5 metre hull - 30 knots - serious off shore passage maker - possible?

Discussion in 'Boat Design' started by flowtech, Oct 2, 2007.

  1. flowtech
    Joined: Oct 2007
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    flowtech New Member

    Hi I am a newbie here so I apologize in advance for my lack of knowledge on the subject. As of today I am on a very steep learning curve about hull design though - and hope someone can steer me in the right direction.

    My design short list of preferences is:

    1/ a 5 metre [15 ft] planing hull made of f/g;
    2/ fully enclosed low profile topsides with perspex viewing bubble for pilot;
    3/ able to cruise at about 30 knots plus [most conditions];
    4/ designed for offshore - predominantly average to below average sea conditions;
    5. power plant 120 HP inboard diesel [110 kg];
    6. endurance 250 litres of fuel [molded under cabin deck stainless steel tank];
    7. load [in addition to 1-6 above] will be an extra maximum of 600 kg;
    8. must be very stable in most conditions;
    9. chine walking or slapping should be designed out [or kept to a minimum];
    10. good steer-ability preferred;
    11. able to make some serious off shore passages;

    Q.
    a/ what are the main technical features necessary for a hull to satisfy the above?
    b/ do reverse chines have a roll in this design?
    c/ can ballooning out of the aft sections above the chine line add some stability and lift when rolling heavily?
    d/ how should I approach weight distribution along the centerline - and what are the principle components that make a boat 'porpoise'?
    e/ what would the hull shape look like?
    f/ what would the draft be?
    g/ is a surface prop appropriate for this spec?
    h/ am I dreaming?
    Regards
     
  2. kapnD
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    kapnD Senior Member

    C'mon Flowtech, pass it around!
    I want to try some of whatever he's smoking...
     
  3. flowtech
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    flowtech New Member

    Come on kapnD - you can dig deeper than that.
    I know your up to speed on all the trick bits.
    Was it the 30 knots that threw you?
     
  4. Geoh
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    Geoh Junior Member

    120 hp=10 gph fuel 1 gl per 20hp or so i have heard...Maui guy here...dont bogard it....
     
  5. Gilbert
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    Gilbert Senior Member

    It sounds like you are looking at least a 3000 pound boat when loaded and for that you would want about 150 horspower to go 25 miles an hour. I would recommend thinking in terms of a 24 to 26 foot boat both for maintaining a decent speed in rougher weather and for load carrying ability. Certainly a 3000 pound 15 foot boat does not make any sense. If you were not talking about carrying a 1300 pound cargo you would not be so far off with a 15 foot boat, although I would still recommend longer.
     
  6. Hotel Lima
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    Hotel Lima Junior Member

    Am I the only person here that would PISS THEMSELVES ROYALY in a 15ft boat OFFSHORE in ROUGH conditions?:eek: :confused:
     
  7. Geoh
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    Geoh Junior Member

    The Hawaiian "stuff" is coming around exhale slowly and in a little while it will sound better....
     
  8. kapnD
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    kapnD Senior Member

    Flowtech, 30 knots in a 15 foot boat sounds pretty cool, that is until you throw in the offshore word. Even at twice that lngth, you are talking about a very specialized hull to accomplish this speed in rough offshore conditions, comfort and safety aside. Item #7 in your list must be 600kg of some very worthwhile commodity that needs to be delivered in an out of the ordinary manner. Whats driving your design parameters? If you can forgo the 15 foot requirement, there are some very interesting discussions elsewhere on this forum.
     
  9. flowtech
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    flowtech New Member

    Now you’re coming around kapnD.
    As a result I have stretched it out to 6.0 metres [20 ft] and stripped off some payload - down to 350 kgs [this includes the fat pilot - 100 kgs].
    The diesel stays the same hp though 120.
    Have been investigating stepped hulls today with trim tabs aft for some efficiency and power economy. Seems like they are worth thinking about if the c of g can be maintained as near as possible to fixed, and some clever feed forward electronics controlling a constant angle of attack via the aft trim tabs. That way, the cambered step can pretty much be guaranteed to perform as designed if it is maintained within the operating criteria - which I gather is what is required for a step to perform with any degree of certainty.
    This hull is totally sealed topsides and deck walk around etc is not a consideration. It allows for a reduced freeboard and some other aerodynamic considerations as well to be employed topside.
    I like the idea of persevering with the stepped hull - as this spec needs all of the help it can get.
    This design has one role only – to bat away in most conditions at about 20 – 25 knots between fuel stops. Other considerations such as aesthetics / comfort / rough weather anxiety / etc [Hotel Lima] are irrelevant.
     
  10. FAST FRED
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    FAST FRED Senior Member

    "I like the idea of persevering with the stepped hull - as this spec needs all of the help it can get.
    This design has one role only – to bat away in most conditions at about 20 – 25 knots between fuel stops. Other considerations such as aesthetics / comfort / rough weather anxiety / etc [Hotel Lima] are irrelevant."


    First thing to locate is a Stidd seat (about $6000) to help you keep your spine intact in the 6 to 8 G loadings that will be common in moderate seas.

    In rough weather the seat will not help in the even higher G loadings.

    Yesterday it was blowing about 20K here in the Gulf, the buoy reported 9ft wave hight with an 8 second period.

    20-25K in that?

    Hardly,

    FF
     
  11. Quietboats
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    Quietboats Junior Member

    Dreamers advance society--Darwin maintians it.
     
  12. juiceclark

    juiceclark Previous Member

  13. messabout
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    messabout Senior Member

    Flowtech;
    Your screen name implies that you have some knowledge of fluid dynamics. Some of your comments suggest that, as well. You can apply some of your knowledge to the elements of the boat that are above water. Perhaps some below water too.

    That said, you are way off base with the 5 or 6 meter length. Ten meters with the 120 HP diesel is more realistic. You will need to moderate your requirement for speed on at least two counts. One is the power limitation and the more serious one is a matter of survival at sea. Fast Fred is right as are some of the other respondents. 30 knots at sea in a small boat is just an improbable scenario except in very large boats as in 30 meters or so. . In fact it is a suicidal one if you plan to go fast in a small boat for more than a 60 seconds at a time. In steep or confused seas you will break your body and your boat.

    Let's consider a boat that could compromise some of your wishes. It will be 10 meters minimum length, very narrow, moderate or deep vee design, kept as light as structurally possible, and you will need to be very conservative about the stuff you load aboard. The key word here is "compromise". At the present state of hydrodynamic technology you can not have all or even most of the attributes that you have described. Not to throw cold water (no pun intended) on your dream boat, but you must lower your expectations. Especially for speed.
     

  14. Gilbert
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    Gilbert Senior Member

    Stepped hulls typically have higher G-forces than warped bottoms under the same conditions.
    Your speed goals are not outrageous, which helps a lot, but certainly there will be conditions that will prevent you from making your average speed goals on some days.
    Go longer. You'd be surprised how much better 24 feet will be.
     
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