Scow Efficiency At Displacement Speed

Discussion in 'Boat Design' started by tpelle, Aug 27, 2021.

  1. tpelle
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    tpelle Junior Member

    I am considering the construction of a simple houseboat scow for use on inland rivers in the U.S. Think of the Ohio River. No rough water beyond the wakes of the towboats and their barges (which can approach the footprint of, say, an Essex-class aircraft carrier. Much less draft, but you've gotta stay out of their way). I am thinking along the lines of a Phillip Thiel Escargot, except maybe a little longer and wider with a forward pilot house having standing head room. Oh, and I would reverse the accommodation plan back-to-front, so that berths were in the stern. The pilot house would be long enough that it would extend at least partially over the galley/head area for standing headroom there, but the aft sleeping cabin would be lower. Thinking along the lines of a single old feller as a live-aboard. I have in mind 21' LOD, 18' WL, and 7' Beam. The cross-section form would be a box. It really would be a dolled-up shanty boat.

    Aesthetically I am thinking in terms a river tugboat, or towboat as they call them here on the river. The pilot house is a necessity, as I am troubled with skin cancer, and would need to be mainly under roof, so the forward pilot house would fit that style. I also have this idea, borrowed from a web log of a guy building a camping trailer, who used salvaged windows from a school bus, of all things. Might be just the thing for the pilot house and cabin if one were building on a budget. The standing head room in the pilot house, with sitting head room aft, would mean that the view aft from the pilot house would be unobstructed.

    The philosophy of use for this boat would be that it be towable behind a V6-powered compact pickup or SUV (my Ranger is rated to pull 5000 pounds), and while on the trailer could double as a camper. Power would be an outboard in the 5 to 10 HP range, possibly in a well for security, protection, and aesthetics.

    I am thinking about a tiller-bar for steering, kind of like the big boys, and the tiller-bar would would naturally indicate the direction of thrust of the outboard. But this arrangement would mean that the throttle and gearshift controls would have to be pretty long, though, and I'm not sure what the max limit would be for this.

    I envision a flat bottom center section with an upswept bow and stern a la the Escargot. I realize that the Escargot was, at heart, a canal boat, but on the river one may have to contend with some wind. I was thinking about possibly an offset center board or lee board to prevent the boat handling like a hockey puck.

    Question 1: At displacement speeds of 5 to 6 knots max, how much speed/efficiency/fuel economy does one give up with the scow bow vs. what, back in the riverboat days, they termed a "model" bow?

    Question 2: What is entailed in rigging a long engine remote control of, maybe 15 feet or so?
     
    Last edited: Aug 27, 2021
  2. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    5 or 5.5 knots from a 9.9 I would think on a scow bow.
    No idea on the other.
     
  3. clmanges
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    clmanges Senior Member

    Look into Triloboats. They use modular size increments to maximize the use of plywood sheets, so you'd be looking at a 24x8.

    If you're not going to put a sail on it, I can't see a need for center- or leeboards. A few sturdy full-length rub strips on the bottom might be all you need to keep it tracking nicely. You'll probably want those anyway to help stiffen the flat bottom and maybe protect it.
     
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  4. philSweet
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    philSweet Senior Member

    Unfortunately, what you describe is just about the worst possible live aboard. The most important performance feature of any live aboard is its behavior at anchor. You want the best hull for anchoring in a river. It needs to lay to the river current and not sail around the hook in any conditions. You may need to leave the boat. Maybe for a year or three. The boat has to be able to take care of itself for years at a time at anchor, and that takes some thinking about. Traditional sailboat hulls are pretty good at this. Round bilge, long keel, shallow draft, rig forward designs work best. You can loose the rig, but you need the aero center of the hull aft of the hydro center, and that is why old sailboats had bowsprits to push the sail center far forward over the hull. The hydro center followed the sail center forward. But when the sails were down, the hull's aero center was aft of the hull's hydro center and it would hang at anchor like a duck.

    So unless you plan to stay at marinas 100% of time, your boat just won't work. Look at the small Fisher motor-sailors. They are very nice little boats for what you describe. Double ended (except for the 25'er), aft pilot house, round bilge, all as it should be.



    upload_2021-8-29_10-16-50.jpeg

    upload_2021-8-29_10-17-10.jpeg
     
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  5. sharpii2
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    sharpii2 Senior Member

    Typically, the mast on a sailboat produces more aeorodynamic drag than the pilot house, if it has one. This is why schooners were so great. They could make frequent anchoring stops without even lowering the largest sail.

    Having the tallest mast aft also had them behaving better at anchor when the sails were furled.

    Also, if the pilot house up front proves to be an issue, the boat can be anchored from the stern. It simply has to be designed for that. Being a vessel used in only protected waters, this shouldn't be a problem.
     
  6. philSweet
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    philSweet Senior Member

    I wouldn't exactly call the Ohio River "protected waters", though. She's a mean old thing. Retiring onto the Ohio is like retiring to Fairbanks or Leadville. It's not for the faint-hearted.
     
  7. sharpii2
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    sharpii2 Senior Member

    A couple of thoughts.

    1.) A scow diverts the passing water under the hull, instead of around it. So, the more gently this is done, the less drag created. This applies to the stern as well, because the passing water has to be allowed back up to the surface. Otherwise, itt will tend to be dragged behind the boat. Your hull will have greater whetted surface area than a more typical hull design, but such makes up a tiny portion of its drag.

    2.) It is probably better that your sides go straight down to your bottom. This is so they can grip the water like shallow keels, helping to keep you from being blown sideways. Yes, this adds more whetted area, but it's worth it.
     
  8. philSweet
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    philSweet Senior Member

    When anchored in a river, it's probably 90% of the drag, and this goes up if there is a wind because the wind turns the boat to a less favorable angle. This is all added wear on the ground tackle and it gets expensive real quick. When under way, skin friction is about 80 % of the total drag of a well designed and sensibly operated small craft. That would be for one like the Fisher I posted. If the skin drag percentage is less than this, you've done something wrong - the other drag factors are too high.

    For a river boat, you can trade displacement for a better hull form and come out ahead because the thing costs you money at anchor due to the wear on the ground tackle. The OP's scow could easily wear out a couple thousand dollars worth of gear in a year depending on where he anchored.
     

  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I am getting confused here, what is the concern with anchoring, the boat jinking around too much making it uncomfortable to live aboard ?
     
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