NACA in encapsulated?

[massnspace]

I am researching different features of boats for an upcoming sailboat purchase.

I have only owned bolt on fin-type keeled boats…..but thinking hard about an encapsulated type.

Question: Is it possible to get a boat with an encapsulated keel that has (or comes close to) the same shape (proper NACA foil section) of a bolt-on keel?

It seems to me from the online pictures that most encapsulated keels do not have a proper foil section……just big “blobs” of material with not much thought about the shape. If so, it seems upwind performance would be severely limited.

Do encapsulated work well upwind? Are there things to look for in an encapsulated type of keel? Do non-naca keels work similar to a naca, just a shade of difference, or is it a whole different creature?

David
Seattle

[Chotu]

It’s certainly *possible* to get a proper NACA foil shape on an encapsulated keel, but do boat manufacturers strive for this like those of us that built our own foils? Not really. They are blobs.

My dagger boards are “encapsulated” in glass and are perfect NACA foils. Rudders too. A company churning out boats for profit just doesn’t have the interest in that level of perfection because it’s not a wise business decision.

That said, I don’t think the lift is awful on them like a big, shallow, full keel. Just not as good as it could be which means less pointing ability.

[wholybee]

NACA is a way to describe an airfoil. It doesn't mean that is is a "good" airfoil. I'm not sure what you mean by "blob", but I am sure it could be described with NACA. So, good or bad, a designer can use NACA to describe the foil they wish to use and have the builder follow that description.

My boat (Morgan 382) has an encapsulated keel and a NACA foil shape. Listed on the Drawing as NACA 64(sub1)-A012 Streamline.

sub1 is a subscript "1", I don't see how to type that on the forum.

[Chotu]

Wholeybee: in boat building, it is used the other way around. The NACA foil shape chosen by the designer (from the standard foil shapes developed over decades of research in aeronautics) is used to *specify* rather than describe the final product on a boat.

Your goal, as a builder of a fast/efficient boat is to get as perfect a shape to the foil specified by the designer as you possibly can.

For me, this meant tracing out tons of NACA foil profiles by hand (from flat sheets of foam), stacking them up on top of each other,, bonding them to each other and then vacuum infusing a thin layer of glass over them leaving an almost perfect NACA foil shape.

Many encapsulated keels are blobs in comparison to the true foil shape. However, yours may not be. I think it’s a YMMV type of situation

[Bowdrie]

Generally speaking, boats that use encapsulated ballast all share a certain sameness.
And it matters not whether the hull comes out of a one-piece mold or a two- piece.
The width of the hull along the center line must be great enough to allow the workers to be able to reach down and do the lay-up along the centerline.
Laying down on suspended platforms, reaching down into the cavity area where the ballast will end-up, whilst working to get slippery sections of wetted-out fiberglass into position, and then making sure it is well rolled-out is a miserable job.
I think you can imagine why, (in plan view,) that boats with encapsulated ballast generally show more thickness thru the keel sections than can be achieved with bolt-on ballast.
An NACA section? I'm not sure, some designers/builders can be pretty creative.

[Tortuga's Lie]

My old Sparkman-Stephens Tartan 37 had an encapsulated keel with a centerboard that had a nice foil shape to it, the last 18 inches was hollow and faired to a fine tip to finish off the foil.

[Bowdrie]

I might add, the building of production fiberglass boats that used encapsulated ballast occupied, (with few exceptions, Pacific Seacraft comes to mind,) a relatively short period of time.
From the later '60s>early '80s many, (most?,) of these boats were adaptations/modifications of earlier designs that would have been built in wood.
It was fairly easy to take a heavy displacement full keel design, build a mold, lay-up a hull and dump the ballast into the space that in wood construction would have been a large keel/keelson/frame structure.

[rslifkin]

Originally Posted by Bowdrie

I might add, the building of production fiberglass boats that used encapsulated ballast occupied, (with few exceptions, Pacific Seacraft comes to mind,) a relatively short period of time.
From the later '60s>early '80s many, (most?,) of these boats were adaptations/modifications of earlier designs that would have been built in wood.
It was fairly easy to take a heavy displacement full keel design, build a mold, lay-up a hull and dump the ballast into the space that in wood construction would have been a large keel/keelson/frame structure.

