Naca Airfoils shape and rocker oft flat bottomed boats

[Dieter51]

Hello,
since I used to work on airplanes, I would like to ask, have inverted NACA profile contours, which are subjected to flow from the trailing edge, been studied for their application in the rocker design of upright sailing flat-bottom boats?

Best - Dieter

 

[Dieter51]

Better said:
Well, I realize this question could have its origins in pareidolia (pattern recognition), but I wonder why almost all flat-bottomed boats, from the Opti to the yacht, have a certain bottom profile that is reminiscent of a simple NACA profile with a large thickness setback.

The catch is stored at the back, so the greatest depth of the boat must be at the back, they say. Chapelle does not elaborate on this either, it's just the way it is. But why is this the case, even for boats with a “sawn rocker”, i.e. with vertical and parallel sides whose saw cut determines the bottom profile- scows.

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[baeckmo]

I have not come across any studies relating the bottom shape to the aft part of a foil, but there are fluid dynamic similarities. In both cases, the fluid is supposed to slow down, with the consequence that dynamic energy is converted to static. The efficiency of this process ( we call it "diffusion") depends on the behaviour of the boundary layer along the surface; the fluid is actually moving from a low pressure zone into a higher pressure. The closer to the wall, the lower the fluid velocity; at a certain limit the flow cannot follow the wall and it detaches. When this happens, the flow losses increase, which for the boat hull primarily manifests itself as increased drag and change of trim, while a foil will suffer from increased drag and reduced lift/drag ratio.
In both cases, the shape of the surface (basically the change of flow area per unit length) determines the risk for boundary layer detachment, which explains why there seems to be a similarity in shape. In general though, the aspect ratio (beam/chord ratio) differs considerably between a foil and a hull, making the comparison a bit shaky.

 

[Dieter51]

This is the pressure distribution along a Naca 0012 profile flowing from the “front and one from the rear” resp. 0 and 180 degree / 10 an 170 degree. I have not found anything else on the subject.

Source: Marchaj, Aero- und Hydrodynamik des Segelns, Klasing

This is the pressure distribution along a Naca 0012 profile flowing from the “front and one from the rear”. I have not found anything else on the subject.

Source: Marchaj, Aero- und Hydrodynamik des Segelns, Klasing

Is anyone in a position to derive a qualitative statement from this as to why the bottom of flat-bottomed boats is designed like a backward-flowing Naca profile? Whereby I know that it is more of a slim body than a wing due to the low aspect ratio < 1.

 

[baeckmo]

Are you looking at the hull from above, or are you referring to the bottom shape, seen from the side? I think the translation is not conveying your question correctly?

Edit:
If you are looking from above, any similarity between the waterplane area and a symmetrical foil is irrelevant, since the hull is operating on the surface between a dense and a low density fluid, where wave-making is occurring, while the wing foil is operating in a more or less homogenous fluid.
The term "rocker" is used for a bottom with a convex shape in the longitudinal direction.

 

[philSweet]

The math for an optimal foil riding on the surface - ie a planing surface, was solved eons ago back in the early seaplane days. They had to build floats that could get on a plane, then on a step, then had to reach takeoff speed. And since they were aero guys, they started with what they knew.


NACA 1932 - https://ntrs.nasa.gov/api/citations/20050242060/downloads/20050242060.pdf
NACA 1934 - https://ntrs.nasa.gov/api/citations/19930094647/downloads/19930094647.pdf

 

[Dieter51]

Agreed, the pressure distribution suggests that such a profile of the bottom indeed offers little reason for a flow around the hull from the bottom to the sides and that the pressure gradient towards the tail builds up very slowly.

But what really happens?

I once built a swept wing with a Naca profile 40 years ago, which lay in the water and served as a fuselage. See picture below. During towing tests, this “hull” went to depth faster than a submarine could. Great amazement. It needed a very large angle of attack to prevent this.

But why then do we not have a sinking stern on “boats” like the Goose even though there is a considerable curvature near the stern?

 

[Dieter51]

ops, I am of course referring to the flat bottom curve *viewed from the side". I guess they call them "rocker" in American - who knows why. It's not the mathematical model I'm interested in here, but whether anyone knows why all the flat bottom boats built from experience have a bottom curve when viewed from the side, similar to an inverted simple NACA profile - a half section.

Some people would expect the bottom to start with a curve at the front, making the bottom very stiff, and to end flat at the back, but it is exactly the opposite. The bottoms all start with a flat run - I don't know the name for it - and end with an “upward bend”, which is expected to be disadvantageous.

Is there a hydrodynamics explanation for the development, which undoubtedly reflects experience? That is the reason for my question.

 

[philSweet]

There're a few reasons.

