Keel / centreboard aspect differences

[seasquirt]

It's a rainy sunday and I was pondering. I'm curious about the difference between long vs deep keels / centreboards. Looking at 2 different theoretical rectangular keels or centreboards of the same mass, weight distribution, volume / displacement, and surface area, given the same front vertical edge rounding / rear edge feathering, flat bottomed treatments, and being almost identical, but one is made to be set as a narrower deep board, and the other is made to be set as a long shallower board. Ignoring improved ballasting effects of the deeper board, and assuming both boards are vertical, not heeled, and subject to identical overall sideways forces, in ideal conditions, and disregarding any hull effects. Which one is most efficient at reducing side slip, from stalled, to speeds under say 8 Kn ? Would forward speed even make a difference ?
I can see that the same wetted area but deeper one would suffer greater frontal resistance when moving forward, pushing through more water due to its length, but I don't think the longer drag length of the long shallow board would give equal resistance when moving forward. The longer leading / trailing edge treatments of the deeper board may make it a tiny bit more 'slippery' to sideways thrust, possibly negligible. As for resisting sideways movement, would the water density difference between say 1 foot deep and 3 feet deep, make any appreciable difference in un-aerated water ? Maybe the slightly less dense, air pressured surface water has the benefit of more 'give' to flow upward, or around the edges to the low pressure side, where the lower down slightly more dense water can not move away so easily, so resists more. Not a weighted bulb style board like ocean racers, but an evenly weighted homogenous regular slab sided rectangular metal plate.

I'm messing about with my small slow sailing dinghy, not making it go faster, thinking of replacing the swinging wood centreboard with a slab of something heavier, in the shape of a vertical lifting steel or ally plate of about 20 - 30 Kg. Not huge. Shaped to take up the length of the existing centreboard case, (so it will be longer to the rear), but not lowering as deeply, thus the 1 foot deep x 2' 10" (3 feet) long, vs 2' 10" (3 feet) deep and 1 foot wide question. I know the extra weight will slow it down a tad, it's already slow anyway, but hopefully will make it stiffer, and also less twitchy on the steering, for better self steering. Making it more like a steady micro cruiser than a tender beach dinghy. So, will the same surface area - aligned long ways instead of deep wise, allow the same side slip, or better, or worse, or negligible difference ? I assume negligible, at that order of magnitude anyway, I don't know, but someone here does. I think the boat might squat a little with the extra rearward weight.

It's a simple job to convert: pull out wood board, plug pivot holes, pull case top, make a new (removeable) case top setup with lock pins, and slip guides, cut plate and shape the edges, 3 lifting points, a stop on top so it can't fall out the bottom, lock pin positions so it can't fall out the top, but can still be locked retracted for the trailer. Lift board up by hand, not that 'heavy' . If it doesn't work on the boat, I'll have a unique barbeque plate.

While I'm at it, is there anything else I could do to the new centreboard, apart from an anode if steel ? Latest edge shape treatments, eg. bullet style ogive, or sharp, or just a blunt semicircle leading edge; full - flat - or hollowed exit edge feathering ? Arguments for or against a thinner steel plate (I can cut weld) or a fatter ally plate (not magnetic, self anodising, more drag) of same weight, in a wood boat.

Thanks for any answers to my excessively long, simple question.

 

[jehardiman]

Looking at 2 different theoretical rectangular keels or centreboards of the same mass, weight distribution, volume / displacement, and surface area, given the same front vertical edge rounding / rear edge feathering, flat bottomed treatments, and being almost identical, but one is made to be set as a narrower deep board, and the other is made to be set as a long shallower board.....thinking of replacing the swinging wood centreboard with a slab of something heavier, in the shape of a vertical lifting steel or ally plate of about 20 - 30 Kg. Not huge. Shaped to take up the length of the existing centreboard case, (so it will be longer to the rear), but not lowering as deeply, thus the 1 foot deep x 2' 10" (3 feet) long, vs 2' 10" (3 feet) deep and 1 foot wide question.

The discussion of the aspect ratio is only remotely tied to it being a fin keel or a centerboard. All else being equal, higher aspect ratio foils have a better lift to drag curve (....to a point, then motions become an issue). More important for comparing lifting between the two (i.e. fin keel and centerboard) is the thickness ratio and root fairing. In low aspect, low Reynolds number foils the root fairing of a fin keel provides a significant advantage as there is not much difference between thickness ratios. As Reynolds number increases, the root fairing becomes less significant. In high aspect foils for most Reynolds numbers, thickness ratios have to sufficient to prevent leading edge separation and the early onset of stall; again most centerboards have a disadvantage because they typically have low thickness ratios in order to prevent a huge slot drag penalty. At higher Reynolds numbers, the thickness penalty is not as pronounced. Regardless of the aspect ratio, you would not want to make a keel just the same as the centerboard. In order to be useful, a centerboard gives up some performance in shape and fairing, mainly at the root and in section thickness. On the other hand, centerboards bring a lot to the table in terms of draft and the ability to balance the lateral area.

Realistically, you seem to want a daggerboard or lifting keel. In this case (get it? case?) you will probably want something significantly thicker that the existing board, but I would not make it as long as the existing CB slot. You would just need to make sure that the location selected for the daggerboard is correctly placed relative to the center of effort. For most small sailing dinghys this would be the same location as the existing centerboard when fully down.

