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enter image description here

OK, the weapon idea is the following:

  • general shape similar to the drone on this picture
  • modular armament, in default configuration with anti-ship missiles (but goes with torpedos, gun, mines, etc.) and serves as missile boat
  • material: frame and hull - fiberglass; empty unused spaces filled with styrofoam to keep buoyancy high even if severely damaged
  • intended to indirectly achieve low radar signature and low magnetic signature.. through use of rather cheap materials
  • intended to be used in packs to take down much more expensive adversaries
  • semi autonomous weapon, not only expendable but no space for crew is needed

Would use of such synthetic materials and small size give it noticeable edge in radar detectability?

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    $\begingroup$ You do realise that your guns and torpedoes will have their own radar and magnetic signatures? $\endgroup$ – nzaman Mar 21 at 15:58
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    $\begingroup$ Congratulations. You have invented the torpedo boat. Next, the adversary will invent the torpedo boat destroyer... $\endgroup$ – AlexP Mar 21 at 16:30
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    $\begingroup$ @Alexander - A void full of closed cell Styrofoam Remains a void 'mostly full' of Styrofoam (and styrofoam is mostly full of air) after a hole is put into it. A void full of only air is at strong risk of becoming a void full of mostly water in the event of a leak. Other partitioning methods may yield lighter options, but are generally more expensive. $\endgroup$ – TheLuckless Mar 21 at 17:07
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    $\begingroup$ @JoeBloggs, Switzerland also has a stealth navy. $\endgroup$ – ShadoCat Mar 21 at 18:41
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    $\begingroup$ @ShadoCat and they got a Carrier! $\endgroup$ – DarthDonut Mar 22 at 7:05
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Gravity (and force in general) is your enemy

One of the many reasons why military ships are made out of metal and not cheaper, lighter, harder-to-see-by-radar materials is that the stuff you're storing or have installed on the ship has weight.

This may sound trite, but buoyancy isn't magic. It isn't a form of anti-gravity. If your engines weigh a ton, that ton is still there, putting stress on the hull and whatever mounts are used to keep it in place. If it produces 1,000 ft-lbs of torque, that torque is acting against the frame of the ship — a frame not made of metal. Does it carry torpedoes?1 That means a pneumatic firing system that pushes against the frame of the ship. Does it have a gun mount? That's tons of weight being born by fiberglass. Fuel, radar, communications... it all adds up to weight and force and your hull (and the infrastructure connecting everything to the hull) must bear the weight and force.

And to top it all off, you have no armor. That means enemy bullets (ship- or airborne) will tear through your drone like tin foil. Sure, with all that styrofoam it'll still float, but what's the point if everything it's carrying has been ripped to shreds?

And while the hull itself isn't causing much in the way of a radar signature, your engine, comms, gun mounts, etc, (the stuff you can't make without metal) all will. It may only have the signature of a dump truck, but it'll have it.

Finally, there's value to all the weight of metal hulls: stability. Due to the weight (because gravity is still trying to pull the ship down to the ground), it's harder for the ocean (or storms) to toss the ship around. Lightening it with fiberglass and styrofoam would cause it to be tossed around in the high seas like a proverbial leaf on the wind.

IMO, such a vehicle would have little use other than short-range reconnaissance. The forces involved to move things long distances quickly and to engage in combat would either rip a fiberglass/styrofoam hull to shreds or, having used enough fiberglass to overcome the problem, make the ship difficult to handle without much improvement in stealth.

The best stealth doesn't deny you your advantages

Modern fighter planes use (basically) two forms of stealth tech: radar-absorbing materials and radar-reflecting shapes. These allow the plane to be built with all the tech (including metal) it needs to operate well for its intended purpose.

But, to quote from Wikipedia:

Just like choices in shaping, the choice of materials affects the RCS of a ship. Composites such as fiberglass and carbon fiber are great blockers of radar and give smaller vessels an advantage in further RCS reductions. However, composites are fragile and often unsuited to larger ships or ships that expect to take fire, though new laminates can partially negate some of the weaknesses. This restricts larger ships to metals such as steel and aluminum alloys. To compensate a ship may include a coating of a radar absorbing material though this can be quite expensive and may not stand up to the corrosive effects of salt water.


1The case of a torpedo is unique. If it's self-guiding or guide-by-wire you could just drop it in the water, but it doesn't change the basic rule for everything else: Newton's 3rd law, equal and opposite reactions.

