Panga Chica

Discussion in 'Boat Design' started by kengrome, Aug 14, 2008.

  1. kengrome
    Joined: Jul 2006
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    kengrome Senior Member

    I think this little 1-2 person boat will plane nicely with low power at moderate speeds in rough conditions without beating you to death. I'm interested in hearing opinions and discussing the boat's design and performance parameters with anyone who's interested:


    Hull Length = 15 feet 7 inches
    Hull Beam = 3 feet 8 inches
    Bow Height = 3 feet
    Lowest Gunwale Height = 1 foot 6 inches
    Transom Height = 1 foot 8 inches

    Design Displacement = 900 pounds
    Design Hull Draft = 7.5 inches
    Freeboard = 10.5 inches
    Waterline Length = 14 feet 2 inches
    Waterline Beam = 3 feet 3 inches

    Engine = 9.9 HP 4-Stroke Outboard
    Hull Weight with Engine = 300-350 pounds

    I made some design decisions that might be called 'unusual' by folks who've been designing power boats for the U.S. market for the past few decades, so I'm hoping they are good decisions. I would like to see boats like this catch on again since they are very fuel efficient and handle bad conditions well.

    My goal for this boat is 20+ mph with a light load in the range of 500-600 pounds total displacement weight. I'm not sure it will hit 20 mph with an engine this size, but I'm hoping it will.

    What do you think?
  2. Guest625101138

    Guest625101138 Previous Member

    Attached suggests more like 20HP to get 20+mph with typical outboard efficiency.

    What is the KMT?

    I feel it will tend to dig the bow in off a wave and be difficult to control running downwind.

    Rick W.

    Attached Files:

  3. lazeyjack

    lazeyjack Guest

    think Ken, light ship will be very tender indeed?
  4. kengrome
    Joined: Jul 2006
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    Location: Gulf Coast USA

    kengrome Senior Member

    Hi Rick,

    Thanks for commenting.

    10 HP engine = 7 HP at the prop so I followed the 7 HP line across your chart until I hit the curve a that's 16 knots = 18.4 mph so already we're pretty close with your numbers.

    I ran that script again with my 'light load' displacement of 600 pounds (Mdepl=275kg) and figures from FreeShip (KMT=0.67, LCG=1.85) and I estimated a narrower 0.7m planing beam and used a beta angle of 10 degrees -- the angle is hard to estimate since the boat is planing not only on the perfectly flat bottom but also on the aft garboards.

    My changes gave me 20.5 knots = 23.6 HP so we're over the hump if these numbers hold true. I think the boat will be in between your figures and mine so that's close enough for me to think about moving on to the next step:

    Either build a prototype, or adjust things first ...
  5. kengrome
    Joined: Jul 2006
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    Location: Gulf Coast USA

    kengrome Senior Member

    Rick, I'm interested in your comment: "I feel it will tend to dig the bow in off a wave and be difficult to control downwind." What would you change to alleviate this issue? Cut away the forefoot some more? Add forward flotation that lifts the forefoot out of the water at slower operating speeds? I hope I'm understanding your concept of the problem correctly.

    Lazeyjack, you're right, the boat's going to be tender at light load. One feature that's not mentioned yet is a 6 inch diameter EPE (Expanded Polyethylene) collar that wraps around both sides of the hull with its top at the gunwale level. If I install it lower as it approaches the transom it would essentially behave like a couple of sponsons back there, thus making the boat's waterline beam a foot wider and probably eliminate most of the tenderness. Not sure I want to install it lower at the transom because it might look funny but it's an easy thing to change and experiment with. The concept would be like a RIB I guess, with the flotation collar in the water aft and high out of the water forward.

    Note that I'm using a similar bottom to Peter Payne's Seaknife and I think (hope) the boat will plane and turn in a similar fashion. I've attached a bunch of Seaknife photos so you can get a better idea of how it performs in turns and at speed in a straight line. It seems to me that the Seaknife uses its ability to bank into its turns rather effectively, and I'm hoping this boat will do the same.

    Attached Files:

  6. Guest625101138

    Guest625101138 Previous Member

    You have my meaning, I do not like the entry being so deep. It will bite in the trough and you run the risk of broaching. It will depend on the size of the wave of course but with a shallower entry the bow will move around easier and the boat controls better. I doubt if there will be much difference in ride quality because the boat is narrow anyhow.

    This is the sort of bow profile I like:
    Any deeper and they want to go where they want to go rather than going where you want it to go.

    Your speed is a bit slower than what I am familiar with in planing hulls so it may not be as bad as I have experienced. It is a nasty feeling when the boat skews into the back of a wave and you are caught between more power for control but the risk of getting it more out of shape.

    Others may be able to give more relevant experience in the size of hull and speed you are contemplating.

    Rick W.
  7. kmorin
    Joined: Apr 2005
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    Location: Alaska

    kmorin Senior Member

    Pitch the Model


    I'm not as familiar with Freeship as Delftship Pro, but if you can pitch the model about a point, or from the 0 station on the grid (?) watch the LCB, CofF and lateral area centers change.

    By making a series of pitched profiles you can plot the movement of the LCB and lateral area centers as the waterlines are increasingly higher on the stem as the model goes down by the bow. (following sea immersed waterline) In Delftship Pro you have to constantly manually adjust the Load Waterline location of the model to keep the same displacement in each degree of pitch change.

    What I've used this method to do in small skiffs is examine the distance that the lateral area center (immersed profile) moves in relation to the other centers to see if a skeg at the stern of the skiff was appropriate?

