is 5.2 M, the design calls for a steel plate board that is hot dipped when shaped.

Paul has made his in steel as per design.
http://users.bigpond.net.au/banjos-backyard/images/DCP_Plate_Paul.jpg
Thats Paul standing proudly next to his home made steel center board, it's 20mm plate and weighs aprox 75 kg.

http://users.bigpond.net.au/banjos-backyard/images/DCP_Plate_Front.jpg
You can see the shaped leading and trailing edges in the shot above.
Paul painted it to stop any rust until he gets around to sand blasting it and get it hot dipped.

I made mine out of laminated wood with a steel insert.
My board is 40mm thick at the thickest point of it's foil so I could fit the 20mm steel plate inside it.
No pics of mine yet for some reason (thought I had some somewhere), oh well I will take some more and update soon.

[merlinron]

a steel board could be made from two sides shaped on a break . maybe 16 ga. black iron, bent in facets into a close foil profile and the two sides welded together along the leading and trailing edges making a hollow foil with a cap welded on the end. it could then be filled with lead to the weight needed and the remainding volume filled with expanding foam. faired with epoxy fairing compound to a true foil shape, the weight would be at the very bottom of the board, it would be as effective as a bulb keel for stability and the close shape would minimize the work and amount of fairing material needed. any decent sheetmetal fab shop or HVAC shop would be able to bend the sides. the pivot could be a short piece of mech. tubing welded to the sides and a poly or delrin bushing inside that.

[MiddleAgesMan]

Fabrication of a foil shaped steel board could start with a length of black iron pipe for the leading edge, say 1/2 or 3/4 ID pipe for a boat this size. Cut the sides from 10 or 12 gauge plate and weld them to the after portion of the pipe, on an angle that takes them further apart from each other.

You would then clamp them together along the trailing edge, using a torch to direct the point where they bend a few inches behind the leading edge. Weld the trailing edges together, close up the top and bottom and then fill with oil or lead shot as necessary to get the weight right. Grind a taper to the trailing edge but leave a flat maybe 1/8 to 3/16 wide.

Use fairing compound to improve the shape then prime and paint.

Such a board would be incredibly strong and it is easy to fill it with whatever you need to achieve the designed weight. If it needs to pivot weld a short length of pipe at the pivot point, with an ID sized for the pivot bolt. Oil or lead shot could be introduced through this hole in the early stages of fabrication.

[andrewe]

Merlinron & Middleagesman, this is roughly what I am doing at the moment.
The design calls for 1" plate ground or milled to a foil,hot dipped, about 340lbs finished.
With the designers approval, I am fabricating a foil out of 4mm. steel. Leading edge is 20mm.round bar, two 40x8mm. `spars´at the point of max. section. The steel is sprung at the fore and aft edges and welded to the bar and each other at the trailing edge. This gets it closer to the dimensions of a naca foil. Hot dip and ballast up to weight. It sound like a lot of work, but I have the cutting and welding kit so it is only a question of my (free) time. The quotes for for one of the other builders where up to
$3,500. He went the ply/lead fill route.
The local galvanizers charge by the kilo, so my `plate´at about 50/60 kg (110/135lb)will be around a third to dip compared to a solid one.
Slightly off topic, the price of scrap lead has doubled here since last april, must be the Chinese.
Andrew

[Clinton B Chase]

Bringin' this one back as I am coming to a point where I have to decide on the centerboard for my design.....

In the intervening time, I saw an article in Watercraft on the construction of Nigel Iren's Romilly. In the article, they took a 200lb steel C/B and clad each side of the plate with plywood. They then shaped to plywood to get the foil...I don't know whether they used a NACA foil shape or not, but I suspect they got something better than bevelled edges that just the steel would allow. This picture got me thinking and it seems like the best solution so far. I haven't drawn out the board yet to see what the limitations would be but thought I'd bump the post to see if anyone out there has used this method of making a foil shaped steel C/B.

Cheers,
Clint