12ft Cat for Solar Splash Competition - first time tips needed

Discussion in 'Boat Design' started by JacobMUSI, Oct 13, 2021.

  1. JacobMUSI
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    JacobMUSI Junior Member


    Here's what I designed with that in mind, I really like it. Endurance Fins.JPG No fins.JPG Sprint Fins.JPG
     
  2. BlueBell
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    BlueBell "Whatever..."

  3. JacobMUSI
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    JacobMUSI Junior Member

    I really don't know how they got that number as that is not my area on the team. The batteries are brand new, but obviously lead acid is pretty crappy for this application. I really am only focused on the hull design.
     
  4. fallguy
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    fallguy Senior Member

    I am a boat builder of a catamaran.

    A couple simple takeaways and gut hunches you can take to the review.

    The boat looks too long forward. My gut tells me it is unbalanced.

    What is loa vs lwl?
     
  5. fallguy
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    fallguy Senior Member

    Also, xps has very poor shear qualities and this allows it to delaminate easily.

    Choose a foam with adequate shear properties that can handle slamming forces.
     
  6. kerosene
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    kerosene Senior Member

    I think I have written elsewhere about experimental cinema... Eager students want to make experimental films before learning traditional film making. Not only will these films that are "breaking the rules" utterly suck but they end up surprisingly similar amongst themselves.

    To break rules and conventions in any field is needed for innovation but one needs to know the rules they are trying to break before doing so.

    Gerr Nature of Boats is a good 1st book. It is an easy read not overly technical with numbers but covers basic concepts very well. You should do some reading before designing more.

    Seamanlike sense in power craft by Uffa Fox is a good book about planing boats.
     
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  7. JacobMUSI
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    JacobMUSI Junior Member

    what foam would be better?
     
  8. BlueBell
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    BlueBell "Whatever..."

    That's too bad as the boat works as a whole system.
    You need to know how much HP you have to make a good design.
     
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  9. kerosene
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    kerosene Senior Member

    there are actual professional core foams: divinycel, airex etc. can't recall names now.

    But xps is not bad as long as you understand it's weaknesses and essentially treat it as a leave in mold not trusting it to provide significant strength.
    Burt Rutan designed aircrafts to be built from XPS and he knew what he was doing.
     
  10. upchurchmr
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    upchurchmr Senior Member


    This is one of the most important issues in design. And it is not only students who don't understand, we get "adults" who make the same mistake.
    Even more upsetting is that most school professors either don't understand or don't teach this point.

    I use to get new engineers in aerospace who had no clue about this. They were upset when they had to repeat "old" or current designs until they finally understood them.
     
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  11. messabout
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    messabout Senior Member

    Jacob; a theoretical horsepower in electrical terms requires 735 watts. Electric motors have a variety of losses that saps up energy. Figure at least 1000 watts per horsepower. Thirty horsepower will then need in the neighborhood of 30,000 watts input. The watt is calculated as volts times amperes. (W = V x A) You have 36 volts to use if the battery is at full charge. The arithmetic says that.........you will need 36 volts with an output of 833 amperes. Consider how long those batteries will last with that kind of load on them. ( Incidentally, you will to cook some wires with that kind of current flow) If you consider that the available HP is only ten, then you still need 277 amperes, or more, continuously, for the duration of the speed trial. Three twelve volt lead- acid batteries are not going to like that much drain. Twelve volt batteries will be wired in series to get the 36 volt output. If so you will not have an ampacity that is sufficient unless the batteries are huge....and beastly heavy. Remember that weight is the enemy.

    I suggest that you consult with some of the EE students to get a better handle on what is actually, electrically, do able. When you establish the real world power available for your boat, your design may need to change considerably. You can design a boat that will plane nicely, but no where near 30 MPH........ with about five HP. It can also work with reasonable electric economy for the endurance part of the trial.

    Do try to abandon pre-concieved notions about what you think will make a competitive design. Some of the most efficient boats are simple and ordinary looking.
     
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  12. messabout
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    messabout Senior Member

    Are you still there Jacob?
     
  13. JacobMUSI
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    JacobMUSI Junior Member

    Yeah, I am just waiting on my electrical team to figure stuff out. At this point I cannot change the design anyway, I am out of time. The bouyancy calculations turned out good though so I am gonna continue. I did switch construction to stitch and glue so that was helpful.
     
  14. messabout
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    messabout Senior Member

    In about two hours of drawing board time, not computer exercises, just drawing board with a pencil paper and a few splines, you can design a small boat to suit your needs rather nicely. In another hour you can do all the applicable calculations about displacement, draft, center of buoyancy and all that basic stuff.

    If you explore the options sufficiently, you would likely come up with a flat bottomed skiff deliberately designed to minimize weight. If you wish to win the endurance contest then diminish the wetted surface with a trapeze bottom. That is important to low speed performance and the sapping of power. If you wish to win the speed contest the flat bottom hard chine model will be the better choice. Worlds better than the model that you have drawn. It is not out of the question that you could win both the contests with a simple flattie skiff. Make it longer, maybe a 15 footer, if you don't mind the necessity of using scarf joints or the ugly board and batten joint system.

    Your budget will of course play into the scheme. If you can afford the cost, use a stitch and glue ocumee structure. Ocumee costs a lot more than fir ply but it is much lighter. Don't bother with fiberglassing the outside of the boat. That would add weight. Every pound of weight is the enemy of performance with limited power. At the end of the construction you will need to spend a lot of energy, determination, and money, (lots of sandpaper of fine grit after a very careful paint job) to make the wetted part of the boat as smooth and fair as humanly possible.

    You will need to do some basic engineering to account for the weight and location of the batteries along with the weight and location of the propulsion unit. The design of the bottom rocker of the boat will reflect your attention to this detail. You make the boat longer to reduce the upward angle of the the after part of the bottom. Remember weight is the enemy. Forget foam. Just build a real little boat that may have some resale value after the contest is done.

    So if you have a few hours left in your quest, stop messing with unique designs that are unproven and have some serious basic faults. Just layout a simple little boat that has, over the ages, proven its' mettle in so many ways.

    I apologize for delivering a mini rant here. I do know what I am talking about after many years of building, design failures, and sometimes getting it right. I do wish you success in your quest.
     
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  15. JacobMUSI
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    JacobMUSI Junior Member

    Ok, I finally came to my senses and designed a 14ft monohull. The waterline is just under 6in at 511lbs which is at least 50lb over what the load should be. The hull itself should weigh ~50lbs. bottom front.JPG Bottom rear.JPG Sidefront.JPG
     
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