Wingboat Design

Windjet Wingboat

Hi Guys, great input to this thread ! I must confess that all the mathematical formulas are way beyond me. I'll leave that to you guys. But, what about the Windjet concept? Although the UK team is breaking new ground (with regard to sailing craft), it relates to the discussion at hand ... (I think).

I'd like to hear your input / analysis of this design related to 'wingboat' / 'tunnel' / WIG designs ? Of course, it's always risky trying to predict the performance of a design, but step out on a limb ... In 'Theory', how will this craft perform? Any suggestions as to how to make the design work better?

http://www.windjetproject.com/index.php?option=content&task=view&id=65&Itemid=74

Cheers

Russ

 

Stability and Analysis and intuition

John David - I sure appreciate your approach for understanding the "analysis" approach so to develop a good "intuitive" feel for design impacts. This is really true for all applications. Understanding how the different forces and moments are affected by design approaches will really help the designer "narrow down" his ideas to those that have a real chance to be successful.

The complexity is quite involved, since the "balance" of forces and moments is resulting from:

1) profile drag, induced drag & friction drag from aerodynamic forces;
2) profile, induced, friction drag from planing (hydrodynamic) forces;
3) aero lift, affected by AR, angle of attack, height above surface, aerofoil shape, location of lift center;
4) hydro lift, affected by AR (varying from low to high) of wetted surface (only), angle of attack, location of lift center, shared lift with aero forces present;
5) dynamic stability issues (pitch instability) that are different at every velocity due to changing lift/drag characteristics of aero and hydro surfaces, and location of lift centers.

The net impacts of all of these factors are very inter-related, making the analysis quite complex. But a good understanding of what things are "in the direction of goodness" and which are not, is sure helpful in considering potential design changes.

You are correct that there are often differences in the angle of attack of aero and hydro surfaces...that adds another variable to consider, but at least it is a "constant" relationship.

The issue of "dynamic stability" is a major issue (problem) very quickly in most tunnel hull/catamaran designs - not just at very high velocities. To design a safe hull that achieves the performance objectives, the stability must be checked at the full range of operating velocities. Because all of the contributing forces and moments are influencing each other, and because each changes it's relative contribution at different velocities, the issue of dynamic (pitch) stability is very complex in itself. It is a particular issue to be addressed at the "transition velocity" or "hump zone", when the balance of lift forces changes from predominantly hydro lift to predominantly aero lift. There are, however, design approaches to minimizing the drama of the changes thru the hump zone. (This complexity is why we ultimately designed software to do the hard work for us!)

To answer your question, a high speed tunnel hull will experience aero lift from 5% to 70% (of total lift).

I have written several Technical articles on the calculation of lift, drag and dynamic stability in tunnel hull design. Check 'em out, if you like.

Cheers!

 

I am looking for a propulsion sistem that I saw on the New Orleans looks like the cross between a jet drive and surface piercing drive. I am looking to build a mini-ferry 14-16mtrs cat, to shuttle 30-40pax at high speeds ( 40-45kts ). The route is a lake in South America, were the water has debris (logs, even floating pipelines!). From the picture it looked like it was possible to manufacture it from scratch (no cast pieces). I know that jets are less efficient and surface piercing propellers are the tiket for speed, but the fact that the debris will not be able to dodged at this speed (it is difficult to do so at 25kts). This option might be a good balance.
Any info is apreciated.
BRGDS
Miguel Smith

 

Jim Boat,
Your "description" of the problem in the form of the list of "variables" is excellent and It is the most complete list I have seen in any one place. It sounds to me as if your computer program is very practical and useful.
I have considered all of the same variables in my analysis, except I can not do dynamic pitch stability. Instead I use simple criterea such as position of the center of lift vs. center of gravity and how these and other forces affect the pitching moment. I must confess, I don't have the mathematical skill to analyze dynamic problems such as porpoising.I do however have a theory relative to it's source. My other analysis weakness is in low aspect planning surfaces. My drag predictions are overstated for these planning modes.
Probably another shortcoming in my analysis is the "piecewise" method which can't "keep all the balls in the air" at once as in a computer.This may lead to missing subtle interactions. So far, though, I've been doing O.K. Since the boats I am building , don't have low aspect planning and are well balanced, my analysis although not rigorous seems to work. By the way, I ignore any aspects of displacement operation and don't consider what goes on speedwise below the last wave drag hump. As a result the first model I built, wouldn't plane . Talk about embaressing! It didn't plane for a different reason than most boats that won't get up on plane. This was an education all by itself.

Thanks for taking time to correspond, in such an informative and meaningful way.---------John David

 

boat design

JD - sounds like you're having fun! Keep up the good work!

 

Jim Boat

As I was thinking about your list of variables relative to performance, it accured to me there is another parameter absent but no doubt included in your program; propulsive thrust. This in turn is dependent upon a host of variables: full throttle engine shaft horsepower, propellor pitch, slip and propulsive efficiency.I find accurate numbers for some of these hard to come by. Since the holly grail of most racing is maximum speed, this is what I feel must be predicted reasonably well. When the rapidly rising drag vs speed curve intersects the rapidly falling thrust vs speed curve, maximum speed is reached (ie. drag equals thrust.) There is, as always, a complication. The pitch that produces the fastest boat also produces the lowest acceleration at some lower speeds.
What to do is a function of how you race. Long leg courses want higher pitch, while short leg courses,with more turns, requiring acceleration afterward , need lower pitchs. Many boat racers have several different props which they change for racing conditions.

