Stern shapes on displacement motor-cruisers

Discussion in 'Powerboats' started by marshmat, Jun 26, 2005.

  1. marshmat
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    marshmat Senior Member

    Looking at the few displacement-hulled motor cruisers around here has me wondering about the various stern forms. Rounded, canoe-stern, full transom, etc.... In terms of maximizing fuel efficiency and stability, and minimizing the wake, how do they compare? (Looking at the 10-30 metre size range mostly.) I've seen claims in favour of all sorts of different shapes, and intuition is at something of a loss on this one. Any thoughts?
     
  2. FAST FRED
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    FAST FRED Senior Member

    The best long distance transoms for low power are all on sailboats.

    They make no pretenses of speeds above displacement, and are very very efficent at low power & low speeds.

    The "problem" is usually the add folks , so "Semi Displacement " and other such hype starts to reduce the boats seaworthyness and range.

    The reason is, although thousands of "trawlers" or other "long distance" style boats are sold , DAMN few ever venture off soundings.

    If your going out , I would look for offshore scantlings in the hull & deck ,and outfitting.

    Quickest check is to see how thick the deckhouse/pilothouse glass is.

    1/4 or 3/8 glass the boat is probably only suitable for brown water use , 1/2 and UP and the rest of the boat might be usefull offshore.

    Good hunting,

    FAST FRED
     
  3. marshmat
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    marshmat Senior Member

    Seems there's two types of sailboats around here. There's some with sleek lines, solid fittings, etc that look meant for real cruising.The usual stern shape here seems to be a gentle curve upwards and inwards, converging in a rounded stern at about 10-15 degrees to the water?

    Then there's the type that are massive fat pigs with flat butts and tiny rigging. The 'slip queens' as they seem to be known, floating cottages that never leave port.... all those businessmen who have lots of money but no time to actually use the boat, they just want to say 'I've got a 40ft yacht down at Bronte".....
     
  4. FAST FRED
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    FAST FRED Senior Member

    Look at the 40+ year old cruising sail designs, for efficency at displacement speeds.

    The old comuters are even better if you want to "go fast & cheap".

    The dock queens are trash for toy folks,
    who want to sail an afternoon with 30 goons as crew.

    FAST FRED
     
  5. yokebutt
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    yokebutt Boatbuilder

    I have to say, I do find freds reasoning a bit simplistic at times.....

    Yoke.
     
  6. mackid068
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    mackid068 Semi-Newbie Posts Often

    Marshmat, you are on the money. You want a serious boat? Check out a passagemaker or an older design, not a "rocketship."
     
  7. xsbuilder
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    xsbuilder New Member

    Maximum fuel economy and stability are somewhat opposing concepts when you're talking displacement hull forms. For best mpg, a narrow, round bilge hull with a minimum drag stern (canoe type?) would be best to keep the wetted surface to a minimum (like a sailboat). But for max stability you'd want more beam and a wide squarish stern with little or no deadrise at the transom (more wetted area, more drag). I think the stern form is more about the accomodations and amount of useable deck space than economy for most displacement crusiers.
     
  8. ABoatGuy
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    ABoatGuy Member

    Marshmat, mackid068,

    Kind of a neat concept . . . check the date on the drawing. If it is old, it is good. Never a really bad design "in the day".

    Spent a good deal of time on rocketships with goons. Some of them were really bad and some are pretty good boats, especially the post IOR boats. It seems many of the good cruising boats were based on the rocketship ideas of a few years back.

    What ever rings your bell.
     
  9. fcfc
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    fcfc Senior Member

    All depend of the indended deplacement speed.
    Look at the evolution of design between nordhavn 46 and 43.

    Both have the same LWL,same engine, and similar weigths.

    The 46 has a canoe stren, the newer 43 has a wide, flat immerged transom.

    The wide transom has probably a lower efficiency in the low speed range (more wetted surface + transom drag), but has more deck space, more weight carrying capability, although ligther (bigger waterplane area), and more stability. What the 43 loose is probably some comfort on movement. (bigger waterplane, less weigth), but it regain some comfort with increased speed, and more space.

    And the 43 has a slightly lower weight, a bigger prop (1:3.79 redution). The 46 has only 1:3 redution. So the prop and the weigth of the 43 will probably have a better efficiency than the ones of the 46.

    And as the speed increase, still in displacement mode, the transom of the 43 will clear of water if not too deep, so will reduce drag, and the more aft LCB / LCF will further reduce wave resistance.

    So, at low speed, say when the transom of the 43 is submerged, the 46 has a better design. But weigth and prop design of the 43 should overcome it.
    At higher speed, bad luck for the 46. The 43 has a better transom + a better prop + less squat + less weigth. See http://www.nordhavn.com/43/enlarge/cruising_1.jpg

    BTW, the 46 is no longer produced.
     
  10. FAST FRED
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    FAST FRED Senior Member

    With todays overcrowded wsaters the shape at the aft deck is probably as important for utility as the underwater shape.

    My preference is for the completly rounded aft deck as seen on most harbor tugs.

    The ability to use the entire stern to spring , or turn with, makes bow & stern thrusters redundant.

    A very handy vessel is far more fun to operate than a lump that needs complexity and expense (with limited operation times) .


    FAST FRED
     
  11. marshmat
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    marshmat Senior Member

    A lot of modern designs do seem to have very squared-off transoms like those of planing boats.... my experience with running planing hulls at low speed is that you get horrendous energy loss as waves, and difficult tracking. Intuition would seem to indicate a long, thin, curved-bilge hull with a fine entry and with smooth (not turbulent) water flow around the stern. I see a lot of these modern square-tailed passagemakers just throwing huge wakes everywhere while not going all that fast. Whatever happened to efficiency? I'll blame it on marina fees favouring short, stubby and square.....
     
  12. mackid068
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    mackid068 Semi-Newbie Posts Often

    You know what modern "euro-designed" boats are? Garbage.
     
  13. marshmat
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    marshmat Senior Member

    They're only garbage when they're underway. Lying alongside the pier, they are gorgeous works of art. It's only when you have to come alongside the fuel dock, or slam through 8-footers, that they're garbage. Is that not right, Mac?;)
     
  14. mackid068
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    mackid068 Semi-Newbie Posts Often

    For sure! Looking at them: pretty, but underway...crap, total crap. Give me a nice, traditional trawler anyday (Nordhavn, American Tug, custom etc.). Then, we've got the issue of the party-barges: big yachts that can't do much more than a harbor cruise with lots of liquor.
     

  15. FAST FRED
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    FAST FRED Senior Member

    "Then, we've got the issue of the party-barges: big yachts that can't do much more than a harbor cruise with lots of liquor."
    __________________


    Most of the "biger" yachts I have seen were 100 to 350 ft long and can make a passage .

    Some of the smaller ones (onder 100ft) are designed to run at 75K (between harbors) and have little range at hi speed .

    The usual 50 ft "party barge" is EXACTALLY the harbor front cottage the purchaser desired.

    Many folks love the water , but have no desire to go ANYPLACE , or acquire the skills needed to get off the dock. SO?

    Its THEIR version of boating , SO?.

    Should they be forced to RV's because YOU think their recreation style sucks?



    FAST FRED
     
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