Requesting Small Sternwheeler hull design and analysis help

Discussion in 'Boat Design' started by phrogjlf@yahoo, Nov 18, 2014.

  1. PAR
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    PAR Yacht Designer/Builder

    75 pounds of thrust per HP is the usual estimate (745.6 watts). It's an estimate because the two (thrust verses HP) don't directly compare. Thrust is subtracted power (absorbed) from a power source, while HP is force-over-distance over time.

    Watts are a different story and it's important to know the distinctions:

    P(hp) = P(W) / 745.6

    It's not opinion about 4 strokes being less noisy than 2 strokes. The EPA and manufactures have conducted extensive testing in this regard. Further things can be done to silence them more, though the result is typically power lose.

    The Club Cadet axle is a Peerless unit and you should have good look at this puppy, because it's not very stout, not tolerant of moisture vapor (of which they're be lots) and "continuous duty" cycles, typical in a boat. With the exception of the pinion gear, everything else in this assembly will rust quickly and the case will get eaten up, because it's the wrong alloy for marine use.
     
  2. phrogjlf@yahoo
    Joined: May 2006
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    phrogjlf@yahoo JL Frusha

  3. phrogjlf@yahoo
    Joined: May 2006
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    phrogjlf@yahoo JL Frusha

    Thank you. Still haven't heard back from Cub Cadet. Not sure that we're talking about the same unit, though. The electric drop-axle isn't the hydrostatic drive from the gasoline units. I mentioned the gasoline one for comparison, since it's the drive-line that is the main difference, between the two.

    I understand the conversion between Watts and HP, but the problem is actually the lack of thrust ratings on gasoline outboards.

    The Elco electric outboards are the only ones, so far, with a comparison and then a thrust rating, such as the EP 9.9 being comparable to an 11-13 HP outboard, and having 240 lbs. Static Thrust.
    http://www.elcomotoryachts.com/ep-99.shtml
    Their EP 5.5 they compare to a 6-7 HP outboard, @130 lbs. static Thrust.

    Since I'm looking for a silent, low power option, by comparing thrust to some of the electrics, there are several of the better electric trolling motors that approach 100 lbs of thrust, while the Minnkota double motor unit, that mounts to an outboard, could be a fixed unit, with the Riptide 160 being roughly equivalent to an 8HP outboard, and significantly cheaper (comparing new to new).
     
  4. PAR
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    PAR Yacht Designer/Builder

    The math is simple, so what you're seeing on sites is clearly advertising "fluff", as there's no way a 240 pounds of thrust trolling motor compares to a 12 HP outboard. Maybe they've come up with some unique way of measuring thrust and are cleverly calling it something other than what reality suggests would be an accurate description, likely in favor of a higher comparative figure.

    Gas outboards don't need to count thrust, particularly with HP being so well accepted and understood. Again, trying to make this comparison is simply not reasonable, given what the to two things are. Small engine manufactures in recent years have started doing similar, by promoting torque, instead of HP figures for their products. The idea is to place a bigger number on the unit, so you think you're getting something more powerful that you actually are. Electric trolling motors could also publish HP figures, but no one is going to buy a 1/3 HP trolling motor, but hey, a 25 pounds of thrust motor might get them to bite. This is an old marketing scheme and it works, though often is less than accurate or truthful.

    Simply put, if you compare apples to apples and bolt an electric to a load sensor, mounted to a pole (prop appropriately immersed) and compare this to a 5 HP Mercury outboard, there's going to be a huge difference between the two. Lastly, look at the cost and weights (not including batteries) $3,000 for a EP-5.5, really? That's a brand new 25 HP outboard and including the batteries for the electric and fuel for the gas outboard, the outboard is considerably lighter. Some (few) companies are trying, like Elco and Teredo, but for the most part, it's just wishful thinking, or in this case marketing team dynamics.
     
  5. P Flados
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    P Flados Senior Member

    Thrust is a function of water volume moved and speed of the water. A bigger diameter prop with a lower operating speed and a lower pitch can give you more thrust per horsepower, however, this limits your max speed available.

    It works like the transmission on your vehicle. In first gear you can pull tons, but not go very fast.

    Most outboards are not set up for high thrust, they are set up for more mid range performance.

    There are high thrust models. They are good for pushing big fishing boats that want to slow troll and they are good as aux power on sailboats for getting in and out of harbors.

