Pedal boat propeller specifications data

Discussion in 'Boat Design' started by MarkX, Feb 27, 2010.

  1. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    MarkX,

    Answering some of the points you've raised, (with hard facts, as you've stipulated......):

    1. an average person can't put out 250W, at least for anything other than a short burst. 100W would be a reasonable working figure for continuous prop power input. There is plenty of data to support this approximation, just Google for it and bear in mind that much of what you'll find will be based on the power output of trained athletes, as that's where the research has been concentrated.

    2. Javaprop is dead easy to use and you can set the constants for any medium you like on the final tab. Obviously water is around 1000 times denser than air and has a markedly different kinematic viscosity, so using the wrong constants will give incorrect data.

    3. As other have already pointed out, you absolutely have to know the hull characteristics to make even a half-decent stab at getting the right prop. At these very low power levels and limited rpm range (more gearing = more losses) you have to know how much thrust the prop is required to produce for any given inflow velocity. As thrust is proportional to resistance, knowing the hull resistance/speed curve allows you to pick a prop pitch, diameter and rpm that best matches it.

    This latter point is markedly different to conventional prop matching for power boats, primarily because you don't have any excess power available. Fitting a poorly matched prop on to an outboard (which is probably only about 20% efficient overall) won't make much difference, all you may need to do is twist the throttle a bit more. Do the same when you only have around 100W to play with and you suddenly find you've halved the boat's speed potential.

    I've made a low power prop using modified APC blades and, as a data point for you, can say that a 12 x 10 prop driving a hull that has a resistance of 30N at 3.5 kts needs to spin at around 600 rpm. An unmodified APC prop will collect weed and debris quite quickly an become useless, hence all the efforts into making folding blades to allow weed shedding.

    Contrary to some of your assertions, several of us on here are getting solid data the hard way, by actually going out and doing controlled tests, taking up a great deal of time and effort in the process. If any of us could have just done a search on the web and got our answers we would have, but it seems that very few people truly understand the nuances of getting low power props to work at low speeds, most research and development commercially has been on higher power or higher speed applications.


    Jeremy
     
  2. MarkX
    Joined: Feb 2010
    Posts: 35
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Azores

    MarkX Junior Member

    Thanks Jeremy. I think for now APC ones for testing will have to do. I can go the stainless or Epoxy-Kevlar way later. Plan there is to then make the final blade and take two moulds from it to join into a two blade mould.

    Having looked at Elastomers (plastics being part of my business) I'm also daydreaming about adaptive props. Maybe the blades could flex about a spar to adjust their own pitch, depending on how much load there is on them.
    A kind of rotary propeller version of the Hobie drive.

    Injection Toolmaking + moulding is one of the things I do for my business and I have some engineering machinery (not CNC though) and small injection moulding machines (not big enough for props). I also did some GRP mouldmaking for a friend's firm (riot gear) and build RC planes from scratch. RC planes is one of my main hobbies and in the Azores the main wood is Japanese Cedar (Cryptomeria) so we use that for lack of balsa. They also make their traditional fishing boats from it. Marine ply, ally and GRP are also available there. (Anyone know if Epoxy is available in Horta?)

    I'm only there about 5 months a year so I have to make technical components here in GB and take them there in the suitcase. Better stop because now I'm starting to waffle...:rolleyes:
     
  3. portacruise
    Joined: Jun 2009
    Posts: 1,214
    Likes: 73, Points: 48, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    weed shedding

    Good to have you back, Jeremy.
    There's probably very few on this list that actually have a working version of a low power HPB or e-boat even though the topics have drawn a lot of comments.

    I believe the main issue holding back low power boats is the issue of weed entanglement. At least that is a major consideration for my small eboat when not operating in clear water. If a completely satisfactory answer had been developed, perhaps demand for low power prop driven boats would have surpassed the Hobie and maybe even cut into the kayak demand by now.

    So what is the best compromise solution to this issue is question I would like to pose. Let's see what kind of feedback we get on this issue. I will give some feedback from experience with my own setup if there is enough interest.

