Prop and motor sizing in outboard conversion

Discussion in 'Electric Propulsion' started by Laphroaig, Feb 19, 2022.

  1. Laphroaig
    Joined: Jul 2012
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: NZ

    Laphroaig Junior Member

    A quick question on propeller and motor selection when converting an outboard to electric. (Sorry if this has been asked/discussed elsewhere)

    In the context of converting an older 8hp yamaha 2stroke.

    In selecting a larger electric engine to the replace the two stroke. I am interested to know others experience or thoughts on oversizing the replacement engine.

    Had been looking at an 8kw peak output brushless dc motor but considering oversizing to 12 or 15kw with or without a different prop.

    The assumption being the max output power at max rpm (in this case 6000rpm) will at somestage be limited by the prop not the motor and putting a larger motors won't alter the power.
    Last edited: Feb 19, 2022
  2. portacruise
    Joined: Jun 2009
    Posts: 1,444
    Likes: 168, Points: 63, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    The gears Within the Yamaha may be beyond the strip limit when going over the design power, as electric motors generally have a lot more torque compared to IC, at any particular RPM. Depending on your boat design and your existing speed limit @ 8hp- doubling the power may not have much of an effect on speed.
  3. SolGato
    Joined: May 2019
    Posts: 353
    Likes: 224, Points: 43
    Location: Kauai

    SolGato Senior Member

    It sounds like you are planning to remove and replace the 2-stroke motor with an electric, coupling it to the driveshaft and lower leg?

    Keep in mind there is likely a gear reduction in the lower leg where the driveshaft meets the prop shaft if it’s a typical outboard.

    A 4 stroke that size for example, might be around 2:1, so a motor that spins up to 6K will be turning the prop around 3K.

    I think you should first try to identify what the gearing is in the lower leg.

    Knowing that and what RPM your current motor head operates at will tell you if you’re in the safe zone as far as wear and tear on parts, and then should give you a good idea of whether or not you could swing a more aggressive prop, etc.., and or if a larger electric motor would be advantageous.

    Of course the hull design and displacement of the vessel you want to propel will also have a say in things.

    Here’s a link to a chart to give you an idea of the variety of gear ratios in outboards. Chart.pdf
  4. Laphroaig
    Joined: Jul 2012
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: NZ

    Laphroaig Junior Member

    Thank you. Thats a super helpful chart.

    RPM 4500-5000 gear ratio 2.08

    The idea is to fit a similar rated electric engine in terms of rpm. The one im looking at is max rated at 6000. And avoid altering the gearing.

    With a larger engine and the original prop the engine will need extra power to get to the higher max rpm.

    With a larger prop the power rating of the engine will be reached at a lower rpm.

    Both max rpm and power input being limited by the controller????

  5. gonzo
    Joined: Aug 2002
    Posts: 16,657
    Likes: 1,617, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If the electric motor has the same power rating at the same RPM, then the maximum torque will be equal. The torque curve will be different though.
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.