Hydrofoil Supported Catamaran designs

The air cushion is maintained by a fan, not by the motion of the boat through the water, I don't see how it's supposed to be more efficient that a Hysucat if both are moving at sufficient speed that the foils generate lift.

 

The owner of Teknicraft, Nic de Waal, is a South African who used to be a partner in a company called T-Craft in Cape Town, South Africa. T-craft was the 1st company to construct HYSUCATs based on Prof. Hoppe's designs in the 1980s. T-Craft went bust on a bad business deal and Nic packed up and set up Teknicraft in New Zealand, initially building more or less the same craft as T-craft did, much to the dismay of Stellenbosch University (owner of the HYSUCAT patents). Patents were not registered in NZ and many other countries so Nic could safely design HYSUCATs without legal repurcussions. THe trademark "HYSUCAT" was registered by Stellenbosch University and hence Teknicraft has never used it. Patents expired in 1999/2000 and the trademark has been dropped in may countries- only to be taken up by others on and off.

To Teknicraft's credit they probably have the largest number of HYSUCAT dsignes onthe water for larger craft (Length=15m+). So are successful applications of the technology and other less so...


Since 2000 there have been a lot of failed projects as designers try to design HYSUCATs without real understanding of the design issues. A classic example is the failed HYSUCAT windfarm service vessel designed by Melvin and Morelli for Mr. Robbert van Rijk of WindCat Workboats (2009). After only a few hours of sea trials the owner took the foils off his boat and trashed them.

HYSUCATS are very unforgiving to design/construction mistakes. However if done properly, the system has plently of benefits:
- up to 40% speed increase
- 20-30% reduced fuel consumption
- 30-50% reduction in vertical accelerations
- 20-30% lower wake wash (wave height and wave energy)
- lower motions (pitch and heave mainly)

This has been proven many times on over 500 vessels in size ranges from 6.5m to 40m.

 

I worked for a bit at a company designing surface effect ships (ferries). The air cushion is a positive pressure support system. It always works (as long as your blower works). We've even lost a finger section of the forward skirt and it still works (the internal cushion pressure squeezes the remaining fingers together, sealing the hole).

Foils are negative pressure (the foil shape decreases the pressure above them, thus generating lift). That makes them susceptible to cavitation if they're too small for the weight of the ship and/or they're running too shallow. The obvious work-arounds to this are: (1) to make the foils bigger, but that increases wetted surface area and drag; or (2) to run them deeper, but that increases the risk of hitting something and increases the damage that results. I suppose there's also (3) make your foils symmetric in cross section and rely exclusively on angle of attack for your lift instead of Bernoulli, but that's less efficient and still creates a low pressure zone above/behind the foil. Pressure changes as your foils move through waves and swells also add a whole new series of pitch- and roll-control problems to solve as well.

(2) is the reason the US Navy killed its hydrofoil program (Pegasus class) - they were suffering an inordinately high frequency of whale strikes and other debris strikes. I guess high speed monohulls make enough noise for the whales to (usually) get away in time. But on foils you're disturbing much less water and the engines are not directly transmitting noise to the water. So the hydrofoil "sneaks up" on the whale and blammo. Boeing designed the foil to collapse in the event of a strike, but then your super-fast hydrofoil becomes an awkward displacement hull dragging a huge foil bent at nearly 90 degrees underwater until it can get to a port for repairs. All of the Navy's high-speed craft are now air cushion vehicles or variants, since they're less vulnerable to debris or whales in the water. (Though to be fair, an ACV's amphibious capability probably played a larger role in the decision.)

That's not to say ACVs aren't without their disadvantages though. The cushion depresses the water underneath it. At low speeds, an SES on cushion is basically a monoholl - it's just using air to displace its weight in water instead of a hull. Because it's air displacing the water there's no wetted surface friction, but the displacement creates wave resistance like a monohull does. Past a certain Froude number, the wave resistance starts to blow up just like with a monohull.

But unlike a monohull, the wave resistance drops soon after. Once the SES powers past this "hump", it acts more like a planing hull except with a lot less surface friction (no wetted area in the cushion*). As you get faster, the wave resistance approaches zero. The cushion is in contact with any section of water for such a short time that there's not enough impulse imparted into the water to displace it to form waves - same as a planing hull. But this does mean getting an SES up to speed takes higher peak power than getting a catamaran hydrofoil up to speed. (*A similar concept has been tried in regular planing hulls, called air lubrication.)

You can also get resonance effects in the air cushion. Air is compressible, and the swells entering the cushion at just the right frequency can cause it to start acting like the resonance chamber of a guitar. Most SES ferries have a ride control system to counter this (vents on the side to exhaust air when the cushion pressure starts to spike). The RCS also helps reduce heave as well, leading to a more pleasant ride.

