Pedal Boat Design

Discussion in 'Boat Design' started by BG_Geno, May 28, 2006.

  1. BG_Geno
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    BG_Geno Senior Member

    Rick W.

    I just waded through all 9 pages of the thread. Most of it was information I have already read as I have followed your work/progress extensively on other sites.

    1. I am fine with a mono hull except I simply don't care for the look of outriggers. I know that seems like a silly reason, but my wife and I enjoy the building process and feeling you get when superior craftsmanship meets with a good aesthetic. A well made beautiful boat beats a fast boat for us =)

    We are also older and would just be more comfortable with the extra stability the wider track provides.

    2. Your flex shaft is simple and an excellent performer. However we are building our boat for use in the sheltered waters of the bay her in Port Isabelle in WAY South Texas. Hundreds and hundreds of square miles of water about a foot deep in most places. Also very weedy at times. The kind of weeds that kill jet ski's in about 3 minutes. A paddle wheel is not as "cool" as a prop (for me it's even a little embarrassing lol) but just makes more sense. In 2 or 3 years when we move to Oregon props will make more sense.

    3. Any design help you can provide would be most helpful. I freely admit that I am a hands guy. Math makes my eyes glaze over a bit.

    I work in SolidWorks for my 3D modeling so if there is some file type that you prefer I can send you drawings of where I am at and after your done laughing and pointing you can point me in the right direction. In exchange, if you ever need anything modeled in 3D I would be happy to help out.
     
  2. BG_Geno
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    BG_Geno Senior Member

    Curtis

    I LIVE in forum threads it seems and I know some people get all bent when a thread strays. I am the opposite--I prefer the mental wanderings because neat ideas sometimes pop out.

    I think and learn the way you do...I am exceptionally good at thinking in 3d but there is just no substitute for really seeing it. A few months ago I started using Solid Works--the dreaded move from 2D CAD--and it has just been amazing. You actually build the parts that make up your doohickey, then assemble them and they actually move as they are supposed to. It handles collision detection, restraints etc. It can do chains and belts, cams, linear motion, gears and pulleys etc.

    When I think of the work a Michael Angelo could have accomplished with the tools we take for granted today...
     
  3. BG_Geno
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    BG_Geno Senior Member

    Rick-

    In simpler terms (much simpler terms please =)

    If a cubic foot of displaced sea water provides say 64 pounds of buoyancy and you have 325 pounds of engines on a 125 pound boat thats roughly about 7 cubic feet of displacement needed at your desired water line.

    That 7 cubic feet of area is going to require xxx amount of actual surface area.

    I have read that hull length to beam ratio is the key to a displacement hulls theoretical top speed. As in a non planing hull.

    If a skulls hull is a foot wide and 24 feet long at its water line (and I know thats a 35 pound boat and say a 200 pound engine) you still need to displace xxx amount and have the resulting wet surface area.

    Looked at another way. That boat with that engine requires xxx amount of energy to reach and maintain a certain speed. We will call that a given. Can I assume that if you took that maximized for efficiency hull and it's 200 lb engine and placed an identical hull in the water with the engines twin brother that either or both hulls would suddenly require 40% more energy to reach and maintain the same speed as just the one?

    Now I know you just rolled your eyes and called me Beppe the 2nd because your point was that if you took both engines and put them in one optimized hull the amount of energy required to reach the same speed drops by 40%.

    It still "seems" to me that theres more to optimize then just the hull/s because you have to factor in the 2 engines etc. when you consider that I still have to displace that original 7 cubic feet and thats going to require the same amount of whetted surface whether its a pair of 18 foot long hulls that are 8" wide or one 30' long hull 16" wide (random numbers on the later).

    I know theres a boatload of complicated math going far beyond my overly simplified reasoning, but it just feels like there has to be some point where the optimum ratio of length to width for a single hull balances out with being split between 2 hulls.

