Stalling behaviors of foils based on aspect ratio or chord?

@dustman , the lift on a moving foil will always be elliptically distributed barring the extensive use of spanwise fences. So it's not that distribution which contributes to good characteristics. The fluid interacting with the foil is always in contact with itself so to speak, balancing pressure, vectors etc. So you always have that regardless. A rectangular planform is just bad at everything except ease of construction. Almost as easy to build, but massively better performing is the simple quadrilateral with the tip 40-60% the chord of the root.

Take a look at the planform of a high performance glider or soarplane. Their l/d ratios are so high the aircraft can stay aloft for as long as the pilot can maintain consciousness.

 

To design a good safe boat there is no alternative to deep knowledge of the conditions it will need to operate in.

Look at a map of ship "graveyards". The obvious common theme is places the water gets shallower, places where currents pass through restrictions, and places with warm water. To understand the danger just look at the path and answer, what is the minimum amount of time the boat could lose control and still be safe? That's how fast you need to be able to recover. Then look at how fast conditions change significantly?

"Foam filled" just leads me to conclude your boat will be a hazard to the environment as well as its crew. I don't know what scrutiny you will face at check-in, but if there is any sensibility they would send your boat back to the US on a ferry at your expense. They wouldn't let you bring a foam filled boat to whitewater in the US.

A catamaran of a thousand pounds capable of sailing around the Bahamas will not just lift a hull, it will go over and need a plan for self recovery.

 

Yes, even though it was hard fought. After all the input then reviewing the literature again and playing with the airfoil tools I have a much better understanding than at the beginning of the thread. It's interesting how many threads there are about the exact mechanisms involved, though it doesn't seem to matter much which you adhere to unless the application is critical.

Just last night I came to the conclusion that the naca 0012 would likely be the best option for keels and rudders, though am considering perhaps going thicker on the rudder for a greater range of aoa.

 

It seems like I may take ~4% hit as far as drag for the rectangular planform on the sails for a 4:1 AR, ~3% for 3:1 AR of keels and rudders.

Also of interest in that book is that a smaller aspect ratio foil can maintain lift to twice AoA of a very high aspect foil approaching infinite. In terms of lift to drag ratio, it seems going much beyond 4:1 sees diminishing returns, especially when taking into consideration stability and structure.

Given the much easier application of junk sheeting and not having to design and create every batten differently for a trapezoidal planform, the rectangular planform seems well worth the sacrifice. I can't point to a specific piece of literature, but I have gotten the inkling that the rectangular planform is more docile in some ways.

My coworker back in the day took me up in his glider, it was an incredible experience! I got to fly it for a bit. Then he proceeded to make a bunch of insane maneuvers, I was sure the wings were going to break off as much as they were bending. I pretty much stumbled back to the car and felt sick all the way into the next morning. It was magnificent and beautiful machine.

 

Ok, I'm going to spit fire for a moment, mainly in self defense, forgive me.

I grew up in nature on the bank of a river, I have put in hundreds of river miles, I've hiked probably in excess of ten thousand miles, spent lots of time in and around the ocean, including surfing, snorkeling, and hiking through and kayaking, rafting, boating estuaries. I've traveled to pretty much every awesome natural place there is in the western united states. The point is, I am no fool when it comes to the ways of nature. Learning the specific attributes of the bahamas or any other natural system will come relatively easily to me. And believe me, I will do no shortage of research.

I have looked all over the bahamas on google earth and navionics and seen many wrecks. I am fully cognizant of the potential dangers, which is precisely why I'm designing a boat that will be unfazed by a hole in hull. My biggest concern is being stuck and battered against a rocky lee shore or exposed reef. I will be traveling almost entirely in the daytime and in the lee of one island or chain of islands or another. This a big part of my route planning, to avoid the potential for hazardous situations. By way of having extremely shallow draft hulls and kick up keels and rudders I already have a huge leg up in regards to many of these hazards in the first place. I will be there february through may, to my understanding the safest time of year.

On losing control: As previously stated, one of the primary considerations in the design is ease of control and redundancy. Two sails with junk reefing, two keels, two rudders, two electric motors, two separate solar/battery systems(I worked for almost 5 years in solar and commonly do electrical work), etc.

Don't pretend to have a better understanding of environmental issues. I grew up in the midst of environmentalism, my father was an avid environmentalist and even published a book on the environmental impacts of a certain industry. Most of the people around me were involved in environmental organizations, not the fake pretender, inefficient, sellout organizations either. I spent the majority of my adult life working in some field related to the practical application of environmental values, or social values. I sacrifice a LARGE amount of income to live by my values. I live EXTREMELY simply in a tiny accommodation. I grow some of my own food. My diet is such that it minimizes my impact.

