Help me design the "Poorqeedo", another efficient electric boat

Discussion in 'Boat Design' started by mental_boy, Jun 2, 2010.

  1. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Windage is definitely on my radar. I live on the San Francisco bay where there are often very strong winds. If I had a brain I'd just buy a sailboat.

    I think at first the deck will just be flat with a chair strapped to it for testing. Something to think about.....

     
  2. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Thanks, I'll try to add more height to the hull and a bit of rocker. I should be able to give you an IGS file. My cad skills are shaky so it may take a bit to get that back to you. I may also make an alternate hull with slab sides to compare.

    Could you give me more information on how much rocker there should be? e.g. cm of curvature or something?

    Also, I'm curious how the power requirements change as the load changes from 400kg to 522kg (the max design weight) for the current drawing.


     
  3. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Sounds like you need a weight budget and some design goals (as I've just learned). Figure out how much the boat is going to weigh, how long you intend to make the hulls and how fast you intend to go. Then you can start to guess at your propulsion requirements.

    This job is made easier since you already have the bridgedeck and you can weigh it (I'm curious).



     
  4. portacruise
    Joined: Jun 2009
    Posts: 1,076
    Likes: 22, Points: 38, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    Will the 6 knot speed will be enough for anything but ideal conditions? Unless this is a one time specialized use design or the conditions are very predictable to the island.... What if you have to get out of the way or wake of a cruise ship? What if the wind, waves or current kick up against your direction? What if debris tangles in the props. At least the two motors would allow quick turns into a sudden wake. Can the motors be protected against salt water intrusion? Is there some way to rig a turbo mode which can be run for a few seconds should a burst of power be needed.

    Greg K's boat has a low streamlined profile, is self righting and has sealed cabin features which allowed for rough conditions. That design keeps water out of the electronics and sailors safe and dry....

    Maybe it is still too early to consider some of these questions or they don't apply.

    Anyway, hope this helps.

    Vic

    Porta

     
  5. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    I was planning on operating this thing mostly in protected channels, especially with 4 on board.

    But I'll definitely try and operate it on the bay with one other person on board, assuming the weather looks calm. Getting stuck in chop is a possibility and in that situation I would expect to get pretty wet. (the motors, batteries and electronics will have suitable protection).

    It tends to be calm early in the morning, and if the wind picks up it will almost certainly be pushing me home since it comes from the ocean 98% of the time (don't quote me on that).

    Chop is one reason more reserve buoyancy in the hulls appeals to me.

     
  6. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Also, I'm not worried about moving fast enough, I'm used to sail boats where there is a possibility of having little or no propulsion.

    Plenty of kayakers get by on the bay with limited propulsion too.
     
  7. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    If I use a straight drive shaft for the propeller I'm guessing the percent losses are something like this:

    (tanθ / (tanθ+1)) x 100 = % loss

    Where theta is the angle of the drive shaft. Tanθ is the vertical component of the thrust and 1 is the horizontal.

    Assuming this is right and there is no influence from the proximity of the hull, a 6% angle would have 9.5% loss...

    That stinks
     
    Last edited: Jun 4, 2010
  8. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 146, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    -tell me about it!
     
  9. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Found this elsewhere:

    "Mounting an efficient propeller at a slight angle will reduce
    efficiency by a small amount. However the blade loading becomes very
    unballanced. At a shaft angle around 5 degrees the the downgoing
    blade will be thrusting with high force while the upgoing blade will
    be doing next to nothing. This condition puts the shaft in high
    bending load and creates vibration. Increasing the shaft angle
    beyond 5 degrees really starts to cause problems.

    You will see powered propeller shafts at higher angles like 15
    degrees but these props have high slip and usually three or more
    blades so the vibration is not as bad. They usually have efficiency
    between 50 to 60%.

    If you have a propeller pushing on a shaft the shaft can be
    completely unsupported as the prop is strongly self stabilising.
    When the shaft is inclined you are fighting these forces.

    It is worth the effort to get spring steel if you can. Machinable
    grade aluminium also works but it needs to be about 10mm thick and a
    little longer as it cannot be curved as tight.

    Rick W"
     
  10. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Come to think of it, people even swim in the bay here:

    http://carrielopez.com/Alcatraz/Images/alcatraz (29).jpg

    The route:

    http://www.swim-art.com/Alcatraz-to-AP.gif

    When the wind picks up the water often looks like the link below, view is of san francisco and Alcatraz. Not conditions I desire to be in on the Poorqeedo, but something I could easily encouter.

    http://home.wlu.edu/~mahonj/SanFrancisco&Alcatraz.JPG

    Greater map of the area:

    http://www.welt-atlas.de/datenbank/karten/karte-7-62.gif

    Alcatraz is clearly visible on the map. A trip I might try if I venture on to the bay would be from Berkeley to angel island, A distance of 6.25nm from harbor to harbor. Again, I'd start in the morning when things are calm, and if the weather picks up on the return leg the wind would be at my back.

    If weather on the Bay isn't good I'm likely to launch in the estuary between oakland and alameda, a 7 nm channel:

    http://farm4.static.flickr.com/3499/3859447061_eb3a2c702a_o.jpg



     
  11. Guest625101138

    Guest625101138 Previous Member

    Start the curve about 2m from the bow and lift say 80mm. Likewise on the stern but with 50mm lift. They will be very gentle curves.

