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. fallguy
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    fallguy Senior Member

    OP - my friend wants to know the largest diameter prop the drives would support.

    I was unable to determine an upper limit from f-p.

    I would assume this would be without the nozzles.
     
  2. rwatson
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    rwatson Senior Member

    The thing that would worry me is the increased drag of two extra hulls.

    Doing anything to mitigate the stern shape would be a bonus.

    The loss of buoyancy at the stern due to re-shaping, especially since he cant/wont extend by more than half a metre, is definitely a major issue.
     
  3. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    I attach the drawing of the pod. I intend to remove the nozzles, so the maximum diameter for the propeller is the outer diameter of the nozzle, that is 430 mm (16.9 in.).
     

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  4. rwatson
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    rwatson Senior Member

    I wouldn't expect an improvement.
    Your prop-tip turbulence will reduce the props efficiency.
     
  5. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    Can you be more explicit? What do you mean with prop tip turbulence?
    By the way, the pod is mounted with the motor housing to the bow, and the propeller to the aft. I would have figured it would be better the other way round, for with the propeller facing the bow, it would receive an undisturbed water flow, but my supplier insisted that the propeller should be turned to the aft, I do not know why.
     
  6. fallguy
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    fallguy Senior Member

    We use bow mounted electric motors here all the time. The pod is, of course, aft of the prop if the motor is in the reverse direction. They tend to vibrate bad when running pod aft. They are designed for the propulsion aft; not for’d of the motor. Yours are same as profile in the water.

    Bow mounteds are a bear for docking, just so you know. Not to be confused with a bow thruster; they are not.

    It is very easy to oversteer. You would not like a bow mount.

    F-P is correct on this bit, but the size is almost certainly wrong. I will get you a size recommendation in the next few days. If I were you; I’d ask F-P for a sizing recommendation as well to reduce your cost on experimentation.

    If you size the props better; I expect some improvements, but I still think your toons are undersized and might work better if you put a flat section in between and took watson’s advice on cleaner exit.
     
  7. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    I intended to ask F-P for a recommendation, as you advise. I am interested in getting rid of the nozzles because the rivers here are cluttered with weeds and I must clean my propellers much too often. For luck, I have installed an actuator to lift the propellers out of the water, otherwise I would have turned mad.
    As far as the toons being too small, I have heard of people who had the same problem with a prototype boat and (if I have well understood) who laid the bottom of their hull (externally) with thick foam (closed cells, I guess!) which provided more buoyancy. Could this be a cheap improvement?
     
  8. fallguy
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    fallguy Senior Member

    I am going to stay away from the foam on the bottom idea; not a fan and never seen it. I think you’d be too incremental [sic].

    My friend gave his prop recommendation...

    He believes you can achieve a speed of 6.2mph or 9.98kph. He also believes you will make 10-10.2kph filling your ballast tanks part or all the way. I have some doubts about that personally; I digress.

    He is recommending Powertech WBG3, the size is 11.62 x 9. This is a high thrust prop.

    PowerTech! Propellers - Official Website - WBG3 http://www.ptprop.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=51&category_id=21&manufacturer_id=0&option=com_virtuemart&Itemid=46&vmcchk=1

    He did ask me to ask what prop you are using. He thought you might be overpropped, but that was a guess. I asked him for independent numbers and he has delivered. If I give him your current setup; he might give you a different idea on full information. He is usually very good at prop estimates.
     
    Last edited: Apr 15, 2018
  9. fallguy
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    fallguy Senior Member

    If you add foam to the bottom; say 4”, I doubt you’d get lift of 4”, so I would be concerned your stern drag issue would be exascerbated. There is no solution that seems cheap to me on the trim issue. Modifying the aluminum with foam or fiberglass core seems unwise. I think cutting and welding aluminum flat sections would be better, but is it worth it for the gain of say 2.5kph?

    Try prop mods first. If you can get close to 10; it might not be worth the money.
     
  10. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    My current propeller is a 14 x 13 with chopped blades to fit the nozzle, as you can see in the picture:

    upload_2018-4-16_21-48-16.png

    I am surprised that he advises a smaller diamer than that I already have. I would have thought of an even larger prop. Also, the mounting does not fit my motor, which is as follows:

    upload_2018-4-16_21-54-55.png


    What does your friend think of this?
     
  11. fallguy
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    fallguy Senior Member


    Will report back. He did say he suspected you were overpropped.

    As for the mounting, I will let him know, but the mounting would not change the size/pitch rec. I thought specifying a prop to the vendor was more than I asked.

    I will try on the pc later today to upload the prop mounting and the chart and advise him the prop you are using and see what he has to say.

    Kind regards
     

  12. fallguy
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    fallguy Senior Member

    Well, my friend has seen all your data and is certain you have several issues.

    First, he said your trim problem is stalling water at your korts. He says you need to test at full ballast tanks and remove the korts and expects a minor improvement.

    He insists your props are too large and told me a bigger diameter prop will prove slower still.

    Of course, getting to dwl is the best solution. Lifting the stern to 400 would probably lower the bow to much closer to 400. To do so requires either a weight change aft or pontoon change.

    Let the forum know if you have any luck and good luck.
     
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