Electric outboard conversion

Discussion in 'Propulsion' started by vaderverdi74, Feb 20, 2012.

  1. vaderverdi74
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Balcan

    vaderverdi74 New Member

    Hello there all,
    I am converting a old johnson 25hp 74' outboard motor to electric outboard motor with etek 48v. I use it on boat which is semi displacement (WL around 4.2m) weight around 500kg (or more) with cabine.

    I have made some testing and get a result as for 48v around 90 amps i got top speed very low as a 3 knot or around 5.5 km/h with original propeller.
    I was wondering how can i improve speed, first thing was changing propeller but to what bigger diameter ok, bigger pitch since i think original it was designed to operate to 5000 rpm or more, and i with etek do around less than 3500 rpm.
    So what is your suggestion to put two pulleys so to get more rpm on shaft or change to different propeller and which one.

    I was thinking to make diy propeller just for testing not actual using so i can see which pitch suites me best, but i know very low on propellers, and i do know it is complicated.

    I also work with polyester and sheet metal so i can form some but i dont have clue to which form.

    At moment I have on boat 4.5 gasoline with real on shaft less then 2.5hp around 5.8 kn kouts.


    Any suggestion idea, invision is more than welcome.
    Thanks
     
  2. CDK
    Joined: Aug 2007
    Posts: 3,288
    Likes: 139, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    You are generating 6.8 HP with the present setup. That should give enough push to reach hull speed plus some reserve. Because the hull is only 4.2 m. long you won't get much more than 3-4 knots no matter what you.
    The prop was meant for 25 HP and planing speed, it will not be very effective at the speed you achieve now. Try a prop with a larger pitch, not to get more speed but to reduce the current so you can extend the range.
     
  3. vaderverdi74
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Balcan

    vaderverdi74 New Member

    I am novice about all prop stuff, i was planing to go with pulleys and prop rated to 10 hp which is same like mine but has less surface area, because i get lots of thrust i think at current setup it is way too high for thrust.
    I am not looking to improve speed a lot just a little bit, to max 5-6 knouts no more.
    http://www.evalbum.com/492
    This guy built same almost like me and did greater in speed than me, prehaps diffrent boat, i think it is almost flat bottom maybe.
     
  4. CDK
    Joined: Aug 2007
    Posts: 3,288
    Likes: 139, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    I'm sorry, but you cannot reach that speed. The link you supplied mentions a 26 ft hull, your is only 14 ft. so at 6 kn it lifts the bow creating lots of drag.

    You give your location as "Balkan". If you are near the coast, observe the Trajekt (ferry), doing 24 km/h in displacement mode, while small craft must plane to go that fast.

    There are no belt drives or gearboxes without loss on this planet, so don't use any unless there is no other option.
     
  5. vaderverdi74
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Balcan

    vaderverdi74 New Member

    Yes i am near coast, like you on adriatic sea,
    well i tried and on this web page with 4.2 meter on water lenght with displacement of around 800 kg, i get hull speed of 9.28 knouts, and to go 5 knouts says around 4 hp.
    Have i mistaked somewhere?
    Please do help
     
  6. CDK
    Joined: Aug 2007
    Posts: 3,288
    Likes: 139, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    This boat speed calculator, using Gerr's formula, predicts approx. 6 kn. with 6 HP input:
    http://www.psychosnail.com/boatspeedcalculator.aspx

    Bear in mind however that this is purely theoretical; there is a considerable error margin, especially for small hulls. But let's assume that your boat has the ideal hull and you can reach 6 kn. at full power.
    You need 8 batteries 12V/100Ah to draw 90 Amps for one hour. The weight is approx. 200 kg. so your estimate of 800 kg. for the boat, batteries and yourself is probably on the mark. But at full throttle the range of 10 km. is a bit disappointing, at least that is how I would feel about it. So you need more batteries and/or a generator, increasing the gross weight to more than a ton and shaving one knot from the top speed.

