Fore to aft rocker.Why is it needed?

Discussion in 'Boat Design' started by ben2go, May 23, 2009.

  1. ben2go
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    ben2go Boat Builder Wanna Be

    All of the flat bottom boats that I have been looking at have a slight amount of rocker fore and aft.Why?I need a little more draft (10 to 14 inch draft) to remain stable.My understanding is that the rocker provides more buoyancy and gets the bow out of the water for faster planing.I'm not interested in planing as much as cruising.It seems to me that at displacement speeds,the bow would pound if it's out of the water and it would need more power to plow the bottom of the hull through the water.Wouldn't it be better to bring the bow down in the water and let it cut through instead of pounding and plowing across the top of the water?
     
  2. PAR
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    PAR Yacht Designer/Builder

    It depends on what type of craft you have or desire. Rocker is typically used in light weight displacement craft to offer maneuverability and reduce weight in the ends of the boat. It can also be used to provide internal volume, again to concentrate weights amidship and keep the ends light. On plane mode power craft, rocker usually exists only in the forward sections, where it will not detract from planning performance. Very much rocker in the after sections will cause the boat to squat too much for an effective planning.
     
  3. ben2go
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    ben2go Boat Builder Wanna Be

    The designs I am looking at have very little rocker but it runs the entire length of the hull.Deepest part of the rocker is roughly at the beam or just behind it.It is a power boat 24' LOA and a beam of 8'6".It's a sharpie type cruiser with a sharp bow and narrowing slightly at the stern.I'm not worried about hull volume,there's plenty without rocker.I was planning a dead flat bottom with a skeg that ran from bow to about 2' short of the stern and a port and starboard skeg for beaching.I will mainly be running at displacement speeds.
     
  4. lewisboats
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    lewisboats Obsessed Member

    If you are running at displacement speeds then you don't need a flat run aft...it will simply create more drag, use more fuel and make bigger waves and water disturbance for no purpose. The rocker aft allows the water to flow aft with as little disturbance as possible...thus not wasting any more thrust than necessary.What you are probably looking at is a Semi-displacement hull, designed to run at a speed somewhere between full planning and efficient displacement speed. these aren't very efficient or fast but tend to be popular.
     
  5. ben2go
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    ben2go Boat Builder Wanna Be

    So what would be the best design that allows the max interior space?The design I'm looking at is like the Bolger Idaho and Mark V 39 crossed.I'm looking for mainly fuel economy.I'd like to be able to get on plane for crossing vast areas of water,but it's not a deal breaker.My main area of operations will be large lakes.I am also planning a trip through the Great Dismal Swap Canal and ICW.
     
  6. alan white
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    alan white Senior Member

    Best design for interior space has plenty of beam. At 40 ft, depth gained by heavy displacement doesn't usually add any cabin height to the ends of the boat since even a light 40 footer has headroom throughout.
    All things being equal, 2 ft of extra beam adds a huge amount of space.
     
  7. PAR
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    PAR Yacht Designer/Builder

    If fuel efficiency in a displacement speed power craft is desired, you want to part the water gently, move it around the boat easily, then allow it to reassemble at the stern, also as gently as possible. This means push the water down and around the hull form, in nice sweeping curves and terminating the same way.

    The biggest factors (affecting efficiency) you'll encounter will revolve around max beam. The narrower a 24' boat you could live with, the more efficient it'll be, assuming it's well shaped. You'll want the bow just immersed, maybe an inch, but you'll desire the stern to lift clear of the LWL as to not drag a huge hole behind you. For example the Mar V 39 has a 4.33:1 beam/length ratio and his 28 has a 3.73:1 beam/length ratio, both well above the norm for power craft. These would be considered "fat" by sharpie standards. Or if you look at Bolger's work, the beam/length ratio actually matches the true sharpie design traits (6:1 beam/length ratio), like the Sneakeasy, which is 6.2:1.

    The exit is also an important consideration. In displacement mode, it should be "clean and clear". In semi displacement you'll need some immersion, which will cost you big when this type of hull is driven at displacement speeds.

    If you want maximum interior space and efficiency in a sharpie hull, you have to use length, not beam to achieve it.
     
  8. alan white
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    alan white Senior Member

    Within a given length, adding to beam adds more space than any other way.
    I owned a Carter 33 with an 11 ft beam. The waterline beam was actually quite narrow. The interior was cavernous. Four feet between the settees, and then upper and lower berths each side. IOR racer nonetheless.
     
  9. ben2go
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    ben2go Boat Builder Wanna Be

    This all making much more sense now.So 24' LOA and an 8' beam equals one fat water plow.LOL
     
  10. alan white
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    alan white Senior Member

    If the 24 ft LOA boat has close to a 24 ft waterline, it will likely need the 8 ft beam to carry a typical sailplan. Ratios of length to width would be relative to length, meaning a shorter boat is inherently less stable than a longer one if the shape is identical.
    Therefore a 24 ft waterline boat might have a 3:1 L/B ratio and a 48 ft boat having exactly the same beam and depth, while having a 6:1 L/B ratio, would be exactly twice as stable.
     
  11. lewisboats
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    lewisboats Obsessed Member

    Actually the stability varies as a 4th power of the lwl so instead of double the stability, it has actually 16 times the stablility. Heeling moment is only a 3 power so that is why you can pile on the sail on a longer boat with a larger LB ratio and it can stand up to those sails. These numbers are from Skene's by Kinney (it was the first one I laid hands on to verify my memory)

    Steve
     
  12. alan white
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    alan white Senior Member

    Read it again. You are correct only if all dimensions are doubled.
    I said, "...a 48 ft boat having exactly the same beam and depth..."
    In other words, the longer boat has thesame depth and beam as the shorter boat and the stability is doubled even when the LB ratio goes from 3:1 to 6:1.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Yes, in short that's correct Ben2go, an 8' beam 24' boat will be a plow all right. 6' of beam will be a dramatic improvement, but still not great efficiency. Anything less then 6' will progressively shown increasingly more efficiency gains, the narrower you get.

    Again, length is the only way to get more interior volume and still retain a high level efficiency.

    Shown is Floom (RYD-29.6a) one of my riverboat designs. The first (lefthand) version is 7' beam on 33' length, making for a boat with nearly 5:1 beam/length ratio. It's very efficient considering what it is. Her sister (RYD-29.6c) is her well fed sibling and 9' of beam. This extra beam does offer a lot more interior volume, but at a huge cost in efficiency. Her beam/length ratio is 3.6:1 and more in keeping with modern trends. Her dry weight alone nearly doubles with this extra beam, you just imagine how much more power it takes to push her.
     

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  14. ben2go
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    ben2go Boat Builder Wanna Be

    Great designs.I think I got it now.It's all about the ratios.
     

  15. ben2go
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    ben2go Boat Builder Wanna Be


    The plans I am looking at are for outboard cruisers,but it would be great to hang a junk rig on the final build.Heck I'd hang one up front and one around center.I worry about sails pulling over the designs I'm considering.I need standing head room due to a back,knee,and ankle injury.I can't duck walk through a traditional sail boat or fold and roll to use the head or cabin.I need shallow draft,but that doesn't kill swinging center boards or dagger boards.I need to run shallow less than 14 inches and max 18 inches.
     
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