I need to add bottom rails... Help!!

Discussion in 'Boat Design' started by Zane, Feb 9, 2012.

  1. Zane
    Joined: Jun 2011
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    Zane Junior Member

    Hello Folks

    Im building this flats boat and need to add some sort of guide rails on the bottom of it to help keep it hold its line. The thing is i already painted and flipped the hull... was that a big mistake :s

    What should i do? Heres a pic of the bottom. She's a 20 footer

    Any and all help is greatly appreciated!!

    Zane
     

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  2. michael pierzga
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    michael pierzga Senior Member

    Personally I hate keels on small craft. They always get buggered up from grounding or trailering and are tedious to repair. . Spray chines forward are worthwhile, but many of the serious fish guys dont like the noise these spray chines make when wavelets slap on them as you fish.

    Id stay clear of anything on the bottom. If your boat is powerful...fast...you may end up having to add some directional stabuilty on the bottom.
     
  3. Zane
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    Zane Junior Member

    Ahh thats what i was referring to... maybe i had the wrong term!

    Basically I'm planning to throw on a 225 or 250HP on the back of this boat... so should be quite fast!!

    What should i add for the directional stability? How and what shape/ size/ length and position?

    Im also planning on putting some trim tabs and a hydraulic motor lift plate...

    Cheers!!
     
  4. michael pierzga
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    michael pierzga Senior Member

    Thats a fast boat...I dont know, Ive never been that fast...time will tell . You know what the keel is for...it up to you to decide.

    Keels as usually something like 25mm square. shaped , Epoxyed on, then biax tapped over. They end about 300mm from the transom and continue forward to the length of what you judge as immersed when at speed...perhaps two thirds of boat length ?

    Spray chines are also nice .

    Some of the speed demons on BD net might have a better formula.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't understand why you build a boat, then decide it won't perform properly, before it even hits the water ? Someone has got in your ear, it seems. Suck it and see first.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Add nothing to the bottom of the boat, you don't need it, except at displacement speeds, which this boat isn't likely to do much of.

    The lower leg of the outboard or outdrive will provide directional tracking. Lifting strakes on a hull shaped like that, are also a needless expense and effort too. The hull appears to be a very modest warped bottom, which will hop up on plane, just peeing off the stern, so don't bother. Be careful how much engine you toss at this puppy, as this type of hull will reach a point of instability, once over 60 MPH. The vertical flanks will also tend to trip her, with aggressive maneuvering at high speed, so slow wide turns if over 45 MPH. Trust me you don't want to trip this girl hard at 45 MPH either, but with luck she'll give you some warning before she does a barrel roll and you can ease the helm or back off the throttle.

    If interested in rub strips, say for protection from sand bars and the trailer, you don't want to bond them on like Michael suggests. These are typically sacrificial in nature and are just lightly screwed to the bottom, over a bedding compound, so they can be replaced as needed, when they get torn up. HDPE or UHMWPE are very good for this, as they don't rot, they're self lubricating, very easy to work with the usual suspects of hand tools, they don't swell or shrink and they don't need paint. If shaped with a crisp inboard edge and a rounded over outboard edge, they'll offer some tracking assistance. Place them parallel to the center line, say 2 on each side, with the outboard one a few inches inboard of the chine, the next, maybe a foot further inboard. Stop the inboard ones a few feet from the transom, but the outboard rails can go all the way to the transom. They only need to be in the after half of the boat, leaving a clean fore foot entry. If you must use wood, live oak is the best of the commonly available stock, white oak is a second choice and don't even think about red oak or softwoods.
     
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  7. Zane
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    Zane Junior Member

    Ahh great thanx much for the replies!!

