Water Dynamic Hull Form

Discussion in 'Boatbuilding' started by Matt.D, Feb 26, 2012.

  1. Matt.D
    Joined: Oct 2007
    Posts: 84
    Likes: 1, Points: 0, Legacy Rep: 6
    Location: gold coast

    Matt.D Junior Member

    Which hull stern is more water dynamic (has less drag) in a forward motion, taking into account the anti foul line is about 50mm above the water line?
    Gavan Construction Photos 004.jpg

    Glass houseboat modern styling 004.jpg
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,107
    Likes: 355, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    They are as bad as each other....not very good.

    But the lower image is better when going VERYYYYY slowly.
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 264, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Gawd, hard to get a clear sense of what we are looking at there. Any more pix ?
     
  4. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    And at what speed ? The top pic looks like it might be intended to plane - but without more info as Mr E said, hard to make a good guess.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    John is correct, both pretty much suck.

    The term "efficiency" is quite relative to a number of factors. What are your target WOT and cruise speeds?
     
  6. Matt.D
    Joined: Oct 2007
    Posts: 84
    Likes: 1, Points: 0, Legacy Rep: 6
    Location: gold coast

    Matt.D Junior Member

    Sorry forgot to mention, non planing full displacement hull average speed 8 knts. Glass houseboat modern styling 020.jpg

    Glass houseboat modern styling 032.jpg

    Modern Houseboat Havana G52..jpg
     
  7. daiquiri
    Joined: May 2004
    Posts: 5,372
    Likes: 239, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    It's Bauhaus applied to boat design! :)

    As Ad Hoc and PAR said, none of the above forms are really efficient at low displacement speeds, the biggest problem being the high wetted surface area and the transom stern.
    But I guess your bauhaus was probably not intended for voyages anyways, but rather for short trips from one (more or less) permanent mooring to another.

    Cheers
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 264, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The one with the anti-fouling certainly would have went better with a shallower angle of departure, or a curved exit, than what looks to be at least 30 degrees at the stern, but pottering around at sedate speeds should be OK. I'd be more concerned about strong headwinds, which should see you doing 2 knots backwards !
     
  9. Delane
    Joined: Apr 2005
    Posts: 91
    Likes: 4, Points: 0, Legacy Rep: 37
    Location: Okinawa, Japan

    Delane Senior Member

    For the purpose of that craft, both cutaways are a waste of time and take away useful buoyancy. Also for draft considerations, mounting the engines just inside of each hull would accommodate.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 264, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The cutaway idea is sound enough, but not the way it was executed. There is just too much turbulence with that abrupt change, which also gives the prop "bad" water to be working in. I don't think the loss of bouyancy is an issue, the thing is clearly not going to plane with the kind of installed power it has.
     
  11. Matt.D
    Joined: Oct 2007
    Posts: 84
    Likes: 1, Points: 0, Legacy Rep: 6
    Location: gold coast

    Matt.D Junior Member

    Thanks for the feedback Mr Eff. so if the transition for keel to transom was greater and it came up to the water line or slightly higher, this would give less turbulence yes?
     
    1 person likes this.
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,680
    Likes: 264, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I wouldn't be so concerned about where the sloped section meets the transom, a little submergence of the transom won't be too big a drag, and you need the prop underwater, at least where it is, otherwise it could leave the water at times. It seems to me that the transition from a flat, level bottom is too abrupt. It would have been better to start rising, preferably in a smooth arc, from somewhat further forward. There looks to be a penalty in hull drag, as well as having your prop working in water being dragged along by the boat, which is not ideal at all.
     
  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Both transition types suck at the speeds you'll be traveling at. The stepped shape being slightly worse then the abrupt tapered shape. Considering what this vessel is, there's very little you can do to improve over all efficiency. Even if you elected to carve up the bottom aft half of each pontoon and reshape them to a more efficient form, there's so much windage, you'll not see much more then a fraction of a knot improvement. In other words, a well done set of pontoons might get you from 8 to 8.1 or 8.2 knots. If this is what you want, then pull out the reciprocating saw and have at it.

    The question at this point would be, "what are you hoping to improve?" With some pontoon shaping, you might get a very modest speed improvement (lots of cutting, very little speed increase) or you could improve your fuel efficiency a tad, but again, the pontoons are just part of the equation.

    For example; if you took the tapered section and started this taper several feet forward of it's current position, you might see this very modest improvement.
     
  14. Matt.D
    Joined: Oct 2007
    Posts: 84
    Likes: 1, Points: 0, Legacy Rep: 6
    Location: gold coast

    Matt.D Junior Member

    Thanks for the input.
     

  15. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Consider that when a hull moves forward it displace water, then as the hull passes, the water has to come back into place. The less wake there is the less drag, the flowing out and the flowing back of the water should be as little as possible and with as little as possible movement of the water itself. Every ounce of water you move takes energy to do.

    You compromise a lot in any way you look, wrt flotation, speed, hull interior, comsumption etc. Nothing is perfect. If there was one perfect solution eeeeeveryone would be using it !
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.