    Once you get some shapes close to what you want to build send the iges files and I will do the drag analysis for different loads.
     
  12. mental_boy
    Joined: Jun 2010
    Posts: 98
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oakland, CA

    mental_boy Junior Member

    Rick,

    I've been thinking about where to mount the motors and I've been eying the hulls. I like the idea of putting them in the hulls because it is already waterproof, it reduces noise from the belt (this is important to me) and it seems like heat would dissipate better than a tiny motor box.

    I'm a little wary of the curved shaft method in terms of robustness and rust resistance since the boat will be operating in saltwater.

    What do you think about lifting up the stern more (say 80mm or more), and using a very long, straight drive shaft to minimize the angle of the shaft? In the drawing below the bow and stern are lifted ~80mm (it looks like more on the rear because the photo is at an angle) and the prop shaft is at a 3.5 degree angle. I realized just now that the tube should be submerged and the open part of the shaft should be in the boat to protect the shaft from strikes:

    [​IMG]

    Also, what do you think about making a stainless circular hoop for a prop guard in this configuration? I'm thinking 3mm by 50mm stainless bar, rounded on the front, tapered on the back and rolled in a hoop. I don't think the prop is much danger to anyone, more like submerged objects are a danger to the prop.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Anything you put in the water adds drag.

    There is no reason the prop cannot operate behind the hull. It has canoe stern so the flow behind it has little disturbance. You can then mount a rigid skeg directly off the stern that takes the prop thrust. If you are worried about people colliding with it then extend a little shroud aft above water level.

    The prop only needs to be submerged about 50mm to avoid aeration. The boat will trim aft down slightly and sink as well once underway so this gives more depth.

    You do not need spring steel for a curved shaft. I use that because it allows the tightest radius. You could use 8mm stainless by making it a very gentle curve. There are different types of stainless - typically 8mm could be curved at 8m radius and achieve infinite life.

    The small diameter shaft is adequate for the torque. It will have good compliance to reduce the likelihood of the blades breaking if you hit something. You can set the current limit in the motor at a level that avoids damage. I now make very simple folding props that overcome a lot of issues associated with solid blades. A few others are now doing the same.

    The other feature to consider is to extend the shaft tube above the waterlevel. This then overcomes the need for a shaft seal.

    Poly pipe makes a good shaft tube and it can be curved to suit the shaft. It also has very low friction coefficient so really does not need bearings.

    To get an 8.5m radius you need to angle the shaft tube at 8 degrees where it comes through the hull. The point of exit needs to be 1.3m from the stern. The centreline of the shaft needs to be 250mm below the surface. Draw this to scale and you will get an idea of what it looks like.

    You can choose if you use a straight shaft tube or curved. If straight you need to make up a thicker section of the shaft through the tube so it does not bend with the moment from the shaft.
     
  14. Ad Hoc
    Joined: Oct 2008
    Posts: 6,126
    Likes: 360, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Oh my mistake, I only saw references to the hull, not rudders.

    If it is yaw moments from steering systems, the basic analytical equation of motion of a ship in a steady turn, is thus:

    Forces:

    vYr (lateral force du to lateral velocity) + rYr (lateral force due to rate of rotation) + ‘delta’RY ‘delta’R (lateral force due to rudder angle)– MVs.r.cos.‘beta’ = 0

    Moments:

    vNv (moments due to lateral velocity) + rNr (moments due to rate of rotation) + ‘delta’R.N ‘delta’R = 0.

    Where:

    MVs.r is the centripetal force
    R = radius of turn
    r = rate of turn
    Y = force
    N= moment
    ‘beta’ = yaw angle
    Vs = ships speed

    You then non-dimensionalise and solve as these as simultaneous equations

    You eventually arrive at a terms:

    [(N’r.Y’v) – N’v(Y’r-m’)] …..which is called the stability index, or SI.

    N' & Y' are the non-dimensionalised forms

    If SI = 0, rate of turn is infinite
    If SI < 0 ship turns the wrong way
    If SI > 0 ship turns the right way at a finite rate.

    It should be noted that ships which may be ‘unstable’ at low yaw angle, beta, may become stable at a larger angle of beta.
     

  15. portacruise
    Joined: Jun 2009
    Posts: 1,076
    Likes: 22, Points: 38, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    The almost constant waves and wind are cold in unprotected SF bay and out when I've visited, so maybe wetsuits might be in order? Should you allow for deeper mounting of props if you will be in waves, otherwise might be gulping air with the props in a rocking boat.... Kayakers are much smaller, but less exposed with spray skirts or SOT design and simple paddle propulsion is less likely to be problematic and safer. Swimmers are even simpler propulsion and are not on the water for extended periods plus have flotilla of support. Both of these latter situations use expertise from extended training, and don't go under severe conditions would be a good assumption. If you are a trained athletic swimmer, that may not be a consideration from the safety standpoint. OTOH Catalina may be warmer and more protected route...

    What kind of wave would it take to flip the cat, if hit broadside, since it is not self righting?

    FWIW, hope it helps.

    Porta

     
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.