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Questioning Hydrofoil Technology


Shadowsage
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Okay, I've become a bit fed-up with Hydrofoils. So I've read up on them. Modern-Technology rarely exceeds 50 knots with the devices, and even then on extremely specialized crafts. Obviously puzzled by the controversy between this and the high-speeds (150 knots +) used in CNRP, I took the liberty of trawling around the internet for some information.

So, I came upon this little tid-bit of information: "The tonnage of hydrofoil type craft is limited currently to about 400 tons, because the weight increase of a hydrofoil craft is much faster than the lifting force from foils when the dimensions of the vessel are increased."

If we look at this statement, the weight increase from added tonnage outstrips the rate at which hydrofoil craft can effectively use the lifting force to get out of the water. If it can't get out of the water, it's not an effective means of propulsion and thusly can't move. But I looked further into the subject.

"The tonnage increases cubically by its dimensions while the lifting force from the foils increases squarely by its dimensions. The power transmission system on retractable foil type hydrofoil craft is very complicated and often causes maintenance and operational problems resulting from many delicate moving elements enclosed in the movable spidery struts."

This part is really easily understood. Tonnage increases by an exponent of three, whereas the lifting force increases by an exponent of two according to its' dimensions. Tonnage thus easily overwhelms the lifting force after a threshold already noted to be at 400 tons. But the power-transmission system for retractable foils is another interesting hurdle; power-systems like the jets on LeVentNoir's own naval vessels are CERTAINLY complicated systems. That many moving parts requires amazing amounts of maintenance, with the wear-and-tear of the ocean.

If your solution is fixed struts; "It is very inconvenient for a fixed foil type of hydrofoil craft to rest at a limited depth of harbor because the extension of foil planes may hit the bed of the harbor: especially for a large tonnage vessel with long and deep fixed foils."

Large-tonnage being entirely relevant, since 400 tons is considered the absolute maximum.

"At higher speeds associated with hydrofoil boats, collisions between the foils and submerged objects are of great concern. Prior art hydrofoils include hinged struts and other methods of releasably rotating the foil support structure, should the foil strike a submerged object. Unfortunately, designs do not provide for safe transition to hull borne operation and the danger of cartwheeling or otherwise capsizing is high; particularly in a smaller craft at high speed."

A pretty large bit of text to swallow. Hinged-struts and what-not are not good enough for maintaining proper sailing. As soon as they rotate, the hull starts to take part of the water and Hydrofoil ships are NOT designed for that. High-speed turns are not recommended, as a result.

"Another problem resulted from the operation of the vessel in heavy seas or other conditions involving substantial wave action. The rigidly mounted hydrofoils previously employed produce rough rides or instability, and it was difficult to mount an appropriate suspension between the main portion of the vessel and the foils in an economical manner."

So, unless the sea is relatively calm, the Hydrofoils start to flounder in the waves amplified by their operation. Rigid Hydrofoils add to the problem, forcing an expensive suspension-system further biting into the ever-present 400 ton maximum. It's also adding MORE maintenance problems to an already touchy craft.

And for the finishing blow:

"Conventional hydrofoil craft have a number of problems which make them difficult or impractical to operate at high speeds. A first problem is cavitation, which is a phenomenon in which vapor bubbles form along the upper surface of a foil due to a low fluid pressure on this surface. Cavitation invariably occurs in conventional hydrofoil craft above a certain operating speed (typically around 50 knots). When the vapor bubbles caused by cavitation collapse in the water, they produce strong shock waves. If the collapse occurs in the vicinity of the foil, the shock waves not only produce unpleasant noise and vibrations, but can also physically damage the foil of the craft by pitting."

So, accelerating past 50 knots, regardless of propulsion system, starts to damage the Hydrofoil itself with cavitation. Unpleasant noise and vibrations are the least of your problems when you consider that your craft is rotting out from under you.

Edit: Source

Edited by Shadowsage
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LVN isn't using hydrofoils

He's using something like giant pontoon rails instead. I dont have the crappy 1990's bad salesman site he used for it, but there are no fullscale models.

Honestly anything but a mono-hull can't take much damage without losing a lot of stability and sinking. The ships he has are to flimsy to take even a single hit in the rails or structs to the rails.

The whole thing will sink once the balance is screwed.

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No they are not, they are twin standard displacement hulls, just extremely narrow, with massive engines stuck on top.

I never said my ships were robust, just they were fast. With todays weapons a single hit will cripple most ships, so why build overly robust ones?

Having made and tested scale models of this type, (scale weights and thrusts) and seeing their performance, I have to say, these speeds are not impossible, but do require specialized design and large amounts of propulsion.

(Um do you know the difference between rails and hydro foils? Rails have no upper surface, and a high fluid pressure on all surfaces, and a lot of your points stop being concerning)

Edited by LeVentNoir
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No they are not, they are twin standard displacement hulls, just extremely narrow, with massive engines stuck on top.

