Tunnel in Hull

Discussion in 'Hydrodynamics and Aerodynamics' started by Bukmaster9, Jun 18, 2021.

  1. Bukmaster9
    Joined: Jan 2021
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    Location: New Orleans, Louisiana

    Bukmaster9 Junior Member

    What is the purpose of a tunnel like this? This boat is an inboard so I’m assuming it serves that purpose. I am changing it to an outboard. Will there be benefits or risks to keeping or removing the tunnel ?
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    There are several reasons why that might have been adopted, certainly to reduce the hull draft would be one. What is the size of the boat, existing power, any other pics ?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I'd say it is rather obvious...

    There is a rudder... check... and what is behind it...a prop..check.

    Thus to aid a lesser draft aft...the hull has a tunnel to allow the prop to sit in it...thereby, not increasing the overall draft.
     

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  4. Bukmaster9
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    Location: New Orleans, Louisiana

    Bukmaster9 Junior Member

    The boat is 24' and I am stretching it to 27 and putting a 400 verado on it. I do not have any other pics it just got picked up today and is being trailered back to the house. Will it matter if the tunnel stays after the boat rebuild is complete with the outboard? I should note the outboard will be on a bracket set back about 30''
     
  5. Bukmaster9
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    Location: New Orleans, Louisiana

    Bukmaster9 Junior Member

    Understood. How will it affect an outboard placed on a bracket set back 30"
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It seems quite flat bottomed, the flow of water to your outboard might be a slightly ticklish problem with such a shape ahead of it, but if you allow some room for engine height adjustment, should be OK. However, a 400hp Verado seems like complete overkill for what is (seemingly, more pictures needed) a relatively modest vee.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    If it were placed directly at the transom i.e in the 'tunnel', then it should be ok. In the sense that it is not doing anything different to the prop.
    But placed back by some 30" = 760mm.. then I would expect some cross flow and eddies/turbulence in the water flow from the shape influencing the flow into the prop.
    How much.. unknown.
    Suck it and see...
     
  8. kapnD
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    kapnD Senior Member

    There was a thread on the same subject here a few months back, I couldn’t locate it, but if you use the search engine diligently, I’m sure it’ll pop up eventually.
    Anyway, it concluded with the tunnel being filled, as no amount of outboard motor height adjustment could set things right.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Technically, you don't need to fill it.
    Just a simple thick plate covering the bottom, flush with the hull, and you can leave the back (transom part) open to flood at slow speed ...
     
  10. TANSL
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    TANSL Senior Member

    As not everyone has the enormous knowledge of a naval architect, we have no choice but to resort to a tool that is available to many of us, common sense: it is very possible that anything strange in front of the propeller of an outboard engine is detrimental to its good performance. The configuration shown in the photograph would surely have advantages that outweighed the disadvantages, which also existed. Therefore, it was decided to adopt that solution. Once the propeller and rudder of the old configuration have disappeared, the advantages of that configuration disappear, leaving only the disadvantages. It therefore seems logical to think that it would be best to completely eliminate this strange configuration of the stern and adopt a more conventional solution. Try to fill the hole, line it with glass, and create a normal outboard engine transom. This is probably the simplest solution, the cheapest and, of course, the most effective. (Should I define "effective"?)
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    regardless of the complications that may arise from the "furrow" left over from the old installation, I don't see any place for a 400hp engine on that hull, unless you want to put it into orbit, unless my eyes deceive me, it is quite a low deadrise boat
     
  12. DCockey
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    DCockey Senior Member

    How does low deadrise at the transom affect how powerful an engine should be used?
     
  13. Bukmaster9
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    Bukmaster9 Junior Member

    The hull new, at 24' is rated for a 350.... stretched to 27' I can't see how it will be an issue.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    a 350 cubic inch motor ? What is this boat, it could be an optical illusion, but looks south of 15 degrees deadrise ? The point is this, to every boat used offshore, there is a "happy" cruise speed, and from what I can see of your boat's bottom, it would not be at all happy cruising in average offshore condition at the 30 knots that would justify a 400hp motor.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless he is running on flat water, up the Mississippi, there is no value in installing huge outboard motors that are geared to use on fast boats that can cruise at brisk speeds offshore without shaking and stirring the occupants too much. No way that boat is in the "30 knot offshore cruise" genre. which it would have to be, to be sensibly installing that engine.
     
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