flat vs convex surfaces

[lucdekeyser]

I realize that your answers will start with "It depends ...". I am only after a sense of proportions.

Given that flat surfaces that make up the airframe of stealth jets cause a loss of aerodynamic efficiency of about 20-30 % within their flight envelope, compared to their more aerodynamic counterparts, is it safe to roughly estimate that a sailboat built from flat surfaces would incur about 10-15 % loss of aero- and hydrodynamic efficiency compare to a more usually shaped boat?

What would be the rough estimation of the weight penalty of using only flat surfaces for a hull built to match the strength of a hull made up from the usual convex planes?

 

[TANSL]

As you said, "it depends." Let me speculate a bit, however, and make the following considerations:

• the hull above the waterline would not necessarily have its aerodynamic properties diminished or improved, as they would be practically the same. (Let's forget about aerodynamics, then.)
• the hull below the waterline could give rise to greater turbulence (if the speed were very high) due to the sharp edges on it, but, as the wetted surface would probably be smaller, it would have less friction resistance.
• as for weight, the surface of the hull panels would probably be smaller, and perhaps the chines would allow you to save some longitudinal stiffener.

There may be many other aspects, which I have not taken into account, and which also depend.
The discussion is open.

 

[Barry]

Given that flat surfaces that make up the airframe of stealth jets cause a loss of aerodynamic efficiency of about 20-30 % within their flight envelope, compared to their more aerodynamic counterparts, is it safe to roughly estimate that a sailboat built from flat surfaces would incur about 10-15 % loss of aero- and hydrodynamic efficiency compare to a more usually shaped boat?

"is it safe to roughly estimate that a sailboat built from flat surfaces would incur about 10-15 % loss"

Not safe to roughly estimate this amount of loss.
Stealth airplanes speeds run between 600 and 1200 mph
A sailboat???

 

[gonzo]

Also, water is about 830 times denser than air. It is also much more viscous. The comparison doesn't make sense. The loss % should be a lot more than with air.

 

[TANSL]

What doesn't make sense is mixing aerodynamics with hydrodynamics.

 

[BlueBell]

is it safe to roughly estimate that a sailboat built from flat surfaces would incur about 10-15 % loss of aero- and hydrodynamic efficiency compare to a more usually shaped boat?

No.

What would be the rough estimation of the weight penalty of using only flat surfaces for a hull built to match the strength of a hull made up from the usual convex planes?

Roughly zero.

 

[lucdekeyser]

Thank you for your insights.
Can I conclude that building a hull with only flat surfaces would have roughly the same weight to strength and aerodynamic properties but that the flat construction surfaces below the waterline must be "foam filled" outward with the "convexities" that together shape into a smooth hull foil?

Thus, for catamaran hulls one could choose an asymmetric foil that is essentially flat on the outside while respecting the convex shape of the other side of the foil on the inner side of the hull. The asymmetry in lift from the hulls would essentially cancel each other out going down wind, and the lift difference when heeling will help counter some of the leeway. Correct?

 

[redreuben]

Didn't Bernd Kohler use asymmetric hulls in his earlier designs ? And I'm sure this is the principle of Hobie hulls and the Hawaiian designers of the 1960's

 

[Ad Hoc]

....What would be the rough estimation of the weight penalty of using only flat surfaces for a hull built to match the strength of a hull made up from the usual convex planes?

Depends upon the Length-Displacement ratio.

If low, a lot, if high, not much. .... in a nut shell.

 

[Barry]

is it safe to roughly estimate that a sailboat built from flat surfaces would incur about 10-15 % loss of aero- and hydrodynamic efficiency

Professional Boat Builder, August/September 2024 , Author Patrick Bray, Naval Architect, in his article "Embracing the Buildable" (a discussion of conical/developable surfaces in boats) writes;
"We have run extensive model testing programs for over 25 years and compiled data from dozens of ocean proven vessels. These results show that, regardless of speed-displacement,
semi displacement or planing-a well designed chined form does not require more power than a round bilged form"

About the Author
Patrick Bray is a naval architect with more than 50 years of experience in design and new construction----doing shop drawings, and ensuring compliance with class societies.
And (short verion) worked designing aluminum commercial vessesls
Doing business as Bray Yacht Design and Research Ltd.