    Lets add a restraint to the mix as a probability test. If we stay with the skulls as our example. One stoker and a 24 foot length. Two stokers and the length jumps up to 30 feet. If you look at the eights those boats are 40 something feet. That pretty much shows your point very clearly as the hull length doesn't double from one stoker to two etc. Now if we limited overall boat length to the initial 24 feet...that means the beam has to go up right? Does that mean that at some point splitting into two hulls for the better ratio starts being a better option? Again, silly, but all design faces limits, and mine is a length of about 18 feet--20 tops.
     
  4. clmanges
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    clmanges Senior Member

    BG,
    It seems as though you may be confusing volume with surface area, or maybe thinking incorrectly of their relationship.
    There is a difference. To illustrate very simply, draw a cube which will encompass 7 cu. ft.; each side will measure the cube root of seven. Now calculate its surface area.
    Now draw a sphere of 7 cu. ft. volume, and you'll find that the surface area is smaller. You can also try other simple shapes, such as a short, fat cylinder as opposed to a long, thin one. Consider this as it applies to your design, and you'll see what Rick is getting at; displaced volume has to stay the same to float the mass of your boat, but wetted surface area can be changed. At a given length, a monohull will have less surface area than a catamaran.

    Since you aren't concerned with ultimate speed, though, I don't think the extra drag would bother you.
    Could you have been thinking of DaVinci? That would have really been something.
     
  5. tinhorn
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    tinhorn Senior Member

    Bite your tongue, my friend! It doesn't get any cooler than a sternwheel! Paddle wheels are the aquatic equivalent of the old school beach cruiser bicyces we see more and more often.

    I'm in good company in this thread - I, too, figure things out by seeing them in action, or at least a picture. Formulas and equations - yeek!

    Speaking of pictures, here's a PDF file of various feathering patents. They're rough - I didn't take time to clean them up or anything, and I may have overdone the file size reduction. If you want a hi-rez version of any of the patents, just PM me. And yup, the last patent shows a prop sneaking up on a paddle wheel. It's purpose was to enhance the wheel's performance. Hahahaha! Can't you just picture the self-administered forehead slap that came after filing that patent?
     

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  6. BG_Geno
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    BG_Geno Senior Member

    AHAHAHHAHAHA...I can't believe I did that. I even looked up the SPELLING!.

    Yes I meant DaVinci of course.

    I understand the difference between volume and whetted surface area (at least I think I do). My point was to displace that much volume won't the whetted surface area be a constant? It's shape can obviously take on many different forms, but the area has a minimum achievable...compression for want of a better word.

    The trick (laughs again about DaVinci =) is to find the most efficient form. The least drag and other variables I am unaware of etc. My thinking was that in simplified terms if the whetted area was a hypothetical 1,000 square inches, and we used the long thin hull/s concept (length to width ratio) and made one hull with 1,000 sq in that worked out to be say 40 feet long and a 12" beam...wouldn't it follow that if you scaled that same design down to a pair of hulls (exact same shape) 20' long and each a 6" beam (the same ratio for all the hulls in effect) would they still take 40% more energy?

    Maybe one of you guys with the hull software could take a 30 foot skull type hull and run the hydro statics (or w/e it's called) on that design. Then just reduce the design by 50% and copy another one next to it 3-4 feet over and run the same test again...

    As I see it the ratio of width to length stays constant. Your 50% of full scale hull also has 50% less draft (as compared to the full sized) because the pair shares the weight load.

    Now I know I am WAY over simplifying this and explaining complex mathematical things to dense people is irritating...but there IS empirical evidence that correlates at least a little. Longer skis with less width are faster then shorter wider skis on both snow and water which matches the length to width ratio thinking for boats. Yet 2 skis are faster then one (ask any snow boarder)
     
  7. BG_Geno
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    BG_Geno Senior Member

    TinHorn

    Some of those are really interesting (they all are really). I have been working on trying to figure out a geared version like the one about half way down thats gears and pegs.

    Several have the look like they were on the verge of becoming propellers, particularly the corkscrew looking one and of course as you pointed out, the last one. And what a kick those LOW LOW patent numbers are. I have a patent pending and the numbers are a lot higher now lol.

