Question for Pat- "SKIPJACK" exhaust: reply...

Yes, I'm putting an engine-driven hydrojet (jetski drivetrain or the like) into a drysub. It's a concept I've been researching since the mid-80's, when I first built a working test model of a closed-circuit engine system; but it got sidetracked when my main focus became the NAUTILUS MINISUB project. Since then, I've resumed work on the "squirt-drive" idea and have a basic hull and a JS-500 at this time. I still call the project by its original designation HYPERSUB (for HIgh PERformance SUBmersible).

Because the first hurdle seems to be getting the drivetrain installed, and making sure it will work without leaks, the idea is to do it as a "momentary immersion" AND snorkel sub. On the surface, this "open system" will take in ambient air through a snorkel, and exhaust externally, underwater at shallow depth. Submerged, the snorkel intakes close and the engine is aspirated from onboard pressure-tank storage. The design allows for a lot of tank storage, and if it only enables the engine to operate at shallow depths for limited times, it will serve the purposes of experimental testing.

I'd planned to vent the exhaust aft of the hydrojet, and into the thrust column itself; the idea being that, in accord with Bernoulli's principle, the increased fluid velocity in this zone should engender reduced water pressure; thereby reducing back-pressure resistance that might choke off the exhaust, and thus minimizing the amount of compensating air pressure I'll need at the carburetor intake. Also, the turbulence in this area would disrupt exhaust resonance sound waves, thereby muffling the engine noise to some degree.

The shape of the hull is a teardrop spindle, terminating in a "stinger" at the tailcone; so, the motion-relative hydrodynamic flow over the hull will accelerate as it moves aft from the maximum diameter zone, thereby "ramming" water into the hydrojet intake, while simultaneously decreasing pressure further aft in the area of the hydrojet thruster and engine exhaust. Thus, by something akin to "hydrodynamic venturi effect", I hope to improve exhaust functionality while reducing dependence upon depth-compensating pressure from tank storage to the engine intake.

The boat will also carry at least one DC electric motor-thruster system enabling conventional operations underwater with the engine off and the intake / exhaust valves closed.

If the "open system" works, I'll modify it to a "closed system' wherein exhaust gas is circulated through a heat exchanger / scrubber, and returned to the engine compartment where it will be enriched with oxygen. On the surface, the boat will aspirate through a snorkel and vent overboard; submerged, the intake and exhaust are closed and routed as described above, and will run in a closed loop. The advantage will be in increased submerged engine run-times, because I can carry 5-times more oxygen than air in the same set of tanks. Cryogenic storage of the oxidizer would also improve this capability, but that's another step down the line. I'm also looking at oxygen-generating methods; but again, that's further off in the future.

Before I go to the closed-circuit system, I'll need to replace the two-stroke jetski engine (which burns a noxious mix of oil and gasoline) to a four-stroke that can run on either propane or diesel (which give off a non-toxic exhaust of water-vapor and CO2, and are more suitable to cleansing through a caustic soda scrubber or the like). In fact, I'm looking for a four-stroke jetski right now, which is a primary reason I haven't cut the hull and installed the JS-500 yet. I COULD get her running using the two-stroke, and convert later to a four-stroke; but in the long run, it will be less labor-intensive to install the one I'll end up running, so I'm shopping for a better drivetrain than the one I presently have.

Another fuel source worth thinking about is ALCOHOL. It burns clean, and requires comparatively little oxygen: big advantages for submarine propulsion. Good enough for a torpedo, good enough for me! ;-)

I've had some questions over the fact that jetski's are extremely loud; and being bottled up inside a tank with one is a sure way to lose one's hearing. It's all a question of how the exhaust is routed and muffled, I think. The ski is only loud when the exhaust soundwaves exit into the air without being disrupted; they can be muffled mechanically, and disrupted in the thrust column. I don't think engine noise will be more than can be compensated for with a set of earplugs.

As far as the VASH project goes; where can I find a picture of it? I doubt I'll be cutting the stern off my boat; I think unrestricted hydrodynamic flow, "aftwash" and the reduced pressure engendered thereby will do more to

extract exhaust than chopping off the tail cone would. In fact, if I understand the concept correctly, it seems such a dissection would actually retard flow, impede the exhaust function, and increase hydrodynamic drag on the hull.

If you've ever owned a van, you know that as it moves through the air, the abrupt and un-aerodynamic shape of the boxlike body creates a partial vacuum behind it: all manner of road dirt accumulates on your back doors; and if the door seals are poor, exhaust fumes can be sucked into the passenger compartment.

If you've ever skydived, you know a body falling stable at terminal velocity creates a partial vacuum behind the jumpers back that extends upwards approximately seven times the width of his shoulders. Approaching from above, one wants to avoid this zone, lest he lose the cushion of air he's flying on, and drop onto his unsuspecting buddy's back. (A sure way to get excluded from the next RW dive!)

Or, consider the physics of a camber-shaped object like an aircraft wing as it moves through the air: the trailing edge produces "downwash" and the aft wingtip generates a vortex that spirals inward toward the wing root.

Similarly, since my hull is a teardrop (essentially a cylindrical camber) the hydrodynamic motion-relative flow is going to meet with some initial drag at the nosecone which will increase as it proceeds to the area of greatest circumference; and then, as it moves aft over the tapering cone, it will accelerate and decrease in pressure.

If I chop off the tail cone, the flow will create an area of reduced pressure aft of the flat tail-plate. Whether that will enhance exhaust extraction or pull the hydrodynamic "aftwash" and surrounding water into that area is the question. At first glance, it seems like it could do more harm than good; whereas I already know that, if I terminate the hull in a cone and/or "stinger", the accelerated flow will reduce pressure in accord with Bernoulli's principle, and that definitely will enhance the exhaust extraction of gasses vented in that area.