New Hovercraft design change ?

[kach22i]

Originally Posted by DocScience

The front top triangular area would most likely have an opening to the top, to stay unpressurized.
Does anyone have suggestions or other ideas about this ?

On my own dream design hovercraft I played/sketched the idea of sending unwanted heave air burst into a series of ducts similar to a muffler trap. Mufflers dampen sound, and their is a lot of similarities between sound attentuation and air pressure attenuation.

Or at least that's what I've been told.

As usual it much easier to draw complex systems than to build them yourself. so when it came time to build a large model I just left most of that extra stuff off.

I do have a couple of bypass duct channels which would work with dampers and sensors. The concept is similar to an overflow bypass channel on a river for times of heavy rain and water flow. However on my full sized hovercraft I'd like to rip out 75% of things I've done because they only add weight and decrease efficiency.

Once you get to building things, you will want to keep them simple for many reasons. Look to start simple, get more complex only if you can see an immediate benefit.

[SeaSki designer]

DocScience.

I hope I can shed some light on your dilemma without adding to it!

I am not an engineer, but have gained a lot of practical experience over the several years that I have worked on my own project, the Sea Ski design. Rather than try to explain my design here I would refer interested persons to my website, www.seaski.com.au, or to another thread on this site called “speed boat on skis”.

My prototype is a 21 foot vessel with a flat under hull area of 150 sq ft, so it is about half the area you are contemplating, but, to my knowledge, it is the only vessel in the world of its kind and I think it has a substantial bearing on your work.

Like your design, mine does not use centrifugal fans to get its “surface effect” performance, and nor does it employ bow or stern “bubble” constrainers to keep the air underneath. It relies purely on its skis and forward momentum to create pressure or lift under the hull, or the RAM factor that you wish to utilise.

Yellowjacket referred you to the site, http://aero.stanford.edu/StdAtm.html, (and very useful it is too) to calculate the dynamic pressure required to support your design in operation. This may be very good science, but it does not equate with my real life experiences.

I must reiterate that I have no “front or back doors” underneath my prototype so once it becomes ski-borne there are only two forces sustaining its action; the skis and the air pressure developing beneath the hull as a consequence of its speed.

Some previous comments have been that the RAM lift required will only come at very high speeds and that the size of the inlet is of little consequence. My experience indicates otherwise, and I will try to step through this as succinctly as possible.

(The following experiences relate to sheltered waters, because running in waves makes it difficult to pick fine performance nuances or fix visual reference points)

First. Getting my vessel ski-borne at given throttle settings is significantly affected by head winds (like aircraft). A breeze as light as 5 knots will substantially reduce the effort necessary to become ski-borne, depending on whether the breeze is bow-on or astern.

This tells me that at my slow launch speeds of about 30 knots, a head wind of one sixth of that speed is very influential and not to be dismissed!

At high speed operations (now using metric in which I normally work), I have twice experienced what was either complete or “near to complete” water-vessel separation at speeds slightly more than 90 km/h (approaching 50 knots).

At other times I have observed the rev gauge fly from a steady 5500 rpm to over 7000 rpm in an instant as load came off the vessel and on one occasion, using accurate GPS readings, I saw my speed reading race from 83 km/h to 91 km/h in less than 20 metres (about 3 boat lengths) or the space of one second before reducing power (I have no insurance and no income, so am prudent).

These experiences convince me that at these higher speeds and with favourable conditions (such as head wind) my skis are doing very little, if any, work at speeds equivalent to about 100 km/h (adding in some wind factor), so I am led to believe the skis are supporting less than 500 lbs of weight, at a charitable guess.

To get back to the dynamic pressure calculator that Yellowjacket referred you to, if I run my weight versus under hull area through the calculator at 100 km/h (91 ft/sec) I get a dynamic pressure reading of 9.8 lbs per sq.ft., which means, theoretically, that the induced pressure is supporting1470 lbs of boat weight.

With fuel, my prototype weighs in at about 3500 lbs so I have about 2000 lbs of boat weight that seems unaccounted for by the calculation, or given my charitable guess above, about 1500 lbs.

My experience with the prototype suggests that at 100 km/h (54 knots) I am getting the equivalent of 18 to 22 lbs per sq ft of dynamic pressure, or about double the calculator amount.

I also modified the bow of the hull to “scoop” as much air as possible under the hull and, while this gave a slightly noticeable improvement, I am not sure how relevant it was because the vessel tends to travel bow up so half the length of skis are often above the surface, allowing “pressure” to escape under them. The reality is the vessel is travelling over the air and squeezing it underneath (in my case) so it pays to have a bow profile that will assist this activity.

I am certainly counting on my prototype experiences and calculations to make our production model workable.....if I can find an investor with the necessary $240,000!

The Sea Ski production model will have specifications very close to what you are proposing in length and under hull area, but will be nearly 3000lbs heavier with full fuel load (1000 litres), but I am confident it will comfortably make the grade, so I am sure yours should too, but I think you have to trust the RAM factor and I am not sure this is possible if the air is not passing freely underneath the vessel.

Good luck.

[BMcF]

Originally Posted by SeaSki designer

With fuel, my prototype weighs in at about 3500 lbs so I have about 2000 lbs of boat weight that seems unaccounted for by the calculation, or given my charitable guess above, about 1500 lbs.

My experience with the prototype suggests that at 100 km/h (54 knots) I am getting the equivalent of 18 to 22 lbs per sq ft of dynamic pressure, or about double the calculator amount.

