What type of prop would be recommended in this case?

Discussion in 'Props' started by Nidza, Oct 21, 2017.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I didn't realize your prop is getting air or disturbed water when I commented to increase prop pitch, but our Swedish friend seems pretty certain.

    If that is the case; then the rpms running too high is a totally different issue. Why do you keep tempting fate and running the engine rpms too high? If the prop doesn't have enough water; that is a serious issue. Can you hear the prop sucking now and then? Usually you can hear some gurgles if the prop is up too high. The only way I can describe it is like a dog licking water from its bowl. Unless you are just in disturbed water from the keel which might not make a sound.

    I'd not invade the hull. I think the keel is the problem and you aren't getting away from that.

    It would be nice to see a side view of the av plate and the transom out of the water.

    I am definitely not an expert on modifying the boat or motor to get less disturbed water; honestly I don't think it can be done.

    Have you considered just trying a hydrofoil? Typically a hydrofoil would force the bow down some and if so, that alone might result in less disturbed water. Before you buy it, give our Swedish friend a chance to comment. I might try it with your rig. If you do it and your rpm drops at WOT; you might have found an inexpensive solution.

    I did notice the design of the boat was for twin 150s. I am guessing this was done to get out of the keel water.
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Nidza1 001.jpg
    Nidza, I think a "fake nose" air-separator would be a good start, see attached sketch (roughly to scale according to your installation) . Easily manufactured from oak or a PVC block. In this case, the forward corners should be given a generous radius. As an alternative, it could be made of alu.
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Very interesting post. I think you ought to try building one in oak. You wouldn't have to spend too much on it. Use white oak-it is tough as nails. Just form it all up and encapsulate it in resin and bolt it on. You wouldn't even need to paint it for testing. You could probably even build one that slid on for testing purposes; although you might need to wire it on from above. Then if it didn't do much for you; you wouldn't have holes in the unit. And if it did work, you could perm mount as shown.

    I put a hydrofoil on my boat last year and it reduced my mph by about 2-3, but it stopped the porpoising and the bow stays down much better and I have safer steering. I expect the real problem was due to the motor change from 2 stroke to four stroke and the weight delta. Downside is fishing in the weeds makes for a bigger mess around the prop.

    I still think a hydrofoil might work here as well by forcing the bow down and getting you out of keel wake quicker, but it might not be enough, so I'd be tempted to try Baeckmo's suggestion.
     
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  4. Nidza
    Joined: Nov 2016
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    Location: Belgrade, Serbia

    Nidza Junior Member

    @baeckmo
    I really, really appreciate your effort for detailed drawing! Thank you. I will try this one as a winter project and can't wait for the results. Not sure yet about the material, but all are feasible for me. One question: Am I right if I think that the difference between this and hydrofoil is that that this air separator, as you have nameded it, prevents "waterless" path to the prop by redirecting the water, while the hydrofoil prevents air from the surface to reach the prop if "waterless" path exists, like bigger extension to AV-plate (hydrofoil)?

    @fallguy
    Do not worry that you did not notice air problem at the beginning, the old thread is long and it really asks for a patience to read it all and the responsibility on the project is only on me, but good and sound advises and views are welcome from everybody. I need to be careful and that is why I ask multiple questions, even though some are even repeated, but some are still not answered. Good point that it is easy to find and try with hydrofoil. Thank you for advises about materials and temporary attachment of air separator by baeckmo.

    Still nobody commented the distance between left and right trim tab/flap from the propeller, as shown in last attached photo. Are trim tabs/flaps far enough from the prop?
     
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Will comment on that in due time, there is a life to live as well.......
     
  6. fallguy
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    fallguy Senior Member

    Can't you just move the tabs to an up position for testing? I doubt they are disturbing the water as much as the boat keel.
     
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  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I think you could mount a camera and then review what you recorded to actually see what is happening.
     
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  8. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    It sounds reasonable, my neck was stiff a couple of days, so life was a bit slower and forums were good fun and smart used time. Well, I have probably overused them, for sure actually.

    Full up position of tabs is almost exactly zero degree, I mean straight with the hull bottom. I could de-attach the motors from the tabs and "tie" up both tabs and the motors (electrical actuators by ElTrim). I presume you meant by this comment to put tabs at angle above zero degree? About the keel, possible, but someone said earlier that the keel seems far enough from the sterndrive. Anyway, the other boat shown in the video with VW engine has sterndrive mounted on some extension so it has a bit more distance between keel and outdrive, but the owner said it was inherited that way, so he kept it that way when changing engine. I do not have optimal view from the side showing keel and sterndrive, but I have attached the best I have and the keel itself when watched from stern. 00d.jpg 00a.jpg 00b.jpg 00c.jpg

    Not a bad idea, I ccould borrow one such camera, but it would have to wait for a longer period of stable river level, since only then the river is clear enough to see through the water far enough.
     
  9. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    It would be OK to see the flow around the AV plate, thus not necessary to see through the water.
     
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  10. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    OK, when weather and free time allow it, I will make a video of flow around the AV plate so we can see what is actually happening there.
     

  11. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Seeing what is happening will end the mystery.
     
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