    As for feeling silly about a "paddle boat" I meant in the context of a PPB or HPB, not the majestic river queens. Clive Cussler books are one of my guilty pleasures and he makes them really come to life. Of course theres M. Twain =)

    Unfortunately, if you say human powered boat, or even pedal boat those are taken seriously. If you say paddle boat now days, those blocky pond kind, or worse, the big swan ones are what people think of. That said, one of the few perks of getting older is we tend to care less what the rabble thinks =)
     
  8. Village_Idiot
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    Village_Idiot Senior Member

    OK, here's a little more out-of-the-box thinking. Since most of the waters that Geno is plying are less than two feet deep, a more efficient propulsion system would be to leverage the substrate of the bay rather than flail about in the water. With this in mind, how about designing twin push poles (one on either side of the boat, or aft), operated by leg power. Use a kayak paddle for the few times you need to cross the ICW.

    On a different tangent, another advantage to using a paddlewheel may be its ability to enhance stability. Correct me if I'm wrong, but the advent of paddles pushing down, through, then up through the water seems like it would significantly enhance lateral stability while underway.

    OK, one more tangent, then I'm done. If we were to use a propeller, would there be an advantage to using a shrouded prop? I know shrouds are a hindrance on performance boats, but they are said to produce a marked increase in thrust at lower speeds - maybe worth investigating on a PPB? Taken a step further, could one create a pedal-powered water pump above the waterline (for drafting purposes), shooting the water out of a pipe at the stern to create forward thrust? Again, I know jet drives are inefficient in motorized craft, but can they be designed to operate efficiently in the PPB?

    I'll stop rambling for now...:D
     
  9. clmanges
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    clmanges Senior Member

    No . . . just do the little solid geometry problems I posted above, you'll see. A sphere has the lowest surface area per volume; any other shape will have more surface area for the same volume.

    Here's a simplified approach to understanding how a single hull compares to two; the math is really simple, so you can do this yourself and see. I'm going to reduce those nice pointy pontoons to plain cylinders. We won't even need a spreadsheet.

    First, you specify some displacement. Let's stick with your 7 cu. ft. You can't go less than that or the boat will sink. Also, we'll fix the length of both kinds of boat the same, say 20 ft.

    Now let's make a cylinder that will displace two times that for our monohull. Two times, because the top half of the cylinder would be above the water and won't count; we throw it away. When we're done calculating this cylinder, we will use half of its surface area as wetted.

    Volume (V) = 14 cu ft
    Length (L) = 20 ft
    Area (a) area of the end of the cylinder = pi*r^2
    Area (A) of the whole cylinder = 2 pi*r*L+2 pi*r^2
    V=L*a
    solving for a,
    14 = 20 a
    a=14/20 = 0.7 sq ft

    now, find radius

    a = pi r^2
    .7 = 3.1416 r^2
    r^2 = 0.223
    r = .472 ft

    Next time you drive past a phone pole, you can say, "Hey, I'll bet that thing has a volume of about seven cubic feet!"

    now, we find A -- well, half of it

    A = 2 pi r L+2 pi r^2
    1/2 A = pi r L+pi r^2
    1/2A = 3.1416*.472*20 + 3.1416* .472^2
    1/2A = 29.66 + 0.699 = 30.36 sq ft.
    This is the wetted surface for the single cylindrical hull.

    Whew, I need a cigarette.
    Now we'll use the same 20 ft length and the same 7 cu ft volume. This time, though, we can calculate for one cylinder and use all of it, since we just split it lengthwise to make the two hulls.

    V=7 cu ft (notice that the first calculation used 14 cu ft, since we had to throw away half of it)

    V=L*a
    7=20a
    a=7/20 = 0.35 sq ft

    a = pi r^2
    0.35 = 3.1416 r^2
    r^2 = 0.35/3.1416 = 0.111
    r = 0.334 ft

    We use the area formula for the whole cylinder

    A = 2 pi r L + 2 pi r^2
    A = (2 pi * 0.334 * 20) + (2 pi * 0.334^2)
    A = 41.97 + 0.7 = 42.67 sq ft

    See that? With both boats having the same length and displacement, the monohull has 30.36 sq ft wetted surface, and the cat has 42.67 sq ft. The ratio of wetted areas for the cat vs the mono is 1.405, about 40% more, just as Rick said.