Although draft is reduced while on cushion, the sidehulls always remain in the water, as do the fore and aft skirts. These contribute to wetted surface area. So a properly designed hydrofoil should be more efficient at high speed than an SES. However, an SES is considerably easier to design and control than foils, and are much less susceptible to debris strikes (they tend to ride over it). Unless there's been some new developments in the last 15 years, the fastest ship in the open ocean was a Navy SES, at over 100 mph (just under 100 knots). Supposedly the Navy LCACs (pure hovercraft) are faster, but their top speed is classified. Without the wetted surface of the sidehulls and spillage from the cushion reducing skirt contact with the water, a hovercraft should be more efficient than a hydrofoil.

 

A great read Solandri - very concise and descriptive.

 

You are confusing a HYSUCAT hydrofoil system with DEEPLY SUBMERGED foils of US Navy /Boing Jetfoil type. These are two different beasts completely. DEEPLY SUBMERGED hydrofoils are susceptible to cavitation but this is not the case for SHALLOWLY SUBMERGED hydrofoils which work in surface effect. SHALLOWLY SUBMERGED




The obvious work-arounds to this are: (1) to make the foils bigger, but that increases wetted surface area and drag; or (2) to run them deeper, but that increases the risk of hitting something and increases the damage that results. I suppose there's also (3) make your foils symmetric in cross section and rely exclusively on angle of attack for your lift instead of Bernoulli, but that's less efficient and still creates a low pressure zone above/behind the foil. Pressure changes as your foils move through waves and swells also add a whole new series of pitch- and roll-control problems to solve as well.

(2) is the reason the US Navy killed its hydrofoil program (Pegasus class) - they were suffering an inordinately high frequency of whale strikes and other debris strikes. I guess high speed monohulls make enough noise for the whales to (usually) get away in time. But on foils you're disturbing much less water and the engines are not directly transmitting noise to the water. So the hydrofoil "sneaks up" on the whale and blammo. Boeing designed the foil to collapse in the event of a strike, but then your super-fast hydrofoil becomes an awkward displacement hull dragging a huge foil bent at nearly 90 degrees underwater until it can get to a port for repairs. All of the Navy's high-speed craft are now air cushion vehicles or variants, since they're less vulnerable to debris or whales in the water. (Though to be fair, an ACV's amphibious capability probably played a larger role in the decision.)

That's not to say ACVs aren't without their disadvantages though. The cushion depresses the water underneath it. At low speeds, an SES on cushion is basically a monoholl - it's just using air to displace its weight in water instead of a hull. Because it's air displacing the water there's no wetted surface friction, but the displacement creates wave resistance like a monohull does. Past a certain Froude number, the wave resistance starts to blow up just like with a monohull.

But unlike a monohull, the wave resistance drops soon after. Once the SES powers past this "hump", it acts more like a planing hull except with a lot less surface friction (no wetted area in the cushion*). As you get faster, the wave resistance approaches zero. The cushion is in contact with any section of water for such a short time that there's not enough impulse imparted into the water to displace it to form waves - same as a planing hull. But this does mean getting an SES up to speed takes higher peak power than getting a catamaran hydrofoil up to speed. (*A similar concept has been tried in regular planing hulls, called air lubrication.)

You can also get resonance effects in the air cushion. Air is compressible, and the swells entering the cushion at just the right frequency can cause it to start acting like the resonance chamber of a guitar. Most SES ferries have a ride control system to counter this (vents on the side to exhaust air when the cushion pressure starts to spike). The RCS also helps reduce heave as well, leading to a more pleasant ride.

Although draft is reduced while on cushion, the sidehulls always remain in the water, as do the fore and aft skirts. These contribute to wetted surface area. So a properly designed hydrofoil should be more efficient at high speed than an SES. However, an SES is considerably easier to design and control than foils, and are much less susceptible to debris strikes (they tend to ride over it). Unless there's been some new developments in the last 15 years, the fastest ship in the open ocean was a Navy SES, at over 100 mph (just under 100 knots). Supposedly the Navy LCACs (pure hovercraft) are faster, but their top speed is classified. Without the wetted surface of the sidehulls and spillage from the cushion reducing skirt contact with the water, a hovercraft should be more efficient than a hydrofoil.[/QUOTE]

 

You are confusing a HYSUCAT hydrofoil system with DEEPLY SUBMERGED foils of US Navy /Boing Jetfoil type. These are two different beasts completely. DEEPLY SUBMERGED hydrofoils are susceptible to cavitation but this is not the case for SHALLOWLY SUBMERGED hydrofoils which work in surface effect. SHALLOWLY SUBMERGED hydrofoils generate most of their lift from the pressure (bottom) side of the foil. The pressure drop on the top side of the foil is minimal and at high speeds when the foil is very shallowly submerged and turbulance levels are hgih, there is an air-water mixing layer of the foil and it is impossible to cavitate. We have run HYSUCATS up to 70 knots and never suffered with any kind of cavitation - something you can never do on a deeply submerged foil.

