2-4 elec. 30-36lb thr, what can it move?

Discussion in 'Outboards' started by mrdc, Mar 23, 2011.

  1. mrdc
    Joined: Mar 2011
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    mrdc Junior Member

    hi

    looking at moving (trolling) a large boat with 2 or 3 or 4 elec minn kota
    ehgines with 30-36 lbs of thrust.

    i haven't bought a boat or the engines yet, just want to know what
    is possible.

    my idea is can these engines move say a 24ft power or sailboat?
    obviously i'm not interested in speed, just wanna get out on the water.

    thanks
     
  2. PAR
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    PAR Yacht Designer/Builder

    You can move a battleship (no kidding) with those motors, but you're not going to be especially quick and range (like always) will be an issue.

    The problem with all electric trolling motor setups is range. To have range, you need storage capacity, which requires batteries. Current technologies have batteries that don't store enough and aren't light enough to offer much range, particularly compared to a gas outboard, for example.

    Again the problem is the weight of the electric storage. If you have enough batteries aboard to provide sufficient range, then you can do the same thing with an outboard for a fraction of the weight (and cost).

    If you insist on electric propulsion, then there are some (costly) options, but don't get to excited about it until you look at the complexity and budgetary considerations.
     
  3. mrdc
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    mrdc Junior Member

    my idea was to take her in and out of the marina and onto the lake, point her in the right direction with a gas 7.5 merc, shut it off and let the elec. motors take over,
    turn around later and return, if the batteries die on return, i have a 7.5 gas to get me back.

    plausable?
     
  4. PAR
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    PAR Yacht Designer/Builder

    I use a 12 VDC trolling motor to move my 23' daysailor around. I don't need to do much more then clear the launch ramp and marina. It's a 2,200 pound boat naked and with a single group 24, with 500 CCA (45 pounds worth of battery) I get a little less then 2 hours at 3 knots or 3 hours at 2 knots. This is enough to bring me in if the wind dies, though it's painfully slow, even for a sailboat. I do keep a second battery aboard, in case I'm really far out and the wind craps out, for a total of 90 pounds, just in batteries alone. Once you count the cables, switch and trolling motor, it's easily more then a 2.5 HP kicker and fuel tank, which BTW will have a lot more range for less weight.

    Calculate about 75 pounds of thrust in trolling motor umpf, for each HP you need to push your boat to what ever speed you desire. Lastly, the props used on trolling motors are designed to push a Jon boat at trolling speeds (about 3 knots). You can experiment with different props, which is always fun, but more speed means less range (higher battery draw).

    In the end, if you dedicate 8 batteries, say 4D's (for example) and hook up a 24 VDC motor to a shaft (and prop and controllers) you could actually have a serious setup, capable of all day motoring in silence, though what to do with 800+ pounds of batteries, the 70 pound motor and the 40 pounds of cable, controllers, etc.

    Buy a good deep cycle battery (or two) and try it.
     
  5. mrdc
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    mrdc Junior Member

    great reply par, thank you.
     
  6. PAR
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    PAR Yacht Designer/Builder

    I once built a little gas power generator. It was a chain saw engine that drove an automotive alternator. The idea was to use this to charge up batteries to extend the range of an electric motor propulsion system. It worked, though there were issues (naturally). Noise was a problem until I built a box, then I had cooling issues so I used a small fan, eating into my electricity production rate. Then the amount of time it took for the alternator to charge up discharged batteries.

    My point is there are ways, if you're the type that can tinker. If not, bring a big check book.
     
  7. pistnbroke
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    pistnbroke I try

    if the alternator is the same voltage output as the batteries ie 12 or 24v then with a 65A output you can charge the batteries and run the min kota.... well on a car you charge the battery and power the lights dont you .......???/

    often these small 12v 36 lb run better on 24v with no problem ...study the min kota catalogue tech page
     
  8. eric le marin
    Joined: Jun 2010
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    eric le marin naval architect

    6 electric trolling motors on a 27ft sailboat ?

    Hello,

    did someone actually tried the 4x36lbs setup ?

    I am ready to start an installation with 6 of these cheap motors, but I still wonder if I try to find bigger motors (80lbs), or if I run the 12V motors in 24V, as suggested above.

    My boat is an aluminium oceanic sailboat of 27 ft, weight approx. 2,5t.
    I have a 15hp outboard : it took me along the african coast, around spain, and along the norwegian coast, facing strong winds easily, pushing my boat at 8 knots on calm seas, but it makes a hell of noise.

    And every sailor will tell you that when you have a nice evening on the fjord, and the wind dies, the last thing you want to do is to sart a noisy petrol engine. The max speed with two guys paddling (hard because of freeboard) is around 2,5 knots. the range is very limited ! ;-D

    So, since the season is starting, I want to try an electric setup, just to keep the serenity of the evening, but I search for the best ratio result/expenses.

    I get the 36lbs motor (salt water) for approx 150$ in the local shop (biltema), and this is the best ratio power/$ I see out here (it proved impossible to import the cheap 80lbs motors from ebay in Norway)

    Question 1 : can I run it in 24V ? (someone tried ?)
    Question 2 : don't you think that having 6 screws will give a better efficiency than having only one ?
    Question 3 : if anyone tried it before ?

    PS: For those who would like to point out the wheight of batteries, I don't care 200kg. I load much more when I go offshore. I currently have 208 Ah in 12V, but I will probably double it.
    PS2: the torqueedo motors and big minn kota are too expensive.
     
  9. pistnbroke
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    pistnbroke I try

    yes many have run the 12v on 24 with no problem provided you adjust the prop so it still draws the same current,* ie twice the power

    Think of a boat with two outboards ...you dont go twice as fast as with one and you dont match the speed of a single ob of the total power ..and you must adjust the pitch of the props on the twin set up to get it to work ...


    so 4 or 6 is a crazy idea ..costs a fortune and the battery current ??

    I have seen electric boats on lakes with two min kota thrugh the floor each side of a central seat ..the controllers made the arms of the chair ...I would not go beyond 2
    so
     
  10. johneck
    Joined: Nov 2011
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    johneck Senior Member

    The key question is wind drag. If you are in a place where there is usually alot of wind, that will be a limiting factor.
     
  11. whitepointer23

    whitepointer23 Previous Member

    is there a chart available which shows thrust of different size outboards. so you could look up the thrust of say, a 2hp outboard to give a starting point for comparisons to electric. i get confused because petrol is measured in hp and electric in thrust.
     
  12. Stumble
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    Stumble Senior Member

    1hp is roughly equal to 60 pounds of thrust from a trolling motor. They actually measure different things, but that will get you close.
     
  13. whitepointer23

    whitepointer23 Previous Member

    thanks stumble.
     
  14. FAST FRED
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    FAST FRED Senior Member

    I always rule of thumb 20lbs of thrust from one gas or diesel HP..
    25 of push on a fantastic setup.

    Why would an electric driven prop be 300% more efficient?

    Are you not counting about 50% loss of most prop's?

    FF
     

  15. johneck
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    johneck Senior Member

    The props are very lightly loaded so they can get much higher thrust per HP.

    Efficiency is not a very good way to measure the performance of a thruster since efficiency is based on work which is based on moving something. In this case the efficiency may well be close to 0, since no real work is being done.
     
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