    Thanks,

    Porta



     
  4. MarkX
    Joined: Feb 2010
    Posts: 35
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Azores

    MarkX Junior Member

    Nice pic of a prop fitted with a weed slicer disk here:
    http://www.prop-protector.com/home.phtml

    Most electric trolling motors have weedless props, I can't see the problem in doing a coarser pitch HPB one apart from the complexity. Are they actually less efficient than straight blades or does the Scimitar shape cancel this out?

    The prop on my boat will be mounted under the pedal by my feet, this means I should be able to observe it through the perspex floor window.


    Another point: Trolling motors are designed for slowly pushing larger boats, so surely a different prop would make them far more useful for sleek craft like Kayaks and Canoes.
    I think that small electric and HPB boating has various gaps in the market, electric and pedal drives with a prop choice designed for this job are rare/non-existant
     
  5. MarkX
    Joined: Feb 2010
    Posts: 35
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Azores

    MarkX Junior Member

    Another propeller link with some data:
    http://www.freeenterprises.net/HPBoats.specs.html

    and one whom Rick helped:
    http://www.recumbents.com/WISIL/hpb/prop/default.htm

    Rick's guide to making stainless props:
    http://www.recumbents.com/WISIL/hpb/Prop_Fabrication/Propfab.htm


    Before I forget... I read something about wind turbine props and some chap who was cutting them from simple plastic tube. By cutting the shape at an angle to the tube the necessary twist was achieved.
    By doing the shaping mostly from the tube inner side a decent airfoil shape might be achieved.


    RICK: Does Java prop allow for the curving of the outline so that the prop can be shaped with a straight or swept-back trailing edge to be more weed-free?
     
  6. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Mark,

    Most, if not all, of the answers to these questions are in the threads already referred to, mainly those where Rick has been an invaluable researcher, experimenter and contributor.

    At the risk of repeating stuff that's already on here on another thread, direct drive systems, like the trolling motors that are cheaply available, are inefficient for two principal reasons.

    1. The limited motor diameter that's possible means that prop rpm is a bit high, and high prop rpm with low boat speed generally means poorer overall efficiency. To get high efficiency and high torque from an electric motor at low speed means having a large diameter, as torque is a function of magnetic field strength and rotor diameter. A large diameter motor would be impractical, so the trolling motors compromise and use too fast a prop rpm for best efficiency. The exception to this is Torqeedo, who use a high speed, small diameter brushless motor and a gearbox to reduce the prop rpm. This gains prop efficiency, but at the cost of gearbox power loss and noise.

    2. Just like low-speed aeroplanes, short fat blades are less efficient than long thin blades (think of glider wings as an efficient example, as that's the best analogy for human powered boat props).

    The trolling motor manufacturers are after a cheap design that works acceptably well for their market. Range isn't a key selling point, as most motors are used by folk who just want to go a relatively short distance under power. Weed shedding is a big selling point, as it's a real nuisance if a prop collects weed easily. The result is that trolling motors use swept, wide chord blades that shed weed well but aren't very efficient. Their props are the same sort of compromise as any general purpose outboard prop, in that they are rarely a good match for the boat they're used on. As efficiency (best range for the size of battery) isn't as important as other factors, the manufacturers don't really worry about this. There are after market props available for some of these motors, though, that are claimed to improve performance for some applications.

    You can only sweep a narrow chord blade so far before it starts to suffer pitch change under load from it's inherent lack of stiffness in bending (think of where the thrust vector is acting and how it will tend to twist the blade if it has sweep). A straight blade in less susceptible to this pitch change with thrust effect. Scimitar shaped blades aren't inherently that much less efficient than straight blades, if they are of the same aspect ratio and thickness, although they can suffer from slightly increased spanwise flow induced tip vortex losses. The problem with scimitar shaped blades is the the blade chord and thickness need to be greater in order to stiffen the blade to counter the pitch change effect noted above. For high efficiency you need a high aspect ratio (the glider wing analogy again) and a low wetted area (for reduced viscous flow induced losses), so anything that tends to reduce the aspect ratio or increase the wetted area will also tend to reduce efficiency.