Deep or shallowly submerged, there is always the risk of hitting something. Most boats run the same risk with their propellers. The only solution is to design the hydrofoils to withstand strikes without damage. Boeing developed design criteria for impacts. We apply similar design criteria to HYSUCATS and our systems can withstand impacts with logs. We have even had HYSUCATs run aground on rocks without the foils being damaged. See picture below. The boat was floated off on the next spring tide and went back into service without any repair work being required.

With HYSUCAT vessels the foils are self stabilzing and they damp out the motions of the vessel in waves. For example, when the bow of a HYSUCAT hits a wave the boat starts to rise up, this reduces the effective angle of attack on the foil, reducing lift and thus reducing the vertical acceleration. When the boat reaches the crest of the wave and starts to drop, the inverse happens and the foil prevents the boat from slamming in the trough. Experimental results show that HYSUCATS have better, pitch, heave and vertical acceleration characteristics than catamarans.

The seakeeping on deeply submerged hydrofoils is unmatched by any high speed vessel except SWATHs (if you can call them high-speed). For passenger vessels the Boeing Jetfoil is still the benchmark for pasenger comfort.

My understanding of the reasons the US Navy hydrofoil program was killed had more to do with high operational cost, no real operational plan for the vessels, reliability problems with complex propulsion system. All of these problems are avoided with HYSUCATS:
- hull remain in water contact with the hydrofoils at keel depth. If the boat hits a whale it will be similar to a conventional monohull or catamaran, where the boat will likely sail over it rather than chop it up.
- foils are fixed with no moving parts so much simipler to maintain
- vessels use conventional relaible propulsion systems (props, waterjets, surface drives, stern drives, pods etc) commonly used on conventional high-speed craft.

I agree for deeply submerged hydrofoils but HYSUCATs dont need any type of control. They are as easy to control as any monohull/catamaran.

Regarding efficiency, SES/air cushion vehicles have very low resistance and can achieve better transport efficiencies than a HYSUCAT or any other high-speed craft except WIG/Ekranoplans. For this reasons they are the prime candidate for the US Navy high-speed sealift program. However they have their limitations:
- higher maintenance costs of rubber skirts that wear out
- difficulties with propulsion due to introduction of air under hulls

There is always a compromise....

 

Obviously my background in foils is not as thorough since we did not build those. Please help me understand exactly what's going on.

Ok, so it's doing the same thing as a planing hull. Water deflected downward generates a reaction force upward, which counteracts gravity's pull of the boat down. Same as a planing hull.

Yes, that's another way to prevent cavitation. But it also completely destroys any Bernoulli lift you derive from the foil, which is why I didn't include it. If your Bernoulli lift is minimal, then the foil isn't really a hydrofoil. It's an extension of the planing hull. I don't mean this to be disparaging. I'm just trying to properly categorize the dynamic forces keeping the boat aloft.

So in essence, almost the entirety of the HYSUCAT's advantage comes from the improved lift to drag ratio versus a planing hull, due to not having any more planing hull than needed to raise the ship? That would put it more in the class of air lubricated planing hulls, rather than hydrofoils. And I'd have to revise my earlier statement and say an SES could be more efficient than a HYSUCAT, since (sidehulls excepted) the former "planes" almost entirely on air, while the latter makes contact with the water.

The low-drag lift generated by Bernoulli forces is what really gives hydrofoils the edge over other methods of reducing displacement and wetted area. (True hydrofoil > SES; WIG > hovercraft).

I'm not sure the operating costs were that high, nor the propulsion that complex (it's just a waterjet, albeit one where water has to go through an S-shaped duct much like a 727 or L-1011's third engine). Several dozen of the Boeing Jetfoils you mentioned were built and sold to various ferry operators. I believe most are still in service (in waters with few whales, e.g. Hong Kong-Macau). I got to ride one between Korea and Japan (Busan-Fukuoka, about 100 nmi apart) in the early 1990s. Very nice ride in 3-5m seas, almost like flying. Ticket cost was only about US$90 round trip, though fuel prices were much lower back then.
http://en.wikipedia.org/wiki/Boeing_929
http://www.kjps.co.jp/english/eindex.html

 

yes roughly the same. THe main difference being a planing hull has a lift to drag ratio L/D = 5.0 and the hydrofoil L/D = 20 so you produce lift far more efficiently.

The lift is not totally detroyed but I agree a shallowly submerged foil is never as efficient as a deeply submerged one. However the shallowly submerged foil remains far more efficient than a planing hull so there is significant benefit to be gained by supporting 40-60% ob the boat weight on high L/D foils and not on the low L/D hull.