    I would settle for a lower speed and try to optimize the propulsion.
    You need an Ammeter, power controller, rpm indicator and a GPS, so you can reliably plot a graph. If the functions of rpm and speed are linear all the way (that means a straight line) the prop doesn't have enough pitch. With the right prop, the line should start to curve near the end.
    Also check the relation between current drawn and rpm. That should also be more or less linear with an offset at the beginning of the line, showing the power lost in the transmission. Use a thinner oil in the lower gear case: the temperature without exhaust gases is much lower.
     
    1 person likes this.
  7. michael pierzga
    Joined: Dec 2008
    Posts: 4,865
    Likes: 114, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

  8. vaderverdi74
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Balcan

    vaderverdi74 New Member

    Postovani CDK,
    i allready have 4 batteries 12/100ah and for 1 hour is 64amp's, for 48v at 64 amps gives me around 3kw or 4hp, for top speed is just like runway when needed, but my goal is to travel around 5 knouts for travel time of less than 1 hour. Boat is rated usually around 500-600kg, so batteriers 30x4=120, gears lets say around 60 kg, outboard maybe 35kg (didnt measure), 2 persons 160kg so in total lest say 1000-1100kg. And i have all items to measure (amp meter via shunt, volt meter, and android with gps:), controller i am making in the moment)

    I saw that page and especialy Terapin great project, i was thinking to make prop just for testing, with larger diameter(13"-14") with different pitch's, maybe it will improve to some point and i will do that with extend of shaft so prop could little bit higher than cavitation plate, and after i surround prop with aluminium ring to improve eff.

    I have just found book of dave gerr reference of props, just started to reading it, cant wait.

    thanks for helping me :idea:
     
  9. nevada14
    Joined: Feb 2013
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: France

    nevada14 New Member

    Out board motor conversion

    I have read with interest this conversion. I beg your pardon as i dont write english very good. I have a 5.20 m boat with a 6hp johnson converted with a 500w 24v electric motor.
    With 27 A (648w) the boat run 5.5 Km/h.
    Boat 500 Kg
    Batteries 200 Kg
    Crew 200Kg
    With a 10 HP Johnson (Gasoline) we can run at 11 Km/h
    I have try a 10 HP converted with a 2 HP electric motor. The boat run at near 7 Km/H with 24v 60A (1440w)
    The good new is the boat can run at 3Km/h at 24v 5A (120W)
    The bad new is he run at 4Km/h with 10A (240w) and 5.5 Km/h at 27A 24v (648w)
     

  10. nevada14
    Joined: Feb 2013
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: France

    nevada14 New Member

    I think you can have a better speed with an other outboard motor with the same electric motor. The 25 HP johnson have a gear ratio of 2 between the motor and the propeller.
    The 25 HP Johnson is frequently sel with a 11 or 12 inch pitch propeller. That's to hight for you boat. This pitch give the speed of*:
    Calcul for the speed with pitch*:
    pitch x 25,4 (to have mm) / gear ratio x RPM x 60 (for cm/h) x Coeff / 1000 000 (to have km/h)
    For this hull, coeff is 0,60
    11 x 25,4 /2 x 3000 x 60 x 0,6 / 100 000 = 15,08
    If you have a 7 inch pitch you have a speed of 9,6 Km/h with less current than 90A, or you can have more speed for less current.
    Unfortunately, it is very difficult (perhaps impossible) to find a 7 inch pitch for 25 hp johnson.
    You can use the 10 HP Johnson who have a gear ration of 2,42. and you can find propeller from 5 inch pitch (Pirahna propeller). A good pitch with this base will be 9 inch (easy to find) an who give 10,2 Km/h or 8 inch who give 9,07 Km/h
    You can also use a 50HP johnson (or Evinrude) who have a 2,42 gear ratio with 8 or 9 inch pitch propeller or the 55HP Johnson or Evinrude who have a gear ratio of 2,67.
    I have read on internet*:
    For the hull speed*: 2500w/ton
    For 90*% hull speed*: 1000w/ton
    For 80*% hull speed*: 700w/ton
    For more the hull speed it is very expansive of power if the hull cannot fly on the water.....
    If you want double the speed on the water, you must use x 8 power. The speed is very expansive.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.