Pocket cruisers

Discussion in 'Powerboats' started by Guillermo, May 13, 2006.

  1. brian eiland
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    brian eiland Senior Member

    How about something like this Leo?....reminds me of some of those old Elco's, ACF's, Trumpy-Mathis houseboats.

    ...a project I am working on
     

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  2. tom28571
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    tom28571 Senior Member

    Don't have time to read back through a lot of this thread right now but though I might add a comment. A couple years go I built a test model of my proposed "two phase" boat and planned to run some comparative towing tests. Things do tend to get in the way of our intentions sometimes and health got in my way. I have models of 4' LOA ready to test and a recently modified Hobie 16 ready to tow them with. One model is the one I originally used in developing the Bluejacket design and the other is identical above the waterline with a 16:1 canoe keel and flat bottom aft upper hull. The plan is to test for resistance and visual characteristics against each other over a range of displacements and speeds. Since I know the full scale performance of the Bluejackets, I expect to get some idea of the full scale performance of the glider hull.

    I don't think you can look at the glider as a single boat but need to consider it in its two separate modes. At L/B of 16:1, the keel will never be in planing mode at the projected speed up to the high teens. The upper hull will have extremely low bottom loading and should also have low resistance in this speed range. I have designed 75% of the total displacement into the keel. I know from the Bluejackets that bottom loading in this range will permit planing of the upper hull at very low speed with low power requirements.

    The objective is to develop a larger cruiser with much greater accommodations and facilities than the Bluejackets and still meet similar fuel use goals at 2 to 3 times hull speed. From talking to many cruisers with trawlers and other basically displacement craft, it appears that this would be like a holy grail for them.

    I should be able to start testing in a couple weeks. I am under no illusion that these tests will satisfy everyone's questions but do think they will provide enough data to give an indication of possibilities.
     
  3. Easy Rider
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    Easy Rider Senior Member

    A pocket cruiser that's a favorite of mine is this Atkin design.

    http://www.atkinboatplans.com/Cruisers/Tang.html

    He says 15mph w 35hp. I assume that's max and she cruises 10 or 12.

    I like the balance of a midships inbd but I have a 60hp OB and would like to use that. I see Atkin's fuel tanks are amidships and I wouldn't want to put them in the cabin.

    But w a modern engine and a good plywood build (otherwise built basically as designed) Tang should make an excellent pocket cruiser. Both economical and I'll bet quite seaworthy. But w a lighter engine and lighter plywood const the boat would be lighter and faster and more efficient but the high deadrise hull may have her chines too high and lack balance. But of course Atkins recommends ballast anyway.

    Does this design have merit in today's world?
     
  4. Tad
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    Tad Boat Designer

    No......"incorrectly described" by who?

    There are hundreds of books and papers on the subject, and lots of controversy over the nuances of each speed regime, but the three forms; displacement, semi-displacement, and planing, are well accepted descriptive terms.

    See Preliminary Design of Boats and Ships(Hamlin 1989) or Hydrodynamics of High-Speed Marine Vehicles (Faltinsen 2005) for more info.

    Or see this http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf
     
  5. troy2000
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    troy2000 Senior Member

    I like it; it's a jaunty little thing. If I weren't so pleased with the one PAR has drawn for me, I might have considered building it instead. It's interesting and instructive to compare the two...

    The overall length and beam are about the same, but yours has more open deck up forward, and a smaller interior. It also has more draft and more displacement. In spite of that, PAR's Cindy Lou gives me the impression of a larger, heftier boat anyway. And of course, I like my cool raised pilothouse....:p
     

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  6. tom28571
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    tom28571 Senior Member

    May have merit for the confirmed minimalist camper. There is no galley or other provision for being aboard more than the occasional one night. Even Atkin says it is not a cruiser. In southern waters after a day of motoring, that hot engine will probably make you think of a tent in the aft cockpit. If I were going to spend that much money and time on a boat, one of the others would suit far better as a real cruiser.

    Its interesting to note the change in hulls that Mark Van Abema went though starting with a long skinny round hull version that he reported rolled too much. Next came the flat bottom of the Mark V 39. The latest is the Mark V 28 where he went with a V bottom and finer entry. Maybe there is a message of experience there.
     
  7. Manie B
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    Manie B Senior Member

    What I like about Cindy Lou is that you can create a "wet room" below the hatch next to the head. This means that when it is raining and you get inside the boat you have a "room" where you can take your wet clothes off and then enter the "main cabin" thru a door. This way the main cabin is always dry and clean.
     
  8. Manie B
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    Manie B Senior Member

    Something like this (wet room?)

