A real off the wall question for Rick Willoughby

Discussion in 'Boat Design' started by BHOFM, Sep 11, 2008.

  1. BHOFM
    Joined: Jun 2008
    Posts: 457
    Likes: 14, Points: 0, Legacy Rep: 247
    Location: usa

    BHOFM Senior Member

    Rick,

    Thank you for taking time to reply to my mad rantings.

    We were sitting in the shop drinking beer, watching it
    rain and talking about the pedal boat thing.

    I had a brain storm, or maybe a light stroke, at my age
    it is hard to tell the difference.

    How about an air propeller? Large paddle blades. Fiberglass
    maybe, with a way to adjust the pitch, not from the seat,
    but movable by a clamp device?

    Sort of a pedal air boat?

    We have a 14' hover craft with a 20hp Honda twin and it
    will go a lot faster than I want to go in the damn thing!

    It has a carbon fiber prop, 36", very light weight.
     
  2. MLampi
    Joined: Aug 2008
    Posts: 74
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Bellevue, WA

    MLampi Junior Member

  3. Guest625101138

    Guest625101138 Previous Member

    The fastest single person pedal boat uses an air prop. Google decavitator.

    Air props are an advantage at higher speed. It was a good choice for the hydrofoil application. Would be possible to get efficiency around 85% with lower drag than an underwater prop apart from using the strutless prop that I have used.

    At slower speed such as a displacement craft you need a very large diameter to get decent efficiency. A 3m diameter prop will get low 70s efficiency. With a water prop you can get mid to high 80s. So a big difference in efficiency and size is more practical using water prop.

    The prop shown in the image Mike provided would achieve about 50% at normal cruise speed. Efficiency might approach 70% at very high power level in a sprint.

    Rick W
     
  4. MLampi
    Joined: Aug 2008
    Posts: 74
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Bellevue, WA

    MLampi Junior Member

    Hi Rick,

    I thought he was looking for something that was more like an airboat, and that was exactly the scenario for which the Crosstrek was built.

    Decavitator would have real problems in the swamps and shallows.

    The humanpoweredboats.com site has photos of Decavitator, and links to the site as well, IIRC.

    Michael Lampi
     
  5. Guest625101138

    Guest625101138 Previous Member

    Mike
    Do you have any performance detail on Crosstrek? It seems it got close to commercial production. I am interested to see how my prediction compares with what they achieved.

    Rick W.
     
  6. Guest625101138

    Guest625101138 Previous Member

  7. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 174, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Since props is an issue, and considering water cannot be compressed, shouldn't one think in the lines of a prop where the water can only be thrusted one way, and which have less slip than when the water is allowed to move sideways off the prop center. If you put a shroud around the prop maybe...

    And I remember seeing a double prop, or tandem prop, they turn oposite directions, the idea was probably for the one to feed the other, making the water pressure more so the second prop can kick harder in it.
     
  8. Alan_VA
    Joined: Oct 2008
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Virginia

    Alan_VA Junior Member

    Shrouded Props

    Shrouding a propeller is one way to increase its efficiency because, as you point out, when there is less chord-wise flow, thrust increases. Well, it also increases because the shroud increases the apparent aspect ratio of the blades..... Contra-rotating propellers improve the situation by other means. The first set of blades, in effect, better aligns the water flow into the second set of blades, improving the overall thrust.

    Alan
     
  9. Guest625101138

    Guest625101138 Previous Member

    Alan
    Most pedal boats use high aspect blades because the thrust loading is very low. Typically less than a medium size model plane. Prop efficiency is already mid to high eighties. The drag from any shrouding in these circumstances more than offsets the very slight improvement possible in efficiency.

    For similar reasons counter rotating props do not offer any advantage. The velocity ratio for the prop is very low so there is very little energy put into the stream from the prop. The added mechanical complexity would likely offset any benefit.

    Rick W
     
  10. 2farnorth
    Joined: Oct 2008
    Posts: 38
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Stillwater, Mn, USA

    2farnorth Junior Member

  11. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 174, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Rick,
    We were working on a more efficient prop setup for your masochist... I mean the paddle boat you have. If we could get it efficient enough you need to paddle once every... or does that rather defeat the object ?
     
  12. Guest625101138

    Guest625101138 Previous Member

    Always aim to go further for less. On a good day most of my energy goes into overcoming viscous drag from water. On a windy day it is still viscous drag but about 50/50 water to air. The air drag could be reduced with better fairing but then I prefer to pedal on good days. Hence my biggest challenge is to reduce viscous drag. The things that work are regarded as pollutants so need something new here.

    Rick W
     
  13. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 174, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Come now Rick, you can be honest with us. We are your friends, remember (in theory anyway).

    Lets just for a second say you get the golden way and this is so efficient, you don't even have to paddle any, the boat just goes and goes.

    I just bet you are going to find something else you can paddle instead :D

    It's like these guys running marathons. I mean this guy has just put his poor body through the worst unthinkable and unnatural horryfying experience for hours on end. In theory this guy is dead, the body quit 150 km back.

    Now ask him - How do you feel ?

    'No well fine... I feel great' :rolleyes:

    See what I mean ?
     
  14. Guest625101138

    Guest625101138 Previous Member

    Fanie
    I am hooked on the endorphin rush. Every Saturday morning is like Christmas for me right now because I can load the boat on the car and spend a few hours at the lake just going up and down and watching the scenery. The body is starting to get better.

    Very windy last weekend. I pushed into 30 - 40kph winds on one leg and then had the benefit of the wind the other way but the waves were too small to keep up so constantly driving through them. Hence the speed variation between consecutive runs on the chart.

    I did the last lap with Ian Cassell in his 15ft pedal boat. We were amongst the little sailing fleet and a few paddlers.

    Rick W.
     

    Attached Files:


  15. Guest625101138

    Guest625101138 Previous Member

    The dip between the 40 to 50 minute mark was spent looking for and retrieving my water bottle. The damn thing came unclipped and fell off. I did not realise it until I wanted a drink. Luckily it had enough air in it to stay afloat.

    Rick W.
     
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.