I never said my ships were robust, just they were fast. With todays weapons a single hit will cripple most ships, so why build overly robust ones?

Having made and tested scale models of this type, (scale weights and thrusts) and seeing their performance, I have to say, these speeds are not impossible, but do require specialized design and large amounts of propulsion.

(Um do you know the difference between rails and hydro foils? Rails have no upper surface, and a high fluid pressure on all surfaces, and a lot of your points stop being concerning)

Have you made videos of such testing? Any records of them? I thought you were a physics major, not a naval engineer.

Besides, if we are going to treat this like a real test between two ideas, we need to provide evidence that everyone can see. It is bad form to trust someone's word that they built something at a smaller scale without error when they are not even in that field. He presented a well written argument, if you don't counter it with REAL evidence I'm going to have to agree with him.

Edited by BaronUberstein
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Considering they are twin-hulled ships, they are privy to the same consequences ships of the type have been forced to deal with. I quote.

"In order that a hull can be considered to have good seakeeping characteristics, it is not only necessary that the motions be small at high speeds in head seas, but also small or within acceptable bounds in other operating conditions."

In other words, high-speed manuevers are undesirable considering that the ship would flounder and rock uncontrollably. Scaling upwards in unsteady conditions will have a far different result than models built and floated in a pond.

Quick Edit: Accepting Pontoon Rails for the moment, consider that Pontoon ships have a nasty tendency to capsize with high loads. The pontoon effect is the tendency of a Pontoon Boat to capsize after sufficiently heavy loads with high-centers of gravity are placed on-board.

Edited by Shadowsage
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Considering they are twin-hulled ships, they are privy to the same consequences ships of the type have been forced to deal with. I quote.

"In order that a hull can be considered to have good seakeeping characteristics, it is not only necessary that the motions be small at high speeds in head seas, but also small or within acceptable bounds in other operating conditions."

In other words, high-speed manuevers are undesirable considering that the ship would flounder and rock uncontrollably. Scaling upwards in unsteady conditions will have a far different result than models built and floated in a pond.

The problem is minimized by placing the rails far apart, and building a low ship, hence minimizing rocking, besides from later in the link, it says that these types of hulls have much smaller movements than monohulls of the same displacement in all types of seas, except when the ship is stationary, when the movements are equal. So, yes you can turn, as this type of design has a much smaller vertical movement compared to a monohull in pretty much every situation.

Have you made videos of such testing? Any records of them? I thought you were a physics major, not a naval engineer.

No unfortunately, I don't. And for the record, I'm an engineer, not a physicist.

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The problem is minimized by placing the rails far apart, and building a low ship, hence minimizing rocking, besides from later in the link, it says that these types of hulls have much smaller movements than monohulls of the same displacement in all types of seas, except when the ship is stationary, when the movements are equal. So, yes you can turn, as this type of design has a much smaller vertical movement compared to a monohull in pretty much every situation.

Placing the rails further apart will result in a lower ship, yes. That only amplifies the ability of any heavy tonnage aboard the extremely low deck to capsize the pontoon. Furthermore, with such a low deck the ship will become swamped in water the instant it hits bad weather.

I have been providing evidence for every one of my statements. Would you be so kind as to return the favor?

Edit: Carrying the missiles is not the point. The question is over the possibility of the ships, as stated, to even exist.

Edited by Shadowsage
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Having made and tested scale models of this type, (scale weights and thrusts) and seeing their performance, I have to say, these speeds are not impossible, but do require specialized design and large amounts of propulsion.

I need to know, you are aware that the scaling down of weight and thrust is essentially meaningless. Boats, like airships and the like, don't work the same at small scale. The resistance of the water would be much larger, and the surface tension would be exponentially smaller for the real boat, meaning the small scale would travel much faster, because it would be on top of the water, barely touching, but at a real scale, the hulls would be deeper, upping the resistance, and decreasing the amount of surface tension holding the boat up.

I would assume with all you talk about with scaling things up or down and how there are a lot of issues with it, I would assume you found ways to make it more realistic of a test.

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One other thing, anyone who knows what the differential on a car does will see the problem with this. As you try to turn, the outside edge will be trying to go faster than the inside edge. The forces involved at high speeds, with a heavy ship, would tear the damned thing apart.

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I think the silly things will just airplane like highspeed racing boats that tend to travel at similar speeds. I raised most of these arguments when he claimed his super carriers could launch b-52 bombers one day.

However since the resource site he used only has one thing of techincal data about these ships, that being the dont sink and are stable in waves, there is little so support claims of massive power.

I think the only actual REAL example of the railed ships they had was a 4 foot balsa wood model they made to putter around a small pond.

btw, here is that site he used for his images, and more than likely info

http://www.hydrolance.net/page12.htm

Bewarned. Its very poorly designed, and has obnoxious sounds on almost every page.

Edited by Tahsir Re
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Placing the rails further apart will result in a lower ship, yes. That only amplifies the ability of any heavy tonnage aboard the extremely low deck to capsize the pontoon.

