Jet drive trolling motor? Has this been done?

Discussion in 'Hydrodynamics and Aerodynamics' started by Maverikk, Jan 4, 2018.

  1. Maverikk
    Joined: Jan 2018
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    Location: Lake Erie North Shore

    Maverikk Junior Member

    Okay guys and gals here is an idea and I am looking for honest answers and math not that it is a stupid idea.
    I am a mechanical engineer by day but true passion lies in fishing.
    I have a 25 foot pursuit center console that has a pair of 150s on the back so I really dont want to hang a kicker on the back also. I built a custom Armstrong style bracket for the back and left a tunnel in the base for the idea. I want to install a seadoo style jet drive in the bottom for forward propulsion. steering is not a super big deal because I can use the 2 main screws as a rudder style set up. my question is can I power it with an electric drive on a variable speed dial and can i run it at slow enough rpm to create thrust. Most of the stuff is in 20+ feet and always fresh water. I just really dont want something sticking out the bottom since it is a trailer queen. I am looking for sustained 1.5 to 6 mph in a 8000 lb boat.
    Like I asked has it ever been done? pros and cons and is it even possible. It is only ever in fresh water and I have already got plans on replacing the transom.
    Second question is will I get interference form the electric drive on the fish finder transducer. Noise is not really an issue and the drive would be under the bracket so no tangled fishing lines or other such complications.

    Thoughts and honest opinions appreciated
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    [​IMG]
    OR consider an electric motor mounted on the ventilation plate of one or two of the outboards. A seadoo jet is designed for high speed inlet water velocities. You will get some thrust but the electric current usage might be high for a given amount of thrust at low speeds
    Additionally, you will not need to cut an intake or outlet hole for the jet in the hull.
    Probably a lot cheaper as well
    a
     
  3. Maverikk
    Joined: Jan 2018
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    Location: Lake Erie North Shore

    Maverikk Junior Member

    BARRY
    Thanks for the honesty. looked at these systems and the problem is I would need the twin version times 2 to get the desired speed out at 8000 lbs in current. And the 48 volt battery requirements are super heavy and take up a ton of room. Love the idea just wondering if there is a low rpm low speed application jet drive or am I completely off my rocker?

    Please any more feedback or suggestions are welcomed.

    ALso have a few options under the bow mount category for steering aid but again that hanging more stuff on a boat.
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Seadoos have engines of 230 to 300 HP. That is the same as the pair of outboards you have added up. You can troll on one engine and save a lot of money. An electric motor that big, and the batteries to run it will probably sink the boat. Further, outboards use thrust to steer. If you want to steer with outboards that are not running, the drag of the propellers are not really effective.
     
  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Just put a trolling plate on one of your engines; no need to recreate the wheel.
     
    Barry likes this.
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    At the Seattle Boat show two years ago, a company was marketing trolling plate mounted on the ventilation plate and actuated hydraulically from a trim tab pump, just push a button and troll
     
  7. Maverikk
    Joined: Jan 2018
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    Location: Lake Erie North Shore

    Maverikk Junior Member

    Gents, I have had trolling plates and trolling tabs. I have trolled with both main screws. One at a time and both at idle. Problem with trolling on the main screws if they are older 2 strokes they bog down. And at 30K each running hours up on the main screws seems counter productive.
    A brand new 9.9 hp or 15 hp installed and rigged properly with troll smart systems and that is north of 5k.
    The whole point of asking was not a cost thing or a a bunch or alternative suggestions is was to open a discussion on the feasibility of a small jet drive powered by whatever at lower speeds.
    A gentlemen posted earlier that the new sea doo were 200+ hp and I get it it that is what you are using it for. a 9.9 hp equivalent is 10 KW I can get that out of a 24 volt motor under 3kg. the whole system weighs about 20 lbs less then a kicker.
    So not to be an *** but nobody has answered the question of hydrodynamics or thrust. There is a company in Australia that makes an electric jet ski. I have seen electric and small gas powered surf boards. So I know the technology is available. I am just asking if someone that is way more knowledgeable and experienced than me can do the math to give me an idea. even if I installed the drive in the bracket........ it is strictly a low RPM vs thrust question and the possibility. Yes it will most likely be a one of and not something that anyone ever does again but as far as discussion goes not what I was expecting from such a diverse and knowledgeable forum. You are all smarter than this and although I have just become a member I ave been reading the stuff on here for a long time.
     
  8. ondarvr
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    ondarvr Senior Member

    Can it work....yes...is it cost effective and will you get the desired speed of 6mph, not easily.

    I'm not going to do the math, but you can easily buy an electric motor and mount it in the tunnel, jets don't typically work that well at low speeds, so it would be more like a bow thruster (prop in a tube), this would get you to 3 mph or so without much trouble.

