Help me design the "Poorqeedo", another efficient electric boat

Discussion in 'Boat Design' started by mental_boy, Jun 2, 2010.

  1. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    So I've decided I want to build an electric catamaran. Torqeedo (the electric outboard manufacturer) modified a Hobie catamaran for a trip to catalina island and I like the concept very much:

    [​IMG]

    However, I can't afford a torqeedo outboard, or a hobie catamaran, or almost any boat for that matter. In comes "Poorqeedo," or the poor man's torqeedo catamaran. I don't really want to copy the torqeedo catamaran, it's just the configuration that I like.

    I settled upon the idea while mulling over the qualities I desire in a boat:

    1. Car toppable (I have an truck with a very strong rack)
    2. Relatively easy to set up and launch
    3. Carries up to 4 people (although not necessarily with optimal performance)
    4. Cheap to operate
    5. Fast and cheap to build.

    To be car toppable the boat would probably need to be broken up into 4 components: 2 hulls, superstructure/seating, and batteries. To launch, the hulls would individually be brought to the dock, the superstructure attached, and the batteries added last.

    For propulsion I have 2 APC 16x16 props and 2 electrocraft 100v, 5.6A continuous motors (38v/1000rpm). I didn't choose these for this application, I just happen to have them laying around. My inclination is to use the electrocraft motors at 48v with cheapo ebay motor controllers and 4 walmart or costco traction batteries. If the gearing is right the motors could yield ~250 watts each in that configuration. Obviously the gear reduction will depend on hull shape, total weight of the craft, and target cruising speed (which are all totally unknown to me at the moment.) I'm hoping to skip building rudders since I can use the 2 motors to steer.

    I'm not sure how I should handle power delivery to the prop. I like Rick Willoughby's spring steel method due to it's simplicity:

    [​IMG]

    But that would make it difficult to attach the superstructure with the hulls upright on a dock. It also makes it impossible to inspect the prop while underway. I imagine I want something like that but hanging from the super structure. Definitely need ideas here.

    For hulls I have only a vague idea of what I want. Something like an old hobie 18 but with a rectangular cross section. Or like a pair of Rick Willoughby's V14 boats, but with more load capacity:

    http://www.rickwill.bigpondhosting.com/V14.htm

    I have 2 sheets of 1/4 inch meranti, 1 sheet of 3/8 meranti, a gallon of epoxy and a 28' by 5' piece of fiberglass cloth which I'll try to incorporate into the design as efficiently as possible.

    I'm thinking the cross braces (aka super structure) will be large plywood box beams that double as seating. They could also be made of triangulated stainless tubing since I am proficient in metal fabrication and welding.

    Anyway, I really need some input on how the hulls should be shaped (i.e. length/beam etc) the overall beam of the boat, the gearing, what kind of speeds I could expect, etc. I'll post more information and drawings after I get some more sleep =)
     
  2. Guest625101138

    Guest625101138 Previous Member

    For regular use you will find it becomes increasingly tedious to assemble a boat like this each time you want to use it. It is worth while thinking about it eventually being trailerable if that is not the case in the first instance.

    The first step is to do a realistic weight estimate. You could allow about 20kg for each hull - they should end up less but there will be other bits you forget. The design speed will depend to some degree on the maximum length. Hence the maximum length is important to think about and set a limit. If you can go to something over 6m then a monohull might become attractive. They can be stabilised.

    The range without charging will depend on speed and batteries. You could eventually use solar panels. The Chinese made ones seem to be used more often and they are said to be reliable and about half the price of ones made elsewhere. So maybe plan on fitting these later. On a sunny day a couple of 200W panels will provide nice shade and give enough power to do a respectable speed.

    Rick W
     
  3. portacruise
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    portacruise Senior Member

    If you can find something along these lines (or at least the hulls) on E bay, Craigs or other such lists at low cost, it might fit some or your listed requirements.

    http://www.easysail.com/england/easyto.asp

    There are also large river cats that may not be as streamlined which will be harder to push and thus lower efficiency:

    http://www.nrsweb.com/shop/product.asp?pfid=1119&deptid=1132&avad=20917_bb4f55d

    Both seem to have a lot of safety features built in and may handle rough water offshore well. Mounting two motors to the frame on the inside should not be too difficult.

    Porta


     
  4. ancient kayaker
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    ancient kayaker aka Terry Haines

    Here's one process for making a rough first stab at the design numbers; forgive me if I pitch it a little too low for your current level of knowledge. I'm assuming from the load you wish it to carry that performance is not the main goal here, just something that will be fun on the water and reasonably useful.

    You need to start off with a weight budget, which will be revised as you work through the design process. Crew and batteries, motors and gearboxes accommodation, supplies will mount up quickly. Also include the weight of the beams and platform to bear this load.

    Then you can start estimating weight for the hulls: these will be far heavier than Rick's number, IMHO. You will have close to a tonne on the go with this boat, I'm guessing, and if it comes to a sudden stop on a rock or a deck you would probably be happier if it doesn't spring a leak. I would use a budget figure of 1/3 of the total for the hulls, or half of what you get from the first paragraph.

