Can it float, and can it get anywhere afterwards?
Land it on water
Flotation is trivial and it is not the problem. A simulator that integrates the real seakeeping, and the finding that a sealed pneumatic float is stiffer than the water it replaces.
Land it on water
Flotation is trivial and it is not the problem. The load resting on the water is the STATIC HEAVINESS, not the weight: trimmed 550 kg heavy this vehicle displaces 0.54 m³ under a 34,271 m³ envelope. It is a cork with a 118 m sail on it, and every consequence is the opposite of boat intuition.
Relieving at 0.40 kPa gauge through 0.33 m² of vent. It cannot push harder than that times its 56 m² of contact, times the 2.5 overshoot the XC-8A actually measured, because a relief valve does not dump air instantly.
Hull speed is 5.2 m/s and the porpoising limit is 11.6 m/s. The hull carries so little weight that it walks through the wave-making hump and runs into dynamic instability instead.
What actually changes when it lands
Three configurations, at each design point's own dimensions. The propulsors tilt to their own authority, the centreboard comes out of its trunk, and the waterline sits where the flotation state puts it rather than where it looks right. Drag to turn it.
Propulsors horizontal, board stowed in its trunk. The configuration the vehicle spends almost all of its life in, holding station against the wind.
It does not slam. It gets picked up.
A floatplane is limited to about 0.3 m of wave because it is heavy: several tonnes have to be stopped in a hull length and the deceleration breaks things. This vehicle puts 550 kg on the water. It is far too light to slam.
What happens instead is that a crest tries to LIFT it. The envelope above is fixed in altitude by 32 tonnes of buoyancy and an enormous added mass, so the whole relative motion goes into the suspension. A rigid hull is a hydrostatic spring with no ceiling: in a 0.3 m sea it feeds 127 kN up the cables against a 56 kN flight design load.
| Sea state | Hs | Rigid hull | Sealed bag | Vented bag |
|---|---|---|---|---|
| 1 calm rippled | 0.05 m | 76% | 535% | 76% |
| 2 smooth | 0.3 m | 226% | 3094% | 100% * |
| 3 slight | 0.875 m | 551% | 8964% | 100% * |
| 4 moderate | 1.875 m | 1043% | 19106% | 100% * |
| 5 rough | 3.25 m | 1729% | 33050% | 100% * |
| 6 very rough | 5 m | 2615% | 50806% | 100% * |
Suspension load as a fraction of its flight design load, with the dynamic amplification from the 62,929 kg effective heave inertia included: the wave has to accelerate the ship AND the air it drags with it. A rigid hull is limited to sea state 1. A SEALED bag reaches sea state none at all, because it is a gas spring at absolute pressure and nearly sixty times stiffer than the water. Only the VENTED bag, relieving at 0.40 kPa through 0.33 m² of vent, reaches sea state 6. An asterisk marks where it is venting rather than transmitting.
Motoring to windward
The question that decides whether marine mode is an escape or a trap. The hull could be towed at hull speed by a rowing boat; what has to be pushed through the air is the entire envelope.
| Wind | Speed made good | Of the drag, air is | |
|---|---|---|---|
| 0 m/s | 11.62 m/s | 56% | limited by porpoising, not power |
| 3 m/s | 10.77 m/s | 67% | |
| 5 m/s | 9.28 m/s | 72% | |
| 8 m/s | 7.05 m/s | 80% | |
| 10 m/s | 5.80 m/s | 88% | |
| 12 m/s | 4.50 m/s | 96% | |
| 15 m/s | 1.80 m/s | 99% | |
| 18 m/s | 0.00 m/s | 100% | blown backwards |
| 20 m/s | 0.00 m/s | 100% | blown backwards |
Above 17 m/s the vehicle goes wherever the wind goes. That is not a failure of the propulsion, it is the ratio of a 34,271 m³ envelope to 8.3 kN of thrust, and the answer to it is the bow drogue rather than more power.
