Need Advise : Flat Bottom + V Seaworthy ?

Discussion in 'Boat Design' started by bingo, May 26, 2013.

  1. Albert Jr.

    Albert Jr. Previous Member

    Boat will do fine in those waves.
    I've rode one myself powered by a 50 hp e-tec.
    Two things though.
    1. Get a hydrofoil (extra long fins drilled onto the lower unit)
    2. Handling can be a bit hard, the boat loves to wobble not because of the bottom but
    because of the flat keel.
     
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    With how many people and what load ... ?

    It seems that all the experienced designers have identified the realities of the situation, but Bingo hasn't updated his optimistic weight table.

    This is a classic case of a beer budget chasing champagne tastes.

    If the boat seller is serious, they wouldn't object to a test, with a realistic load of 5 x 90 kilo divers plus lead belts, suits, lunch, water, cameras, fishing gear, face masks, fins ...

    Also Bingo, if this is a money making concept, a bigger boat will hold more people, more comfortably, and payback the investment a lot quicker, if that helps.
     
  3. Albert Jr.

    Albert Jr. Previous Member

    It was me with my dad so around 380 lbs.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    As Tad has mentioned and I hinted at, a reasonably accurate PPI (or KPC) for this boat is a fairly simple formula (LWLxBWLx.75= water plane area then x 5.34 for pounds per inch or x 10.25 for kilos per centimeter of immersion). So, assuming a 16' LWL and a 5' WL beam, that's a 320 pound PPI or a 615 kg per centimeter.

    5 crew is 900 pounds or 408 kg, plus another 200 kg or 440 pounds in gear (sounds light) and now you toss in ground tackle of at least 100 pounds (45 kg), fuel, engine controls, steering, coolers full of beer and well, she starts to look less inviting for anything more then puttering around a well protected lake.
     
  5. richard gray
    Joined: Mar 2013
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    richard gray Junior Member

    drag?

    how do you estimate the drag a hull would have? a flat bottom versus a cat ( versus a boston whaler hull) on a 20 ft boat?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    With a 20 HP outboard on this boat, with the anticipated loads it'll have, drag on this boat would be the least of his concerns.
     
  7. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    I agree with the others that 20hp on that boat is underpowered to the point of being dangerous. Good power = good maneuverability = better seaworthiness in the hands of a skilled captain. I've surfed many 3'-4' rollers in a small aluminum flat-bottom boat, but good power is a prerequisite.

    Have you considered a RIB? They tend to offer better carrying capacities for their size/power requirements.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Multiply by 2 and you will have something to push the load you have in mind... Talking about loads, you will probably need to make your calculations with some 750 kg of payload, plus 250-300 kg of engine (either outboard or inboard) and fuel.

    But I frankly don't see where do you think to find a space for all these persons and their diving equipment. Even with an outboard it would be problematic - let alone a 100 HP inboard engine like Volvo Penta D3-110, which weighs alone 300 kg and occupies approximately 1050x720xH750 mm. You have to leave some space around it for air passage and servicing, so the engine box becomes a cubicle of at least 1100x900x800 mm. Even a smaller 75 HP moror requires a box of around 900x750xH750 mm. Try to find a cardboard box of that size and put it on the deck. It will give you an immediate perception of how little space will remain for all the rest of payload and equipment...
     
  9. bingo
    Joined: May 2013
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    Location: Karachi

    bingo Junior Member

    Test Report

    First I must Thank all of you for providing valuable advices.

    Yesterday we took the boat for stroll. Here's what we found,

    The boat felt under powered and gave us an average of 7.5-8 Knots.

    As for the boat it behaved well, 3 People on-board with fuel tank and extra fuel in bulge hold along with a lot of crap and safety equipment, must be weighing over 150 Kg. There was minimal rocking as compared to V hull & on occasion the skipper had to actively steer it through when the waves were getting us from stern.

    We defiantly feel it needs a 40 Hp engine, but for this season we got ourselves a dive boat ....

    Thanks again.
     

    Attached Files:

  10. bingo
    Joined: May 2013
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    bingo Junior Member

    Boston Vs Flat + V Vs Shallow V

    In the past few days we have tested a lot of boats ... with the same engine on.

    RIB 14' : We have too many sharp things to sink it :) ... So no no

    14' Boston Whaler : It gave us too much drag. 14' boston without the deck was it was under construction was doing 7-8 Knots and a very very bumpy ride, after we got off we didn't felt our bottoms for atleast 15 minutes.

    13' Shallow V : It went through the waves and into them :) ... handled well but seemed too tiny and scarry for a craft everyone was completely wet within 5 minutes. Rocked like a cradle on steroids.

    18' V + Flat : In all of the small boats we tested this one satisfied us if not completely but to an acceptable level. At 8 Knots the Front V cut thorough the waves softly and the flat didn't gave too much drag.
     
  11. bingo
    Joined: May 2013
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    bingo Junior Member

    This is a private boat for exploring our coastline, there will be max 3 divers with minimum gear and we make 8 hours trips and usually don't carry food.

    Essential dive gear + 3 divers + Compact camera + Other things
    Divers : 55+65+80= 210 Kg (checked)
    Small Compressor = 75Kg (checked)
    Diver Gear : 60x3=120Kg (checked)
    Fuel + Extra Stuff : 200 (Max)

    Total : 450-500Kg Max, we wont exceed over that.

    The concept is budget dive boat for shoreline exploration within 10 NM of shore.

    I am now convinced that i need a 40Hp but 20 will take me but we have to be weather weary.
     
  12. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I am amazed that you originally wanted to carry 5-6 divers on that boat. Good to hear you have changed your idea.

    But even so, when I read about your plans to carry 3 persons (something tells me it might be even 4) on an 8 hrs trip with no food, 10 NM from shore on a heavy loaded boat powered by a 20 HP motor, I can't help wishing you sincerely a plenty of good luck.
     
  13. 100islands
    Joined: May 2013
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    100islands Junior Member

    This is a nice looking boat for a run-about how can i get in touch with the builder? Thanks!


     
  14. bingo
    Joined: May 2013
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    Location: Karachi

    bingo Junior Member


  15. bingo
    Joined: May 2013
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    Location: Karachi

    bingo Junior Member

    Re-powered with Yamaha F75

    Today we re-powered this boat with Yamaha F75.

    Will soon upload the test results, I hope this solves all our problems.
     

    Attached Files:

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