Designing a 360cm(11ft) powerboat hull, suggestions?

Discussion in 'Hydrodynamics and Aerodynamics' started by Glattnos, Jul 14, 2018.

  1. Yellowjacket
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    Yellowjacket Senior Member

    Why the extensions on the transom??? That will tend to push the bow down by creating lift aft. If you have a lot of weight aft then maybe that's a good idea, but the hull is pretty heavy and the CG is already forward. At high speeds you're going to want weight aft, but if you're optimizing for lower speeds it may make sense. Your motor isn't that heavy and if the nose is coming up too much you can move load forward to do the same thing. I'd keep it simple and get rid of them. If you find out you need planing flaps then put some on, but otherwise skip the extensions.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't see any value at all in a pad or plank on a shallow vee hull, it is already not that far removed from flat in the aft sections, and you are also losing a little bit of lateral plane, where you need some for directional stability.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    The OP is seemingly fixated on a fine bow shape, which is basically poison in a small boat, You want to maximise the stability in a 12 foot boat, an average person stumbling around in one, is like a grizzly bear in a 20 footer, in terms of "rocking the boat". Carrying the beam well forward, is a must, in my opinion.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Here's another from the same stable as the Magnum 12, I posted earlier ( I have no interest in peddling the brand, this is strictly for illustrative purposes). Once again, should be a dry and stable little thing, though you must get hammered in a chop. ( although that basically holds true for any 12 footer that will plane readily with 9.9 hp). I'd have probably gone a tad deeper in the vee, running around on near flat bottoms is more akin to skating. The boat would require a name change in Sweden, tropical it ain't ! Tropic 12 https://bowdidgemarinedesigns.com/tropic-12/
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Not all 12 footers have flat bottoms, but the power requirements are greater than 10 HP. A 50 year old pic of a 12 foot racing " Haines Hunter" deep-vee ( the boat may have been designed by Raymond Hunt, if not, certainly derived from his designs) powered by the old Mercury 50 HP 4-cylinder. Brave people to be haring around offshore in that thing.
    gary+haines+1967.jpg
     
  6. Glattnos
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    Glattnos Junior Member

    The originally "narrownes" of my design is due to its self-righting ability. As I said before, it is a covered boat that was intended to be self-righting. There is two seats so there is nothing like "stumble around in it" :)
    The "extension" on the transom is to add buoyancy so the swell is a bit less likely to wash over the motor when slowing down and to make the waterline a bit lower in the aft when stationary. See it as a cut-out for the motor instead of a extension.
    I may already have lost the self-righting ability and it is not the final solution.

    I was testing Dingos spreadsheet and it tells the power needed for different dimensions so I can probably work out some good numbers there, thanks for the advice :)

    The question is still about what shape is god and if strakes, pad, inverted strakes etc are beneficial? If a flat V is the only answer thats fine :)
     
  7. fallguy
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    fallguy Senior Member

    I have only some crude advice. I was reading the thread and two things I would caution is a narrow little boat is very reactive to a person moving in the boat.

    Mr E notes this and you discount it immediately based on his word use. What good is self righting if you don't like using the boat?

    A pleasure craft is not fun if a person on the forward seat leans over and the helmsman bitches cause the boat turns or if you are drinking a cola and someone else moves and you spill.

    One of the best designs in 12' is a Lund aluminum vee.

    The other crude remark I have is the bow tip looks low to me vs the WL. I could be flat wrong, but in rough seas or wakes a low bow in a tiny boat becomes wet fast. And a wet boat it will be...so you'll need something for splash and I would cover a portion of the bow; even if just a foot or 18" to more if the space is too narrow to use. It will strengthen the boat and maybe allow for a handle and a nav light mounting. If you have not been in a boat with waves coming over the bow; you would not understand, but the shorter the boat; the easier to bottom into the next wave.

    Look at the Boston Whaler 13 Sport. A proven design and not a wet ride. Totally different, but food for thought.

    And the Bw13 has a cutdown transom for the popular short shafts of the era. This is another thing that can result in a wet boat; especially if there is ever any backing. Of course, you say you won't back, but you end up needing to now and then and I dread it in my long shadt boat to the point of always wearing fishermans waterproof pants. The BW13 has a helm, but the short shaft design is not good on a tiller boat is my point.
     
  8. Glattnos
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    Glattnos Junior Member

    Thanks for your reply and it was not my meaning to offend Mr E, I listen to all your advice.
     
  9. fallguy
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    fallguy Senior Member

    Splash rails will be needed.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    This self-righting ambition has me intrigued, I really have no idea how this narrowness is aiding righting ability, it makes no sense to me, you want the boat to stay largely upright at all times, and importantly not to develop a crazy lean with passengers shifting position in the boat. The problem is, that one person is such a big % of the weight of the whole ensemble, that stability calculations that might be valid for a scaled-up version of the boat, don't really apply the same way.
     
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  11. Glattnos
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    Glattnos Junior Member

    It is a covered design with two seats, people can not shift place in the boat. It is the third self-righting boat I build. It is hard to make a self-righting boat that is wide but because it is also hard to make a small planing boat narrow there is a bit of a problem, but I am working on it. Self-righting is not totally needed but when the boat is a covered one, it is a huge advantage when it comes to safety on those small boats.
    The top-design and interior is quite similar to those: Photos | Seabreacher | The Ultimate Diving Machine https://seabreacher.com/gallery/photos/
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Right, well, this is getting into an area that most people are probably not familiar with, real self-righting power boats are not usually associated with recreational boating, more rescue craft forced to operate in difficult conditions. I am not seeing much application, beyond that, really.
     
  13. Glattnos
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    Glattnos Junior Member

    Yeah, I know but my question in the thread have not much to do with how stable the boat is when static or how to make it self-righting. I just ask for advice on the hull-design(the bottom of the boat) to get a reasonable balance between planing-ability, fule-efficiency and wave-handling and if there is any cheap/free program for such simulations. Hydrostatic stuff like waterline, C.G, C.B, righting moment, load-changes etc. I can simulate in the CAD but hydrodynamics is normally not supported by normal 3D-CAD programs. Dingos spreadsheet was a really nice advice that I am using now to check power/speed calculations.
     
  14. Will Fraser
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    Will Fraser Senior Member

    Just to give you an idea of performance on the wide-flat-short end of the spectrum:
    I recently measured 14kts with a 5hp two-stroke on a 3.4m cathedral hull. Beam is approximately 1.3m and static LWL around 3m.
    Total displacement during the test was 200kg.
     

  15. Glattnos
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    Glattnos Junior Member

    Thanks for that :)
    That sounds quite much like what I am after. I will have 9.8hp and 220kg so 14 knots should not be impossible then on a bit under full throttle.
    How do you like the cathedral hullshape? I have been looking at that and it seems to be a quite good shape for smaller boats, can you notice any specific advantages or disadvantages?
     
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