Custom 19' all weather, minimalist, strip plank composite 'go fast'

Discussion in 'Boat Design' started by socalspearit, Sep 2, 2021.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    In the real world, boat builders have largely desisted from self-bailing cockpits in smaller trailer boats, because it is a safety issue of compromised stability, besides whatever other problems it creates. That alone, condemns this idea, in my opinion. Sure there are beamy, rectangular plan types of hulls that can be excepted, but this boat is neither beamy or rectangular plan.
     
  2. socalspearit
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    socalspearit Junior Member

    Respectfully, the most obvious issue is that it isn't meant to be a boat in the traditional sense, ie a vessel designed and engineered first and foremost to keep its contents dry and out of the water. It is a surfboard with a shallow V hull, some low sides, and a gas engine. I also did model the first interation of the hull (which is narrower than what I'm considering now) in Orca and move weight around, etc, and it behaved much like I'd expect it to given the thousands of hours I've spent in a long narrow hull in our waters, which again is much more stable and seaworthy than people would expect. Orca does not account for freesurface effect and weather, etc, so it's not certainly not perfect and especially when I was doing this I was not very educated on the math of side of naval architecture, but the software does move the model on the waterline plane.

    I am going to build a 15% scale physical model though since I have less experience with a long narrow shallow V hull and I did not do computer tests of overloading it on one side, although again, I know it's counter intuitive but based on a lifetime and current profession that involves a great deal of scrambling in and out of the water on a variety of non-traditional boats such as kayaks, inflatables, SUPs, jetskis, etc, a long hull can be much more stable than anyone expects. Canoes and kayaks not so much though because again, they typically have very fine ends so co-efficient of waterline plane on those vessels is very low and thus they have lousy initial stability. Jet skis are not great, they have very high COG and relatively small waterline area.

    Your description of the WAYFAIRER dingies' self tending valve sounds almost identical to the giant sock scuppers on my friend's old Zodiac 16' RIB. They were wonderful and would drain many gallons of water off the deck in just a few seconds once we got underway (Venturi forces). We never flooded or swamped that boat significantly but used to manually open the scuppers for a moment when leaving the island just to let the deck drain, and I always assumed that was just how it was supposed to be done, but thinking back to those scuppers and that deck, they would most certainly open from water pressure if the deck were to actually be swamped. Those scuppers were very simply designed and more effective than any pump could ever be.

    Very much agreed about the cockpit sole height. At maximum displacement of 1800 lbs this cockpit sole would be pretty darn close to 2" above the waterline! I would likely never want to load this design that heavy although if entirely swamped it might approach that weight. ABYC standards would require the cockpit sole to be minimum roughly 4.5" above max displacement waterline (FOR RATED CAPACITY) for a boat of these dimensions. ABYC standards are of course for commercially sold boats so I may just forget about that, and this design would far exceed minimum US Coast Guard requirements.

    The 'airbox' idea is again a very interesting thought experiment. Bigger airboxes near the transom make the most sense so that they guarantee level floatation against the weight of the engine (and due to 'well' back there there is no foam under a floor at the transom), and if/when the boat onboards water they would keep a significant portion of water over the centerline of the boat.

    Airboxes off the back of the transom are also something I should think about you're right... my inflatable is of course made that way and it never really squats but I always attributed this to the fact that it was long and narrow, a semi-displacement hull that planes effectively just because it's so light. The lack of squat might have much more to do with the tubes extending past the transom than I realized.

    re: self bailing. Again, I suppose 'self bailing' is somewhat in the eye of the beholder. My inflatable is 'self bailing'. Not in the sense that it's ever dry, just in the sense that when we haul ourselves into it after diving, there's about 3" of water in it. Considering that the 'freeboard' of that vessel is not even 10" that mean's it's nearly half swamped. Once it starts to run, mild bow rise and interia pushes all the water towards the transom and it drains. Not as fast I'd like but faster than you might expect due to venturi forces sucking the water out of the too small scupper and as the boat gets lighter it rises and planes nicely. A higher deck would be drier of course but also raise the COG so in the new design I'm not thinking to make it 'self bailing' the way many sport fishing boats are self bailing with a deck still 12" above the waterline when stuffed and hapha which on a boat as minimal as what I'm designing would be a ridiculously high COG.
     
  3. TANSL
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    TANSL Senior Member

    Interesting discussion, more based on impressions than numbers. But what is it and how is it calculated the instantaneous righting moment?. Thanks in advance.
     
  4. fallguy
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    fallguy Senior Member

    Have you calculated ppi?

    getting the scupper height right when loaded is the magic

    otherwise, if you pretend the scuppers can be 1/2" above dwl; then load 5 people and run dwl to 2" deeper, its a wetter ride still
     
  5. socalspearit
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    socalspearit Junior Member

    Sorry, but what is ppi? I am not familiar with that term and googling was not helpful. I will explore it though once I know.

    DWL according to Orca is practically at the chine with a full tank of gas and one person. Orca put the boat sitting higher than I even expected but that would be the bouyancy of the V which I don't have on the inflatable (but it is a lighter boat). This boat will be heavier than first imagined with more structural reinforcement, etc, but it should still be ultralight by powerboat standards.

    This is perhaps unconventional but the scuppers will mostly be underwater all the time since that's how it is on my inflatable, and being below the waterline, venturi forces will drain the boat very quickly if underway. The inflatable has an airfloor (inflated to high pressure so it feels like sitting on a surfboard, not squishy inflatable) so depending on loading the top of the airfloor/deck sits an inch or two above the waterline, and the scupper is in the bottom of the transom, always submerged. Scupper has a plug and originally had some kind of janky one-way diaphram that many moons ago went missing and that never worked, but I never plug it. What this means is that seawater comes and goes in the boat through the scupper and collects under the airfloor (again though mild bow rise and venturi forces suck most any water out once underway); it's just kind of an open bilge area.