What? Your craft is clearly supported in the largest measure by its planing skis. Where is there any mystery in that?

A simple u-tube manometer set up to measure the static pressure on the craft's wet deck will put the other 'magic' to rest pretty quickly.

Have you any video or other description of what your craft does when operating in head or following seas? I can imagine things would get interesting as soon as you bury the fronts of the skis in the face of a wave?

I have to go dig up a great old technical paper..."The Skiers Paradox" or something like that it was titled. It well explains why/how water skis work as they do.

[Yellowjacket]

Trevor,

Most of the weight of your craft is being supported by the skis. Think about it for a minute. A single water skier weighing 200 pounds will plane on a single ski at 30 mph using only the last two feet of the ski. Your skis are much longer and wider, and you are running at higher planing angles.

Do a simple Savitsky calculation and you will see that, even at the highest speeds you are getting most of your lift from the skis. The reason that your craft rides very smoothly is that the planing area does not substantially increase as you go through waves, allowing the planing depth to momentarily increase without a large increase in vertical force.

The reason I provided the total pressure calculator was that the concept being presented included an aft transom that sealed the "box" and the pressure under the craft is the ram pressure. This does not relate to your craft, your craft is getting lift from a wing and that is totally different than an air cushion vehicle. At speed there is also an effect of the pressure above the craft, which can be lower than the free stream pressure, but at low speeds this isn't an issue. The pressure differential is pretty much just the stagnation pressure at low speeds. Don't assume that this concept relates to your design, because it doesn't.

Aerodynamic lift is the difference between the pressure below and above a surface. In your case you have not stagnated the air below your craft since you don't have a rear transom to block the flow. That means your lift (as soon as you get up on your skis and start to allow airflow to escape aft) is based on that of a wing and not the total pressure of the free stream. In order to be a proper WIG vehicle, you need a more efficient lifting surface, since the airflow separates after your cab and that prevents your design from being a good airfoil.

As BMcF noted if you measure the pressure differential between the area below and above the deck you will able to determine the amount of aerodynamic lift being created. The reason it isn't much is that you haven't designed your craft to be an efficient wing. If you did so it would create more aerodynamic lift, but it might also be less stable, so that is probably not a good idea unless you consult with someone who can better assess the aerodynamics and can insure a stable machine.

If you have 150 square feet of surface area, a wing could lift 2500 pounds at 50 knots, but it wouldn't be very efficient because it would have a high angle of attack. A Cessna 182 has 174 square feet of wing area, and it lifts 3100 pounds, but the most efficient speed for the aircraft is much higher (91 knots). At the low angles of attack that you are running and the shaping of the upper surfaces of your craft don't allow you to gain as much lift as an efficient wing.

This means that you can have a wing that helps lift the vehicle, and you are getting lift from your design, but because of your shaping it is not very efficient. If you had a model in a wind tunnel you could get lift and drag coefficients, and that would allow you to better determine what amount of lift is being contributed by the aerodynamics of the system.

Don't let this assessment dissuade you, I think you have a very neat concept and it is a relatively efficient way to travel on the water. Just don't assume that the air is doing most of the work here, because it probably isn't. Trying to get more aerodynamic lift may not be a good idea anyway, since it brings up nasty stability and control issues that you don't have a problem with right now.

[SeaSki designer]

To BMcF,

Thank you for the thoughts regarding the manometer.

Certainly, at my low ski-borne speeds of 60 km/h (37 mph), the skis are doing nearly all the work (I estimate they are supporting 90%+ of the vessel weight), but as speeds increase to above 90 km/h (56 mph) there is a considerable shift of weight away from the skis to the air-supported underside of the hull, which led to my comment that I thought only about 500lbs of the prototype’s total mass of 3500lbs was loaded on the skis at these speeds. (The reality is, if you go fast enough, that flat surface can make the vessel airborne at which time there is zero ski weight, but not a desirable outcome .)

There is some video footage of the prototype in seas at my website: www.seaski.com.au. If you watch closely enough you will see instances of the prototype “air floating” with only a small amount of the rear of the skis engaged.

The front of the skis do not bury in the face of a wave as they tend to be higher than the wave crests, but if they do, they pierce the crests where the wave has its least energy. These experiences relate to seas of 0.5 to 1.7m as measured by the wave rider off Point Danger (near Gold Coast, eastern Australia) at speeds between 70-80 km/h.

To Yellowjacket,

Thank you for your very good description of the dynamic pressure issue as it relates to my craft. I thought there must be something that I was not taking into account! Obviously my interpretation.

I also very much appreciate the trouble you have taken to explain the aerodynamics – I actually did some of my earlier modelling along very similar lines to what you quoted above, but using the Cessna 172 with the same wing area (175 sq ft) which could support a gross weight of 1111kg (2444 lbs) at just above stall speed of 50 knots, close to my prototype speed. Also did water skier evaluation, but with barefoot skiing!

The “above wing” lift was something I did not want to get involved with for the stability reasons you stated (I actually shortened by flat cab roof to discourage this lift) and for other reasons such as wings being cumbersome, their angle of attack issues, and producing docking problems. It is easier to use motor trim to get the desirable ride.

True WIG craft also suffer other disadvantages in terms of cost for payload and requiring relatively calm waters for takeoff and landing.

Thanks for the Savitsky calculation suggestion, and all the encouragement.

To DocScience

My apologies if my contribution was amiss. Would it hurt to run a foil between your ridged sides near the toe of you front skirt?