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Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

    Ian
    You are right about longer life if you do not force the initial curve.

    Ultimately you want the shaft to have infinite fatigue life so it should not matter if the curve is forced initially. However I am always pushing the limit so inevitably they break and I do not like to lose props I have spent time making. Also my strut is semi-rigid to keep the prop away from the hull. Your prop is further aft so has better clearance to the hull.

    On the new boat I will make the strut removable and I will do some performance comparisons.

    I have the stabilisers and main hull for the 5m version of V15 at the finishing stage. Mitrpak are making the square drive boxes for me. I am looking forward to testing it when the boxes get here. I am pleased with the way it looks so far.
     
  2. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    V15

    Rick
    Be interested to see your new boat when it hits the water. Let me know when its ready to go.

    Ian
     
  3. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Rick,

    When you say that Mitrpak are making "square drive boxes", do you mean that they are making gearboxes with input shafts that will take standard bike cranks?

    If so, then I'd love to have some further details, as this may be an option for a friend's project.

    Jeremy
     
  4. Guest625101138

    Guest625101138 Previous Member

    Yes. I worked with Mitrpak on a square drive version of the T-122 box for Greg Ks drive per attached photo. He has a T-122 at the top directly connected to an R-082 having an extended shaft that sits under the water. It has 1:4 overall. Apart from a rolled shear pin breaking when it was slammed into reverse the leg has worked very well. Greg has a few videos of himself and others in the boat and you can hear the quiet hum of the leg.

    After that boat was made I asked Mitrpak to give me a price to modify their T-102-M box for me with square drives on the input shaft.

    The ratio for the three I have ordered is 1:2 but Mitrpak have said they may be able to do 1:3. I would only want this if it did not involve reduced input power. The T-102-M does not have a lot of margin but it OK for my continuous power and the occasional sprint. I have asked for solid shear pins.

    I have used a 1:2 ratio before swinging a big diameter prop. For the current boat I am making a 500mm diameter folding prop. It will be located directly behind the hull so I do not need to worry about it hitting the hull. It will also make the kayakers wary of drafting too close once they see a big prop reflecting in the sunlight.

    Both Ian and I have one of the Involute 1:3.3 boxes with square drive but they weigh 4kg versus 1.8kg for the Mitrpak. That said the Involute box could handle an Olympic class sprinter. They are the boxes used on George Tatum's Wavebikes as shown in the photo. Ian has various photos of his if you go back through this thread. I think I have one of mine back somewhere.
     

    Attached Files:

  5. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Many thanks for the info, Rick, I'll pass it all on.

    Jeremy
     
  6. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Thanks for the update. Interesting approach with the 500 mm that folds down to maybe 200mm estimated from your previous pics. Gives efficiency and ability to coast sections of shallow draft/obstacles. I suppose this will require a thicker diameter flex drive shaft as well.

    One thing I found with going with a stiff strutless drive shaft and large slow props is some prop instability (out of thrust line circular wobble) on gradual start. But, I was running very lightweight RC props at much lower power with solid aluminum unyielding shafts. I wondered what forces were at work. Maybe my shafts were not perfectly true with the output power shaft. Or the gyroscopic stabilizing component of the prop was not sufficient because of the light weight. Maybe even a harmonics effect is at work from a flex shaft that allows self stability. Sometimes I still wonder why a flex drive shaft works at all....

    Vic

    Porta

     
  7. Guest625101138

    Guest625101138 Previous Member

    Vic
    The shaft will be a combination of aluminium through the hull in a straight thicker run that is mated to an 8mm spring steel curved section. I will try it without bearings in the shaft tube initially.

    I will also use a tension strut initially. If the strut gets fouled then I will operate without it. I expect the hull will push most of the weed aside. That was one of the issues with the side mounted shaft. It was easy to clean but was more prone to collect the weed that passed down the side of the hull.

    The only time I have had the wobbles on a shaft was when I was pushing an 8t catamaran with a 9mm diameter shaft that was 2.7m long. I ended up making a 4-bladed prop that worked better but the thrust was sufficient to cause the shaft to buckle and wobble until the boat gained a bit of speed.

    The V15 hull will require very little thrust to get it going so I do not expect the wobbles. I used a 1:2 ratio on V10 with a curved shaft and it was stable.
     