HYSUCATs advantage comes from more efficient lift and also reduction of wetted area of the hull. Thirdly there is some beneficial interactions between hull and foils that further improves the efficiency of both. The simplest of these, is the end plate effect of the hulls which means the effective asepct ratio of the foils is significantly higher than the foil dimensions alone.

I went and researched through my papers to find the reasons for the PHM project being killed. It seems to have been a political decision more than a technical one according to the attached paper. There were however some technical problems if you piece the story together from different papers.

Boeing Jetfoils are still in operation in Hong Kong. Even thoug they are 30 years old they are still kept in operation there because they offer high speed and still the best passenger comfort. The more modern catamarans of +-twice the size can't quite reach the same comfort levels.

It is interesting to read the accident reports on the Hong Kong Marine department website. Theyd escribe a number of accidents where the foils have hit something over the years and the effect. Interesting there has never been a very serious accident even though HK waters are very crowded and there is lots of debris in the water.

 

Not clear what you mean by "Bernoulli forces", Solandri, it seems you are referring to the pressure reduction over the upper surface, is that correct?

If so, your statements on lifting forces on a hydrofoil are wrong. There is a velocity change over the pressure side as well, and with a profile suitable for ventilated flow the Lift/Drag ratio of the foil is far higher than the L/D of a simple planing surface, even if there is no pressure reduction over the ventilated top surface. When operating within a depth of one to two cords, however, there is certainly a measurable lift from the upper side.

A hydrofoil working in "near-surface-mode" is selfstabilizing, the trick is to understand, and design for just that operating environment. This means that the rate of change of the lift with varying depth (equals degree of ventilation) has to be understood aso. This has been a key to the success of thousands of Russian rivergoing hydrofoils, since the pioneering days of Mr Alexeyev and his design team.

Even the ACV at speeds above "hump" speed is still a "planing" object from a hydrodynamic point of view, its displacement carried by the change of impulse of the deflecting surface. Now, the rate of change (or "longitudinal pressure profile") may not be trivial, but have an influence upon the L/D ratio, just as with a "physical" foil.

Taking all design parameters in consideration, I believe there is much in favour of a hydrofoil assisted catamaran in certain operational environments, but as Sottorf points out, there are some delicate optimizations to be done; witness a heap of failed quick-fix, add-on projects.

Oops, Sottorf, you beat me in time here.....

 

Thanks for the paper. Interesting read.

The story I got from the engineers at Maritime Dynamics (company which did the tow tank tests for the Navy's SES200, as well as made its RCS) was that the Navy's SES program ended up dying for political reasons too. The SES200 was made as a lightweight high-speed test platform after the prototype SES100s. The engineers working on it knew that you needed to keep it lightweight to preserve its performance. But the Navy brass tried to attach weapons, armor, larger fuel tanks, and all sorts of other heavy toys onto it. The additional weight resulted in the SES having only slightly better performance than a regular catamaran, and the SES was written off as ineffective. (In our experience, performance in heavy seas is also not much better than a regular cat. The swells reach high up the sidehulls, eliminating much of the advantage of the air cushion lifting them out of the water. The corresponding drop in speed increases wave resistance from the cushion, slowing you down even more.)

I defer to the experts such as yourself and sottorf on this. As I said, we didn't work with foils so my background with them is much more rudimentary. My understanding was that lifting forces from a foil can only come from momentum changes in the water (deflecting it downward via AoA, "planing" as it were), or from pressure differentials (caused by Bernoulli's principle due to higher velocity over the top side).

 

There was a fatallity in March 1999 when a Capt took an old route and hit a submerged object. 1 dead and 100 injured.

In January 2008 2 jetfoils collided, caused 133 to be injured and some 43 or so seriously.

It is not totally unblemished.

 

Its a foil between two hulls !!read this http://www.hydrofoildesign.com/

 

See also
http://www.icarusmarine.com/index_files/hysucat.htm
http://www.hysucat.co.za/

Also attached some technical papers on HYSUCATs

 

I was referring to hydrofoil strikes with floating objects. Hitting a submerged object (a rock presumably) or another vessel is catastrophic for any vessel and this should not been seen as a fault of hydrofoils.

 

We helped design and build others (for example, the 40m SEMO SES' in Korea, and most notably the Skjold and Oksoy class naval SES in Norway) that proved to be quite succcessful and superior for cats in most aspects related to performance. In both cases, the generally better seakeeping, small choppy seas perhaps excepting, was a key factor in selection of the type.

The SES-200, by the way, started life as a Bell-Halter BH-110 SES crewboat that was lengthened very crudely by inserting a 50' "plug" amidships. The effort was completed soley for R &D in to the performance of higher L/B ratio SES'. The craft continued in its testing and R&D role for a number of years, reconfigured as a waterjet-propelled craft toward the end of her life, one that achieved 47 knots at one point. A succesful testcraft..but never the "prototype" for anything.