    At this point, even if you deduct crew weight, I think you will appreciate the wisdom behind Rick's suggestion that you treat this as a trailerable boat, but at least you will have a better idea of what you are trying to do.

    p.s., welcome aboard and good luck on this interesting project!

    Each hull should have enough buoyancy to support the entire all up weight and then some; not more than 33% immersed when balanced and not more than 50% immersed if all the people decide to rush to one side.

    These are the values I would use but I''m not an NA and others may know better. Finally you will have reached the point when you can decide on dimensions. Cats typically have long narrow hulls, Rick's are extreme and intended for human powered boats which are a special case with a beam/length ratio of almost 10:1 and very shallow draft.

    At this point you could get an approximation, at least a place to start the hull design process, by taking a length/beam ratio of 6:1, typical of canoes and kayaks which are fairly efficient things, allow for rather more draft, say 1/3 of the beam, and then you can get a rough idea of length, beam and draft, which will lead to hull height. I'm guessing you will end up around 14' x 2' square or maybe a bit more. Design is an iterative process so you'll likely have to do all this over again, maybe a few times, so put your calcs into a spreadsheet so you can copy everything to a new page each time you revise your concept.

    Most power cats have less than efficient hull shapes. You can try for better in which case I think Rick would be your best authority. Remember that crew/passengers will want to move around so you cannot go for a delicately balanced equivalent to a Swiss watch here; it's going to have to be tolerant of imbalance, safe and tough. With perhaps 1 HP available you are not going to break any speed records: hull speed might be about 7 k, so you can do some guess work on gearing allowing for some prop slip and can get an idea of how much torque you will have to accommodate. Then you can get the nasty surprises over cost and weight of gears ...
     
    Last edited: Jun 2, 2010
  5. messabout
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    messabout Senior Member

    Both Rick and Ancient K are good sources of input. Rick is more than correct in that you need to think of this as a trailerable boat. Way too much assembly/disassembly involved as a car topper. Too much of that kind of preparation at the launch site means that you will use the boat less and less. I base that statement on the long term history of overly complex car top types all over the world.

    If you are convinced that you need a catamaran then...OK. ( I may argue the point later.) If it is to be a cat it should be long and the hulls narrow. Try for a lenght/beam ratio of 8:1 or more. That means that 16 foot hulls can be 2 feet wide. 18 inches width would be even better but you are constrained by the need for bouyancy enough to hold the load that you anticipate. You'll have to fiddle up the dimensions to accomodate desired displacement. AK is correct. The first order of business is to determine how much total weight you intend to carry.

    You can get a ball park idea of the speed that you might attain by taking the square root of the waterline length. This is ballpark mind you. Thus a 16 foot WL would yield a square root of 4. Four knots, then, is about what you can do. 4 knots equates to about 4.4 MPH. That is about the speed you could expect from the power that you have available. That figure is less than theoretical hull speed but you are going to power the boat modestly.... and intelligently, I might add. I rebut AKs estimate of 7 knots unless you are very lightly loaded and the boat is 20 feet or so in length.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Agreed: 4 k likely nearer the mark.
     
  7. pistnbroke
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    pistnbroke I try

    if its poor as in $$$ then have a look at Hannu's boatyard (GOOGLE)and his boat Dug ...I have seen two made into a catamaran ..I feel you want something bigger and not in 1/2 ply but the idea is there...put a deck on and fill with foam etc etc
     
  8. mental_boy
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    mental_boy Junior Member

    Hi guys,

    Thanks for all the good information and the kind words.

    I'd like to trailer the boat, but I have nowhere to store the trailer at the moment. It's definitely possible in the future. I like the catamaran since I can break it down and store it in a narrow space, and it's a stable platform that I can stand on. Point taken on the ease of deployment issue, but I think I will press on with the original idea.

    Solar panels are also a possibility in the future.

    I've come into some free curtis 36v controllers, so assuming I use the electrocraft servo motors I mentioned, I'll have even less power than I was thinking before. Assuming proper reduction, power would be 193w per hull, making the grand total for both hulls 1/2 hp. I could also do some sort of wonky setup where I run the controller at 36v for low speed/maneuvering/reverse and then switch to 72v direct from the batteries for 1hp. I'll have to think about that....

    Anyway, on to the first order of business: Weight estimation. Here's what I've come up with so far:

    Batteries 44lb x 3 (105 ah 12v)
    Motors 10lb x 2
    Drivetrain 10lb x 2
    Hulls 85lb x 2
    superstruct 85lb x 1 (just a guess)
    epoxy 10lb x 1
    glass 5lb x 1

    So approximately 450lb for the craft plus 350 to 700 lbs of crew brings the total to 800-1150 lbs.

