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

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

  1. fallguy
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    fallguy Senior Member

    The Skoota 18 has power in the nacelle and you could literally make steps on each hull. Remove the day cabin and trade it for some boxes for seating and stowage. Your narrow boat idea has me not wanting to set foot in it, btw.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    That Panga is relatively narrow, with a beam of approx 8'9" - yet this beam is still more than twice what the OP's design has.
    He seems to be absolutely determined to build it 'as is' - when he does find out that it will happily 'fall over' without much provocation, then he might consider converting it into a Skoota cat - slice it down the middle and have two asymmetric hulls.
    Or build another hull the same way, and have a symmetric cat. The cat would still get relatively good fuel economy compared to a monohull of the same displacement.
     
  3. fallguy
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    fallguy Senior Member

    hull conversion to a cat is like sayin I'm gonna change my donut into pancakes

    guy writes a great sor, then bombs on execution over a dive platform in a California boat slip? Something smells fishy.
     
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  4. Blueknarr
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    Blueknarr Senior Member

    Me thinks he determined the width by measuring the inside distance of the air chambers not the outside.

    What slip is ever designed for four foot max width.
     
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  5. bajansailor
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    bajansailor Marine Surveyor

    That is an excellent analogy, thanks!
    One should really have 'proper' cat hulls from the beginning.
    But if the OP builds the boat, tests it out, and finds that it falls over too easily, then he has to do something to fix it.
    And the only options I can see then would be -
    Plan A - fit an inflatable collar around the gunwhale, and convert it into a RHIB, or
    Plan B - convert it into a catamaran.

    Or Plan C - build a couple of folding outriggers and convert it into a trimaran? Or just have one outrigger, and have a motor proa?
     
  6. fallguy
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    fallguy Senior Member

    plan #1

    Use the sor as intended to determine the right boat, not a 4' beam small panga for two..
     
  7. bajansailor
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    bajansailor Marine Surveyor

    Absolutely! :)
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    My experience in small boats says 4 feet of beam is not near enough on the open sea. You can get away with a lot in protected waters, that should not be chanced on the "main". It is quite possible to have a good seaworthy boat using a 60hp outboard, but three people would be about the practical limit offshore. You want reserve stability to deal with what doesn't happen 99 times out of 100. It is the hundredth that can get you on to the 6 o'clock news.
     
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  9. socalspearit
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    socalspearit Junior Member

    re: convert to catamaran. Yes, it's donuts into pancakes.
    re: narrow boat slip. I have a 'regular' slip intended for a 20' boat or so but half of it's width is taken up by competition freediving platform (think small wood decked barge with a boom arm on it) built on top of a jet-ski float that gets towed out for use once in a blue moon for competitive freediving.
    re: diver's standing. Why do you think a freediver would stand? Fishermen stand because they want to cast. Scuba divers stand because they have to stand to put on tanks. Freedivers, I mean what's the point? You're seated in the boat, you put on your fins (much easier to do sitting than standing) then slip over the side into the water. Out of many hundreds of hours of commercial use nobody has ever stood in my inflatable in open water. My dive buddies and students just aren't that dumb, and if they try and stand they figure out pretty quick that they will fall off before boat itself is even remotely unstable. I know the shape is different than the design I'm working on but I've stood on the edge of the tubes of my inflatable plenty of times on my slip, the boat barely barely heels because even though it's so narrow you have the bouyant force of 15' of edge. Taking a dump off the side (long days on the island), using the utility ropes with my *** and a good part of my weight over the edge, it barely heels.
    re: a 4' boat in the open sea. Yes, my inflatable is actually less than 4' wide. Most local boaters were also scandalized when I started years ago running it originally from LA to Catalina (about 25miles each way) with a 6hp engine after many hundreds of coastal miles. Current iteration has been modifed for a 15hp engine. The inflatable has three chambers so you're 3 x 2mm PVC from sinking the entire time but besides that it's unsinkable. If it sinks I'd hate to lose my outboard but in a modern freediving wetsuit I could 100% swim 26 miles if my life depended on it. Most of my students could not so I don't run them more than a few miles out from the coast when crossing the bay, and currently I don't run the inflatable to Catalina since I have more friends with boats these days. Why does it matter how wide the boat is if it can't sink and the occupants don't care about staying dry? I'm not completely dense, I know it's not good to have occupants ejected from the boat, etc, but my inflatable and this design are such that the occupants would fall out long before they can cause the freeboard edge to submerge, and my occupants are not typical fishermen or tourists. Boarding seas and free surface effect complicate things but that's any boat in my opinion... I have done plenty of dumb things but these days I only buy new outboards and keep them carefully maintained.
    re: guys needing help over the side. That's a consideration, I already realized I may need to build a removable ladder for some students. The inflatable is of course very low to the water but while a few have struggled I've never once out of many hundreds of students had to actually help anyone over the side. The population that freedives is very different than the population that scuba dives. You can be 200 pounds overweight and smoke 3-4 packs a day and be a great scuba diver, instructor even. Guys and girls that already do or aspire to shoot fish 7 stories deep on a breathhold are considerably more athletic.
    re: dive panga. That's designed for scuba divers. I don't want a big boat because I can't use it as a dive float and I can't tow it where I want it if I'm in the water. In the wind it becomes a sail. Scuba divers typically want/need to spend, much, much more time sitting around on the boat than freedivers or spearos. For what I do, this design type is the best I've found (up to a point--obviously range is limited by safety and ultimately comfort too--running that tiller sitting crosslegged sideways for long periods becomes exhausting). I also don't want to haul a big boat full of divers. There's plenty of guys that do that already. I don't want take more than 4 passengers max because that's as many as I can responsibly supervise in water, and my customers come to me for that skill not the boat.

