Planing vs. Semi-displacement

Discussion in 'Downeast Boat General Discussion' started by PatriciaLynn, May 16, 2019.

  1. PatriciaLynn

    PatriciaLynn Senior Member

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    Hiya,
    Someone recently told me that downeast boats ride so well because they are semi displacement and not planing. I disagree. Maybe the boats of the 60s and earlier, but modern skeg built lobster hulls are planing hulls, plain and simple.

    What say you?
     
  2. Blitzen

    Blitzen Captain

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    Yes, most with enough power they are average planing hulls. Not fast planing. If you run a prop calculator you will see that displacement doesn't work as the boats are achieving better than hull speed for a given length and beam and getting over the first wave fraction due to lift.
     
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  3. Yesac13

    Yesac13 Senior Member

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    Hence the phrase "Semi-Displacement". It's kind in the middle term. Downeast hulls do plane but... Downeast boats can operate very well as displacement hulls if you put in a gutless motor. Can't really say the same as most planing V-Hulls. Most people are familiar with planing V-Hulls... that's what most hulls out there are like. I mention this because don't want to mix V-Hulls with normal planing hulls which are flat at the stern but is not Downeast.

    I drove many V-Hulls, hate them at hull speeds or just above. They start going nicely at 18 knots or above. There's a 9-18 knot hole area that most V-Hulls just do terribly... bump the throttle a hair, you're going 18. A hair off, down to 9 knots. A hole there. Some call it the "Hump". That is how I got interested in Downeast hulls... often I want to just go 15 knots. Most V-Hulls simply cannot do 12-15 knots. Well, you can with some but you're pushing lots of water, bow pointing to the sky, etc. Just crappy.

    If you always are going 20 knots or faster, V-Hulls are great! Better than Downeasters at that speeds, actually. But... that hole area I mentioned earlier means V-Hull owners just don't want to go below planing speeds so they just go pounding hard at just above planing speeds leading to the Downeast smooth riding myth. I said myth cuz if you push a Downeast hull faster than 25 knots in rough water... the V-Hull would be nicer to ride in!
     
  4. PatriciaLynn

    PatriciaLynn Senior Member

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    Thanks for the thoughts. Sounds like we all agree, modern hulls are just low deadrise planing hulls. Not great at fast or slow, but great at going at moderate speeds.
     
  5. backman

    backman Captain

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    TANSTAAFL however. While a DE can cruise comfortably in that 9-18 knot “hole” it loses its fuel efficiency one you go faster than hull speed.

    In my NB36 for instance I’m 2.5 GPH at 8 knots, 4 GPH at 10 knots, then 9-10 GPH at 12 knots and pretty much 1.1: from that point through high cruise.
     
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  6. Ben Zehnder

    Ben Zehnder Member

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    In my non-technical view, which is all I seem to have, it feels as if the hull doesn’t get up over the water so it doesn’t impact coming back down. I don’t miss the days of coming home from a long day fishing in my old Steiger 23 ( which was a great boat) with a sore lower back.
     
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  7. PatriciaLynn

    PatriciaLynn Senior Member

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    The wide, flat transom helps keep the bow down. I would guess that downeast hulls are significantly heavier on average and has the weight further forward than something like a steiger, which helps slow the up and down motion of the bow.
     
  8. PatriciaLynn

    PatriciaLynn Senior Member

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    What hulls do better than 1mpg at 20 knots or above? Not many in the 35' and up range. I would bet some of the 40 something single engine carolina hulls can do that, but not do much better.
     
  9. backman

    backman Captain

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    It’s actually 1.1:1 , I burn 18 and change to go 20 knots. An equivalent 36 sportfish with twins is about .67 to .75 nmiles per gallon!
     
  10. south shore

    south shore Senior Member

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    BenZ
    In my non-technical view, which is all I seem to have, it feels as if the hull doesn’t get up over the water so it doesn’t impact coming back down. I don’t miss the days of coming home from a long day fishing in my old Steiger 23 ( which was a great boat) with a sore lower back.

    My 23 Steiger Chesapeake was a couple of hundred pounds heavier than my 25 Northern Bay which is much larger hull but has more composite panels making up the build process . as far as the ride in tough conditions there is no comparison even up to the 18 knot mark and at 18 knot I am burning
    7 GPH = 2.57 MPG if its really snotty out I can slow down to 10 to 12 knots and just chug along. On a calm day push it to the pins and I can get low to mid 30 knot range but that puts me at the 25 GPH mark so we don't go down the path much. The Steiger was a hard riding boat at most speeds and the Northern Bay is a great ride a most conditions with the correct speed and most of all my back thanks me every time.
     
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  11. RickC137

    RickC137 Member

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    A layman's point of view: At rest, the lowest point of my bilge is probably just forward of midship, where the bilge pump is located. This is also the area where the "round" chine is most pronounced. Unlike more common V hulls where the lowest point at rest, and on plane is the furthest aft part of the bilge.

    In the 14 or so hours I have operating my new boat, I have found that the round hull design seems to be the key to the relatively easy ride in a head sea. There is little if any pounding. There is none of the usual "bow rise" associated with a v type hull.

    It is however, "Wet", and lively in a beam sea.
     
    Last edited: May 26, 2019
  12. PatriciaLynn

    PatriciaLynn Senior Member

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    Ha, good point. I know this feeling. The older designs, with their minimal flare can be very wet. The soft chines and narrow sterns of the older designs definitely seem to make for more tender running.
     
  13. Bruce Rudin

    Bruce Rudin New Member

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    My MJM 40z does 1.1 mpg at 37 knots. At the 23 knot cruising speed of my old Grand Banks 38 it does 1.4. Twice the efficiency of the GB.
     
  14. eyschulman

    eyschulman Senior Member

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    Just saying an MJM is anything but a typical DE. A great deal of effort and expense goes into getting good fuel #s. A boats weight and beam,and wind-age also affect fuel burn it is not all hull shape.
     
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  15. Bruce Rudin

    Bruce Rudin New Member

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    Typical in style but not execution.
     
  16. Shark49

    Shark49 Member

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    I have a Sea Way 24 Hardtop sport. We put a 135 Honda an a full Armstrong bracket. My favorite thing about this boat is the ability to cruise under 10knots. Boat just cuts the water, my 20 foot Key West WA pushed water at that speed. When I bump it up to around 12 knot the boat comes on plane but without the annoying bow rise. At 4300 I get around 17 knot and 5500 is WOT for 26-30 knot depending on trim and current.
    At that speed the stern gets loose.
    I see a lot of planing hulls smaller then me with much larger motors. My wife and I will not go back to a planing Hull. Up in the North East there are not a lot of days when I need much more then 20 knots. I’ll take this happy medium and the fuel burn.
    Long and short, I can run any speed just as good as a deep V up to mid 20 knot, but when it snots up I can slowdown and run nice.
     
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  17. PatriciaLynn

    PatriciaLynn Senior Member

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    The MJM looks like a downeast hull above the waterline, but it does not look like a "lobsterboat" below the water. They sure are pretty, but i would be hard pressed to call them a lobsterboat style hull. The cutaway forefoot, deadrise astern, hard chine, lack of a keel, IPS drives and narrow beam are a bit different than a grand banks or a typical lobster hull for comparison. Great hull for pleasure cruising, especially at higher speeds.
     
    Last edited: Jun 2, 2019
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