    The crude hull I based the weight calculations on is 20 feet long, 1 foot wide and 18 inches tall at the center:

    [​IMG]

    The scale on the graph is 6 inches per square. It just happens that to immerse both hulls 6 inches it takes 853 pounds. After that point the immersion rate is 160 pounds per inch. Total buoyancy is 1386 lbs per hull, 2772 lb for both hulls. Wetted surface area for both hulls is 60 square feet. That's as far as I can get using basic geometry...

    This doesn't have the 50% reserve buoyancy that AK suggested if everyone rushes to one side, but there would still be about 350lb of buoyancy left in that situation.

    Am I on the right track, or would it be better to make fatter, shorter hulls? I'm guessing regardless of proportions there should be some fullness in the ends of the hulls to avoid pitching.
     
    Last edited: Jun 8, 2010
  9. mental_boy
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    mental_boy Junior Member

    Found something interesting here:

    http://www.hobiecat.com/community/viewtopic.php?p=13445&sid=ca7271b3be79bd9498bd97412c6e5e12

    Brian stated "I have a Getaway, we motor it with a Minn Kota 36 motor. Full draw pushes us along at 5 knots measured on the GPS. Half throttle is 3 knots."

    MCdenny's tests found that the 36lb minn kota motor draws 200 watts here:

    http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html

    This boat will have double the crew weight and double the power with similar boat weight (the getaway is ~400lbs). In addition the motor and prop combination should be significantly more efficient, so 5 knots or more is clearly attainable with full crew.
     
  10. luckystrike
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    luckystrike Power Kraut

     
  11. mental_boy
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    mental_boy Junior Member

    Not too sure about using an old outboard, I imagine the gearbox would eat a lot of power. If I had a nice etek pancake motor I'd consider it.

    Here's some more detailed information on the torqeedo hobie cat, which also happens to be the getaway model:

    http://www.navagear.com/2009/11/tor...eeks-to-prove-open-water-electric-propulsion/

    The article says 5.6 mph at 1000w, and 10.2 mph at 4000w on the open ocean. I may need 72v after all....

    The part of the article where they mention 1000 porpoises is ludicrous.
     
  12. luckystrike
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    luckystrike Power Kraut

    Fatter, shorter Hulls for a cat is no good idea as resistence will be higher. In my opinion you should consider the "ilan"- concept with one, long but slim hull and two small stabilisers made from shaped styrofoam and glass.. This will have the best ratio of wetted surface and wave resistance. You could make the mainhull to be broken down into two or three pieces to carry all the volume needed

    A three-piece main hull sounds silly in the first impression, but it makes some sence. You can have a big center piece (made from your existing 1/4' plywood) for the accomodation and all the technics and two pieces made from styrofoam or styropor for the ends. Built in emergency flotation all inclusive. Light and very cheap! The foam will be free and the glass will be cheap if bought on Ebay.

    Grrreetings from the North Sea Coast, Michel
     
  13. luckystrike
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    luckystrike Power Kraut

    Forget about the gearbox, you will use only the vertical shaft drive, the 90° force redirection , the buit in ball bearings and the streamlined outside with its fixation to the hull. Wait for the sketch. It will make your project a little simpler and cheaper.

    Michel
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Some points to consider:

    Resistance is a combination of wave resistance and skin friction. Wave resistance increases rapidly as hull speed is approached. The concensus is that with your power levels you are not going to approach hull speed. Skin friction increases with increased area and I think it will dominate, so you will go faster with shorter hulls IMHO. There are some formulae and data easily found with a google search.

    As hull length increases and hull width and depth decrease hull strength (and weight) becomes a challenge - a long skinny hull is more likely to break due to load concentration or impact with a fixed object.

    The design of the craft that is emerging from this discussion is suitable only for calm conditions. With the limited power available it will have problems pushing through waves caused by weather or other power boats. If extra sources of power - like an outboard - are taken along, these should be factored into weight calculations and the extra stress on the hulls and platform taken into account in construction.
     

  15. Guest625101138

    Guest625101138 Previous Member

    I have attached the linesplan for the lowest drag hard chine hulls for 6kts and 400kg.

    These hulls will require a total of 290W to achieve 6kts in calm conditions.

    With the very best drive and prop set up you could build, expect an overall efficiency of 70%. Hence you will draw at least 414W to do 6kts. No allowance for windage - small at 6kts and calm.

    Using the 105Ah at 36V and 80% discharge you get 55nm range in calm conditions.

    Hulls are a little over 6m long and 400mm wide at the deck. As drawn the fully immersed buoyancy of each hull is 407kg. You might want to go a bit higher on the sides. You also need to consider deck clearance. With an overall beam of 2m you would want the bridge to be about 300mm above the waterline to avoid it clipping waves regularly.

    Buoyant ends increase the response to waves so this increases pitching. Hence you may want to lower the line of the ends. The flat bottom increases pitch damping. This needs to work with the bridge arrangement.

    Just for comparison I looked at a 7m long 400kg monohull for 6kts. It requires 203W. It gives an idea of the advantage over the cat but then it is 1m longer.

    Rick W
     

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