 

[revintage]

No reason to slow your boat down even more with a metal plate. Why not just buy a hollow aluminium NACA section? If you want, you can fill it with lead granules to make it heavier and if you find 0009 a little thin, you can expand it to 0011 by inserting a 1 inch tube.
Windknife Aluminum Extrusions https://duckworks.com/windknife-aluminum-extrusions/

 

[seasquirt]

Thanks for the reply jehardiman.
It would be impractical, (in my case), to install a heavier vertical daggerboard, since when up, it would be fouling the vang, and maybe the boom, (I didn't think about that earlier), as well as reducing stability greatly, whereas a horizontally longer shallower plate would lift to not much higher than it already does, keeping the weight lower, not taking up much more room, and not being as much of an inverted pendulum when raised. And I think I understand now that a flat plate no matter the edge treatments, will be more turbulent than a fin/wing shaped cross section. I don't want a weighted swing centreboard, stressing the pivot area, and potential kinetic swinging damage into the hull and case. I can put shape into the existing bottom slot, reducing the gaps, to take a more shaped foil than just a flat plate. So now I'm thinking of some sort of backyard composite, if flat plate is such a drag, despite being simple and easy and cheap and multi-function (BBQ plate). Maybe sheet metal bent around a diameter, doubled back, squished to shape, back edge welded, bottom plate welded fixing the cross sectional shape, top attachments, lead in the bottom, structural foam filler on top, sealed with resin. That's getting complicated, and expensive, and it's only a 10 feet long sailing dinghy. How bad can a modified flat steel plate be, I'm not doing the America's Cup.

Hi Revintage, the NACA section looks very nice, I'd only want a metre length if making a daggerboard, or half a metre if making a stretched shoal draft version. It looks expensive though, I'll look it up. Thanks for the input.

 

[Tops]

I did an internet search for 'centerboard section comparison' on the Duck-Duck-Go which led me to an image off a thread on the Woodenboat forum. My connection died so I lost that link but here is the picture. My 8' boat has simple , both sides foiled plywood + fiberglass cloth and epoxy resin for the rudder and a single leeboard, the C-scow parts I have left over from a scuttled boat are flat aluminum plates that are beveled instead of foiled. It had 2 leeboards so one side of each leeboard is flat.

 

[seasquirt]

Thanks for the info Tops.
While looking up NACA profiles and Reynolds No's info, I found a site with rudimentary info for smaller boats, which basically said a NACA foil is probably going to be 'crabbing' anyway, and not so efficient at the low speeds a small sailing dinghy will do, (around 4 Kn) and so will be barely more efficient than a flat plate anyway, for that size, but bigger and easier gains can be made by using NACA profiles on rudders instead.
This may be true for some of the time, except when correcting your heading, which is almost continuous. It's much easier to modify rudder blades though, than widening a narrow centrecase for a 12% fat foil shape.
My existing centreboard is 14 inches broad, extending vertically 28 inches out from the hull. The case slot is 3/4" wide, (21mm), and if 21mm = 12% , then I can only fit a 175mm broad foil in that existing slot; I'd need to at least double the slot width to get ~equal foil area. Which means too much work retro-fitting and strengthening a 10' plywood fun dinghy.
So it still looks like a shaped flat metal plate is the easiest compromise solution, with semi-circle / bullet shaped leading edge, and elliptical bottom to reduce drag, (like a spitfire wing the article said, similar to Tops's info illustration), and a long tapered exit to a thin rear edge. Then re-shape, or replace the rudders for a NACA shape.
If I tried to use 3/4 of my 80cm centreboard slot length for a shoal draft NACA profile, 60cm long needs a 7.2 cm wide slot, nearly 3 inches, which is a huge amount of work. And it will still never win any prizes for speed.
Inefficient, draggy, flat plate given the best surfaces, probably gives a better fun : work ratio, I think, in the short term.

 

[Doug Halsey]

I did an internet search for 'centerboard section comparison' on the Duck-Duck-Go which led me to an image off a thread on the Woodenboat forum. My connection died so I lost that link but here is the picture. My 8' boat has simple , both sides foiled plywood + fiberglass cloth and epoxy resin for the rudder and a single leeboard, the C-scow parts I have left over from a scuttled boat are flat aluminum plates that are beveled instead of foiled. It had 2 leeboards so one side of each leeboard is flat.
View attachment 182818

FYI: That figure is from Marchaj's Sailing Theory and Practice.

 

[Doug Halsey]

Thanks for the info Tops.
While looking up NACA profiles and Reynolds No's info, I found a site with rudimentary info for smaller boats, which basically said a NACA foil is probably going to be 'crabbing' anyway, and not so efficient at the low speeds a small sailing dinghy will do, (around 4 Kn) and so will be barely more efficient than a flat plate anyway, for that size, but bigger and easier gains can be made by using NACA profiles on rudders instead.

There are literally hundreds of "NACA foils" with a wide variety of shapes & characteristics. Whatever you look up should be more specific which ones they're referring to.

 

[Tops]

FYI: That figure is from Marchaj's Sailing Theory and Practice.

Thanks Doug for noting the source. I have yet to read any Marchaj for myself.