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    $\begingroup$ Norway has a stealth navy $\endgroup$ – Joe Bloggs Mar 21 at 17:26
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    $\begingroup$ Just saying that sentence makes me inordinately happy... $\endgroup$ – Joe Bloggs Mar 21 at 17:30
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    $\begingroup$ "And to top it all off, you have no armor" modern warships generally don't. Their structural materials are tough, but they can't actually keep out anti-ship weaponry. $\endgroup$ – Starfish Prime Mar 21 at 21:54
  • $\begingroup$ @StarfishPrime, wellllll..... compared to fiberglass and styrofoam, modern warships are all armor. $\endgroup$ – JBH Mar 21 at 22:06
  • $\begingroup$ @JBH still won't keep out weapons. Compared to a cotton tshirt motorbike leathers are all armor, but they won't actually keep out bullets. $\endgroup$ – Starfish Prime Mar 21 at 22:11
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Large GRP or fibreglass vessels are primarily used as minesweepers rather than general purpose military vessels, and have been used as such since the 1950s. Historically mines were magnetically triggered, so what amounts to a plastic hull wouldn't set them off so easily.

You may struggle to make the hull strong enough to take traditional large ship guns, but for all other purposes they shouldn't have a problem.

They don't have any advantage in radar detectability (that I'm aware of) as that has more to do with profile and surface coatings than structural materials.

The biggest quirk that I've heard of is that the much lighter hulls cause a greater issue with seasickness.

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  • $\begingroup$ Normally small fiberglass / wooden boats are even asked to carry a corner reflector, to make them more radar visible. The part that I'm unaware is whether that would actually matter for any military application. $\endgroup$ – Shadow1024 Mar 21 at 16:14
  • $\begingroup$ @Shadow1024, that's true of all small boats regardless of what they're built of. It's entirely about profile. $\endgroup$ – Separatrix Mar 21 at 16:44
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Back when stealth fighters were first made public, there was lots of excitement in some circles about stealth fast attack craft. "Impossible to detect, impossible to defeat, the perfect killer." Turns out that excitement was premature.

Compare the 1988 Arleigh Burke class and the 2016 Zumwalt class. The Zumwalt is more stealthy, it is larger, it is more expensive. The radar cross section of the Zumwalt is "more akin to that of a fishing boat" but fishing boats are not invisible. Is the stealth worth it? Probably, in some missions.

Compare the 1982 German Gepard class and the 1999 Norwegian Skjold class. The Skjold is actually smaller and faster and probably a better attack craft overall.

  • Radar stealth helps against radar detection, but there are other sensors. An effective stealth ship also has acoustic and IR stealth. That still leaves wake-homing sensors. Any warship must expect to be attacked and armor and countermeasures take space.
  • For ocean operations, size brings speed and range. Yes, small boats can be faster than large ships, but only at a huge endurance penalty.
  • Larger ships will be a more stable weapons and sensor platform. Imagine the ship in your picture in a gale.

So think of what aerial drones do today. Some are stealthy, some carry weapons, but they cannot replace a manned fighter or bomber yet.

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A smaller vessel will be harder to detect with radar, simply on the basis of a smaller cross section returns a weaker signal. The additional element of using slightly lower 'signal return' materials also helps, but at some point you're reduced to systems and machinery being the larger part of your signal reflection.

But the smaller vessel will also have to address other compromises:

Biggest issues for a small vessel become its sea keeping abilities and longevity at sea. Removing the need for crew does reduce your size and energy requirements, but you can only take reductions in size so far before the vessel becomes unable to make useful headway through rough weather.

As you lower the size, you increase the impact waves will have on a boat's motion:

  • A short hull will fit 'between' a greater number of waves on average, which and reduce how effectively it can make forward motion, and will put it at risk of being 'pushed around' by smaller storms than a slightly larger boat.

  • Too small of a vessel will also not be able to carry the fuel to travel far, making rough weather that much worse. [Not only can it not go far, but it can be at risk of easily getting pushed beyond its own range to reach its target.]

One point that is readily overlooked with reducing the size of a boat to make it harder to see: You also make it harder for that boat to see for itself. Tiny boats may not have the stability to support its own radar array as well as a larger ship would - It would be lower, therefore have less range, and prone to more violent motion, which degrades its ability to establish and maintain clean target locks.

As you continue to shrink a design's size, you eventually reach a point where you have less of a 'boat', and quickly begin to approach something that better resembles a "smart sea-mine with a gun".

However much of these issues may be overcome with a combined arms approach, or working around them in other ways.

  • Tender/Mine-layer approach: Rather than expect the small boats to operate straight from port on their own, operate them off larger tender vessels that can move a flotilla at speed over long range before deploying an area-denial screening force around itself. Rather than an independent ship, you can treat them as a 'pre-fire weapon platform'. (A tender/carrier ship might even be designed to be able to deploy at least some of its drones even in the event of taking ship-sinking damage...)

  • Renewable energy: Give them sails for non-combat movement, and accept that they're slow and clunky to deploy them anywhere, but make up for that in ability to deploy vast numbers of them.

Well networked and with well designed controls, small vessels could form a deadly mesh of data gathering and weapons deployment points that would be difficult to detect with much confidence. Combine them with the trash that is already out there (how does a radar tech confidently tell one of these drones-boats from a lost shipping container full of rubber ducks?), and possibly with scattering cheap dummy targets around the area on your own, and you end up with a region of ocean that would be extremely dangerous and expensive to operate a traditional modern fleet in.

