Fitting a anti roll tank to a deep vee planing hull

Discussion in 'Stability' started by FishStretcher, Dec 28, 2013.

  1. FishStretcher
    Joined: Oct 2011
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    FishStretcher Junior Member

    I am considering fitting an anti roll tank tank to a small and somewhat tender cuddy style deep vee power boat. It has a fairly fine entry, a variable deadrise planing hull and a 26 degree deadrise at the transom. LOA is 24'10" (7.56m) and beam is 8 feet (2.4m) even. Displacement would be 4500 lb (2000kg) lightship, and about 6200lb (2800kg) fully loaded. Power is a single 300HP (225kW) counter-rotating I/O.

    It is said this hull was designed for a bit more mass low and forward than it has. A NA who owns a few fitted them with add-on chines with a narrow tall air pocket to aid with stability at rest and the compressible air doesn't cause slamming, it is said.

    It will cruise at a bit under 30knots and top out at about 40 knots.

    The cockpit sole is below the rear deck by perhaps 8"/ 200mm and that continues into the cabin.

    The cockpit sole is near full width, but short, fore- aft. 20"/ 500mm.

    Standing in the cockpit, there coaming is about shoulder height. Perhaps 5 feet (1.5m)

    With a narrow deep vee, it likes to roll at rest. It came with wooden boxes made to stand on on either side of the cockpit sole for shorter captains so the can see over the instruments properly. I think I can use these volumes for the two tanks of a passive anti roll tank system, with perhaps 13 gallons of water in 26 gallons total volume for about 100lbm/45kg or about 2% of displacement. (A guideline I found online.) If I fit the system with bilge pumps and filled with fresh water, I could empty them if necessary in an emergency or use the water for deck washdown.

    This seems reasonable in scope to try to me, and removable if I want.

    Has anyone tried such a system on such a small craft?
     
  2. FishStretcher
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    FishStretcher Junior Member

    The red lines are the lines of the hull in the first image. I will try to locate a line drawing of the cockpit placement, but the 2nd picture is mid-rehab of the cockpit sole.
     

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  3. Tad
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    Tad Boat Designer

    Unfortunately those small tanks down low in the boat won't do anything, they need to be up high(on the pilothouse roof or at least above the rail) to have any effect.

    But I suspect the roll period of this boat will be too fast for them to work at all. Typically small vessel Flume tanks are rectangular with the long dimension full beam of the boat (or close). What sort of connection between the tanks are you intending?
     
  4. gonzo
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    gonzo Senior Member

    If you add the amount of water needed to stabilize the boat, it will not get up on plane. Tanks are used in relatively slow boats where the extra weight is not a critical issue. A flywheel system would be more adequate. However, anything you install will take a huge amount of interior space.
     
  5. TANSL
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    TANSL Senior Member

    Any weight you put reduces the ability of planing.
    Have you thought about bilge keels?. Only increase friction and are very effective at low speed or boat at rest.
     
  6. NavalSArtichoke
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    NavalSArtichoke Senior Member

    You really haven't discussed why you think your boat needs anti-roll tanks. What kind of unusual rolling are you experiencing?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I am surprised at some of the answers here ! Ballast tanks are not that uncommon on tender vee-hulls. Naturally, they are designed to drain quickly once power is applied, and they can empty quickly if the vent is large enough, so planing is not delayed too much. They need to be air-vented so they fill (and drain) quickly when the boat drops off the plane. But they also have to be substantial to make a discernible difference, and there is usually a fuel tank located where the ballast tank would be best centred. Here is one manufacturer's slant on the problem:


    http://www.barcrusher.com.au/technology/quickflow/
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think anything less than about 100 gallons would be worthwhile in a boat this size, and it is no small problem to retro-fit in a boat it wasn't originally planned for.
     
  9. gonzo
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    gonzo Senior Member

    100 gallons of water will need at least 240 gallons of tankage and piping. Add to that the structure to hold it all in place and a pump to fill it. How do you fit that in a 24' boat?
     
  10. Tad
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    Tad Boat Designer

    Ballast tanks and anti-roll tanks (Frahm or flume) are two very different things. Ballast tanks in deep-vees are on centerline, usually free flooding, and let the hull sink to immerse the chine and aid at-rest stability. Anti-roll tanks use water moving across the vessel in opposition to motion to reduce rolling.
     
  11. FishStretcher
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    FishStretcher Junior Member

    References I have found online indicate ~2% of boat mass as design target. Which makes 100lb /45kg of water a good starting point, if I believe resources like these:
    http://www.neely-chaulk.com/narciki/Anti-roll_tank

    With a payload of 1600lb, and this using 100 lb plus a bit more in tank and pipe mass, I think I can get on plane with the existing 300hp I/O.

    This is something I can mount amidships fore-aft and on average at the waterline at rest. Some calculations last night led me to believe that it could be plumbed with 4" /100mm plastic pipe and not have a problem with fluid friction in the interconnecting pipe. And I still like the idea of it being used as a reservoir for post fishing deck fresh water washdown.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, I follow you, I assumed (wrongly perhaps) that the OP was really talking about ballast tank(s) and not something to slow down the deep-vee flop, just reduce the amplitude of it. I'm not entirely sure now what his plan is, but I'm sure 2% of the displacement as ballast would be ineffective.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, I don't see that it needs any volume beyond the 100 gallons, the whole lot flooded at rest. But as I said, the hull interior would basically need to be designed around it, retro-fit would not be easy. But it seems we may not be talking about ballast that is shed underway in this case, but some much smaller volume that gets carried everywhere. I doubt to any avail though.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Agreed in that it will not be as effective as one would think. A boat with that much deadrise is just going to be what it is, without amas, flopper stoppers or something.
     

  15. gonzo
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    gonzo Senior Member

    Mr Efficiency: 100 gallons of water needs a 100 gallon tank on each side plus piping. That is 200 gallons plus.
     
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