About Flat Bottom Boats.....?

Discussion in 'Powerboats' started by jfnewell1965, Aug 12, 2005.

  1. jfnewell1965
    Joined: Jun 2005
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    jfnewell1965 New Member

    I hear that flat bottom boats pound in chop.......could someone define "chop"?

    Or....could someone tell me what to expect from a flat bottom boat?

    I would plan to run the boat on lakes and rivers, but there are always waves, wakes, etc.

    I get the impression that I will be beat to death in a flat bottom boat. Is this more or less true?

  2. KCook
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    KCook Senior Member

    True for a light flat bottom with wide beam. Heavy designs with long length to beam ratios ride somewhat better. Good examples of advanced flat bottom hull designs can be seen in some tournament ski boats. They do what they do very well. But they also demand pretty mild water.

    Kelly Cook
  3. jfnewell1965
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    jfnewell1965 New Member

    This boat would be 23 to 28 feet long, and 8 feet wide.

    I have not settled on a design yet. I wanted to learn what to expect, before I made a final choice.
    Last edited: Aug 12, 2005
  4. PAR
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    PAR Yacht Designer/Builder

    All boats will pound to some degree if over driven for the conditions, regardless of the bottom shape.

    Small craft suffer from this because they are generally light and have a high beam to length ratio to provide accommodations. This creates a fat boat that will have bad manners sometimes.

    Flat bottom craft can be most efficient, but need to be designed and operated so.

    A lot depends on your requirements in the vessel. Fuel economy, cruising speed expected, top speed desired, accommodation wishes, general function of the boat (fishing, partying, sight seeing, etc.) conditions of the locations you'll be operating the vessel and a number of other issues will demand difficult decisions made in regard to the design. Some of these will be in direct conflict with others on your "wish list" so a balance will need be struck.

    The design selection process is a painful one, because of the compromises we must make in the boat to get all the things we want. Don't pick a design without very carefully going over your needs. You can build a really bad (for your conditions or requirements) boat as easily as a well suited one, so the selection process is the most important task of the build. Once you've got a design, build her to the plans, so you end up with what the boat is intended to be, not a basterd incapable of doing what you ask, because of changes to the engineering, built into the design.
  5. wdnboatbuilder
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    wdnboatbuilder Senior Member

    How shallow do yo need to go. if you are looking at 2" draft the why go flat bottom, personally I like a warped bottom you get the benifit of a good entry to break the waves yet you can keep in shallower water ( in most cases) and gives a flatter running surface. Thats just whats been beat in my head for a long time, leave the water as you find it.
  6. cyclops
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    cyclops Senior Member

    I can give you some actual flat bottom boat ride conditions. Saltwater bay that runs the southern 1/2 of New Jersey. Channel is 4' to 8' deep. Flats are full of drop offs and are from 20' to 1/4 mile wide. boats are 25' flat bottom and flat curve up of the bow. Flat as possible. 4 or 6 cyl. engine, trans., shaft, prop and a rudder. Fantastic group boat. 6 people on one side all catching fish or crabs, diving or swiming. Food and drink for a weekend. waves less than 1' ok. -----1' to 2', should we go in?----2' to 3', run it around the down wind side of a island before we sink. At + 2' waves the boat has little foward speed and each wave slams the hull hard enough to knock down most non-boaters. -------1 hand on the boat and knees bent slightly is the position home. Some times it was 1 1/2 hours home.------ Get a V hull and you only get wet going home sitting down all the way. ---------Oh, more than 1 we almost ran out of gas in a storm because we made such slow foward speed. Plan ALL trips in GOOD 2 days of weather, if flat bottomed.
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Chop is the condition of a lake, river or bay with a little breeze blowing and a bunch of boat wakes, currents, jutting out land masses, etc. disturbing the water. Small, confused waves, traveling in every direction is chop. At sun up, these same waters will be much smoother, the waves will be smaller and traveling in the same general direction if there is (or was) any wind blowing. If no wind is blowing, these waters will appear quite smooth, even glass like sometimes.

    Flat bottom boats get a bad rep, because there are a lot of over powered or over driven stories told and the blame is placed on the boat, rather then the driver, who could have avoided the bad ride. All small boats will pound, unless it's exceptionally heavy or driven at displacement speeds. This is, as I said earlier, a result of light weight craft being driven too hard or poorly for the conditions. A 25' center console doing 15 knots in a 2' chop is much less likely to spill your beer (regardless of bottom shape) then if it is dancing on the surface at 40 knots. If your boat starts to fly off waves, you're going to fast for the conditions. Some people think this is cool (it can be fun for short periods) but it is beating the hell out of the boat.

    It's true that some shapes are better in chop then others, but it has more to do with mass and entry exposure then the generic, cover all the bases, the flat bottom term implies. A narrow entry angle and carefully shaped forefoot sections will do much more for the ride in a chop then the amount of dead rise ("V") a hull has. Now all the radical "V" hull form folks will roar in. The fact is, the flat bottom craft is the most efficient planning form going. Planning, by it's nature in powerboats, isn't a gentle thing. The boat is literally bashing and bouncing its way across the water, subject to all the surface imperfections (chop) it may have, which doesn't help the ride much.

    So here we are again, what kind of boat are you interested in? Size, speed, accommodations, duties, local conditions, etc.? All boats, every single one (canoe to super tanker), is a difficult set of conclusions, reached by tough decisions and compromise. Ever watch off shore powerboat racing? How much design effort went into a hull shape concerning the ride quality? Not much, they wanted all out speed while trying not to have her go airborne. Your boat will be different, ride will be higher up the importance list. It's difficult to make a recommendation for a boat without the basics questioned above. Personally, I have a 27' lapstrake utility that has an uncommonly soft ride in a chop. It's a round bilge boat (the softest kind) that has 185 HP which gets me to 37 knots. The shape of the boat pretty much prevents me from going much faster. 400 HP might get me to 45, but why bother, more then doubling the HP for 8 extra knots (another one of those compromises I mentioned)

    For protected waters, which is what you seem to be interested in, the flat bottom boat will serve you well and be a bit easier and less costly to build too. If you expect to be in rougher conditions most of the time, then a "V" bottom or round bilge boat may be more to your needs. Both of these types have good points and bad as well, just like most every other element of yacht design.
  8. glassman
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    glassman Junior Member


    if the water has ripples your going to pound, flats key word . have a 18 flat bottom boat great to get around the back bays, for me just i use the wind ,if i go s. winds out of the w. run the w. shore,
  9. Tim B
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    Tim B Senior Member

    You could use a flat bottom aft, but I'd strongly suggest at least a little dead-rise forwards, otherwise you'll have a very raw arse in more than a 6" high wave.

    Also boats with flat bottoms tend to slide around corners, you'll either need a skeg to counter that, or 5 to 10 degrees deadrise aft, increasing forward for ride quality. If you're running the boat in rivers then handling and longitudinal stability (course holding) are a major consideration.

    Tim B.

  10. Deering
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    Deering Senior Member

    Look at the classic dorie design. Flat bottom with a curved-up bow. Shallow draft, easy on plane, and excellent stability. For the length you're talking about you should be able to handle most of the chop encountered in a smaller lake or river.

    Here's an example design: http://www.boatplans-online.com/studyplans/OD18_study.htm
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