Turning a planing hull into full displacement

Discussion in 'Boat Design' started by fiftysixer, Oct 23, 2009.

  1. fiftysixer
    Joined: Oct 2009
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    fiftysixer Junior Member

    Could a steel planing hull be turned into a full displacement hull by welding on a bow bulb?
     
  2. JLIMA
    Joined: Oct 2009
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    JLIMA crazed throttleman

    i wouldn't try it planning hulls are meant to be their most stable while on plane, besides most boats of the planing type are too small to benifit from a bulb in anything except a flat calm. Imho you would be better off not trying to do such a conversion. How ever if you still want to go ahead, i'm just saying i wouldn't do it.
     
  3. gonzo
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    gonzo Senior Member

    There are displacement and planing speeds for hulls. The design takes into account the target speed. The bow is only one of the design parameters. However, a bow bulb is not even close to converting a hull designe for planing speeds to be efficient or effective at "displacement speed"
     
  4. apex1

    apex1 Guest

    In short NO
     
  5. Homefront
    Joined: Oct 2009
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    Homefront Junior Member

    You would also have to address the water flow at the stern. No small task, but it could be done if you don't mind increasing the OAL.
     
  6. apex1

    apex1 Guest

    Jaja, and what would be the OAL then?

    lets see it so: you can make a Porsche pull a trailer like a Truck, you can make it going slow like a Truck, but you can not transport a fridge, without destroying what makes it a Porsche.
    Sell the Porsche, buy a Truck! Or be fine with the given limits, anything else makes you unhappy and wastes your money.

    Regards
    Richard
     
  7. messabout
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    messabout Senior Member

    Apex; SO you are advising the OP to sell the planing boat and get a proper displacement boat if displacement is his choice. I heartily agree.

    Although I have never seen a Porsche pulling a trailer, your analogy is sufficient.
     
  8. apex1

    apex1 Guest

    No messy, I´m not recommending to sell the boat. I did comment in deep at his other thread.

    I´ve seen Porsche´s pulling Horses and boats on trailers, does that count?;)
     
  9. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Turning a planing hull into a full displacement

    fiftysixer,

    Yes. Homefront is on the right trail. Change the quarter beam buttock line (QBBL) of the hull. In a sentence, change the shape of the aft part of the hull below the WL so the bottom ramps up from about 1/3 fwd to the transom at approx the static WL. Basically it must be a hard chine boat. Exotic composites and ferro-cement would be difficult but wood, steel or aluminum and fiberglass should present few problems .. mostly just work. For example: a 36' GB w two 120hp engines burning 6gph at 8 knots, if her stern was turned up so the transom was out of the water would go 7.5 knots w two 55hp engines burning 1.5gph. I was once hot to convert an old Chris sedan (34'). Would have been ideal for the mod. A fairly narrow boat w a nearly flat bottom aft. You can choose how "displacement" you want it to be by adjusting the height of the bottom edge of the transom. If you want the above GB to go 8 knots and only burn 3.5 to 4gph leave about half the amount of submerged transom of the original configuration submerged. In quarter beam aft seas the resulting boat will be considerably easier to control directionally. If fiftysixer has plenty of time and not much money this mod could be a good move. The expense in money and time would need to be ballanced w the advantages of the end result. I am only an amateur NA so anything I say needs to be considered an opnion but I believe the above to be true.

    Easy Rider
     
  10. apex1

    apex1 Guest

  11. Easy Rider
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    Easy Rider Senior Member

    Apex1,

    Am I to assume a Roamer is too old and rusty to weld?

    Easy Rider
     
  12. apex1

    apex1 Guest

    Of course not!

    I restored ships more than twice that age.

    But I can see little sense (in fact none), to convert a good functional boat, a sort of oldtimer actually, into a krook. Not capable of what the owner wants in the end.
    Leaving it as it is, provides the trolling speeds he wants, though not perfect, but it does. And it gives the opportunity to go home fast and comfortable.
    A conversion as planned (or discussed), may end up in a more fuel economic vessel at trolling speeds, but the speed wil be gone, and it would cost several times the (now wasted) fuel over hundred years of service.
    I doubt the boat will see much more than another hundred years of service.

    Let me know if time has proven me wrong, ok?

    Regards
    Richard
     
  13. fiftysixer
    Joined: Oct 2009
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    fiftysixer Junior Member

    I bought the fiftysixer in August without even seeing it out of the water. My goal is to figure out what type of hull I have to figure out optimal cruising speed for the best economical results.

    At first I assumed that it was a planing hull. After logging some hours on the water I am not convinced. Before I had to shore her for the year i ran some speed to rpm tests.

    @ 1200 rpm = 6.26 knots 1500 rpm = 7.65 knots at these speeds the boat felt 'right' kinda gliding through the water with no sense of strain. Then I pushed her to 2800 rpm = 18.25 knots then 3100 rpm and could not break 20 knots and the boat felt very strained and had some vibration.

    The boat has twin 350 cubic inch crusaders - model 270. Below are some pics I took today when she got pulled. Can someone please help and tell me if I have a porsche or a truck?
     

    Attached Files:

  14. apex1

    apex1 Guest

    You have a planing boat with a moderate hull form and according performance. A heavy boat btw. What did the crane gauge say?
    A Mercedes 220 Diesel!
    A nice one.

    You said something like 28 knots full throttle in one of your former posts, I do´nt understand the difference now?

    Richard
     

  15. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    What you have is a Chevy. Those boats give you a nice ride at moderate speeds. At the time, their speed was respectable.
     
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