Advice on old aluminum boat

Discussion in 'Metal Boat Building' started by Thegimpcb, Sep 24, 2021.

  1. Thegimpcb
    Joined: Sep 2021
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    Location: Tampa FL

    Thegimpcb Junior Member

    Hello
    I have a 1958 aluminum prototype brown water river boat.
    It's a 26ft/8.5 meter semi displacement hull.
    It's a one of a kind odd boat that over the years it's been modified many times.
    It was originally a straight inboard then it was converted to an I/O 454 7.4 l I have converted to an outboard with a bracket because the water I run is very shallow until I break 8 miles off shore.
    The boat weights in at 4100lbs without motors. 7k fully loaded.
    My question is How much power do I really need?
    I know its not a speed boat I'm hoping to get a Cruse of 25 knots.
    If I put twin 150hp outboard motors on her is this to much?
    Also the bow dips below the aft plane so it should ride like a lobster boat but this is all a hypothesis.
    Thank you for any input.
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Any more pictures, particularly of the stern ? Are you sure it is that old ?
     
  3. Thegimpcb
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    Thegimpcb Junior Member

    I'm not sure of anything lol the paperwork states that it was built in navy shipyard #2 in 1958
    Here are some stern pictures
     

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  4. Thegimpcb
    Joined: Sep 2021
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    Thegimpcb Junior Member

    Here is the way I found it
     

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  5. Thegimpcb
    Joined: Sep 2021
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    Thegimpcb Junior Member

    I have spent weeks diging foam out of the boat and everything I've read says a boat this size does not need floatation foam
     

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  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    It does not look like it would need 2 x 150 even, being quite flat aft, a couple of things I would be concerned about, corrosion, and how that would handle in a following sea, it doesn't have much grip on the water aft.
     
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  7. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    She looks nice - if you managed to buy her for a song (ie not a lot). then even better.
    And if she was really built in 1958 then that should help to silence the doom sayers who love to tell everyone that aluminium boats fizzle away in sea water very rapidly! :)

    She appears to have a fairly shallow deadrise, hence I would think that 2 x 150 hp O/B motors should push her along reasonably well - I concur with Mr E above re how she might not 'need' this much power, but it is useful to have, and you might well end up saving fuel by running bigger engines at a lower speed, rather than smaller engines at higher revs, in order to obtain 25 knots.

    Re floatation, if you can create a number of sealed watertight compartments then so long as they are watertight (ie not breached), then they will do the same job that the foam did (when it was new) re helping to keep you afloat if the hull is breached or filled with water.
    Have you noticed any signs of corrosion in the bilges where you removed the foam, or where they all very dry?
     
  8. Thegimpcb
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    Thegimpcb Junior Member

    I feel like the bow will drive into and above the water I agree she may be pushed with following seas im hoping with almost 2 thousand pounds down her center line she will act as a displacement hull she has a 200 gallon fuel tank and 100 gallon water tank all centered
     
  9. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re how you want her to act as a displacement hull, do you mean travelling at 'displacement' speeds?
    This would be a fairly slow speed, probably 6 or 7 knots at the most.
    If you want to go faster, you have to get her up on the plane.
     
  10. Thegimpcb
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    Thegimpcb Junior Member

    There was some issues under the foam but nothing that was very deep most of it was surface reaction to salt water I'm having the hull sand blasted next week. so I can see any issues.
    The bottom plate in 5/8th thick it's built like a tank.
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Good grief, is it really? That is 16 mm in metric (I prefer metric) - more like a battleship than a tank even!
    You certainly have a huge margin for corrosion here - 1/4" or 6 mm would usually be considered to be more than enough for the bottom plating on a boat this size.
    And I see that you have some longitudinal frames as well as the deeper transverse frames.
     
  12. Thegimpcb
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    Thegimpcb Junior Member

    I feel she will plane like a lobster boat with a wide shallow aft she should pop up quickly I've done the numbers and she should plane around 12 knots
     
  13. Thegimpcb
    Joined: Sep 2021
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    Thegimpcb Junior Member

    She is reinforced so much so I feel they added foam to offset the big block motor placing the outboard motors on her she should balance about level understanding that I want a efficient vessel do you feel I could power her with twin 140hp?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    A boat like that, 20 knots would be a good cruise speed offshore, it is not a boat to be putting into a ballistic trajectory and re-entering the water, the boat might handle it, the passengers doubtfully. 2 x 150 seems a reasonable choice, I'd be inclined to select the engine with a little more reduction in the gearcase.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    With two engine legs in the water, broaching is damped a bit, but it does look like it would benefit from a well-placed skeg, if it does not have one presently
     
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