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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    dustman Senior Member

    Methinks there is a lack of understanding of the concept that I am pursuing. I am not a rich guy building a rich guy boat. This will basically be camping on a fancy raft. I have figured the weights on the materials and 1100-1200 lbs with motor, but otherwise unloaded, is quite feasible. Water will primarily come from distillation and rainwater catchment, with backup filter and enough storage to account for a survival scenario. I'm pretty sure I'll be getting much greater than 3mpg. Apparently this pontoon boat, Qwest 818 Adventure Cruise, gets 7.5mpg at full throttle with a 40hp yamaha, 1500lb dry weight. Obviously minimally loaded in ideal conditions... Many small power boats report around 3mpg going well over 20mph. I'm betting on close to 10mpg for my setup at 9-10kts, if not better. As Ad Hoc so perceptively observed, I am here for trends in hydrodynamics(and aerodynamics).
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    As I mentioned, and assuming your are going to have twin engines, outboards, the ability to run on one engine will be a significant range extender, at the cost of a little speed. What size engines do you have in mind ? You really need high-thrust engines for this speed range, and there does seem to be a bit of a gap between 9.9 and 40 hp with their availability, and 40 hp is way too much here
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    Without such trends or guides all other issues are moot.

    Engines, props/jets, etc etc etc .. is a totally different question, and one that has not been asked.

    NA is an engineering discipline.
    As such, answer the question at hand, not one that has not been asked... no matter how itchy one is to do so, or to appear knowledgeable on such.
    The 'next' question, tends to be driven by the conclusions of obtaining data from the first... not the other way around.
     
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  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    It is far simpler than that. You just select a hull form that accommodates varying displacements and knowing what the result is of such variables.

    Once the basic are understood.. the 'length' and all its influences, per se, matters little. Since one uses said data to then 'design' the objective.
    Hulls are easily scaled, called geosims. The data scales accordingly.

    MORE displacement?...how so, and what is the concern?
     
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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Being able to chew gum and walk at the same time, I can see he needs to be aware that suitable outboards for such a vessel are not well supplied in the gap between 10 and 40hp
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I'd rather not mess up the thread commenting on my boat. I just find it interesting that the OP did NOT scale. He created a decision between two hull forms that offer the same displacement, but almost certainly not the same weight. Then, he asked whether it ought to be a planing hull.

    I am a guy building a cat and I love his wishes! But I am wholly confused on how someone can be posing the question on how long should the boat be and then asking if a planing boat is right.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Again, irrelevant.
    It does not address the question at hand.

    That's the difference being professionals and amateurs.
    Amateurs just want to keep answering question on subjects they know about or like to talk about, not the question being asked.

    It is like asking,
    what is 2 + 5?
    And you answer well, don't forget if you also add another 5 and then dived by 3 the answer will be 4.
    When the only answer is 7.
     
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  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    I dont claim to be a psychologist, just a NA. So I can only answer the question at hand.
    What the OP does with said answer , or what they are thinking.. is anyone guess, unless is it clearly stated... again, I can only answer the question at hand. Not guesses at what I think they think...
     
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  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Well, this comment is helpful. A 24 foot cat is about the smallest you'd want to make the crossing to the Bahamas. A planing cat is typically not much beam either and you'll want some beam for such a crossing. Two 20hp engines and start batteries are going to weigh about 260#. Add a bit for steering and controls. Two 20 gallon tanks and lines and fuel go say 140 each or 280#. This is 540# for engines and fuel. So, keep that in mind..

    Skimping on dual engines for a Bahama crossing is not so wise. But people do buddy up for the crossings. Another place to avoid skimping is on short shafts. Want to keep the powerhead up some..

    Gonna be hard to land around 1100, no?
     
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  10. bajansailor
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    bajansailor Marine Surveyor

    I know that Fallguy will agree with me when I suggest that you have a look at Richard Woods 28' Skoota for some very good design ideas - he has often crossed from Florida to the Bahamas with his Skoota in the past, and has lived on board for months at a time.
    Sailing Catamarans - Skoota 28 transportable minimum live aboard cruiser https://www.sailingcatamarans.com/index.php/designs-2/6-powercats/264-skoota-28

    Sadly Richard's Skoota was destroyed in the hurricane that hammered the northern Bahamas a few years ago - she was laid up in a yard in the Abacos.
     
  11. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I plan on using a single 20hp outboard(maybe a suzuki but further research is needed). That should be enough to get me up to 15kts, which is an acceptable top speed for me. I will choose a prop that has appropriate pitch for that speed at a little below redline, that should leave the engine at a pretty good rpm for my cruising speed. Then I will have a trolling motor backup, about a KW of solar, and 4-5kw of lithium battery storage, most likely a tesla model s module(55lbs). Then I will have a quite small sail, just enough to keep me moving in the general direction I need to go if everything else fails. If I have really short passages between islands will probably use the sail and/or trolling motor only.

    By the way, if anyone knows of a higher pitched prop that could be easily adapted to a trolling motor please let me know.
     
  12. dustman
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    dustman Senior Member

    This was my train of thought. The design displacement for the hulls would be about 1800lbs. Given the semicircular form this will increase wetted surface a bit but worth it for the extra buoyancy and capacity.
     
  13. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I believe you are reading too much into the question. I was just pondering if planing hulls, semi-planing or otherwise might be a viable option. Honestly full displacement hulls at only 24' seem a little short for true displacement hulls at 10kts. I think Ad Hoc alluded to this, but concluded that the resistance hump was small given the fineness of the hulls, which seems like a reasonable conclusion to me. I am scaling in my mind all the time because I dream of building much larger boats and crossing oceans in the future. My questions are much more than about this particular boat, I want to learn principles, then I can extrapolate from there.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Planing is simply not on the menu with a 20hp outboard, I see you having problems running a single outboard on such a boat offshore, either keeping it in the water, or sufficiently out of the water that it is not dragging too much, and the engine mentioned is only a 20" shaft
     

  15. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I realize that from researching. This project is realistically a couple years out at this point, so plenty of time to learn more and gather what I need.
     
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