Constant deep V deadrise. Powering up angle.

Discussion in 'Powerboats' started by cyclops2, Sep 18, 2011.

  1. cyclops2
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    cyclops2 Senior Member

    I am hopefull that it will lift up almost level at all power & speed settings.

    Yes. No. Maybe?

    Major factors, assuming it floats level at rest?
     
  2. Easy Rider
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    Easy Rider Senior Member

    What on earth are you talking about?
    Are you asking if your deep V will run level at all times? I hope not. You won't get any lift if it does and it won't "lift up" as you say ...at all. You need angle of attack to create lift and the deeper your deep V is the greater angle of attack you will need and the less lift you will have. If you need a boat to run level get a flat bottom boat, balance the cg and run it fast.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Eric, how much you want to bet this poster is/has "designed" a monohedron powerboat hull and wants to know if it'll "get up"?
     
  4. Easy Rider
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    Easy Rider Senior Member

    Hi PAR,
    Well whatever "upness" one gets it comes from angle of attack.
    Not sure what he means.
    Still have your "Digger" on my downloads but I'm leaning toward a longer L/B ratio. I'm really taken by the Atkins boat Tang. Not an easy build though.
     
  5. PAR
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    PAR Yacht Designer/Builder

    How much longer a B/L ratio do you want? Why? Digger is currently 2.83, at 20' she'd be 3.33, though most would find a 6' beam 20' boat a tad skinny. Her HD scantlings could handle a 20' station stretch.
     
  6. Easy Rider
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    Easy Rider Senior Member

    3.4 to 3.7 would be ideal.
    And a speed of 12 to 16 knots.
    Beam a bit over 7'
    Length about 26'
    The long and narrow boat is required to ride level enough to run efficiently at low speed.
     
  7. PAR
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    PAR Yacht Designer/Builder

    A 7' beam boat with a 3.7 B/L will be ~25' 10". The speed would be an easy task, how much power?
     
  8. tom28571
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    tom28571 Senior Member

    PAR, Easy,

    I assume you know this but everyone may not. Are you talking about WL B/L or aspect ratio? Of course, deadrise distribution fore and aft as well as deadrise aft plays a part in determining aspect ratio (B/L) and trim angle.
     
  9. Easy Rider
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    Easy Rider Senior Member

    Tom,
    Good point but I'm looking at boats that vary a lot that way. But I'm able to swing the design choice one way or another depending on disp or deadrise as the power is almost fixed. PAR, I have an 18' FG boat w a 60hp Suzuki and don't really like the boat. Want to sell the boat and trailer and get another boat for the engine. But I'll have the engine on the boat to demo the boat for sale and if the buyer offers me enough I'd sell the whole thing and then having the power option I may choose a different boat but probably not much different. I like the Atkin Tang the best but my own design and PAR's Digger are still on the table (among others). So 60hp is the power PAR. So a flat bottomed boat could be heavier and a deep boat could be smaller but then I would be so close to my present 18' boat I may as well keep it. And Tom I think beam/length ratio, aspect ratio (aren't AR and B/L ratios the same) and water line are expressions 99% of us should be familiar with. I personally hate acronyms and think they should be kept to a minimum. Especially due to the fact that there are so many people here that are amateurs (including myself) that aren't familiar w all boat design terms and expressions. But with such an extremely wide range of knowledge and abilities I think we amateurs just need to sink or swim at times. But for the most part accomplished designers here are very tolerant of lower level designers. Like me.
     
  10. tom28571
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    tom28571 Senior Member

    Easy, aspect ratio is an attempt to look at the same parameter as B/L but it is not the same as B/L since it does take the shape of the hull bottom at the waterplane into account. In that sense I think it is the more accurate way to look at the hull. Aspect ratio will always be a lower number than B/L except for rectangular barges. There are some hulls where the aspect ratio will be very different from B/L and a far more accurate representation of the waterplane. These would include boats with a long sharp bow and a relatively short length of the wider aft sections. In tubby blunt bow hulls, the two ratios will approach the same value. And yes, I am also one of the amateurs here although I study and experiment a lot.
     
  11. Easy Rider
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    Easy Rider Senior Member

    Tom,
    Then you're saying B/L is the aspect ratio at the water line? I thought either one was on deck dimensions unless otherwise stated. Yes, from a standpoint of hydrodynamic hull functions the waterplane dimensions are clearly preferable. Thanks for the response Tom.
     
  12. cyclops2
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    cyclops2 Senior Member

    Been away.

    I asked the lifting question because the British M T B that was sent over to the USA Electric Boat company, definately DID have that acceleration characteristic.

    My boat is 4' beam & 19.5' LWL. 32 degrees. 1500# wet weight.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Cyclops, Easy Rider is correct - you need a trim angle to get a lift from a planing surface. In particular, when going trough the "hump" speed range the trim will go through its peak value, then will level down again (if the boat is correctly designed for high speeds) as the speed increases into the planing zone.
    This behavior is less pronounced for very skinny (high slenderness ratio) hulls, as their resistance hump is very small, but it is still observable.
     
  14. tom28571
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    tom28571 Senior Member

    I had thought aspect ratio was better known than this Easy. Most all planing powerboat waterplanes can be seen as a rectangle covering the aft part of the hull and a triangle over the forward hull out to the WL entry. Beam is the AVERAGE width of the aft hull part and length is the sum of the length of the aft rectangle and 1/2 the length of the forward triangle. Thus the shape of the hull is taken into account and B/L means more to me than just the LWL and max WL beam.

    Many, if not most people do use the simpler version but then, a lot of people still use "hull speed" which is often meaningless and becoming more meaningless all the time as both sail and power boats, along with designers often ignore it.
     

  15. Easy Rider
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    Easy Rider Senior Member

    Tom,
    Do you think talking about hull speed on a non full disp boat like a old Grand Banks 36 w a squarish stern and a nearly flat run aft is basically meaningless? I do.
     
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