    I do appreciate the feedback because it is making me do more research and study. Up until now my knowledge of boats, surfboards, kayaks, canoes, and platforms has been empirical and based on experience. I get it, it is hard to understand what I do with my boat and why... But, this is a boat design forum. I may change some design aspects but advice is suspect if comes from anyone who doesn't seem to understand that bouyancy is a function of volume.

    IMG_20210512_141501.jpg

    By the way, I found this the other night. It's one of the better texts I found of practical boat and stability calculations for vessels of my scale: https://www.dco.uscg.mil/Portals/9/DCO Documents/5p/CG-5PC/CG-CVC/CVC3/references/Stability_Reference_Guide.pdf

    This one also, more math: https://www.gerrmarine.com/Articles/StabilityPart1.pdf
    That article also concerns sailboats primarily but there's this, which is again explained by CWP: "We’ve seen that stability increases as the cube of the
    beam, but it also increases directly with length. A
    boat that is otherwise the same as another, but has
    been stretched to be 1.2 times longer will have 1.2
    times the stability. It also has something else—a longer waterline. The longer the waterline, the higher the hull’s speed
    potential. Given the same parent hull, you could, say, increase speed by making the hull wider and shallower. This would
    increase sail-carrying power so you could install a bigger rig and go faster. But, you reach a limit at hull speed (roughly 1.4
    x the square root of the waterline length in feet, in knots). After that, the boat has to be light enough and flat enough aft to
    start some planing. Alternately, you could make the boat 20 or 30 percent longer. This would increase stability by 20 or
    30 percent, which would allow a larger rig. The longer waterline would give higher top speeds before planing, and higher
    average speeds in general. Such a longer and proportionately narrower hull could still plane in the right conditions, given
    the proper run (underbody aft). "
     
    Last edited: Sep 6, 2021
  10. socalspearit
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    socalspearit Junior Member

    While I don't entirely agree that 4' of beam isn't enough I would also mention that I was never intending to take this design on an island run with me + 4; for what I want to do that's coastal, me + 2 is perfect for running to our islands.
     
  11. socalspearit
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    socalspearit Junior Member

    I am going to throw together an 8' long plywood shell that's based on the middle sections of my intended design and float it in the harbor, put some weight in it at deck level and maybe something on top of the transom and see how it responds if I climb in and out of it. Being only 8' long, it should have in the neighborhood of half the stability and likely be quite tippy. Most boats of this beam are about 8' at the most and are quite tippy of course but it's been a while since I've tried to climb over the side of something like that. I've got enough scrapwood and could use the practice fiberglassing.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    So what was the maximum number of people aboard that you are contemplating ?
     
  13. TANSL
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    TANSL Senior Member

    There is no relationship between the length and stability of a hull. The proof that you are raising is not going to be of any use to you. So save yourself the trouble of doing it.
    It would be preferable to start a process that allows you to determine the dimensions, proportions, and shapes of your hull. That is, what some insist on calling "designing a boat" that responds and optimizes the SOR that you undoubtedly have.
    If any of the dimensions are already fixed for any reason, or there are any other conditions, clearly indicate this in your SOR and let the design process lead you to create the appropriate shapes. Doing the opposite is not helpful, and don't forget to draw a General Arrangemen plan, even if not in Rhino: a freehand pencil sketch is sufficient.
     
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  14. fallguy
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    fallguy Senior Member

    There is no reason to build a 4x8 sheet oxf plywood to prove you can climb on.

    I didn't realize you were freediving; perhaps it was mentioned, your posts are long and it may have been missed. It only changes that people stand less; not that they won't, nor that they need to board.

    Any chance you could make your boat be the platform. Seems an awful use of space to keep a not oft used barge in a costly California slip. Or make a barge that can be stowed on the vessel..

    Every now and then we get a person on the forum who is very stubborn and not really interested in understanding or modifying their plans, often to their own detriment. I'm getting a sense of that here. I'd be making that Skoota18-20 and getting rid of the floating barge in my slip.

    The idea for a composite boat in carbon sounds cool, but the boat is all wrong and you'll find that out over time.

    Best of luck.
     
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  15. bajansailor
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    bajansailor Marine Surveyor

    Socal, please do listen to and take in the good advice being offered very politely to you in the above posts - everybody offering you their opinion most definitely knows what they are talking about.
    I would agree with Fallguy - get rid of that barge that takes up a lot of space in your slip, and build a cat instead.
    And I think you will find that the cat will have excellent fuel economy, while functioning as a barge and as a transport vessel for your free diving guests.
    TANSL has again mentioned the importance of a general arrangement drawing - please do sketch out something for your long skinny boat (it does not have to be fancy - a pencil and a ruler is all you need really) and I think you will quickly find that things do not look right - especially if you do a section view, and draw in a diver or two to scale as well.
     
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