Naca Airfoils shape and rocker oft flat bottomed boats

[Dieter51]

Sorry, there was a typo. The boats are 6,5 m long. I corrected that.

Another way to explain the prominently raised stern of the early flat-bottom sailboats would be to assume that the boats were given this raised stern so that the flat rudder could be accommodated beneath the boat's bottom. This design allowed the boat to dry out or glide over shallows without the risk of damaging the rudder. However, this does not account for why these boats, despite the strong curvature of the boat's bottom at the stern, demonstrated good speed and could even glide. It is no coincidence that we still see this bottom configuration in boats that have a deep spade rudder today.

Translated by ChatGPT 4.0

 

[CarlosK2]

[1] Royal Navy Destroyer and Sailing Dinghy that hoist 30-40 squared meter per Ton

We want the bow to go through the bow wave, we want the bow not to go up, we want the stern to drop as little as possible.

We want to minimize the forward Resistance.

[2] Small (6-12 m) relatively slow sailing downwind. We want the stern not to rise. We want the bow not to sink. We want the bow to produce very little lateral force and quite a lot of vertical force. We don't want to bump into the bow, because that tires the helmsman or burns out the autopilot and even the sailboat turns violently and knockdown, which is quite unpleasant and saps the crew's morale, and in the worst case capsizes.

We are looking for Comfort and Safety.

 

[CarlosK2]

Ludwig Prandtl and Max Michael Munk (1924) in two kicks

High Pressure (H) depends on Angle of Attack (AoA)

Low Pressure (L) depends on the Curve (the arrow of the arc)

The velocity of a small sailboat going down a big wave is equivalent to the airspeed of a Cessna plane.

Cessna 172 - Wikipedia https://en.m.wikipedia.org/wiki/Cessna_172

It is criminal to disregard the Hull Profile of a Sailboat for the purpose for which it is intended.

 

[CarlosK2]

If we are looking to minimize drag in the zone 0.40-0.80 Froude or if we are looking for high speed (Froude > 1) then we do not want Low pressure (Suction) at the stern.

But if we are thinking of a small sailboat relatively slow in relation to the speed of the waves coming downwind, then we want Low Pressure, Suction at the stern.

We might think about moving the crew and cargo aft ... But, no, there is no comparison to doing it right and designing it right with HydroDynamic Forces.

 

[Dieter51]

I agree, the classic flat-bottomed boats were quite safe as they were used professionally.

 

[CarlosK2]

The curious thing is that this is the first in the first series of the long-running Delft series.

It's funny: the first one still reminds of sailing boats sailing at sea: bow that creates little lateral pressure and creates vertical pressure, and stern that creates low pressure, Suction, so that it sits well in the Wave.

 

[CarlosK2]

And it's funny that in a humble side branch of sailboat design the good* forms have been preserved.

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*: For small and relatively* slow sailboats that hoist 10-15 square meters of sail per Ton

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*: In relation to downwind wave speed

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The funny thing is that a slow but well balanced small sailboat can reach a peak of 20 knots safely Surfing a big Wave.

 

[CarlosK2]

1924

Ludwig Prandtl

Ludwig Prandtl - Wikipedia https://en.m.wikipedia.org/wiki/Ludwig_Prandtl

 

[CarlosK2]

The beauty of the matter is that it is self-regulating: the bigger the Wave, the more the brave little tailor boasts.

The Brave Little Tailor - Wikipedia https://en.m.wikipedia.org/wiki/The_Brave_Little_Tailor

 

[Dieter51]

Carlos II, they show the typical rocker curve. For flat-bottomed boats, this curve is approximately the same as the sheer line curve, which Phil Bolger often chose. And if the shown boat is a "Bolgerbox," then it will also exhibit this bottom curve. I especially enjoy looking at these boats, which so drastically follow the principle of "form follows function and makes it easy for the amateur builder." I particularly appreciate how these boats are reduced to the essentials. Indeed, just like the brave little tailor in the fairy tale, Bolger's boat designs have amazed many with their performance and reliability. And just as the tailor had a profound training, so did Bolger. He too could "sew royal garments."