Pearson built some encapsulated fin keels in the 80s as well. Not sure who else did.

The Pacific Seacrafts I'm familiar with have a nice, deep keel stub, but the rest of the keel (and all of the ballast) is bolted onto that stub, not encapsulated.

[Bowdrie]

Originally Posted by rslifkin

The Pacific Seacrafts I'm familiar with have a nice, deep keel stub, but the rest of the keel (and all of the ballast) is bolted onto that stub, not encapsulated.

You're right, having that "stub" was what was needed to get a good structure that would still allow a good hydrodynamic shape.
Look at the early S&S designed Swans, a keel with decent lift bolted onto a stub that's shallow/wide enough for easy lay-up of the glass.
I seem to remember some Pacific Seacrafts with integral ballast, but it's been decades since I was at the facility.

[DougR]

The answer to your questions are : yes and yes.

Yes, you can buy an encapsulated keel with a proper NACA foil shape, and yes these keels can work very well going upwind.

Of course, how do you know whether the boat you are looking at with an encapsulated keel has a well selected foil shape? You don’t unless you get a look at the lines plan. But then again, how do you know what the foil shape is for a bolt on keel? You don’t know that either....


One of the advantages of a well designed NACA encapsulated keel is that all that
labor in building and fairing the keel is done only once, on the plug. Thereafter each boat that comes from the mold has that same uniform, fair keel.
Compare this fair encapsulated keel to cast iron, or cast lead bolt on keels. All you have to do is walk around a boatyard in the winter time and look at bolt on keels made of cast metal. Course, rough castings, sometimes with cast in numbers still showing on the surface, flat spots from shrinkage, poorly fitting at the keel stub, zero or minimal fairing. Really ugly.... I’m not saying that all bolt on keels are like this, but lots of them are.

I have owned several boats with bolt on lead keels and my current 39’ boat has an encapsulated keel. Far and away the encapsulated keel is smoother and more fair than the bolt on lead keels were. But what about the shape of the keel? That I leave to the vessel designer. All of the boats I have owned came from the boards of S&S or Lapworth, neither one slouches. The encapsulated keel is a NACA 0010 foil with the max thickness at station 4. Not what you would call a “blob” shape, but more likely one optimized for use in moderately high performance fin keels. And built into an encapsulated keel.

[wholybee]

Originally Posted by Chotu

Wholeybee: in boat building, it is used the other way around. The NACA foil shape chosen by the designer (from the standard foil shapes developed over decades of research in aeronautics) is used to *specify* rather than describe the final product on a boat.

Your goal, as a builder of a fast/efficient boat is to get as perfect a shape to the foil specified by the designer as you possibly can.

For me, this meant tracing out tons of NACA foil profiles by hand (from flat sheets of foam), stacking them up on top of each other,, bonding them to each other and then vacuum infusing a thin layer of glass over them leaving an almost perfect NACA foil shape.

Many encapsulated keels are blobs in comparison to the true foil shape. However, yours may not be. I think it’s a YMMV type of situation

I understand what you are saying. But it doesn't change the premise. Whether the designer draws a keel and calculates the NACA code to describe it, or looks up a keel they like in a book and uses that code doesn't really matter.

The point is that NACA doesn't guarantee good performance. NACA codes are mathematically derived such that they can be used as input in a simulation. The code *could* be run through simulations to measure the performance. But being an NACA coded keel shape doesn't in and of itself mean very much. It just means that you could more easily evaluate the keel in a simulation. I.E., a really crappy keel shape can be described with a NACA code.

Water has a much higher density than air. A really fat foil that provides good lift in air, would have more drag in water. I think that is what the OP is getting at, that encapsulated keels are thicker. (I still don't know what blob means). But such a thick keel can still be selected from a NACA catalog.

The rudder is a key component as well. A keel is symmetrical. In order to provide lift, a foil needs to be asymmetrical. The water flows over the keel and rudder together, and a slight angle to the rudder (as a result of correcting weather helm) creates the camber needed to provide lift. This is why a small amount of weather helm is important, and why not trimming the sails to have the correct amount of weather helm has a large affect on performance.

