What hull will be most efficient between 8-12 kts?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Oct 9, 2021.

  1. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Senior Member

    I'm keeping the beam at just under 12' regardless of the hull form(unless I decided on a monohull, which will never happen), as wide as I can make it but still be able to put it on its side on a car hauler and take it down the road without a permit.

    One 20HP engine, long shaft(110-120lbs, steering/throttle assembly/wire(10-20lbs), 20-30 gallon tank(150-200lbs full), the tesla model s battery module(5kw, 55lbs) I will likely be using can put out 750amps, plenty for starting(probably add a second small high amp 12v battery for backup) . And will have 2 other means of backup propulsion(<100lbs), 1 KW of flexible solar/wiring(50lbs).

    And believe you me, I will be paying very close attention to the weather. As much as I love the ocean I am equally terrified by it.
     
  2. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Senior Member

    Yes, I've considered this, and am working on a solution, part of that is not mounting it all the way aft(I know that will affect my maneuverability).
     
  3. dustman
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    dustman Senior Member

    Thank you, I have looked over some of his designs. I've already decided that I'm not building with wood.
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I built in foam and it is not budget friendly to do well. But you may find some weight savings in foam done with vacuum. Plan for some losses of epoxy. If you wet bag; epoxy losses are around 25%, a bit less for table infusion I'd say. Lower still for half or whole hull infusion. But that is risky for first timers. I opted away from the risk and wet bagged. Do again; it'd be table infused.

    One engine will help with budgets of weight and time. The tradeoff is maneuverability and redundancy. You will need something fairly robust to mount the engine; not light foam. It will be in a nacelle perhaps? Getting the engine height right will be a bit of a trick. Best to plan for some possible variation.

    I'll enjoy watching and hearing more about it.

    What the devil are you going to make the beams from? Wood is rather obvious here, but can be heavier.
     
  5. dustman
    Joined: Jun 2019
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    dustman Senior Member

    We're thinking like rich guys again, no offense.

    But I will try to explain and my method isn't perfect but it will work just fine. I will be using square aluminum tube as crossmembers, bent at the ends to go down and meet with longitudinal tubing that will be embedded in rigid foam, the hulls. When looking in plan form the hull forms will be made in the desired shape from that perspective. Pour foam. Once the foam has cured I will use a 1/4 circle form which will have sandpaper on the inner radius which I will use to shape the hulls into a semicircle, and as you work to the tapering end of the hull and keep it on the same vertical plane the result will be a perfectly transitioned entry. Then I will simply cover it in light fiberglass, reinforced wherever it may encounter impacts or wear from beaching, etc. No bagging, no fancy machines, no worries. Easy to repair and it really wouldn't matter if you put a hole in it. Basically the hulls will have an internal skeleton and a light exoskeleton. Wherever there is a major structural joint between aluminum pieces I will weld additional aluminum plate to further bond them, make them one, the joints would be stronger than the tubing itself. The motor would have its own lower mount welded onto the back crossmember with some kind of additional bracing, not quite sure yet the how that's going to look. The weight of all the aluminum would come to somewhere around 300 lbs. The motor would be under a sound damping housing because I don't want to hear a loud drone for thousands of miles. Then I slap a deck(foam/fiberglass composite) on it with a small cabin(with aerodynamics in mind) to fit all the necessary stuff with room to sleep and sit and prepare food. Sitting room only. A lot more details of course but I really don't want to explain them all at the moment. Redundancy will come from the trolling motor and small sail. There will be almost nothing that can happen to keep me from moving. Well, except for getting flipped, but that is incredibly unlikely with the journey I'm planning and all the modern weather prediction.

    Forgive the roughness of this response, it's very late and I've had a few beers.
     
  6. fallguy
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    fallguy Senior Member

    What is the pour foam getting poured into?
     
  7. dustman
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    dustman Senior Member

    Simple thin plywood forms treated with release agent, with stringers on the outside to hold its shape.
     
  8. fallguy
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    fallguy Senior Member

    I'm not sure what the shear properties of pour foam are precisely, but you'll need enough laminate to deal with it. A layer of 6 oz woven probably won't be enough. But outside my wheelhouse to spec it. I just see a potential issue.

    I suppose you can get a higher than 2# density pour foam that may have better properties.
     
  9. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    If you have actually followed my threads on this, you will see that the structure weight for a cat does not follow the scale factor cubed, but the scale factor to the fifth. Even something as simple as a tramp Hobie gains weight to the fifth on length. Subtracting out the 200-300 lbs for engine and fuel does not leave anything for habitability. You need to do a detailed weight study.
     
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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I think what is missing here are the finance details. Boats and shoe-string budgets have an unhappy relationship, that can involve unsatisfactory compromises. I think if you name the figure you have to spend, it will be a good start to working out what is sensibly practical to do.
     
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  11. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Contrary to the view that you design the boat first then worry about engines later, I think in this case, not such a great idea to do that. Seeing the speed requirements are quite modest, I would select a couple of Yamaha 9.9 XL leg high-thrust motors as the starting point, and work from there, you need a reserve engine that will make headway in adverse conditions, a puny electric trolling motor is less reliable to be able to do that. If they sound too expensive, then I'm afraid it does not augur well for this project. The reason I specify those motors, they are closest to optimal for the boat size/ speed mentioned, the next bracket of engines that are most suitable for non-planing boats are the 40-60hp high thrust engines, but they are best for bigger boats, and slightly higher speeds.
     
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  12. fallguy
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    fallguy Senior Member

  13. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I've done all that and I don't think there is an understanding of how simply I'll be living. If I go a hundred or two beyond expected outcome no big deal because the actual design displacement of the hulls and structure will be several hundred lbs beyond that.

    Just a reminder that people used to cross oceans on sticks of various sizes lashed together with substandard rope. I think I'll be alright going to the bahamas with weather reports on tap.
     
  14. jehardiman
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    jehardiman Senior Member

    True, but not many survived with the length-displacement ratio you propose. Traditional vessels are what worked in their particular situation...not what didn't. As I have said before, there is a reason the Polynesians didn't discover the "Stormy". Tread carefully here.
     
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  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Its a long way from Arizona to Florida, you really would spend a lot getting an over-size boat transported, I think you need to scratch that from the list, considering the budgetary constraints. Some kind of modular construction might allow a boat that is a good beam on the water, and really you do need beam with a slowish cat, if keeping it easily driven is paramount. Bonus a more stable boat.
     
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