    A quick search for "high thrust outboard" found

    http://yamahaoutboards.com/outboards/High-Thrust/overview

    http://www.suzukimarine.com/Product%20Lines/Outboard%20Motors/Products/DF9_9PT/2012/DF9_9TH.aspx

    I also saw several on Ebay for about $3,000 new for a 9.9 hp and a lot less for used models.

    A single gas 25 hp will cost a whole lot less than 2 of the 9.9s.
     
  6. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

    That's just it, though. I can get something like 2 of the Minnkota Riptide doubles, with 160 lbs thrust/set, cheaper than that one single 10 HP (comparing new-to-new). I'm not trying to put this on plane. Not even sure it WILL plane. but not even trying to. Hull speed SHOULD be around 15 knots (guessing at 14-16 ft waterline), but to plane, I would need even more power. She's a piddle-fart-around and have some personal space/just enjoy the attention when folks are around, kind of boat, not some haul-@$$ across the lake, or bay, just because I can... Absolutely NO intent on speed. I DO need power to battle current and wind, as necessary.

    I can move 2 pair of batteries around, for balancing, once she's built, use solar on the roof as a trickle-charger/maintainer. No gas, no noise, just add rudders, which I intend to do. She's for puttering, not ANY sort of competition.
     
  7. gonzo
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    gonzo Senior Member

    You should compare the 2 Minnkotas to a 2HP outboard, maybe a 1.5HP for apples to apples. Otherwise, it you want to compare a 10HP outboard, count 10 to 12 Minnkotas.
     
  8. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

    Thing is, if you build anything that is dependent upon a propeller, you have to design for thrust. Horsepower is a rating to do work, but thrust is actually doing work. That outboard has to push the boat, but it doesn't do it with HP, it uses thrust.

    See the frustration? VERY few small outboards seem to have both ratings, and then everyone seems to disagree on the results...
     
  9. PAR
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    PAR Yacht Designer/Builder

    Jeff, we don't have a problem with the figures generated by the manufactures. Again. the math is easy enough to do. The problem is you just don't have a good grasp on the physics and dynamics of it all yet.

    For example, assuming a LWL of 15', you'll be in plane mode at 11 MPH (S/L 2.5), which is on the light side and doesn't have any reserve, so figuring a S/L of 3.0, you'll be in full plane mode at 13 MPH. For some reason, you seem to think hull speed will be a few MPH above this, which is a clear indication that you're not doing the math. Propulsion isn't difficult to figure out, unless you go weird and then it's simply a set of engineering issues, before you can get back to the basic calculations again (power, weight, slip, loses, drag, etc.).

    Lots of industries have these conflicting output figures. Prop driven aircraft have engines rated in HP, but jet driven aircraft are pounds of thrust, from the engine. Again, not hard stuff to work out, but you do need a firm grip on the physics and dynamics.

    So, get a firm idea on the weight and drag of your proposed project, so you can scale the propulsion to suit, regardless of what that may be. It would be wise to shape the hull to maximize propulsive efficiency, so you can use smaller, cheaper to buy and operate propulsion. From the little I've gathered from you hull concepts, you're not that interested in efficiency, so expect to need more power (possibly a lot more) to get away from a dock.
     
  10. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

    Just trying to get a safe enough speed when moving. Believe me, I understand the aircraft thing, but they also recommend a specific prop/engine combo for a specific thrust. Last aircraft engine I worked on was a radial. Magneto wouldn't time out, couldn't just replace the points, but I finally figured out the arms of the points were bent. smgdh My next question was "Just HOW did you manage to bend a set of points?" Been more than a few moons ago.

    I was guessing, based on a supposedly standard formula of 1.37 X Lwl. I don't expect to get this on plane, and am not even trying to. May hit 70 mph on a trailer, but I don't ever expect it to be faster than 10-15, short of going downstream/with the current, and a tail-wind.

    Comparing outboards to outboards should BE 'apples-to-apples', at all sizes. Seems there ought to be a formula, chart, or calculator, like there is with aircraft.

    I REALLY hate beating myself over the power, but I REALLY want it to operate silently, rather than have a rattling outboard that I'm trying to muffle. That's also why I am hoping the electric drop-axle drive is a viable solution. It's just too close to what I think I need, to let it just go.

