Hull extension..now outboard ventilation issues.

Discussion in 'Boat Design' started by Brother Bill, Jun 27, 2020.

  1. brendan gardam
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    brendan gardam Senior Member

    I was about to say exactly that. The hull extension is turned downwards. You can see it in the photos. Get the fabricators to straighten it and your problems should be fixed.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If you are worried about a "hook" you should be able to bog that flat.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    What photo shows the extension has more angle of attack, Brendan ?
     
  4. brendan gardam
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    brendan gardam Senior Member

    The 3rd photo down looks like the extension is turned down towards the rear, unless its an optical illusion. See what you think. I might be seeing things.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    It doesn't seem much out of whack, to me.
     
  6. brendan gardam
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    brendan gardam Senior Member

    Yes, but a little bit makes a difference with all that surface area. The op says he picked up between an 1/8 and a 1/4 of an inch with the straight edge.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Still not a lot, and I'm not exactly sure what he meant.
     
  8. ondarvr
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    ondarvr Senior Member

    Nothing can be done to correct the hook.

    Back to the fabricator to fix his mistake.
     
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  9. Barry
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    Barry Senior Member

    You know that if you raise the motors 3 more inches that the props will ventilate easier.
    You know that if you drop the motors 3 more inches that the props will more than likely not ventilate

    So the path should be clear before you make a bunch of COSTLY changes to the hull extension. Move the engines down.

    As the jack plate is not working, though it may at 40 - 50 mph with a different prop, you don't need it. You should be able to build a manually adjusted, by that I mean just a series of holes into a rectangular tubing, to be fabricated by
    your guy, for very little money.


    The 1/8th trim down should not affect the trim much and this "rough " measured difference may be suspect. Depending on the thickness of the hull and the ORIGINAL transom to hull bottom weld, there exists a good chance that
    the weld BUMP that will present itself on the bottom the hull of the original transom to hull bottom bead, (print through as a crude description) MAY cause this small 1/8 inch discrepancy when measured say 2 - 3 feet away from the bump

    So before removing the extension, you really should absolute ensure that what you thought that you measured as "trim down" on the extension exists.

    A couple of ways to do this would be to sand down the weld print through to ensure a completely flat surface and then run the same straight edge and measure the trim down to see if it exists or take say a 2 inch flat bar of aluminum
    say 5 feet long. Starting from one side for these measurements, leave the first 24 inches straight , recess the next 12 inches say 1/4 inch (cut with a skill saw as compared to say a cut off blade to keep the heat influence at a minimum) , leave the next 24 inches straight. So you can span the area around the original transom to hull bottom weld joint area.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    A small amount of "hook" makes not a lot of difference at low speed, say at the point the boat is just planing cleanly, if he is experiencing the problem at that speed, then I would say 1/8 to 1/4 inch of hook, whether it be a "hollow" in the bottom panel, or just an extension that has a slightly greater angle of attack, is not going to a huge factor. I recall having a boat with some hook aft, and it only kicked in when you really throttled up. It will certainly be a problem then, we see the same even with generously sized trim tabs, slight application at lower speeds is hardly felt.
     
  11. ondarvr
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    ondarvr Senior Member

    A trim tab the size of that extension angled down 1/8 to 1/4" will have a huge effect. And would be just as he described.
     
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  12. Barry
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    Barry Senior Member

    Before cutting up the extension, I would purchase a level indicator of some quality. Stabila make them ($50)and you can check the running keel angle when on step at your cruise speed. If the deadrise is around 18 degrees, you would want a running trim angle of about 5 degrees for the best fuel efficiency at that particular cruise speed.

    If it is within a few degrees of this, then the issue is still the ventilation plate height.

    I have attached a poor sketch but
    1) The bottom sketch is a method of taking the hull line and carrying it over any inflection/bump at the original hull to transom intersection. ie the gap/relief does not contact this area. Note that this straight edge MUST be parallel to the keel, EXACTLY
    parallel or you will see a discrepancy, down turn or up turn that is not really there

    2) The channel, "B" is the piece that would be built to attach to the existing mounting profile that is on your extension. I would use something like 1/2 inch thick material but as I dont know the weight of the motor or the dynamic loading
    that the engines could place on this piece, you would have to make the call on this thickness. Judging by the thin material on the jack plate, this should be overkill.
    After B is built, then A would be welded to B to make a tubing.

    Really important here. I don"t know the diameter of the mounting bolts so the center to center spacing needs to be addressed. Touch base by PM and I can walk you through the spacing issue. With a proper starting measurement
    for the top hole on the B channel and a different starting measurement on the A plate, you can produce say 1/2 inch increments of height change of your motors. As compared to running the same measurement on both and only having say 1 inch
    changes.

    The idea of the channel is that you would use short bolts to attach the mounting assembly B plate to your existing mount area and short bolts to attach your motors to the A plate. Use larger flat stainless washers, 1 inch diameter prox and stainless nylocks as compared to lock washers

    The channel permits access to get a wrench on the bolts to tighten things up
     

    Attached Files:

    Last edited: Jul 10, 2020
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Not as much as you'd think, the key thing is the length of the leading edge, not the area of bottom. Anyway, it isn't established that the entire thing is at a different angle of attack to the old hull, it sounded more like a local "hollow", but I can't see much sign of anything untoward in the picture. If it is a case of an abrupt change of angle of attack at the junction of the extension and the old hull, that is where the lift will be concentrated. But whatever it is, even a quarter inch, over 30 inches, is 1:120, barely perceptible.
     
    Last edited: Jul 9, 2020
    brendan gardam and Barry like this.

  14. brendan gardam
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    Location: east gippsland australia

    brendan gardam Senior Member

    Makes sense. Will have to wait and see what they find I guess.
     
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