Sizing Prop on a Prototype

Discussion in 'Boat Design' started by BlueBell, Nov 16, 2017.

  1. BlueBell
    Joined: May 2017
    Posts: 92
    Likes: 1, Points: 8
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Building a solar/battery powered race boat for a school challenge.
    We are at the prop sizing stage of design.
    What we have:
    Motor rating: 500W, 467RPM loaded ( it's got a 6:1 reduction gear built in ).
    Two power sources: one is a 180WattHour lipo battery only for the 25 minute endurance race.
    It should deliver 400watts ( via a throttle control and watt-meter ) for 28 minutes.
    With ~75% rated motor efficiency that's about 300watts at the shaft.
    The other power source is ~200watts from solar panels only, so ~150W at the shaft.
    We are allowed to change props between heats, fortunately.

    None of those fancy software programs like Javaprop work on my old computer.
    Can you help us?
    Simply looking for diameter and pitch of a few plastic R/C propellers to test.
    Our set up is long-boat style with the prop on the end of a long shaft.
    The motor pivots horizontally only, for steering.
    We'd like to test surface piercing with only the bottom half of the propeller disk submerged.

    Cheers and thank you to all respondents.
     
    Last edited: Nov 16, 2017
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,005
    Likes: 91, Points: 48, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    In order to come up with something in the right ballpark, it is necessary to have an idea about the speed potential of the hull, preferably drag asf of speed. This since the propeller characteristics (thrust versus speed) and the hull characteristics (drag versus speed) together determine the operating point (i.e. possible speed and endurance). The first is generally falling off with increased speed, and the latter increasing.
     
  3. BlueBell
    Joined: May 2017
    Posts: 92
    Likes: 1, Points: 8
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Right you are Baeckmo, thank you.
    I returned to provide just that info.
    7 knots, 3.5m/s
    I know that may sound optimistic.
    It is a 6m hull with a 40cm beam (0.4m)
    Very light weight skin-on-frame with a 25kg pilot!
    The battery 1kg, motor 6kg (loaded RPM rating 467)
    Calm sea conditions in fresh water.
    Thank you

    Edit: I should note: in the 25 minute, battery only, endurance heat, we are allowed to remove our solar panels, 25kg.
     
    Last edited: Nov 18, 2017
  4. BlueBell
    Joined: May 2017
    Posts: 92
    Likes: 1, Points: 8
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I have a bit more info now:

    Boat weight for the 25 min endurance, battery only heat, will be 55kg and 65 kg.
    There are two different pilots and we require a driver change at the 10 minute mark.
    Hmmm, should we run the heavier pilot first or the lighter...?

    The other two heats are short, like 45 seconds each for the slalom and sprint.
    These are with the panels only. No driver change so 80kg.

    Thanks
     
  5. Richard Woods
    Joined: Jun 2006
    Posts: 2,040
    Likes: 120, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

  6. BlueBell
    Joined: May 2017
    Posts: 92
    Likes: 1, Points: 8
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Thanks Richard.
    Looks like he's right up our alley with those props he made for R2AK!!
    I wonder what happened to Baeckmo...
     
  7. BlueBell
    Joined: May 2017
    Posts: 92
    Likes: 1, Points: 8
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Nothing back from Alec...

    Baeckmo, I'm sorry I don't have drag numbers for you.
    I would think something in the 60 - 80 Newtons range, no?
     
    Last edited: Nov 20, 2017

  8. baeckmo
    Joined: Jun 2009
    Posts: 1,005
    Likes: 91, Points: 48, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    I wonder what happened to Baeckmo...

    ..Gone walkaround hunting for a couple of days, to break in a young search dog (lost my old, long time companion "Ivar" a year ago). I'll be back to you soon.
     
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