    I'd really rather go all-electric and rewire a 2kW portable genset, than fight with silencing an outboard. It's just THAT big of a nuisance. Trying to keep everything above about an 8"-12" water-level to hull bottom, is a PITA, but I need that kind of clearance, with this muck. Neighbors had a flat-bottom that stuck in it, didn't bother to work it loose and, last I saw, it was still submerged in the river, being slowly swallowed by this crud.
     
  11. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

    I doubt she'll cross 1600 lbs., not counting batteries and load. As for hull efficiency, it's a jon-boat... I have no way to do a CAD, nor any sort of hull analysis, which is why I asked for help, when I started this thread.

    The hull will be close to these, with sponsons/wheel arms trailing:

    (this one approximates the rise to allow clean flow to the wheels)
    [​IMG]

    [​IMG]


    This one shows a better transition, but lacks the shallow 'V'/low deadrise I need, and also lacks the wheel arms/sponsons
    http://www.atkinboatplans.com/Misc/LeveeBelle.html

    [​IMG]
     
  12. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

  13. PAR
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    PAR Yacht Designer/Builder

    Your hull speed formula is incorrect. Hull speed on a 15 LWL craft is 6 MPH. Yep, that's it. You only need a few HP to get to this speed if the boat is less than a ton. To go much more than this, you'll need huge increases in power and a boat shaped to do so.

    You are making a lot of unreasonable assumptions, such as flat bottom boats getting stuck in the mud. This is only possible if the skipper was clueless, as a 15' LWL boat will trim several inches in either direction if the skipper simply farts. Hell, with the draft of a 15' boat, you can just get out and push it off, for God's sake. If this was truly the case (your assumption about flat bottoms) there'd be sharpie's and jon boats littered all over the world, in every tidal flat. It's not that you can't get stuck, but that just walking to the stern (in a 15' boat) will usually free the grounded bow and you can back off. This is especially true of displacement speed craft.

    When we talk about the weight of the boat, it includes the engine, tanks (typically half full), full crew, stores, coolers full of beer, Fidel the wonder dog, the whole bit, not simply a dry hull with nothing in it. Calculations are based on what the craft has to propel, with a normal load. Most of us do calculations on a slightly heavy load (full tanks, etc.), if only to make our speed predictions look better on launch day.

    Your hull form choices are dramatically different, though from a novice's point of view, I guess they sort of look similar. Your assumption about "flow to the wheels" isn't sound with the dramatic rising stern quarter hull. The Atkins will do, though considering when it was done, there's much to improve this 80 year old design now.

    Outboards are compared to outboards and trolling motors to trolling motors. The two look similar, but they perform different roles. Even if you find a really hefty trolling motor setup you can live with, you'll do 4 MPH. This is what they're designed to push at. You can fool around with custom props, but if you think the motors are expensive, just try custom props.

    Your comment about "just a safe enough speed" seems like you have minimal experience in displacement craft. A safe speed is more of a skipper responsibility than the boat's top speed potential.

    You can go all electric if you want, but as has been repeated by several, the costs will be considerably higher and range will be a big issue. This is just the way it is currently. It's getting better, but storage capacity and range are going to continue to kick engineer's butts, for some time to come. A 4 stroke in a well insulated box isn't something you're going to hear. I've put a few dozen boxes over outboards, some (most) simply 3 sided affairs and the noise lowers to the point, that skippers try to start the already running engine. If this isn't soft spoken enough for you, then hook up a washing machine motor to a huge battery bank, for a hour and a half of cruising time.

    In a nut shell, you've kissed off an efficient hull, for some odd belief in stability, elbow room and water flow. You've decided to take the one of the least efficient propulsion routes and drive this, with the most costly setup, next to atomic power, that happens to offer the least amount of range. It seems you've convoluted your SOR a bit.
     
  14. phrogjlf@yahoo
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    phrogjlf@yahoo JL Frusha

    Damn, I'm tired... Saw this, remember looking at it and still drew a blank.

    Estimate 1 lb of thrust, for every 10 watts. 1 HP for every 750 Watts, or 1 hp for every 75 lbs of thrust...

    Now THAT, I can make sense of...!
     

  15. WindRaf
    Joined: Oct 2014
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    WindRaf Senior Member

    progolf @ yahoo.,

    the boat that you've drawn, has a hull like a speedboat, and is the opposite of the lines must have a boat designed for low power.
    The stern dipped, at the hull speed, sucks a lot of power with the vortices.
     
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