1. The initial bow wave(and some of the rest) of a moving vessel is largely an unrecoverable energy loss - It's all entropy. So you minimize the entropy wave with a fine entry. It's why the cutwater of boats don't look like the bow of a submarine. Note that well down, bulbous bows are fine, but the cutwater needs to be sharp. This becomes less important as you go slower and as boats get bigger, because the entropy loss doesn't scale with displacement.
2. In boats with a raked keel, there is a large volume being taken up by the keel, and you simply subtract that volume from the aft sections to preserve the correct curve of displacements.
3. As you pass the midship section, the water's boundary layer thickness expands rapidly and you have a substantial volume of stalled water with a big momentum defect, and you need to make room for this. This is particularly true for boats with a raked keel.
4. With sail boats, the stronger the wind blows, the more the bow is driven down because the center of aerodynamic force is quite high above the center of hydrodynamic force. So you want a prying moment to get the bow up in strong wind.
5. Engine and prop installations often require a bit of a bustle aft to shorten the propshaft and maximize room below.
6. Some boats want to maintain a mor-or-less constant forward draft as they load cargo. On a sharpie, you can load the hold starting at the front and working backwards and the bow never changes, it just sinks by the stern. This is quite important for the performance of the vessel over a huge range of displacements.
7. In yachts, it gives you better motions back where the important people are.

Which isn't really seven reasons, it's just one reason. All are to control the shock wave losses. That's why the engine and important people are at the back, that's why we pry the bow up running down wind. It's why raked keels (and cut back keels) perform better.

 

[jehardiman]

Pretty much what Phil said. The waterline should look like a NACA 0000 foil going backwards...the centerline canoe hull profile should be closer to one going ahead. So the sectional area curve should look similar to a sine curve with the peak about 50-55% aft.

 

[Dieter51]

Yes, PhilSweet, the displacement of a boat, its buoyancy, represents its potential energy, which manifests through the addition of motion with outgoing waves, as it cannot manifest in lifting ability (apart from planing boats).

I can also accept the other points, although the presence of a "thick" keel, like in van de Stadt's Stormvogel (long ago), would rather suggest a coke-bottle design, which one typically avoids.

Bolger would probably say, "since flat-bottomed boats like Sharpies should always carry the bow a few centimeters above the water surface, ballast (cargo, fish) can only be stored aft, with more draft = possible volume there and not in the middle."

The thickness of the boundary layer is also an important influencing factor for profiles. Therefore, in airplanes, the profile is often not pointed but sharply truncated. The goal is to "suck" the thickened, sluggish boundary layer into the wake.

So, I agree with all the points. What I, as a hobby designer of a flat-bottomed boat, want to clarify is ultimately why flat-bottomed boats (like Michael Storer's Goose), which have a strong upward curvature at the stern (like classic scows), glide very well and with a smooth wake. This does not meet the expectations, not at all, but it is the case.

The fact is that for many years, and still today, flat-bottomed boats have been built with an almost straight bow (up to over 50% of the length), then reaching the deepest point of the hull, followed by a significant upward curvature. In aerodynamics, this would lead to large-scale flow separation and Coanda effects (wandering suction peaks), which in boats, according to the general 15-degree rule for stern curvature, should result in the loss of planing ability.

However, this is not the case. Flat-bottomed boats apparently do not show this or can overcome it with sufficient propulsion.

 

[Dieter51]

A rough example, not to scale or build:

 

[luckystrike]

Hello Dieter,

sorry, Iam a little late to your thread, but here is my opinion...

In a modern flat bottom sailing boat, a so called sharpie, the rocker (profile view) has nothing to do with some Naca profiles. The depth of the curve is a function of the target speed the designer wants to achieve with his design planing and how much of the displacement mode speed he is ready to offer. The flatter the curve, the faster . The aft curve shoud be a continious curve, not a progressing one. The position of the the deepest point of the rocker curve, as well as the same in plan view is a function of the weight distribution. Modern boats have today a slightly aft trim (position of crew, keel and rig).

Typical modern sharpies are the I550 Sportsboat or the Goat Island Skiff. Looking at the lines will explain what I mean.

Bolger, Michalak or similar designers are no reference for modern sharpies any more, because they were educated and working at times where the design of planing hulls was no preference and all was concentrated on displacement hulls. Anyway, the forefoot free above the water is still valid today.

If you want to chat directly and in german language send me a pm.

Have Fun, Michel

 

[Dieter51]

Oops, I did not see your answer, Michel.

Of course, the rocker curve of the Sharpies does not originate from a NACA profile - how could it? But inverted NACA profiles are simply profiles on which similar curves of flowing bodies were examined.

Albeit for completely different purposes, because today those are mostly backward-calculated profiles. A desired pressure distribution is specified and used to calculate the shape that could produce it. And in some foils, which showed a a strong curvature in the last third of the airfoil, the post-stall dynamics were also examined. And they were surprised that the expected strong stall did not occur. (It's been a long time since I read anything about this).

I just meant that it could be such a phenomenon that causes Michae Storer's GOOSE to have such a wonderfully smooth wake instead of "boiling water". But please, I'm not knowledgeable enough in this area. Just an idea.

I'll get back to you with a PN next week, for now we're at a family reunion in the Palatinate (Neustadt an der Weinstrasse).

Best - Dieter


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[CarlosK2]

"The bottoms all start with a flat run (...) and end with an “upward bend”, which is expected to be disadvantageous.

Is there a hydrodynamics explanation for the development, which undoubtedly reflects experience? That is the reason for my question."

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This results in High Pressure at the Bow and Low Pressure (suction) at the Stern.

And the sailboat adopts a bow-up attitude, which is beneficial for example when sailing downwind in waves.

Logic in different length hulls https://www.boatdesign.net/threads/logic-in-different-length-hulls.68753/page-5#post-957925