 

Thrust too!

John David - Yes you're right about Thrust (which must balance drag, primarily); While mostly dependant on shaft delivered HP, is also affected by propeller pitch, delivery angle (which both affect 'slip' or 'efficiency'); and location of thrust points. My software accounts for all components of thrust, as it is an important part of the overall dynamic balance. My software PropWorks2 helps determine shaft rpm, hp and slip characteristics for various outboard and inboard propulsion systems.Performance Boat Design Software does all the analysis for performance, design and stabilty results for high performance powerboats of all sizes and syles.

There was one more thing that you mentioned in this thread, John David, that was very important...that was 'designing for safety and stability'. When boat designers for competition racing are looking for the best combination of speed, acceleration and handling, one of the MOST important factors that I have found is to deliver the 'feel' of stability throughout the full velocity range. The boat that 'drives' without a 'feel' of instabilty to the driver, at any speed, will almost always finish first - even ahead of the radical boat design that might be 'capable' of higher speed! I've designed both syles of boats - stable-fast; and radical-fast. The key point here, is that it is not sufficient for the boat to be 'capable' of high speed - the driver must still be able (with confidence) to drive it to the high speed.

 

Check out the ground effect hovercraft sites on Google. They have stubby wings and fly a few feet off the water or ground. And... they do have longer wing plans for those with a pilot license.

stump

 

All 3 modes of operation were acheived in the 1960's by thr Convair Sea Dart. It sat in the water, taxied, came up on plane, used the ground effect to get enough speed to convert to jet flight, last tests were at 550 mph. It was a pure delta wing configuration. Was the end of the seaplane fighter era. Almost everything today is a little piracey of old designs. Todays computers are resonsible for the flight envelopes. Any single wing vehicle is unstable to the point of useless. That is the reason for tail surfaces, and or foward canards. The farther apart the 2 sets of wings are, the more stabile the craft.

 

Richard,
Here is the link for the ground effect hovercraft plans etc. http://www.du-groundeffect.com/ It was interesting to see what they are doing with "boats"
They have wings and a tail section.
stump

 

Richard,
Here is another winged hovercraft sight... full sized this time, not model boats.
http://www.hovercraft.com/
stump

 

A important fact seems to be sliding by. At about 80 mph most planes take off. Also at about 80 mph most boats try to become planes when lightly loaded. If you need to go faster, get a plane. 100+ mph boats have always ended up killing the best of the unlimited drivers. Only so many chances .

 

Going faster

They go by this motto:

"Build it light, make it fly"
" If you want to go faster, get out of the water"

 

Talking about facts sliding by:
1. planes take off/land at 80 mph because designers have added flaps and slats and what not - designers would love to do away with these complications if only they could make it safe taking off/landing at speeds close to the cruising speed at ground altitude.
2. to get the L/D efficiencies comparable to those obtained in ground effect your plane needs to get in thin air at 30.000 feet (and add power to climb high, pressurized cabins etc...) There is a definite place for WIG's filling up an obvious gap in the Von Karman - Gabrielli diagram as depicted on http://www.se-technology.com/wig/html/main.php?open=commercial&code=0
3. 100+ mph boats kill because at these speeds they are in the worst situation control wise: the waves come in too heavy and too quickly while the vessel is on the fringe of hydrodynamic controllability; the vessel is still too slow for significant aerodynamic control. That is exactly what WIG's are about: at these speeds WIGs choose to leave the water all together and opt for sturdy control in the air and staying close to the surface for efficiency. However, in the twilight zone when taking off/landing WIGs face the same dangers as fast boats. That is why the HeliFerry is so safe: the twilight zone is reduced to a period of a couple of seconds using the kinetic energy in the overrotated rotor. No protracted high speed chase to mount the hump drag.
4. airplane people think it is suicidal to cruise at skimming heights, because they have been taught, and rightly so, that altitude is time to recover. A stable WIG design buys time with the exponential increase in GE lift as altitude drops. Remark the word stable. The HeliFerry has, in addition, an overhead rotor that acts like an instant parachute or boosted control surface in case of catastrophic emergency. No safer "boat" at these speeds.

Luc De Keyser
http://users.telenet.be/heliferry

 

The conversation on this subject has gotten much more practical. I would like to add a little more of such pragmatic thought.
One of the ideas making people think that Wig boats are better is that they believe drag due to air lift is much lower than drag from planning lift.This idea has been fostered in some recent boating literature. For example, a recent issue of TBPN news letter had a discussion of air lift for improving boat performance.There was a note: "The drag generated by a lifting surface in water is MUCH more than the drag generated by a lifting surface in air." This statement was supported by pointing to the density of water as 800 times that of air and since a density term is in the drag equation,water drag is "obviously" much greater. In reality one can find the same density term in the lift equation, so by the same token lift is also much greater.To produce the same lift, planning needs only 1/800 th of the area of the air surface.It is not at all obvious from the density which medium produces more or less drag.
The actual fact is that air lift does not inherently generate less drag but can create more or less depending on the specific geometry.
The angles of attack and the lontitudinal positions of the centers of lift relativve to the CG are the important parameters.Certain combinations do indeed benifit from the substitution of air lift, while others show no gain or even increased drag. In many cases where improvement is noted, the gain is derived from a change in longtitudinal moments that in turn change the angle of the planning surface to one nearer to optmum ;and not from the mere substtution of air for water lift.