flow of water pass transom question

Discussion in 'Boat Design' started by choppy, Nov 17, 2011.

  1. choppy
    Joined: Oct 2011
    Posts: 16
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: SC

    choppy Junior Member

    Are there any websites that would have diagrams showing the different ways water will flow pass different types of transoms?

    Thanks,
    choppy
     
  2. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Qualitative Investigation of Transom Stern Flow Ventilation by Maki, Troesch and Beck discusses different flow regimes past a transom as a function of Froude number based on still water transom immersion depth.

    What are you expecting when you say "different ways water will flow pass different types of transoms"?
     
  3. choppy
    Joined: Oct 2011
    Posts: 16
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: SC

    choppy Junior Member

    related to creating an eddy or drag

    Thanks,
    choppy
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,126
    Likes: 498, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Choppy, it might be more advantageous if you described more completely what you are trying to find out, possibly establishing some basic parameters. In other words, there are so many possably variables, that offering much more then a generic "you'll get bubbles" is all you can expect from such a broad based question. Transom shape in profile, plan, immersion and hull form, Froude number, etc. will narrow down the possible offerings from discussion participants.
     
  5. eyschulman
    Joined: Jul 2011
    Posts: 253
    Likes: 8, Points: 18, Legacy Rep: 77
    Location: seattle Wa USA

    eyschulman Senior Member

    Choppy JayBenford in his book Small Ships has a nice simplified write up on pg 36 about the effects of different stern configurations.The article is titaled The Hight Of Fuelishness
     
  6. cyclops2
    Joined: Sep 2010
    Posts: 242
    Likes: 8, Points: 0, Legacy Rep: 94
    Location: New Jersy

    cyclops2 Senior Member

    In the Saint Lawrence River we have possibly every shaped hull going by from small freighters to ocean goers.
    Same for pleasure boats.

    Problem is, the ones with no wake crests along side the hull. They look harmless. They are the ones that create MASSIVE SUNAMI waves when the more powerfull underwater pressure wave hits a ledge 10 feet down. 1/2 to 1 mile away. Almost sank my 19' bowrider. Head on.
     
  7. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Choppy,
    For boats like the Grand Banks I've always thought it was stupid to have a highly submerged transom like a planing hull and then run the boat at 7 knots where a full displacement hull would be the obvious best design but in many conversations I have to admit the difference in efficiency is'nt as great as I had once thought. I'd still want my old GB 36 with a full disp stern and think it is the only correct stern for the boat w a single 120 hp engine. But I have an idea now why the flat and submerged transom is'nt as bad as I thought it should be. On a fairly steep mountain wind on the surface can be traveling up slope on both sides. A north wind will cause an up slope wind on the north face to be sure but on the lee side of the mountain a rotor wind frequently occurs. The air going over the mountain goes up and then back down. Part of the air goes south and part of that air goes back toward the mountain and up the south face. This "reverse" wind is called a rotor and that does (I think) happen at the stern of a planing hull w a flat submerged transom. You can see the water over the stern looks like it's in a washing machine. Evidence of lots of turbulence and the obvious drag associated with it. However at a speed considerably less than planing speed a rotor may exist similar to the mountain example. The water over the stern looks like it's jumping up and down more than rotoring but the rotor effect could be taking place and to some degree pushing the boat forward. Other than the eco's in my head I've never heard of any such thing but consider it a possible explanation why the efficiency of a planing hull at disp speeds is'nt as bad as I think it should be.
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,126
    Likes: 498, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Most of the boats that you are describing need a really wide stern, just to hold up the aft cabin, washer/drier combo and twin diesels. They must have this bearing area aft or they point their bows to the clouds. You can "tuck up" the buttocks aft to make better flow, but this generally requires space restrictions in the stern (bye bye aft cabin headroom) and more length for the same internal volume. Since slip fees are based on length, every effort is made to pack 10 pounds of boob in a 5 pound bra.
     
  9. choppy
    Joined: Oct 2011
    Posts: 16
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: SC

    choppy Junior Member

    Thank you for taking the time to post. Those photos were really helpful. Thank you for providing the link.

    choppy
     
  10. choppy
    Joined: Oct 2011
    Posts: 16
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: SC

    choppy Junior Member

    Sorry that it took me so long to reply. Thank you for taking the time to write down your thoughts. Your reply has made me feel better about my design. You have described my design exactly. My first goal is a boat that easily planes but I don't want it to be a "dog" when moving under planing speed.

    Thanks,
    choppy

    choppy
     
  11. choppy
    Joined: Oct 2011
    Posts: 16
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: SC

    choppy Junior Member

    Wow! Those photos are really interesting. The more I look at them the more stern ideas start to pop in my head. Would you know of any other sites that would show additional photos and stern designs?

    Thanks for all the help,
    choppy
     
  12. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 111, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    My first goal is a boat that easily planes but I don't want it to be a "dog" when moving under planing speed.

    Then don't immerse the transom.
     
  13. Ad Hoc
    Joined: Oct 2008
    Posts: 7,773
    Likes: 1,678, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    It is all about the energy of the wave system they produce, not a single amplitude wave,per se.

    Those boats that PAR mentioned are all low Length Displacement ratio...terrible. Very high residuary resistance....as noted by their wash.
     
  14. Yobarnacle
    Joined: Nov 2011
    Posts: 1,746
    Likes: 130, Points: 63, Legacy Rep: 851
    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    No wave crests along the hull? Answer is obvious. There are 3 types of resistance. The greatest is wave making created along the hull moving through water. The faster she goes, the further apart the crestlets get stretched. Eventually at some speed, there is one crest at the bow and another at the stern. This is hull speed and longer the waterline the higher velocity when reaches hull speed. To exceed hull speed means driving the vessel up hill over its own bow wave. Surfing on the face of its own bow wave is called planing. The vessels you see with no crests alongside are at maximum speed, hull speed. Any additional power they are applying gets dumped into a wake forming turbulent pressure wave astern. You are observing bad handling and ignorance on part of whoever is conning vessel. Notifying their company will get them educated. They are also wasting fuel ie: money. Companies disapprove of wasting money.
     

  15. PAR
    Joined: Nov 2003
    Posts: 19,126
    Likes: 498, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

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.