efficient 10m displacement powercat (build thread)

Discussion in 'Boatbuilding' started by groper, Apr 15, 2012.

  1. groper
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    groper Senior Member

    Well get your *** up here and give me a hand already! :D i got a pregnant wife, 4yo boy and a full time job to contend with also!

    My biggest concern i have about this hull, is the sea keeping ability and how well it will handle choppy seas as its divergent from the typical deep forefoot, shallow immersed transom shapes that are so common. I think the aforementioned shape is so popular because its the best compromise in terms of a more balanced slow and fast operational speed regime and kindly seakeeping etc... ive tilted the bar a bit further toward higher speed rather than lower speed, and im most likely to get more banging from the bottom of the hull slapping waves as the forefoot is shallow like a sailing cat...

    high speed displacement hulls (i dont like the word "semi-displacement") are characterized by high length:beam ratios (my boat is 16.3:1 and Incat/Crowthers high speed wave piercing ferries are around 20:1 for example) and very aft LCB around 0.4LWL from stern.
    Coupled with a light displacement or DLR, you get best efficiency at higher speeds when compared to a more conventional catamaran hull.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    So you have opted for less sponson depth forward to reduce wetted area ?
     
  3. groper
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    groper Senior Member

    No, not so much due to wetted area, but If you try to construct a slender hull with lcb at 40% from stern, you will find it difficult to shape it with anything but a deep transom and shallow forefoot... make sense? You would have to end up with very hollow waterlines in the bow to keep the bouyancy aft, which would not only look strange but might also give rise to severe bow steering issues. The overall draft of teh hull would have to increase for the same displacement and the wetted area would increase with it etc... so its all a compromise one way or another, i chose minimum resistance at high speed above all else...
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, I guess you could have a deeper footed boat with a narrower waterline forward, do you anticipate much dynamic lift from your design ? If you look at boats like the BW Seacat, it has a chined displacement hull with full stern but fine, deep bows, and the kind of speed numbers you are aiming for.
     
  5. groper
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    groper Senior Member

    No, very little dynamic lift... the hulls are very slender. Another way of saying the same thing with regard to lift is, very low aspect ratio, so generated lift will be minimal...

    I have seen the BWSeacats... they have the conventional hull type i was referring to previously, along with good seakeeping qualities but reduced high speed efficiency...
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Anyways, the die is cast and your build is well underway, all the best with it and I look forward to your progress reports.
     
  7. groper
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    groper Senior Member

    Did you notice that the BWseacat are using twin 175hp to get the 29kts? so they have double the installed power... i have no doubt that style of cat would be a much nicer ride tho...
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Yep, but she is a weighty boat, the 11 metre with inboard diesels is quoted at 8.5 tons. That is a workboat rather than a pleasure boat.
     
  9. groper
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    groper Senior Member

    Yeah, another part of their site says the blue one with the full island wheelhouse has twin 320hp diesels and manages 27.5kts... so one cant really draw too many conclusions?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Seems an anomaly doesn't it. Forumite "Catalyst" seemed sold on the boat after a test run described here:
    http://www.boatdesign.net/forums/powerboats/efficient-power-cat-designs-29598-2.html

    Sounded like she wasn't straight as an arrow downseas, but no more heard since 2009, I'd have been interested in his later experiences if he did indeed go on to have one built.
     
  11. groper
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    groper Senior Member

    All depends on what your after, are you looking to buy or build or design something like this?

    If so, lets talk it out... make a decent thread of it... i like doing all this stuff, even if its just for the fun of it...
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm more than likely going to build, I can't see much around that would answer the description of the type of thing I'd like. Around 30', has to have at least a 20 knot sprint to cross bars safely, high tunnel clearance, cruise in the 16-knot range as a minimum, be firm in pitch as I couldn't abide hobby-horsing, or high vertical accelerations etc. Will have to be twin outboard (high-thrust) as I can't see being able to shoe-horn diesels into narrow sponsons. Shallow draft so able to go ashore without a tender. At the moment I am entertaining the idea of a modular construction that all bolts together, some solid glass, some foam sandwich. I have the freedom of not having to work, so a self-build is feasible in a reasonable time frame, and I have built in foam sandwich before so not starting as a green-horn. I've had planing cats and am sold on the twin hull vessel, but I'm not a speed freak and 20 mph is very respectable progress in my thinking, so the displacement cat idea appeals. But obviously I don't want to create something that doesn't meet expectations, so I'll be consulting fairly widely before committing. May even find some plans that suit, but I'm not thinking flybridge, and internal fit-out of the luxury kind does not interest me, I would prefer just a utilitarian, functional set-up more akin to a work boat.
     
  13. groper
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    groper Senior Member

    damn, sounds just like me when i started this thing, pretty much exactly... it changed a bit as things progressed, but started the same...

    If i could do all this again, i would;

    1. Supply drawings and use pre routed duflex panels for the bulkheads, but still make my own larger panels for hulls and bridgedeck etc via infusion. Its not very efficient in both time and money infusing small panels.
    2. Design it with minimal compound curvature so it can mostly be built from flat panels and folded single curvature panels infused on a full length table- i only have a half length table atm. Minimising secodary bonding and tabbing is worth considering in the design, and strip planking and fairing of compound curavtures is just painful when youve seen a flat panel done in 1 shot and ready to paint... no frames, or strongbacks needed etc...
    3. seriously look at importing materials from overseas, prices here in aus are just ridiculous to what the rest of the world is paying, especially foam core materials...

    Got any drawings or specific ideas?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Haven't progressed to more than rough sketches and general dimensions, but I want to keep it simple and straightforward. I will see what I can do about putting up some sketches in the next few days. Interesting comment about importing materials, I wasn't aware of the disparity with the core materials, but it doesn't really surprise, seems to be the case across a wide variety of goods. The modular idea is basically the only way I could reasonably proceed, i couldn't get something off my property that was 12 feet or more wide, the thinking is the sponsons to the level of the tunnel top would be one module each, and moulded in the same female mould ( obviously we are talking symmetrical ) and the rest of the thing several more separate, mainly sandwich construction, parts, bolted together. If that has an inherent problem, I'd be happy to be advised on it. Obviously everything in the water will be one-piece, all joins well above waterline. Before anything proceeds of course the calculated weights will have to match the displaced volumes, at this stage it is all early days. I will have to consult with the experts about optimum sponson spacing at an early stage as well, once the shape and dimensions of them is settled on.
     

  15. groper
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    groper Senior Member

    Have you played with any design software yet?
     
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