 

[lucdekeyser]

Didn't Bernd Kohler use asymmetric hulls in his earlier designs ? And I'm sure this is the principle of Hobie hulls and the Hawaiian designers of the 1960's

and also the lee hull of pacific proas ... But these have flattened sides where the bow ends still curve some towards the middle. Would there be an important penalty for just staying fully flat and have only the other side curve fully to the flat side? I presume that making two curved surfaces meet perfectly is more difficult in construction than just have one curved surface meet a flat surface.

 

[lucdekeyser]

Depends upon the Length-Displacement ratio.

If low, a lot, if high, not much. .... in a nut shell.

So, for sailing catamaran hulls this would mean that the penalty is not much and maybe not worth the construction complexity, all other design considerations remaining comparable ?

 

[lucdekeyser]

Professional Boat Builder, August/September 2024 , Author Patrick Bray, Naval Architect, in his article "Embracing the Buildable" (a discussion of conical/developable surfaces in boats) writes;
"We have run extensive model testing programs for over 25 years and compiled data from dozens of ocean proven vessels. These results show that, regardless of speed-displacement,
semi displacement or planing-a well designed chined form does not require more power than a round bilged form"
.

I was surprised to learn from recent YT vlogs documenting the construction of aluminum sailing catamarans, how many manhours of manual metallurgy skill are required for "developing" the hulls. Taking your nut-shell insights together, these designs may be curve overkill.

HDPE boats are less curvy but then often resort to a crown of large diameter tubes to mimic the copy of the inflatables they aim to replace.

I presume that welding hulls together using only flat surfaces and straight stringers would mean more straightforward construction for a very large part of the hull, opening up the opportunity for more simple automation and quality control .

 

[rob denney]

Thank you for your insights.
Can I conclude that building a hull with only flat surfaces would have roughly the same weight to strength and aerodynamic properties but that the flat construction surfaces below the waterline must be "foam filled" outward with the "convexities" that together shape into a smooth hull foil?

Thus, for catamaran hulls one could choose an asymmetric foil that is essentially flat on the outside while respecting the convex shape of the other side of the foil on the inner side of the hull. The asymmetry in lift from the hulls would essentially cancel each other out going down wind, and the lift difference when heeling will help counter some of the leeway. Correct?

I'm in Fiji teaching villagers to built boats for transport and fishing. Ease of build and low cost were the primary considerations. Everything is built from flat panels, attached at right angles where possible. Box hulls, no rocker, flat decks, etc. Compound curved panels are stiffer than flat ones, but harder to build. And it is easy to stiffen a flat panel if required.

Performance wise, the attached video shows a 6m water taxi/barge motoring at 4 knots with 3 people on board on an overcast day. The motor is a 40 hp petrol outboard with an electric motor replacing the petrol, but still using the too small petrol prop. The power was from 2 x 315W solar panels. ie a little less than 1 hp. Maybe not a record breaker, but certainly good enough to justify the low cost/time build technique.

Re the assymetric hulls pulling the boat towards the curved side, presumably the same thing happens with rocker pulling the hull deeper into the water? A flat bottom is superior, as long as it pitches up so the water flow lifts it.

 

[gonzo]

Thank you for your insights.
Can I conclude that building a hull with only flat surfaces would have roughly the same weight to strength and aerodynamic properties but that the flat construction surfaces below the waterline must be "foam filled" outward with the "convexities" that together shape into a smooth hull foil?

Thus, for catamaran hulls one could choose an asymmetric foil that is essentially flat on the outside while respecting the convex shape of the other side of the foil on the inner side of the hull. The asymmetry in lift from the hulls would essentially cancel each other out going down wind, and the lift difference when heeling will help counter some of the leeway. Correct?

No, you are not considering the basics of fluid dynamics. To start with, aerodynamics and hydrodynamics are fundamentally different. Water does not compress to a significant amount, while air does.