Jet drive trolling motor? Has this been done?

A propeller dragged behind the boat will create a huge drag at higher speeds. You should design it to trim out of the water when underway. As an engineer, you understand optimization of systems. This route is not optimal, but complicated and expensive. However, if you are looking at it like a hobby, it will keep you busy for quite a while (note: this is not sarcasm, we all have our quirks). Also, the shaft with a propeller sticking out the back may interfere with the outboard at maximum turn and will definitely get fishing lines snagged. I think that if you are looking for a new design, modifying one of the outboards may be an option. Have some way of decoupling the engine from the shaft and turning the shaft with an electric motor while trolling. That would also make steering much better.

 

the option of a folding propeller might solve the drag issue? but I do very much appreciate the quirk. Still chuckling a bit. Using the tunnel in the bracket I could still conceal the the prop from any lines and tangling. I think I have found a foldable prop in the 10 in diameter range

 

Sounds like that 9.9 was your best option.

We have thousands of boats here that troll for salmon and tuna. They all use either the main or kicker, it's just the easiest and most cost effective way to do it.

I should say that we do use electric trolling motors in some fisheries. I use a Minn Kota Terrova iPilot when possible, but at higher speeds it doesn't last that long. At 1-2 mph or so it will last 8+ hours, but that's on a boat much smaller than yours.

 

The reason you're having difficulty with the jet concept, is a lack of understanding about the physics and engineering involved (no offence intended). Lets try a different approuch. Go grab a common, homeowner grade 3/8" drill, a 1/4" length of all-thread, two nuts and two pieces of 3/4" square wood (one 3" long, the other 6" long). Make the all-thread 4" long and spin a nut on about half way down, then measure carefully and drill a 1/4" hole in the center of both the 3/4" square stock pieces. Slide the shorter wood piece on the all-thread and spin the other nut down, to lock it in place. Now, spin the drill at speed, which will be about 1,200 to 1,500 RPM. Note the sensation and drill speed, which will have a little vibration, but not bad and the drill will easily spin the 3" length of wood without bogging. Great, now replace the 3" wooden piece with the 6" and spin her up again. Note the vibrations and RPM drop, which will be significant enough to make holding it in your hand difficult and the drill motor is bogging, having lost a good bit of RPM too.

This is why engines, motors and props or impellers are matched for duty service, which I think you'll understand. Now you can decrease the size of the prop/impeller (the wooden piece in this example) and the RPM's come up, but you've also lost area to bear against the air (in this case, which is about 750 times less viscous than water), so you've lost torque (work). You can change the gear ratio, so the prop spins at a reasonable rate, but now the engine is out of it's effective operational range and it's bogging or over revving, neither of which is good. Okay, so you change the prop pitch to get more or less bite, but again it's a dog chasing it's tail thing. The problems are; piston speed (internal reciprocating) or armature speed (electric), gear ratio, prop pitch and prop diameter.

For a jet to work efficiently, it must be spun up quite fast, just to move enough flow (water or air) to get the thrust you need. This is just the way they work, much like jet engine on an air craft. It has to suck enough volume, compress it, then spit it out with enough torque (thrust in this case) to do the work. Spinning it slower or changing pitch and diameter on the impeller, does have an effect, but it's hell to pay, once you drop below (or above) certain speeds and diameters. The operational range over certain speeds can be fairly wide, but below, they just don't do enough work, because of physical limitations built into the process. This is why piston powered, propeller driven aircraft max out around 500 MPH. The same set of physical laws come to play, piston speed, gear ratio, prop pitch and diameter have limits and above and below these ranges, the performance envelop drops off a cliff. Now a jet aircraft can easily do 500 MPH (hell 4 times this plus), but this is the max for a prop aircraft. Try to land a jet at 60 - 80 MPH and you're falling from the sky, because the jet can't produce enough volume through it to keep the thrust up enough to fly, so they land at 120 - 180 MPH instead, but a prop aircraft easily lands at these low speeds. Try to land a prop aircraft at 120 - 180 MPH, which it's easily capable of attaining and you'll likely ball her up in the middle of the runway, because she's way outside of normal (efficient) operation ranges.

This is a round about way of "Cliff Notes" for the basics, with much left out. In a nut shell, a jet needs lots of volume to work, even moderately well, but has great potential at higher speeds. A prop has considerably less and max's out well before a jet and there's no easy way around the basic physics.

A straight shaft setup (reciprocating or electric) will do and you'll have many more options, but (again) the same principles apply (hyrdo and aerodynamics are the same, just different viscosity fluids). A straight shaft will need some steering issues addressed, but is simple and very common, if draft concerns aren't a problem. A tunnel can fix this, as can an outboard on a jack plate or adjustable bracket. Forget about the two strokes, as at low speeds, these just don't do well. A 4 stroke will do much better, though if you operate for hours at 2 MPH, it'll likely foul plugs unless it's a new FI, CC outboard, which adjusts the mixture on load and speed. I'd recommend you use a "high thrust" version, because these can operate at low RPM's for extended periods, without fouling or eating plugs.

An electric outboard is an option and a good one, though range is an issue with most, on a boat of that size. In the end, besides the "exercise" of this exploration,you need to bake the decisions that will eventually force your hand in the products, engineering and approaches you must take to get reasonable targets ticked off your SOR. As we work around the "design spiral", things just get naturally "discounted" so you move to another possibility or option. This is only practical with a well defined SOR set of targets.

 

THANK YOU PAR

Moving from jet to the straight shaft option. Again SOR can be covered and I have not given up on electric yet. Straight shaft drive with the right propeller will work. Its deciding if the drag placed on it under full power is worth it or do I have enough room to tunnel a folding style prop