    The biggest issue is the battery capacity to try and plow along a 6mph, you could get to that speed, but the battery draw is huge, so you won't be there for long. How much space are you willing to give up to batteries, and how much $$$ do you want to part with to buy batteries that are big enough, and enough of them to do it?

    Even if you went with a small gas motor you need something bigger than a 9.9 to move you at 6mph with all the gear out and a little wind and waves.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can find older 40 and 60 HP jets and this is what you'll need, with some reserve power to scoot past things and steering response, though as has been mentioned, jets need to operate at high RPM and the engines twisted up pretty good, meaning you'll be well out of the usual operating environment for this little engine and it's jet. To simply answer your question, about the feasibility of a small jet operating at low speeds, well it can be done, but nothing about it will be efficient (jet and engine) and over heating, bogging and fouled plugs will likely become close friends with this setup. Yeah an electric motor can be substituted, though weight climbs exponentially, defeating any efficiency gains, though you'll save on plugs.

    In the end, you'll need to decide how much you'll use this setup, to justify the inefficiencies it'll bring to the table. If you're on "station" for an hour or two, maybe worth considering as the battery weight will not be that bad, but if you need more time, well . . .

    Your sounder will not likely be affected by the usual run of electric trolling motors, though a custom setup may cause problems, until you learn how to shield things.
     
  10. Maverikk
    Joined: Jan 2018
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    Location: Lake Erie North Shore

    Maverikk Junior Member

    Thank you gents this is more of what i was looking for. Thought was to use a 36v drive motor similar to that in a golf cart for a marine application. whole setup with jet would be less than 70 lbs obviously without batteries. SO very similar to a 9.9 to 15 hp kicker. I have used a 9.9 4 stroke to get to 6 mph for fish such as muskie and trolled around all day.
    This whole thing started because of the plug fouling issue and bog associated with 2 stroke main screws that don't like the slow speed. I am basing this on the availability of stuff like the Honda 40 hp jet outboard and companies such as minn kota, torqueedo and Rhodan marine having 36v trolling motors and electric anchors.

    I get that inefficiencies of the jet but figuring an impeller change or something similar would have potential. if the need to run at higher rpm is the only drawback one could add something similar to a baffle or "trolling plate" to the jet or am I way off base. As much as I hate to say it I may just have to do it and figure the troubleshooting and solve the TOO Much thrust problem after the fact.

    I do very much appreciate the time guys are taking to answer this. Please keep em coming.
     
  11. ondarvr
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    ondarvr Senior Member

    What boat did you have the 9.9 on when trolling for Muskie? 6 mph with a 9.9 seems fast for trolling on a 25' cc with twin large motors unless the water and wind are calm.

    We run 9.9s on boats of that size here all the time, but at lower speeds, 6mph is wide open.

    I also have jet boats, so I'm very familiar with how they work at lower speeds.
     
  12. gonzo
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    gonzo Senior Member

  13. ondarvr
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    ondarvr Senior Member

    They don't have good thrust at low speeds, so while you can troll with them, the RPMs are high, and control is poor. And you'd end with a 20-30 hp motor hanging on the back.
     
  14. ondarvr
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    ondarvr Senior Member

    What are the main motors you have on it now.
     

  15. Maverikk
    Joined: Jan 2018
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    Location: Lake Erie North Shore

    Maverikk Junior Member

    had the 9.9 Suzuki on the back of the 25. Lake St. Clair and lake Erie. no issues most days. The motor would be about 3/4 throttle. 6 mph on GPS. troll for walleye between 1.1 and 1.8 troll for steelhead from 2.8 to 3.6 and muskie or salmon we push her up the speed index a bit. usually from 5 to 6.2 depending on the time of year.
    I have 2 150H.P. evinrude saltwater (fichts) on the back and currently have 5 deep cycle batteries on board mounted forward in the console seat at the front of the helm. I know that is way overkill on batteries but was working on centre of gravity for possible 36 volt system. on the scales with all the gear and loaded for a day and 3 guys in the 250 lbs range she is 10500 to 11100 on the trailer (all aluminum) full of fuel.
    So factoring a twin axle trailer at 1500 to 1800 lbs puts here in the 9000 lb range.
    She tops out about 42 to 45 mph on the twins at WOT.
    With the bracket the new coast guard rating puts her with a max HP of 500 even though she came with 300.
    Transom totally rebuilt and the stringers completely rebuilt and the addition of Coosa core replacing all the wood. I have a new console and a new t-top waiting to go on but power plant first.

    Since you guys have all but dismissed the jet drive how about a horizontal mounted straight shaft prop. What does it do with drag if i just let it hang straight out the back like sailboat? could power that with whatever!
     
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