    (I love Cindy Lou - cant wait to see it come together)
     

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  9. brian eiland
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    brian eiland Senior Member

    I know these terms are SO OFTENED used, but can you tell me what differentiates a semi-displacement hull from a planning one,....not necessarily in the shapes of the hulls, but in what the hull is doing differently between displacement and planning??

    PS: (I'm going to take that referenced link of yours to the airport with me)
     
  10. troy2000
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    troy2000 Senior Member

    I'll have to do something along those lines, especially since I plan to replace the hanging locker with a shower. My wife (Cindy Lou's namesake) is still claiming she wants nothing to do with the boat. But in spite of that, she's informed me in no uncertain terms that I will add some way to close off the head and shower from the rest of the cabin. :D

    It may wind up being something as simple as just a drape that can be pulled across, if I can't fit a pocket door in somewhere that shoves clear out of sight. I'm not a big fan of swinging doors in small spaces.... or of chopping up interior spaces in our hot climate.
     
  11. tom28571
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    tom28571 Senior Member

    Difficult question Brian. The speed range from displacement to full planing is often so blurred that looking at a single point most often results in looking at what a particular boat is doing at that speed.

    A thread of unlimited length on this forum resulted in a no-decision on the definition of planing with no consensus agreement on anything meaningful. Some boats simply cannot get past their "hull speed" no matter the power applied. Some can go 2 or 3 times hull speed but require huge amounts of power to go faster. Some easily go from displacement to planing with only a very small transition hump in between. Some boats get up on full plane at speeds below their theoretical hull speed and have no transition interval at all.

    Defining the actions of these very disparate types in easily seen categories is, in my opinion, just not possible.

    Nevertheless, some will insist on definitions but I think they tend to inhibit understanding rather than clarifying it.
     
  12. Easy Rider
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    Easy Rider Senior Member

    Cindy Lou looks like a FULL disp hull at less than hull speed. But when she is driven past that point some "squat" will happen and the new and different WL will seem to change her into a semi disp hull.

    The V 28 is pure planing as I see it.

    The layout of the V 28 has great appeal as someone else already pointed out but she lacks the visual balance of Cindy Lou.


    tom28571 being from northern waters I hadn't thought of heat. And Manie B we know all about the wet room in Alaska. Most people all wear high topped rubber boots even in the summer. I didn't have a "wet room" on my Willard but I didn't have carpets inside either.
    I was basically trolling for comments on the hull.
     
  13. troy2000
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    troy2000 Senior Member

    According to Paul, even at a casual cruising speed Cindy Lou should rock back on her haunches and lift her nose a bit. He took that into consideration when he laid out the rocker in her bottom. But of course, she isn't designed to ever plane. Instead she'll slip along at a dignified pace, quietly and majestically, while sipping on fuel instead of sucking it down. :)

    Any 'wet room' space will be there mostly so folks can change out of wet swim suits in privacy, or get dressed after showering. We don't get a whole lot of rain in these parts.... But I do want a small wood or charcoal-burning stove, to take the chill off on cold winter mornings until the desert sun does its thing. I plan to move the doorway of the head to the inboard bulkhead. That should give me enough space to insert a small stove between the galley and the head, and should also make it simpler to close off the aft end of the cabin when warranted.
     
  14. Tad
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    Tad Boat Designer

    How something is defined will always depend on what it's being defined for.....

    Faltinsen defines the differences as "The pressure carrying the vessel can be divided into hydrostatic and hydrodynamic pressure. The hydrostatic pressure gives the buoyancy force, which is proportional to the submerged volume (displacement) of the ship. The hydrodynamic pressure depends on the flow around the hull and is approximately proportional to the square of the ship speed."

    "the buoyancy force dominates relative to the hydrodynamic force at Fn < 0.4" and these hulls are known as displacement vessels. "When Fn > 1.0 - 1.2, the hydrodynamic force mainly carries the weight, and we call this a planing vessel. Vessels operating with maximum speed in the range 0.4 - 0.5 < Fn < 1.0 - 1.2 are called semi-displacement vessels."

    Fn is Froude number, equal to speed / sq rt (g * L)

    Another way to look at it......

    At rest every vessel's center of gravity is a given distance above or below the waterline. As the vessel starts to move through the water, the CG sinks relative to the static waterline. At some point the CG stops sinking and starts to rise (hydrodynamic lift).....At some point this lift becomes planing.....This is not measurable in real life, but it is measurable in a towing tank. This helps with the concept (that Tom mentions above) that every hull is operating somewhere on a long continuum, and all are capable of infinite speed, and there are no hard boundaries......
     

  15. El_Guero

    El_Guero Previous Member

    Tad,

    Thank you for the description.

    wayne
     
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