All and all, I would be much less concerned about a keel being bolt on or encapsulated, and (If I want best performance) look towards newer boats where this type of engineering is applied. I think in the 1980's designers were just starting to think about this sort of thing, and while some great efforts were made, it wasn't really done right until later.

[Jammer]

Not all winglike shapes are NACA airfoils. The NACA nomenclature can only model certain shapes. To Warren's point, of course, not all NACA shapes are equally useful/peformant as keels.



I think what the OP is trying to get at is that certain boats have keels or rudders that have been designed without any meaningful effort to follow a high-performance hydrodynamic shape, whether one from the NACA series or not. Instead, they are shapes that are chosen for strength and ease of manufacture.


The question then is whether (or to what extent) the manufacturer used a keel shape with known hydrodynamic properties and with performance in mind.


The performance penalty for a reasonably shaped "blob" may not be that great. Typically the production keels I see don't taper down nearly thin enough aft, even on racers.

[Lee Jerry]

Originally Posted by wholybee

I understand what you are saying. But it doesn't change the premise. Whether the designer draws a keel and calculates the NACA code to describe it, or looks up a keel they like in a book and uses that code doesn't really matter.

The point is that NACA doesn't guarantee good performance. NACA codes are mathematically derived such that they can be used as input in a simulation. The code *could* be run through simulations to measure the performance. But being an NACA coded keel shape doesn't in and of itself mean very much. It just means that you could more easily evaluate the keel in a simulation. I.E., a really crappy keel shape can be described with a NACA code.

A NACA foil has documented performance (by a government agency many decades ago). That’s what distinguishes it from (some) other foil shapes. Whether it is ‘good performance’ or not is a mater of opinion.

I don’t know what you mean by “NACA code.” You’re using it in an unfamiliar way.

Water has a much higher density than air. A really fat foil that provides good lift in air, would have more drag in water. I think that is what the OP is getting at, that encapsulated keels are thicker. (I still don't know what blob means). But such a thick keel can still be selected from a NACA catalog.

It also has more lift. The lift-to-drag ratio is the same regardless of the medium. (The suitability will vary, of course.)

The rudder is a key component as well. A keel is symmetrical. In order to provide lift, a foil needs to be asymmetrical. The water flows over the keel and rudder together, and a slight angle to the rudder (as a result of correcting weather helm) creates the camber needed to provide lift. This is why a small amount of weather helm is important, and why not trimming the sails to have the correct amount of weather helm has a large affect on performance.

No, it doesn’t. It just needs an angle attack. A flat plate can generate lift.

[Chotu]

Bowing out so as not to get into a big debate but there are some incorrect ideas in this thread regarding the NACA foils. A great part of what i studied in the design of this boat has to do with the foils. There are definitely some misunderstandings here.

[Bowdrie]

Originally Posted by Jammer

Not all winglike shapes are NACA airfoils. The NACA nomenclature can only model certain shapes. To Warren's point, of course, not all NACA shapes are equally useful/peformant as keels.



I think what the OP is trying to get at is that certain boats have keels or rudders that have been designed without any meaningful effort to follow a high-performance hydrodynamic shape, whether one from the NACA series or not. Instead, they are shapes that are chosen for strength and ease of manufacture.


The question then is whether (or to what extent) the manufacturer used a keel shape with known hydrodynamic properties and with performance in mind.


The performance penalty for a reasonably shaped "blob" may not be that great. Typically the production keels I see don't taper down nearly thin enough aft, even on racers.

You've done a better job of explaining what I was try to get at.
The vast majority of so called "encapsulated" keels on boats from a Westsail to untold makes/models from Tiawan are nothing special in their hydrodynamic shape.
Indeed, most boats that we commonly refer to as "encapsulated keel" designs actually have relatively little in the way of a "salient" keel at all, they're quite thick thru what you might call the garboard area.
The process of trying to lay-up fiberglass into a nice hydrodynamic shape that extends down 3 Ft. or so into a female mold gets real dicey.
Can you hire lay-up personnel with 4 Ft. long arms that can reach down and work in a space that's perhaps only a few inches wide at the bottom?
On a lighter note, I gotta keep watch on my terminology.
I suppose that if I covered the bolted-on fin of a Beneteau with fiberglass that I could say that I had an "encapsulated keel".
The "keel" is what we see as the external shape, what is really "encapsulated" is the ballast.