    BARTENDERRR!! QUICK!
     
  10. clmanges
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    clmanges Senior Member

    Tinhorn, that's some collection. Sprockets and levers and chains, oh my! Some of those things look like they'd sink any hull they were mounted on -- all that cast iron and oak planking!
    Some of those old patents are really hilarious. The best one was that thing that looked like a very modern kayak hull, but with that enormous whirlygig thing sticking up out of the deck. What were they thinking?
    Great fun, thanks for the laughs.

    Curtis
     
  11. Guest625101138

    Guest625101138 Previous Member

    If length is constrained there is a point where two hulls becomes more efficient than one. For a single person it is around 12 to 14 ft depending on design speed. For heavier displacement it is going to be longer.

    Generally you can take rowing shells as being close to the optimum length for very powerful people. My V11 hull is not a lot different to a women's rowing scull. It is designed for 90kg at 12kph. The rowing shells are lower displacement with higher design speed.

    If you have a length constraint then a cat can be the best selection. It also works well with a paddle wheel.

    To get an efficient paddle wheel you need large blades.

    If you send me your hull drawing in igs I can tell you what size wheel will give what performance.

    If you tell me the design total boat displacement and some indication of the engine fitness level and engine size I can give you the optimum hull/s for the application.

    Rick W.
     
  12. BG_Geno
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    BG_Geno Senior Member

    clmanges

    The real problem is that on the first day of my algebra 101 class many moons ago I saw my first well filled angora sweater. She was a vision I remember to this day. Sadly everything else going on at the time didn't register lol.

    You only took the first part of my admittedly simple statement:

    I understand the difference between volume and whetted surface area (at least I think I do). My point was to displace that much volume won't the whetted surface area be a constant? It's shape can obviously take on many different forms, but the area has a minimum achievable...compression for want of a better word.

    A minimum achievable compression. There is some point at which you have reached the smallest possible whetted surface for the minimum displacement you need right? Obviously that might not be the best hydrodynamic form, your sphere for example.

    I know you guys are right as the math never lies, even if some of us don't understand it. Just a bit depressing to know the cat design I was in love with is inefficient. I just found out the girl in the Angora sweater is a commie =)
     
  13. BG_Geno
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    BG_Geno Senior Member

    Village'

    The sprockets digging into the bottom is certainly thinking outside the box lol. I would feel guilty for tearing up the bottom everywhere I went though. I already see these big sweeping arcs all over the bay where guys in fishing boats have had to punch the throttle and sweep and arc to get the tunnels on their flat bottom boats filled up. Takes months for them to go away and I doubt they are good for the local ecology.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Curtis
    You get the right answer but you have glossed over a couple of points. You have assumed the same length for both boats. This is not the case for the optimum hulls.

    If you use hulls of identical form but scaled for half volume then the area difference is only 26%. But drag has viscous and wave components. Once you optimise for the minimum combined drag the catamaran might be 20ft long while the monohull will be 24ft long.

    Point is, arriving at an optimum for both cases is a complex task and it just happens that the power difference is around 40% for the same speed.

    Your analysis gives a first approximation and as it happens it is very close to the final result but the hulls of the two optimum boats will have quite different form.

    I wanted to make it clear that I did not arrive at my answer using simplifying assumptions that could be easily challenged. It is arrived at using Godzilla with in excess of 30,000 iterations per boat.

    Rick W.
     

  15. BG_Geno
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    BG_Geno Senior Member

    Rick-

    Going to try to attach the file here.

    The engines total about 325 lbs and I hope to have the boat weigh in at about 125 pounds. As we would occasionally bring a dog and supplies for a day trip figure 525 pounds.

    As for the engines. Mid 40's who has spent too much time in this chair on the computer the last few years. Thus the decision to build the boat this winter. I would rather build it for engines in good shape then get said engines to that point then for weak engines.
     

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