  8. Dennis A
    Joined: Aug 2009
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    Location: Amersham bucks uk

    Dennis A Junior Member

    Tight prop Strut Bearing

    I drilled out the bearing by about 1.5 mm, reassembled and tested by hand cranking under dry conditions. The drive system was very free and could be taken up to 120 rpm so I think the problem has now been cured.
    The boat was tried out and found to reach about 10 kph, so I will now start undertaking some fine tuning.
    The folding prop worked well and I was very pleased when I tried the boat in reverse, the fixed stops which prevents the prop from shutting fully up leave, just the right amount of the blades showing for a gental reverse action.
    Additionally I have to find the pitch slipage factor as I use a cycle computor for my speed and distance readings.
    The river outing was cut short as one link in the chain sprung and a link extracture was one tool that I had not taken with me.

    Dennis
     

    Attached Files:

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  9. Choosethisday
    Joined: May 2010
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    Location: Washington State, USA

    Choosethisday Junior Member

    Hello everyone, I am new here but have been watching with interest for quite some time. I have acquired the composite pontoons from an older Seacycle and have begun the restoration. Because I only have the pontoons and no frame I have had to adapt the hulls to fit aluminum tubing. I can cut the tubing for any width so I am looking for the optimum spacing. The specs for the Seacycle are for 70 inches overall width. Since the hulls are 15 1/2 in width each the centerline spacing of the hulls would be about 54 inches. From what I have read the optimum spacing for a catamaran style boat is about .41 percent of the wetted length. But using the 54 center to center would only be about 30 percent. So is the factory Seacycle W/L not optimum or am I missunderstanding the concept? Also, does one really give up much in efficiency by reducing the width below the optimum. I have an old Aquacat trampoline that would save me quite a bit of time if I could use but this would involve reducing the overall width to perhaps 60 inches. Would this really represent much of a loss? Anyway, I do appreciate any feedback.
    Thanks
     
  10. Guest625101138

    Guest625101138 Previous Member

    Dennis
    For some reason I thought you were making a cat. Apart from the transom it is not too different to my V14 boat. Why did you choose the wide transom.

    Using the cadence to determine speed is not particularly accurate because it will change a little with wind and and extra drag on the hull.

    For development purposes there is great value in getting one of the little Garmin Edge GPSs. These can record speed, heart rate and cadence. This gives hard data that you can relate to changes. Current is then the only variable that is not easy to measure.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Without doing the analysis for the specific hull it cannot be said for certain but typically there is a weak optimum related to spacing of the hulls.

    The performance of the Seacycle is constrained primarily by the hull length and to a lesser degree the beam of the hulls. Length is the dominant factor in limiting the performance. Adjusting the spacing of hulls will make little difference compared with getting longer hulls.
     
  12. Guest625101138

    Guest625101138 Previous Member

    Dennis
    I am interested in a bit more detail on your experience. Just anecdotal stuff like:

    Is this the first time you have been on a pedal boat?

    Were you happy with the way the boat went?

    Did it meet your expectations?

    What areas do you see for improvement?

    Did you get any comments from casual observers?
     
  13. Choosethisday
    Joined: May 2010
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    Location: Washington State, USA

    Choosethisday Junior Member

    Rick, thanks for getting back to me. I realize the hulls are not as long as desirable but my goals are to end up with a boat adequate for two that is able to keep up with the faster canoes but is pedaled rather than paddled. My wife and I are reasonably experienced paddlers and we have a reasonbly efficient canoe. However, one of my rotator cups is getting weak so leg power would work better. Also the inherent stability of twin hulls appeals to us. We are looking for an all day cruising speed of at least 3.5 knots and a sprint speed of perhaps 7 knots, so my hope is these hulls will work for those goals. Once again, thanks
     
  14. Guest625101138

    Guest625101138 Previous Member

    If the hulls are the ones Clems pictured back at post #859 then your targets should be achievable. It does depend on fitness level of course but 3.5kts is not ambitious for these hulls. I think Clem gets a bit better than that.
     

  15. Choosethisday
    Joined: May 2010
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    Location: Washington State, USA

    Choosethisday Junior Member

    OK, sounds good. Time contraints will not allow me to move very fast on the project but once I get the boat at least useable I will try to post some pics.
     
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