It does not slam, and the load is a bracket
This vehicle carries 550 kg of a 25 tonne weight on the water and floats on 10 mm of draught, so it comes clear of the surface on every wave in every sea state. That is the finding, and it is also why there is no single number for the load: rho g A is the restoring force of a float that stays immersed, and this one does not, so the contact is one-sided and the honest answer is a range.
| Sea state | Hs | Wave period | Ratio | Suspension load | Re-entry |
|---|---|---|---|---|---|
| 2 | 0.30 m | 3.5 s | 0.68 | 30 to 85 kN | 0.22 m/s |
| 3 | 0.88 m | 4.5 s | 0.51 | 54 to 247 kN | 0.21 m/s |
| 4 | 1.88 m | 6.5 s | 0.34 | 55 to 529 kN | 0.12 m/s |
| 5 | 3.25 m | 8.5 s | 0.26 | 56 to 916 kN | 0.08 m/s |
| 6 | 5.00 m | 10.0 s | 0.22 | 62 to 1410 kN | 0.08 m/s |
A seaplane slams because it is heavy
A floatplane is limited to about 0.3 m of wave because several tonnes have to be stopped in a hull length and the deceleration breaks things. This vehicle puts 550 kg on 56 m² of waterplane, which is 10 mm of draught. It comes clear of the water on every wave in every sea state, and re-enters at tenths of a metre per second.
That is an order of magnitude under a seaplane, so it does not slam. It is not nothing either, and the impact pressure goes as the square of it.
So stiffen the suspension, not soften it
Vibration isolation says soften the mount to put the natural frequency below the forcing. Here softening lowers the coupled mode and walks the resonance up the sea state table, towards waves the vehicle will actually meet.
- 0.05 MN/mresonates at 2164 mm
- 0.10 MN/mresonates at 1171 mm
- 0.30 MN/mresonates at 510 mm
- 1.00 MN/mresonates at 280 mm
- 5.00 MN/mresonates at 202 mm
A soft suspension puts the resonance in a real sea. A stiff one moves it down to about half a metre of significant height, which is a slight sea rather than a ripple: there is no stiffness that puts it somewhere the vehicle will never go. This is the correction that mattered most. Treating the envelope as ground, on the grounds that its inertia is far larger, gave a resonance at 26 mm and the conclusion that the sea never enters. Whether a body acts as ground is set by its inertial impedance against the stiffness connecting to it, not by a mass ratio, and by that test the envelope is a nearly free mass the suspension drags along.
Where it can actually go
Everyone answers this with the side area: 2,705 m² of sail against a few tonnes of displacement, so obviously it cannot make way. That reasoning is wrong by a factor of 40. BOW ON THE HULL IS NOT A SAIL: the complete vehicle's drag coefficient is 0.045 on volume to the two thirds, an equivalent area of 49 m². Beam-on it is 1,949. The vehicle that cannot make way is the one lying across the wind, and one with enough tail never is.
Fixing the flight stability fixed the boat
The tail on this vehicle grew from 405 m² to 825 m² when the yaw stability check was corrected, and the marine consequence was not the point of that change. With the small tail the propulsors had to hold the heading and had nothing left to drive with. With the corrected one the fins weathervane it unaided and the whole installed thrust goes into making way.
One part decides whether boat mode exists at all
Holding a heading and travelling along it are different things. At an angle to the wind the envelope makes an enormous side force, and a hull sitting centimetres into the water resists almost none of it. The sensitivity to immersed lateral area is brutal and then it saturates.
| Immersed lateral area | Leeway at the beam | Usable cone from dead upwind | Upwind speed |
|---|---|---|---|
| 0.5 m² | 34° | 20° | 5.6 m/s |
| 2 m² | 27° | 45° | 5.6 m/s |
| 5 m² | 21° | 65° | 5.6 m/s |
| 10 m² | 17° | 180° | 5.6 m/s |
| 18 m²fitted | 14° | 180° | 5.6 m/s |
| 30 m² | 12° | 180° | 5.6 m/s |
| 50 m² | 10° | 180° | 5.6 m/s |