[PSUBS-MAILIST] "Exhausting" Post Script...

After reconsidering the comments of Dave, Alec, and Jon while on my way to work the other night, I wrote a post script to that thread the next morning. Thinking I'd emailed it to PSUBS properly, I deleted it completely from my files. It never turned up, so I guess I made a mistake; and I'm now going to do my best to reconstruct it, and respond to some of the other input I've received as well. Here goes...

"Breaking the vacuum": Dave mentioned that the line running from the thruster to the bilge in my JS-500 might be intended to draw air most of the time, and bilgewater only when present. I never thought of it that way, but that's exactly what it does. And this sure makes it look like there really is a need to provide some type of vacuum relief at the thruster. Hmmmm. Got me thinking about it now!

"Chopping off the tailcone": Jon mentioned the VASH BIONIC DOLPHIN was so-dissected to act as a "flow spoiler" that would relieve backpressure in the engine by engendering a motion-flow-relative area of reduced water pressure aft of the hull into which the exhaust would more readily flow. There is no doubt that spoiling flow creates a partial vacuum, but I initially wondered whether that partial void would be filled by exhaust or water. After thinking more about it, I believe Jon is right: exhaust would be more prone than water to fill the void for a number of reasons, including: (1) gas molecules are lighter in weight than water molecules, and thus more apt to move into the void under the coercive force of a partial vacuum; (2) the exhaust would be injected directly into the low pressure zone; and (3) the pumping action of the engine (as well as any additional compensating air pressure derived of tank storage) would assist the exhaust extraction process. So yeah, Jon; I see what you're saying, and I think you're right.

In comparison, the "SKIPJACK"-shaped design I'm working on now (actual designation is HYPERSUB, short for HIgh PERformance SUBmersible) is a "lumbering whaleshark". Looking at the 35 knot submerged speed the DOLPHIN achieves using a 6-cylinder engine and a radical screw (YIKES!!!), I can see my one-ton steel sub isn't going to be nearly as fast as I thought it would be using a jetski drivetrain. I'd be lucky to get 10-to-15 knots. This and other factors has me reconsidering the propulsion source at this time.

At this point I'm thinking a diesel or propane-fueled automobile engine would provide more power; and also be better suited for use with a caustic soda exhaust scrubber in a closed-circuit system, as the combustion by-products are CO2 and water vapor: much cleaner than the oil-and-gasoline mixture burned in the two-stroke jetski. I might add here that the concept of this kind of closed circuit engine is not new, and has been used with great success in submarines of all sizes for more than 50 years.

About using flow-reduced pressure zones to enhance exhaust functionality: while the shape of my hull would aid this process somewhat, my real intent was to inject the exhaust into the thrust column of the hydrojet itself, just aft of the nozzle. The shape of the hull will accelerate water to ram the jet intake; and also lower the pressure around the hull tailcone area. But also injecting the exhaust into the thrust column itself, through a flattened vertical "balogna-sliced" pipe which would produce minimal drag interference with the thrust jet, while creating a "spoiler effect" and subsequent downstream low-pressure zone of it's own; would work on the same principle as the "bobbed tail" of the DOLPHIN, but (I think) be more efficient over a wider range of cruising speeds because it is not dependent on the comparatively slower relative-flow rates generated by vehicle motion through the water; but instead, makes full advantage of the higher flow-velocities present in the thrust column ejected from the nozzle itself. Also, there would be a direct correlation between power output and the amount of low-pressure generated: the higher the engine power setting, the greater the thrust and simultaneous exhaust extraction enhancement. All factors of the equation could work in near equilibrium over a broad spectrum of power settings. But again, this concept is not new: torpedoes have been built which exhaust through their counter-rotating propellor hubs in much this same way.

The question of lost engine RPM between a 4-stroke and 2-stroke engine came up; and yes, the 4-stroke would probably turn at a slower speed, and not be as suitable for use with a hydrojet designed for higher RPM. But then, why not change the whole system? A 4-stroker at slower rotational speeds might yield great horsepower and torque; which, if coupled to the right prop, could generate more thrust. When we're talking about boat-drives, the pitch of the screw is extremely important. A strong engine turning a radical screw is a proven source of great power.

As an illustration: say we're trying to drive two wood-screws into a 2 X 4 with the same electric drill; a "course thread" screw whose threads make, say, five complete turns over the length of the shaft; and a "fine thread" screw which makes 20 turns over the same length. As long as the drill motor has the power to turn both screws easily, the coarse thread screw will bore into the lumber faster (less rotations over a given distance; or more distance per rotation) than the fine thread screw will. By the same token, a "radical" (coarse pitch) prop will "bore" into the water faster than a prop possessing a lesser pitch angle.

Hydrojets with nozzles are great; I'm not knocking them. But a radical screw is the prop of choice on most high-speed hydroplanes I've seen. I don't want to get sidetracked in a discussion of which one is better, because it's kind of like comparing apples to oranges. But either way, as long as the power source and thrust device are well matched, one can expect reasonable thrust performance. Conversely, if they're not, you won't. (You can have a strong engine and a wimpy prop, and you're not going any faster than that prop can accommodate; and vice versa.)

So what I'm saying here, in response to the query about 4-stroke versus 2-stroke engines, is that the relative merits of both need to be considered; and ultimately, the thrust device must be compatible with the power output and RPM of the engine. Right now, I'm leaning toward 4-strokes for reasons of cleaner combustion and "exhaust scrubbability". But there's a lot to be said for a hi-RPM 2-stroke screamer pushing a direct shaft drive, too.

In a closed-circuit engine, the question of endurance is important. How long will it run underwater on a limited amount of stored air or oxygen is relative to engine displacement and horsepower? Generally, the bigger the engine and higher the output, the greater the consumption of air-fuel mixture per horsepower generated. I think the main factors to consider in selecting an engine are displacement and the production of useable torque over the optimum range of RPM-required power settings. A big engine that produces a lot of torque at lower RPM might use less oxidizer while producing great thrust when geared to turn a prop at high speed. Then again, a smaller high RPM engine might use less oxidizer when turning an ungeared prop or jet, and yet still generate a goodly amount of thrust. So the choice of components in a closed-circuit engine propulsion system is effected by many variables, and they must all be considered before an educated choice can be made. But closed-circuit engine propulsion in submarines having respectable underwater endurance is not only possible, it is being done every day in the subs of countries who use the Stirling AIP system and the like.

And Paul, you mentioned "aluminum powder and salt water" in terms of an advanced propulsion means, but didn't explain what that was or how it works. To me, this sounds like it might be some kind of fuel cell for the generation of electricity to drive a motor? At any rate, it's a new one on me; and I'd appreciate it if you'd either expand on the concept a little more here, or steer me in the direction where I can learn more about it.

One other thing I'd like some help with: Jon gave me a lead to the company that produced the F-16 canopy used on the DOLPHIN: he said it was TexStar. I've tried pulling up the URL without any luck. I searched, but the only company of that name doesn't do this kind of work. I've got a copy of TRADE-A-PLANE coming to me; but in the meantime, does anybody know where I can buy a canopy from an F-16 or the like?