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Thread: Backup motor tips

  1. #1

    Backup motor tips

    Hi,

    I'm new to boating. Are there guidelines for having a second motor? I have an 18' half cabin. Main motor is 80 hp. Previous owner said he had 60 hp.

    What motor size would be appropriate for second motor? I'm mostly using in the river.

    Was thinking of using battery trolling motor with 50-70 lbs thrust. If petrol what would be minimum hp?

    Thanks.

  2. #2

    Re: Backup motor tips

    Dakr i am likely going the same route, a couple of months ago i looked into the backup motor and found a 24volt electric motor would push my boat i found the Haswing brand had the cheapest motors without needing to spend 3-5k

    What i found out from searching the internet for my 18.7ft boat i need the following

    60lb of thrust would just get by
    70lb would be suitable
    80lb would be the best

    But what i found those motors are likely 12volt

    Once u go up to 100lb its 24volt from my understanding a electric motor will only go 10kph no matter its pound output capacity but as you go up in pounds of thrust once u reach the optimum level and start to go over your usable amps come down as thrust goes up

  3. #3

    Re: Backup motor tips

    I wouldn't bother if you're only in the river, but if I had to, an electric would be my choice.

  4. #4

    Re: Backup motor tips

    If you only want it as an emergency back up, maybe get an annual marine rescue subscription. saves all the weight and install hassles. Or you could then put the money towards a newer more reliable main motor, going by your previous post!

  5. #5

    Re: Backup motor tips

    as purely an emergency motor....you dont need a lot of hp because youre not going to be able to push the boat onto the plane anyway...so your max speed becomes a function of the waterline length and you will get no extra benefit from increasing hp from say 8hp to 15hp..There are however a host of other issues to consider....reliability ..cost..maintenance ...steering...security. separate fuel supply .Will it actually work when you want it to?

    Used to run a 2nd emergency motor on an 18ft boat...but nowadays you rarely see them..for good reason ....generally they are waste of time and money.Putting the dollars into a modern well maintained motor makes more sense.If you really were to need two motors for safety...split the main engine into two motors...and if your eyes start watering at the cost..thats probably the answer.

    A bow mounted electric motors might well be useful for river fishing...and provide some sort of back up..but to get something suitable that is really functional aint that cheap.I fitted an older one....freebie..onto the 18 footer..it works but just makes you wish it worked like a good one!

  6. #6

    Re: Backup motor tips

    Thanks all. Very helpful comments

  7. #7
    Ausfish Platinum Member
    Join Date
    Mar 2015
    Location
    Kalbarri, WA

    Re: Backup motor tips

    ^^^What Inveratta said. Auxiliary or backup motors are really a relic of the days of the old , less-than reliable 2 strokes. So many were ineffective anyway, due to poor mounting, unless it was flat calm. Not to mention the number of transoms dsetroyed by the mounting of the bracket. Manufacturers mount main engine properly, seal outbopard mounting holes the right way from new. Pig-ignorant DIY'er fishermen then decides he needs an auxiliary, obtains bracket, drills through transom to mount, "seals" the holes with silastic. And the rot begins. pet hate of mine, rant over. Electrics are a viable alternative nowadays, but not cheap.

  8. #8

    Re: Backup motor tips

    You can get some really good run times out of a 100ah battery if u pick your moter correctly, a 12v 60 pound motor will pull around 55amps, if u pick a 100-120 pound 24v motor it'll draw around 15amps for the same speed

    15amps 100ah thats a lotof hours of run time

    There are guys on youtube running watt meters on these motors

  9. #9

    Re: Backup motor tips

    Everything in perspective Gazz. Try running against strong wind and tide and you will run our pretty quick. Been there done that.

  10. #10

    Re: Backup motor tips

    1hp =55 to75 pounds of thrust its all prop dependent as most outboard props wont let a engine rev out at displacement speeds

    thats at 100 percent throttle on the electric, which will drain around 50 amps per hour
    so on a 100 amp hour lead acid battery you will get 1 hour of run time equal to 1 horsepower

    a small 4hp extra longd shaft with a sail boat prop would be the best option for actually being usable for the intended role
    or something like this
    https://www.yamaha-motor.com.au/prod...ur-stroke/t9.9
    https://www.tohatsu.com.au/node/1678

    a 55 pound unit is just sufficient on my 4.2m heavy tinny, Gazza you will need a 24v 36v 80-130 pound unit on your boat to be a useful tool and just not wasted money

  11. #11

    Re: Backup motor tips

    Quote Originally Posted by stevej View Post
    1hp =55 to75 pounds of thrust its all prop dependent as most outboard props wont let a engine rev out at displacement speeds

    thats at 100 percent throttle on the electric, which will drain around 50 amps per hour
    so on a 100 amp hour lead acid battery you will get 1 hour of run time equal to 1 horsepower

    a small 4hp extra longd shaft with a sail boat prop would be the best option for actually being usable for the intended role
    or something like this
    https://www.yamaha-motor.com.au/prod...ur-stroke/t9.9
    https://www.tohatsu.com.au/node/1678

    a 55 pound unit is just sufficient on my 4.2m heavy tinny, Gazza you will need a 24v 36v 80-130 pound unit on your boat to be a useful tool and just not wasted money
    Been in that situation when had problems with the outboard 300 m from the ramp on the way out.
    What has not been calculated by Gaza was the type of battery used and respective DOD. I have a 120 amp AGM on my 12V 55lb Leccy and while at anchor or pottering around for a couple if hours I then needed to get home, around 1km or so and running my 55lb on a 4.2 WB dory on setting 9 (1 short of 10 as way less power used) my DOD was reached surprisingly quickly and then I used my starter battery and it got to DOD basically as I got to the ramp.

    I have an inline power consumption meter so was aware of usage.

    Wasn't fishing far from the ramp and wasn't taking a risk as 2 other boats were out around with me and I could have got a tow in.

  12. #12

    Re: Backup motor tips

    On reflection...... firstly .good luck Dakr 123.. enjoy your time on the water....another bit of a hint...your first boat...have fun ..make mistakes ....we have all been there ..and we all do still not do things perfectly ..well every time....dont over invest in the boat ..use it...


    .in the end we probably arent saving you money ...just trying to help you spend it in the "right" places!

  13. #13

    Re: Backup motor tips

    Yeah how it works is efficiency, bigger output, bigger battery 1x12v apposed to 2x12v for 24v

    100lb thrust 24v at 1/2th throttle will still put out about 6.5kph @ 15amps

    60lb thrust 12v at full power 7kph @ 55amps

    120lb thrust would be even more efficient at less throttle

  14. #14

    Re: Backup motor tips

    Definitely need a 24 volt system as a minimum on that size vessel but unless you will get extra benefits from it (such as using it to fish), I would say save your money. Bow mount will basically be a necessity as a stern mount won't have sufficient push in any sort of breeze to allow effective steerage unless you are giving it the beans - which will result in lower run times. If you are in an area that is sparsly populated or has no VMR support, a small petrol auxiliary will be far more useful as an actual back up motor if that is what you are after. If you are in a populated area I probably wouldn't bother.

    That said, on my last boat (4.5 metre) I did use my auxiliary to get me out of an issue when I had a heap of fishing line tangled around my prop while night fishing in a high current area. Not wanting to risk falling over the back and disappearing into the night, I used it to potter over to an island to clear the prop. I had a 7.5hp but could never run it any more than about 1/3 throttle as it cavitated. It pushed the boat along pretty well though - 4 to 5 knots sort of thing.

  15. #15

    Re: Backup motor tips

    ppppppppppppppppppp

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