Opinions on the performances of my boat and advices, please

Discussion in 'Hydrodynamics and Aerodynamics' started by Jean Marc Delaplace, Apr 8, 2018.

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  1. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

  2. fallguy
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    fallguy Senior Member

    I don’t know if the tap fins are proprietary; so I would not sketch them. But there are pictures on this link as well as contact for the inventor. Simple strakes are also used, but the article says the Conrad TAP fins are better.

    I only know lifting strakes and just happened on this piece trying to show you a strake.

    You can easily find pictures of lifting strakes. My familiarity with them is in flats boats designed to keep the hull up off rocks. The hull gets a little extra lift underway and draft decreases. The concept can be easily used by you with those 3 hulls without any extension. A strake is simply an edge of hull mounted trim tab so to speak.

    As for designing it; not your guy. I would give this guy a call and let others here coin in. I am mostly a boat lover; others here have much more experience.

    http://www.conradmarine.com/
     
  3. fallguy
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    fallguy Senior Member



    Here are another type of pontoon strake.

    Look close. The strakes are in the water. As they accelerate, the hull rises a bit. What portion is due to the strakes, I cannot answer, but this is a large manufacturer here using them.

    If you have a trim issue, these would help. I assume they are applied on an angle across dwl or so. They are usually just aluminum angle.

    Again; not your expert.
     
  4. fallguy
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    fallguy Senior Member

    They are also called lifting fins.

    Not sure if they would help at the speeds you are achieving..

    Another thing is you have a planing hull right?

    A planing hull performs horribly near the hump. More power is needed than at plane because of the water the hulls push. For your design; you cannot trim the hulls with the prop/drive. And you want to perform near the hump. Right?

    So in addition to strakes. You might also consider some other means of adjustable trim. You could start with hydraulic trim and then add strakes if you find good results getting the bow down.
     
    Last edited: Apr 9, 2018
  5. fallguy
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    fallguy Senior Member

    The picture/video rwatson linked show lifting strakes; fyi.
     
  6. fallguy
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    fallguy Senior Member

    I am curious about the nozzles, too. Someone told you it would reduce stern walk.

    When would stern walk be a concern at these speeds?

    Maybe someone can explain that to me because I only thought it to be an issue at high(er) speeds.

    Not a fan of the nozzles, but I could be wrong. I would modify the props first to a higher pitch. Add a hydraulic trim or test a fixed trim. The strakes might not be useful below planing speeds as much as trim tabs.

    I’d only trim the two outside hulls as well. But that is just me.
     
    Last edited: Apr 9, 2018
  7. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    Here is an explanation about propeller walk : Propeller walk - Wikipedia https://en.wikipedia.org/wiki/Propeller_walk.
    On regular boats, the rudder is useless when going astern and the boat has a strong tendency to turn in one direction, which makes manoeuvering uneasy.
    Actually, since I have no rudder and the propeller are orientable, the walk is much less of a concern.
     
  8. fallguy
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    fallguy Senior Member

    Thank you. I had confused stern walk with chine walk; not prop walk.

    I know a guy who is very good at prop sizing. Would you like an independent opinion? He would return the expected speed of the craft as well.

    Need the diameter of the toons, motor rpm max, vessel weight, dwl or draft of the toons..

    He might have fun and worst case he can’t to some software limitation. And it is free, of course.
     
  9. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    Fallguy, I would definitely very interest in such a help. I think I have given the required data above, except the mav rev of the electric motors that is 1250 rpm. I intend to get rid of the nozzles because in rivers they badly catch any
    Yes, I definitely would like to have such a help. The required data is given earlier in this thread. The max rpm of the motors is 1250. I am considering getting rid of the nozzles, because they catch far too easily the weeds and our waterways are cluttered with weeds and algae, so much that last summer I was blocked in the middle of a canal, unable to move for as soon as I had cleaned the propellers, they got blocked again with weeds. I had to throw an awser to someone on the bank to get out of this situation.
     
  10. fallguy
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    fallguy Senior Member

    I did not see the diameter of the toons. He will need that and the dwl or actual draft of the toons or both if they vary. I will give him the available and see if he wants more, can do it, etc.

    Thanks.
     
  11. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    Please refer to my third post from the beginning of this thread, posted on sunday. There are pictures of the boat under construction and all the dimensions.
     
  12. fallguy
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    fallguy Senior Member

    Oops.

    Got it.

    Wow! Any chance you can move any weight for’d?

    You have excaserbated the hump effect and you are pushing way too much water. It is like a big giant hole shot full time. It will be very inefficient. I have experienced this in a different boat and we had to put the boat on a serious weight budget. We were able to find 50 pounds to eliminate. You need much more; so think big, but even small stuff matters nearer the stern. If you have a plywood wall aft; for example; changing to foam would be a bit. If you have no cockpit aft; you might even shorten a room.

    In addition to the prop questions which I hope to get for you; I suggest you review every single element of the fitout aft of center and see if you can either move the item for’d or eliminate it or modify it to reduce weight.

    Even something as silly as a toolbox might need to go forward.

    It is easy to overbuild and overload.

    Sometimes another person might be helpful with this effort.
     
  13. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    It will be very difficult to remove anything from the rear, except perhaps one of the two 24V batteries (50 kg). One thing I can do is adding weight at the bow. I have ballasts just behind the bows that I can fill with water. This is for the case I have problems with bridges that could be too low. Of course this would increase the total weight, but it would reduce the angle. So shall I expect any benefit this way?
     
  14. fallguy
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    fallguy Senior Member

    It would not help. The section in the water increases only. Four inches below planned WL aft and two inches above for’d is a LOT.

    This six inch issue is going to require a prop mod and trim assistance and weight mod and will still affect performance.

    I will report anything I get back from the friend.
     

  15. rwatson
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    rwatson Senior Member

    Fair enough. If 12 knots is all you are after, then strakes would just be extra drag.
    Refer to those two hull examples I posted.

    You could get an aluminium section built with really smooth and long exit lines, you could just have it welded on. In the end, aluminium would be a better solution.

    If the extra length is a problem, you could cut some of the bottom of the existing hull and add a maximum of 500 mm. The longer the underneath curve the better.

    Hulls2.png substern.png
     
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