    Rick has done some sterling work on designing and evaluating the performance of a folding prop design that retains high efficiency but which sheds weed as well. This is why so many of us are now using a similar approach.

    This thread seems to be trying to duplicate a lot of information that is in the pedal boat thread. In my view it would be better to have everything in one place.

    Jeremy
     
  7. MarkX
    Joined: Feb 2010
    Posts: 35
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Azores

    MarkX Junior Member


    That's why I said in the first post that most of us know all the theoretical problems and compromises already, it's all everyone talks about elsewhere, so let's just have REAL examples of what has ACTUALLY been done, with real numbers, as per title. :rolleyes:
    Turns out that my 16x16 APC plane prop guess at but at 5-7 to1 gearing is THE starting point for the majority of Canoe/Kayak type general purpose HPBs. People can then tweak from there if they feel the urge.
    Let's just have practical usable data from now please, so future readers don't have to wade through what's beyond amply covered elsewhere.

    (BTW Blade flex isn't a problem as long as it's at the right pitch under load. If anything such springiness can be an advantage, as used by nature in variable real life conditions, away from abstract mathematical theory.)
     
  8. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    OK,

    I'll stay off "your" thread from now on.

    Jeremy
     
  9. portacruise
    Joined: Jun 2009
    Posts: 1,214
    Likes: 73, Points: 48, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    Response to weedless FWIW

    Hope you don't take responses above in the wrong way, they are meant to help. Sorry about the caps. Some readers can't find responses written within and they are meant to speed up the process.

    Porta
     
  10. portacruise
    Joined: Jun 2009
    Posts: 1,214
    Likes: 73, Points: 48, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    To summarize my experience with weed shedding, there are things that help but nothing I have tried solves the issue under all conditions. Even power boats not on plane are sometimes stalled when they entangle something in their prop. That's why we have jet skis with shielded impeller drives, I suppose. Impellers won't work with low power like human or electric because of ineffiency. My tests with folding model props did not solve moderate weed issues when used with a flex shaft in an electric drive. There was no stalling, but loose balls of weed accumulate on the shaft and increase friction unless removed. Nothing accumulates on the blades themselves, though. Folding props may work better with a below the water troll motor arrangement though. They may also work better under human power where someone can peddle back and forth until the balls shake loose. Still that will cost some time going forward. Just my humble thoughts, offered in a spirit of helping...

    Porta

     
  11. Guest625101138

    Guest625101138 Previous Member

    Vic
    My experience is that the time lost is negligible with the folding prop to stop and give a half turn backwards. The blades have always been clear. The action to clear the prop can be done in a fraction of a second and the boat coasts - not like a fixed bladed prop. There is no perceptible reduction in speed. The time lost is less than a kayak paddle takes between strokes.

    I have noted elsewhere that the strut still collects weed. The only way guaranteed to beat this is to remove the strut, which creates other problems of course.

    Rick
     
  12. MarkX
    Joined: Feb 2010
    Posts: 35
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Azores

    MarkX Junior Member

  13. unclenos
    Joined: Mar 2010
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Malaysia

    unclenos New Member

    This is where I got my 16" x 16" APC propeller from.
    http://www.centralhobbies.com/props/props3.htm
    I will stick to Rick's setup for me;
    at 90rpm cadence, 1:5 gearing.
    I'm 49 years old now, don't know how hard or how easy I will spin when my boat ready soon.
    At least I know where to start.
    Thank you very much Rick Willoughby,
    I'll keep in touch if I can't find the answer.
     

  14. portacruise
    Joined: Jun 2009
    Posts: 1,214
    Likes: 73, Points: 48, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    Pattern APC are the most efficient from my measurements, especially the WIDE versions. If you can get a stiffer carbon BOLLY in the 16 X 16 size (if they ever made it) it would be better. Bolly is out of business but some model shops still have stock, at clearance prices. The electric slow turn with big hubs are wider than pattern type, and a little better in very low power (30 watt) applications, but will deform at higher power draws and become inefficient. The Sport, Competition etc, have a different shape which is considerably less efficient.

    Hope this 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.