The load is placed in between the pontoons, so unless you flip the ship, the pontoon will not capsize. I don't see your point, unless you think this is like an out rigger canoe, instead of a catamaran. Second, Lowering the centre of gravity of anything makes it harder to flip.

Furthermore, with such a low deck the ship will become swamped in water the instant it hits bad weather.

Its relatively lower, and yes, you may get some wave wash over the rails, but it will run off.

I would assume with all you talk about with scaling things up or down and how there are a lot of issues with it, I would assume you found ways to make it more realistic of a test.

I'm not saying it is exact, I am just saying its not impossible.

One other thing, anyone who knows what the differential on a car does will see the problem with this. As you try to turn, the outside edge will be trying to go faster than the inside edge. The forces involved at high speeds, with a heavy ship, would tear the damned thing apart.

They outside edge will go faster, as they are able to move through the water at different speeds. The forces would not need be that much, you increase the radius of the curve, and the force decreases with F = v*v / r. Besides carbon nanotubes and other exotic materials already exist in CNRP, why can't I make my ship from them?

I think the silly things will just airplane like highspeed racing boats that tend to travel at similar speeds. I raised most of these arguments when he claimed his super carriers could launch b-52 bombers one day.

That involves the hull rising out of the water, and my ship does not have that, as the main superstructure is designed to counter the upwards lift possibly generated by the higher speed air moving over the top considered to the bottom. To counter, curve the bottom down the same amount as the top is curved up, and the air speeds on both sides will be the same, so no lift or suction is generated.

Edited by LeVentNoir
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Lowering the entire center of gravity makes it harder to flip, perhaps. Lowering it regardless makes it more susceptible to the ocean. I somehow doubt the entire load is placed between the pontoons; there just wouldn't be enough surface area to go around for all the weapons systems you have aboard your ships.

Stealth also doesn't lend itself well to flowing through the water. That reduces your efficiency.

As for spreading the pontoons...

"The overall goal of the above optimization process is to generate, with the least possible computational and experimental effort, seakind catamaran hull forms with low weight and resistance characteristics. From the naval architectural point of view these requirements are contradictory, because a seakind catamaran requires moderate stiffness (low to moderate metacentric height), thus small separation distance between the demihulls, what has the additional positive effect of low structural weight, whereas the low resistance (and especially, for fast catamarans, low wave resistance) requirement, that indirectly calls for reduced machinery and fuel weight, suggests a large separation distance for the demihulls, for avoiding the negative interference effects on resistance, that can easily triple the single demihull's resistance [6]. Therefore, a formal optimization procedure for fast catamarans should be looking for twin hull arrangements exhibiting the least possible separation distance for the demihulls, thus disposing reduced structural weight and moderate stiffness in roll direction, without compromising on the increase of the sum total of the demihulls' single resistance."

The simple version of this excerpt? The relevant sentence tells us that more spread out hulls increase structural weight and stiffness for rolling direction, as well as reduce the resistance to force the hulls as a whole have. Spreading out the hulls thus results in a ship incapable of turning quickly, overweight and thus even lower in the water (already in danger of seawater getting into the necessarily delicate mechanisms) and much more susceptible to breaking due to force on the ship.

Source

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Lowering the entire center of gravity makes it harder to flip, perhaps. Lowering it regardless makes it more susceptible to the ocean. I somehow doubt the entire load is placed between the pontoons; there just wouldn't be enough surface area to go around for all the weapons systems you have aboard your ships.

Stealth also doesn't lend itself well to flowing through the water. That reduces your efficiency.

Look, its a RL ship which does just that?

<snip>

Thats a catamaran, a ship with two monohulls stuck together. My design is like this this minus the large underwater displacement, allowing for higher speeds.

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The load is placed in between the pontoons, so unless you flip the ship, the pontoon will not capsize. I don't see your point, unless you think this is like an out rigger canoe, instead of a catamaran. Second, Lowering the centre of gravity of anything makes it harder to flip.

You say here that your ship is a Catamaran.

Accepting even that your ship IS the same displacement-wise as the Sea-Shadow (with a top-speed of 28 Knots, I may add), a SWATH has its' own limitations. Going back to the original site which disproved hydrofoils...

The quest to improve seakeeping led to development of the SWATH hullform. Utilizing submerged submarine hulls, wave-piercing struts and an elevated platform, the SWATH hullform has a low waterplane area that is less affected by waves than its predecessors. The result is increased stability in high seas. SWATH hulls, however, are still restricted to lower speeds. This lack of speed limits the effectiveness of SWATH, and to date, ship designers and operators are faced with the dilemma of choosing either speed or stability.

Source

SWATH ships thus have to sacrifice either speed or stability in order to come up with a viable design. As an additional proof, take a look at this site. Every link to the SWATH hulled-ships shows a design with LOW speed, in order to get stability. This relationship prevents a stable, fast SWATH ship the likes of which you have used.

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