There are probably thousands of boats zipping around the bahamas at this very moment guzzling fuel with more floatation foam in them than will be contained in my entire vessel, to say nothing of the other resources used to create them. My boat will contain a tiny fraction of the environmentally damaging compounds as all these giant sailboats that are mostly motoring around the bahamas anyways.

What do you do? Drive a Prius and make the extreme effort of walking to your recycling bin?

So yeah, you were saying?...

My boat will be far more seaworthy and stable than any of the modestly sized motorboats out there. It will be considerably more controllable than all the large condomarans out there. It will have the ability to seek shelter in many more places than larger deeper draft boats.

And since you told me I was "dead wrong" before I will assume I can be granted the same latitude. "A catamaran of a thousand pounds capable of sailing around the Bahamas will not just lift a hull, it will go over and need a plan for self recovery." This is dead wrong. As you well know that comes down to the righting moment vs heeling moment. You can do the calculation yourself based on the figures I think I gave somewhere. It would be as stable(or more) than many catamarans out there. And again I must reiterate that easy reefing is going to be a top priority. And having freestanding masts is going to be another feature that will add safety in that regard. This boat will not be recoverable from a capsize without outside assistance.

Breaking waves are another matter that cannot be solved without building a larger vessel.

While I appreciate all your inputs thus far, I don't appreciate the fearmongering or erroneous assumptions about my knowledge or abilities. I fully understand the seriousness of what I'm getting myself into, thank you for your concern. If you have further well founded concerns I am all ears. Perhaps we could have a conversation or start another thread specific to these aspects of sailing or boat design in that particular environment.

 

Dusty,

I'm not sure what you're doing here,
you've read all the books,
have all the knowledge,
and a huge attitude.

Are you familiar with the Spitfire wing?

 

"I'm not sure what you're doing here"

Trying to understand the behavior of foils.
Defending myself against abrasive, condescending comments.
Being told how my boat is going to fall to pieces by people who probably don't have callouses on their hands.
Basically being told I can't possibly comprehend the nature of the environment I'll be traveling in.
Confronting tradition.

"you've read all the books"
I've read a lot of them, and am coming up against the limitations of my mathematical abilities, and the counter-intuitive nature of some of these problems.

"have all the knowledge"
I don't, and that's why I'm here. But there are things that I do know.

"and a huge attitude"
How am I supposed to respond to, for instance, your comments in this thread?

"Are you familiar with the Spitfire wing?"
Case in point.

 

Thank you to everyone who made positive contributions to this thread. You have helped me to overcome one of the major stumbling blocks in my understanding of the nature of foils. And yes, I still have a ways to go, I'm gonna keep at it.

If you have real concerns about specific dangerous places or conditions in the Bahamas then by all means list them or share some links to good learning material.

 

Please read this entire post before responding.

Induced drag (drag due to trailing vorticity including tip vortices) depends on span only, not on chord, for a given planform shape. Induced drag for a given lift is reduced if span is increased. That means for the same amount of lift and a fixed span the induced drag will not change if the area and chord is increased and the aspect ratio decreases. The induced drag is only affected by aspect ratio if the area is fixed and the span is changed to vary aspect ratio. (Confusion arises when looking at induced drag coefficient vs lift coefficient with the coefficients based on planform area.)

The drag also depends on viscous/skin friction drag, which is a function of area and angle of attack. For the same angle of attack lift and viscous drag increases with increasing area. For the same area viscous drag increases with increasing angle of attack. For the same lift there is a tradeoff between angle of attack and area. Short version: the area for minimum drag is almost always less than the area needed for other reasons.

Bottom line: Maximize the span of the centerboard and rudder based on draft constraints, structural considerations, etc. Select the area based on resistance to stalling, ventilation, structural, etc considerations. Make the area as big as it needs to be. If performance is a concern don't make the area bigger than it needs to be.

 

Induced drag dependence on planform shape graphs usually do not include free surface effects, Wave making resistance of a keel or rudder is decreased if the area is decreased at the root/ top and increased at the tip/bottom. But too much of a shift of area to the tip can increase induced drag. Bottom line - the drag increase with a square tip compared to an elliptical tip may be less than shown on a graph, depending on whether the graph includes free surface effects, the shape of the hull, etc.