    By adding the appendage, even a reasonably deep bow pitch will slow the separation of these centers as the bow gets deeper. Using different lengths and depths of 'skegs' or 'keels' therefore different areas of lateral resistance aft the LCB/CofF reduces the distance these centers move in relation to one another for a given degree of change in pitch.

    I don't get a quantitative 'anti-slewing' factor from this exercise but it has helped to see if a few inches of bar under a metal rowing dory would help reduce its [following sea] tendency to broach in the troughs.

    It is a simple experiment to see the effects of any small boat keel-like appendages might have and reasonably simple to do in a few minutes; in DShipPro. I just screen dump (Windows screen dump) to the printer and look at each series of pitched shapes, I don't try to plot the info, or otherwise manipulate those screen captures.

    I'd say the skiff shown was too narrow for public market as the tenderness requires too much "good boat manners" and the US public, at least, won't have anything to do with it.

    As for how it will behave, without underwater skegs or keels I think it will slew bow to stern and roll in the trough because her speed would be so high she couldn't bleed speed before she 'tripped'; unless the crew were sitting on the absolute hull bottom. Then with the CG back (down) inside the hull she'd be less likely to roll gunwale down, but I think with the very steep garboard plank shown; she'll be tender if anyone even leans over the the gunwales.

    Another way to help reduce the potentially dramatic negative effect of the deep forefoot traveling at speed is to flam the topsides above the second chine- forward. In plan, she could use (in my eyes) plenty more sheer rounding, fullness forward, and the lines would reflect two or three times the angle from the upper chine to sheer in body plan.

    What happens to skiffs of this size with a sharp forefoot and very flared topsides is that the rate of immersion slows dramatically with very flared topsides. Those waterplanes well above the normal load lines that are only wetted when the boat is pitched down by the bow 8 or 10 degree would have radically increased areas if there were more flam in the topsides above the upper chine.

    When a skiff's motion off a swell into the trough begins to bury the bow, the rate of change of waterplanes areas can slow her descent quickly in a flared/flammed shape- but she gets much deeper in the same hull with more plumb topsides. In the first instance the bow slows her forefoot's depth of immersion more quickly than the latter. So the first instance skiff pitches less and the unbalance in lateral area is less dramatic and the stern ahead moment reduced.

    The price you pay is that burying the bow with all that flam will slow the skiff to a near stop as the bow wets and begins to pitch upward fairly dramatically. In extreme cases the crew can be hurled forward to 'chew' on the stem, but she'll keep her head by comparison to the plumb sided skiff you've shown.

    If I were drawing/building her, I'd make the dory shoe panel, on the bottom, widen continually from bow stem intersection to transom, reducing the two garboard planks as they returned aft; to make more flat surface to plane and to widen and harden the chines where the motor is mounted. As drawn I think she'd run down by the stern unless loaded with about 30% all-up displacement in the forward 1/3 of the hull.

    Hope you don't mind my comments,

  8. eponodyne
    Joined: Aug 2007
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    eponodyne Senior Member

  9. kengrome
    Joined: Jul 2006
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    kengrome Senior Member

    Thanks for the comments guys, please keep them coming!

    I know I'm 'jumping the gun' with the following website which anticipates production of these boats in the future, but it's going to be easier for me to give you the URL than to repeat what I've already said there, so please take a look at this page to get a better feeling for my concept and goals for this boat:

    I'm in the process of redesigning the bow so it won't have the problems described by Rick and kmorin, and I will discuss this and post new pictures in a later post, but right now I want to point out another very important issue. I'm hoping to build these boats in the Philippines and ship them to the USA in two stacks of 10 boats each in 20 foot international cargo containers. This imposes certain design restrictions:

    1- It limits the boat's width to about 3 feet 9 inches including gunwales.
    2- It requires enough flare so the hulls can nest inside each other.
    3- It limits the use of factory-installed thwarts, bulkheads, etc.

    I'm currently assuming that each boat will sit 6 inches higher than the one it rests inside of. Given the fact that the bow height of the bottom boat in the stack is 3 feet off the floor, the height of the bow of the top boat in a stack of 10 will be 7'6" high and I think this is about as high as I can go in a standard cargo container.

    In addition to its 400 pounds of flotation, an important reason for me to use the flexible EPE flotation collar mentioned on the web site is that it will make the boat a foot wider at the gunwales after the boat is delivered, but it will not make the boat any wider during shipping. This over-sized 'pool noodle' basically takes the place of airtight flotation compartments built into the hull, which might be nice but they also make the stacking depth a lot more than 6 inches per boat, which means higher shipping costs and this is something I'm trying to avoid.

    I can always offer these boats as semi-finished kits -- complete on the outside but unfinished inside -- and then ship pre-cut flat panels for thwarts, bulkheads, flotation boxes, etc. so owners can install these features themselves if they like. But with strong outwales and 3/8 inch (9mm) plywood sides on such a small boat these internal features will never be needed for strength, that's for sure.

    I'm in a bit of a hurry now so I don't have time to respond properly to the last few posts, but I'll get to it later today or tomorrow ... and kmorin I really appreciate your thoughtful feedback, you never have to worry about posting exactly what you think with me ... :)

  10. EStaggs
    Joined: Aug 2007
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    Location: Spokane, Wa

    EStaggs Senior Member

    Ken, please take the time to do a fast lauan prototype. Shes going to be tender like a GB dory, so make sure its not a nauseating or dangerous sort of tender. I love the idea of narrow, moderate speed low hp boats, but boy shes lookin a little overly tender with the bilge panel angle. She looks to lose stability drastically when heeled over 10 degrees or more, which would be easy to do when fishing or hauling gear over the rail.

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