[Do they fire on EVERY small radar contact they pick up? And even if they fire on it, have they damaged it enough to completely disable it? - Think of the effectiveness of Fairey Swordfish in WWII, cloth biplanes that could have holes punched through them without impact due to how little of importance much of the plane actually had... How many resources must be expended on each and every drone boat/smart mine/radar decoy/floating garbage spotted to remain safe? How large of a loss is it really if the enemy does take out one, or a dozen, or a hundred of this small unmanned boats?]

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You could make the aquatic equivalent of a predator drone.
Simple frame of fiberglass and foam that's just big enough to hold one or two torpedos, maybe a few other weapon systems, a battery bank, a motor, and some electronics.
Put some solar panels on the top surface to extend the time it can be out.

Stealth:

It could be mostly submerged, with only the top foot or so above the water, meaning that it would have almost no radar profile, even without anti-radar coatings and geometries.
Given the right motor it could be run with a very small acoustic footprint, making it more stealthy, and being near the surface with a minimal draft would make it hard to spot with active sonar too. Active sonar works when the sound bounces off the transition between water and air, not the hull itself. Since the surface is also a transition between water and air, anything with a very shallow draft is going to get lost in the noise from waves.

Weapons:

Torpedos that are either self guided or wire guided would work, and wouldn't need tubes as the torpedo bay could be flooded so they could just swim out on their own. A pump may be needed to empty the bay again to restore buoyancy.
It may also be possible to do without a bay, and simply have the torpedos mounted between two pontoons, or possibly on either side of a single pontoon, though this would require that both be fired at the same time to maintain balance.
Having the torpedoes in the water instead of a bay can actually help with the weight issue, because torpedoes are neutrally buoyant, and so would be self supporting instead of putting stress on the hull frame.

Another option would be missiles, though no current anti-ship specific systems (that I found) would work on such a small platform. However, such a system could be developed based on a shoulder mounted rocket with the idea that they could get very close and so not much propellent is needed.

.50 cal machine guns could be mounted to be used against smaller targets and personal.

Lastly, they could operate as kamikaze drones. If the styrofoam is replaced with a foamed plastique explosive, then the entire drone could become a warhead once the other weapons are used up. A mechanical safety system where the detonator is not inserted until needed might be a really good idea.

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First, a clarification must be made about radar. Radar, like light photons, is NOT reflected. Rather, the energy is absorbed by the object, and then retransmitted. Materials that conduct electricity are far better at retransmitting the radar signal than are materials that do not conduct. Fiberglass DOES sent out a very poor radar signature, compared to a metal hull. Stealth aircraft are, in part, made stealth by coating them with substances that do not re-transmit the radar signal, but rather absorb it. The energy is dissipated in some other form, usually heat. See Radiation-absorbent material

If you provide a very thick layer of radar-absorbing material between the exterior and any machinery, the radar image would be severely degraded. Even if the radar signal reached the machinery, it would still have to get back out. Turns out, some foams are a really good RAM.

And, of course, since you do not need any interior crew compartments or crew spaces, you can fill the entire superstructure with foam.

Dissipating the heat is made much easier, since the boat is riding on a very large heat sink. The ocean would quickly dissipate the heat, with proper cooling fins.

Now, back to your boat. Several answers allude to structural integrity. Foam-filled fiberglass does have a lot of structural integrity. The stresses are spread throughout the entire volume of the foam, so although on a cross-sectional basis, foam is a poor structural member, when the entire volume is filled with foam, the member becomes highly structural. That is the basis of packing large screen televisions in solid foam. The stresses are distributed throughout the box, not just along structural members.

Laminated foam and fiberglass sheets are an even stronger structural member. That is, when spraying the foam, embed fiberglass sheets and rods into the body of the foam, not just the envelope.

It is not unknown for larger fiberglass boats to have a steel frame, bonded to the fiberglass and embedded in it, to add critical reinforcement.

I would suggest using a common ship-building technique - a strong keel. If the keel is made of metal, below the water line, this would go a long way to giving you the structural integrity you need. A keel that forms a typical truss beam down the center of the boat, even better. Think open web trusses. Maybe two or three cross trusses midships.

Since radar is usually from the side, not from above, then using flat steel trusses in the horizontal decking, especially if just below the waterline, would keep the radar image to a minimum.

I would suggest making it all electric, using solar power and batteries, to eliminate any heat signature from combustion gases. Again, the heat generated could be dissipated in the water, not the air.

TL:DR

Reinforcing the foam with steel structural trusses, and embedded fiberglass sheets and rods, would give you the structural rigidity you need. With no need for any crew spaces, the entire body can become a solid structural member. Electrical engines would reduce the heat imprint, and placing as much weight below the waterline would give good stability. Since there are no crew members, sea sickness is not an issue, so let'r'rock'n'roll.

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