 

[CT249]

View attachment 195952

[1] Planing Dinghy that hoist (Jib+Main) 30-40 (!) (OMFG) squared meter per Ton ... and maybe even 60-80 with Spi

[2] Sailboat that hoist 10-15 squared meter per Ton ....And that it is designed to navigate in the Sea, a place where I assure you that there are Waves, although I know that nobody believes it.

H: HydroDynamic High
L: HydroDynamic Low

Why on earth do you claim that no one believes that there are waves on the ocean? Of course we know that. We also know that it is ridiculous to claim that small boat navigation has been "destroyed"; there are many small boats at sea at the moment.

You may have trouble steering a yacht offshore downwind, many of us do not. Maybe you should not blame the tools for your problem?

 

[CT249]

I agree, the classic flat-bottomed boats were quite safe as they were used professionally.

The boats used by working sailors were often far from safe; the death toll was horrifying in many situations. I'm not sure how many people were killed sailing the old working sharpies, but a search of old newspapers and magazines such as "Forest and Stream" shows regular fatalities due to capsizes of the old catboats and similar flat, beamy open boats. About 60 people a year were dying in them; in the worst incident 27 women and children died when a 36 foot catboat capsized in the USA but similar stories were seen in other areas, like the nine men who died in an 18 footer that capsized in the narrow waters of Southport, Australia.

The US sharpies don't seem to have had the same tragic death toll as the catboats and Australian open boats but a quick check on the US Library of Congress search engie for old newspapers shows that they certainly did capsize.

As far as rocker lines go, the old sharpies were working craft, designed to be loaded down with oysters etc. They were also driven by sails made from stretchy cloth and therefore less efficient. Furthermore it was common in working craft to ensure that you reached back and forth to the working grounds, rather than having to beat. The sailor were pros and therefore highly experienced and they seem to have had very different practises when it came to sailing in foul weather; in some areas they went out in it while in other areas they didn't work in winter. They were also happy with far less comfort than a typical leisure sailor. The needs of a working sharpie were therefore quite different to those of many modern sailors. Few of us would see a 1905 working truck as a model for a 2024 campervan, even if the delivery drivers of 1905 thought it was the best machine they had ever used.

 

[CT249]

Carlos II, they show the typical rocker curve. For flat-bottomed boats, this curve is approximately the same as the sheer line curve, which Phil Bolger often chose. And if the shown boat is a "Bolgerbox," then it will also exhibit this bottom curve. I especially enjoy looking at these boats, which so drastically follow the principle of "form follows function and makes it easy for the amateur builder." I particularly appreciate how these boats are reduced to the essentials. Indeed, just like the brave little tailor in the fairy tale, Bolger's boat designs have amazed many with their performance and reliability. And just as the tailor had a profound training, so did Bolger. He too could "sew royal garments."

Did Bolger have profound training? In what?

I think the owner of Hogfish Maximus has pointed out that Bolger wasn't a particularly experienced sailor. There are also quite experienced sailors, such as some of those on the Wooden Boat Forum, who have owned Bolger boats and found them to be very disappointing.

Personally I find Bolger's claims regarding the rocker and curve of sharpies to be rather illogical. There are many flat-bottomed craft around that have been the arena of extensive development by many designers that Bolger had never sailed and probably never seen. These craft don't have the features that Bolger claimed to be important and yet some of them handle and sail very well, so why does Bolger assume that he was right and these designers whose work he had never sailed are wrong?

Yes, Bolger did some very interesting designs but when one removes the anecdotes and hyperbole, where is the actual objective or independent information on which to base the claims that he was right and others were wrong?

 

[Paul Scott]

Geary Sailboat Flattie — The Center for Wooden Boats https://www.cwb.org/livery-boats/geary- an older friend of ours, now in his late 70’s, used to waterski behind a flattie when was a kid (not bad for a boat designed in the 1920’s). The boat also handled Lk Washington chop well, and went upwind very nicely.

 

[Paul Scott]

https://sailboatdata.com/sailboat/geary-18/