    I want to do something like that with this design but a higher floor and more contained "well area" (on the inflatable water--up until we're underway--is just sloshing around under the air floor, or pooling into the boat over the airfloor when loaded heavy). This design is such that the deck--unless 3x overloaded--stays above the waterline and the well is just open to the sea, and even if horribly overloaded you just get water in the back of the boat. I would cover the well with a loose grate but here is what I mean:
    SpearIt_One_v2 v14_e.jpg

    Here is a cross section showing the deck height. Besides the fuel tank, sealed battery compartment, a couple longitudinal stringers, and very small internal bilge (for cable access/etc), there'd be foam under the deck. Again, it's less boat and more just big surfboard with low sides and gas engine.
    SpearIt_One_v2 v14_f.jpg

    I am building the wooden model tomorrow. I haven't changed the computer model yet but next iteration I will add some beefy foamed knees (ala 'airbox') at the transom, 3 cubic feet total at least.
     
  6. socalspearit
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    socalspearit Junior Member

    Again, while I have a been a pile of different types of boats, most of my time of small boats is my inflatable or various RIBs and fishing type center consoles. I think some of what you're noticing is the 'tenderness' or 'stiffness' of the righting moments. An inflatable rights itself very tenderly. When I think back to the Honduran water taxis (long skinny design, V hull), they have a very stiff righting moment and thus feel tippy even if their ultimate stability is excellent. I'm not going to pretend I already knew this but one of the articles I linked to earlier goes way into this; per the article the tenderness or stiffness of the righting moment has nothing whatsoever to do with ultimate stability or even initial stability, just the way the boat feels when it moves.
     
  7. fallguy
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    fallguy Senior Member

    How many pounds it takes to sink an inch.

    3' average width?@18' is 54'sq/12 inches/ft is 4.5 cubic feet per inch*64#/cubic foot is 288#; seems a bit high; not sure the waterplane is 54 square feet...any error in your displacement or dwl calcs or extra weight(ppi) will result in water coming back in via scuppers

    a bit above my paygrade, the metric system is kg/cm
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I remember seeing a little boat way offshore once that was very low to the water, it had a small cockpit behind high bulwarks, with wide side decks that looked like they could become awash fairly easily. But would have made it very easy for a diver to get in and out of the water, albeit in a two-step process. The boat was painted a brilliant red, whether in anticipation of trouble, I can't say !
     
  9. fallguy
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    fallguy Senior Member

    Also, if the boat is underwater at rest; in a slip, it is probably gonna sink without buoyancy foam. Any leak in the sheathing will result in plywood blowing up.
     
  10. cracked_ribs
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    cracked_ribs Senior Member

    Personally, I think half of what is going on in this thread is people not understanding that what you are interested in is not a conventional boat.

    The one thing I'd be concerned about would be whether it'll be prone to stuffing the bow, particularly on the way back in when you'll have following seas that you're outrunning. My question would be about how it'll handle if you have to slow down so as not to spear it into the back of a wave, and you're suddenly getting overtaken... I'd wonder if it would be prone to broaching. If the back end starts to swing around, no beam to fight the roll.

    But I have to say that given your application, overall I don't mind the idea.

    There's a boat builder I can think of you might want to talk to...he built really narrow, fast boats and ran them hard in wild places off Alaska. His 25 footers had a 6' beam and the 31s were a 7' beam. If you're interested I can put you in touch.
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Socal, please bear in mind the free surface effects on your stability of water sloshing from side to side in your boat - and there will be water doing this, especially if your scuppers are underwater.
    It is possible to calculate reduction of stability based on free surface effects of liquids in tanks that are not pressed up full.
    Your cockpit will have a similar effect to a tank that is not pressed up full.
    If you would like to have an analogy, try carrying a frying pan with a 1/4" deep layer of water in it - it will slosh from side to side and feel rather wobbly, no?
    But carry the same amount of water in a mug, and you have no issues at all.
     
  12. sharpii2
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    sharpii2 Senior Member

    I would say you are somewhat correct. But that all depends on two design decisions:
    1.) how deep the dead rise is, and
    2.) how high up the cockpit sole is.

    I recommended that it be just a few inches above the maximum load water line.

    Of course, there is no reason the cockpit sole needs to be above the maximum load waterline if the air boxes are there and have enough total buoyancy. Then, in the case of a swamping, the cockpit will fill up with water to the point where the air boxes immerse enough to support the total weight of the boat and crew.

    And this is not entirely accurate, because the submerged parts of the wood hull will provide some buoyancy of their own. Not only that, but if the water that comes on board is several inches high, and the crew sits on the cockpit sole, the onboard water itself will support some of the weight of the crew. The more water that comes on board, the more true this will be. Eventually, equalibrium will be reached. And I expect that would happen long before the boat sinks.
     
  13. TANSL
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    TANSL Senior Member

    But the total weight of the boat does not change for this reason and, as has already been noted, the effect of the free surface decreases the stability (the effect is similar to a rise in the CoG).
     
  14. fallguy
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    fallguy Senior Member

    I wonder how well the sea critters will love a boat open to the sea on the inside. I am sure this comes off as snark, but it isn't. I think.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    I am sure that it will be a beautiful symbiotic relationship, with man at one with the sea and her critters. :D
    (sorry, I could not resist)
     
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