    PAR what do you mean by trip her over? and a barrel roll sounds scary and deffo don't wanna be experiencing that!! :O
     
  8. PAR
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    PAR Yacht Designer/Builder

    Relatively flat bottom boats are very efficient at getting up on plane. If they have nearly vertical sides aft and you're going fast enough, through a tight, high speed turn, at first the boat skids sideways a bit, but then the chine digs in and acts like a brick wall, while the momentum of the boat continues to push the boat sideways. If you're really screaming into the unstable range (around 40 - 45 MPH) and this happens, she rolls over sideways, at speed. Having done this, trust me, you don't want it to happen. The usual result is a sunk, probably heavily damaged boat. At lower speeds you'll feel a vibration, hammering or skipping sensation, which is the boat's way of saying, "if you were going any faster, I'd try to toss you out". Race boats experience this all too often, with the net result typically being a boat, that is all bit little pieces floating down from the sky. Of course, these are also driven well into the unstable region and any little wave can upset them, not just turns. In short, there's a speed range for all hull shapes and venturing over them, thinking you'll go faster, is not only a lack of hydrodynamic understanding, but potentially quite dangerous. With the general dimensions and weight of your boat (full up) I can provide a general guide as to the max speed potential you might have with your boat.
     
  9. tom28571
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    tom28571 Senior Member

    Got to disagree about not needing something on the bottom for lateral resistance. PAR is right about lack of directional control at low speed, especially in cross wind but, and its a big but, this boat will slide all over the water in a high speed turn. There is nothing for the prop thrust to act against to make it turn. This almost flat bottom boat will turn sideways under steering but will just slide. That can bring on the chine tripping followed by a barrel roll. Guaranteed to ruin your day. Best thing is a skid fin about 30 to 40 % forward of the transom but for minimum draft a keel will work, if not as well.
     
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  10. tunnels

    tunnels Previous Member

    Bit late to add rails but rails would only be to sit on and nothing to do with the planing or performance two each side would help to protect the bottom a little !!:p;)
     
  11. SamSam
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    SamSam Senior Member

    Like Tom was saying it looks capable of sliding sideways, and then as Par was saying the sides dig in and flip the boat or damage the occupants. Or the possibility of the forefoot catching and the back breaking loose creating an instant 180 degree spin.

    Unless you have something to guide you, such as some plans or another very similar boat, at speeds like that you're into the "experimental" stage. The size, length and placement of them can have a big effect. They can not only help to keep going straight, in turns they can form a pivot point, and a pivot point in the wrong place at high speed can be too exciting.

    Some commercial fisher guys here had made a 26' flatbottom boat, like a Carolina Skiff or a jonboat with no guide rails. Trying it out, in a high speed turn it broke loose, the flare of the sides kept it from tripping but it did a few 360 spins.
     
  12. Zane
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    Zane Junior Member

    Ok all this talk of spinning and flipping is not sounding too nice :S

    But all this could happen ONLY if i take a fast turn correct?
    As long as I'm in a fairly straight line things should be fine... or am i wrong?

    Whats your take on running aluminum L channels from the transom forward on either side of the keel?
     
  13. PAR
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    PAR Yacht Designer/Builder

    At the speeds the 250 HP is going to drive this puppy, no skeg or runners are going to help really. Admittedly, if the hull was powered appropriately for it's shape and configuration, a skeg or runners would help directional tracking, but the lower leg will also help in this regard as well and again if powered appropriately and driven with common sense, I don't see the need for runners or a skeg (in a broad sense and permitted best possible reasonable speed ). I've found straight shaft boats with rudders, do skid dangerously without some form of skeg or runners, but outboard and outdrive powered craft, with vectored thrust do much better in turns, skidding some, but eventually biting and taking a "set" just before they plow off on the new course.

    Picture this hull at speed, say 50 MPH. There's not going to much hull in the water, just a triangular patch of water around the transom, maybe 4' long and an outboard leg. A skeg might be kissing the water occasionally. The slight V should let her "set" in a turn (flop over to the inside) at which point the leg (now canted and vectoring it's thrust) will try to make her skid on the basically flat aft section of the inside bottom panel. The wetted surface will pick up dramatically, speed will drop off and (hopefully) the outboard bottom panel's deadrise will be enough to keep her from tripping, as the skid slowly turns into a carved turn, once the bow starts to come up and the stern squats from the shaved off speed.

    Yep, I agree it'll be hairy at times, but I think this poster wants to go as fast as possible, but within a practical range. With a skeg he'll have better directional stability and turns will be safer at speed, but he'll also lose some top end. The boat will tell him when he's "pushing it" a little to far and if he's got any sense, he'll ease the helm and/or the throttle. My quick estimates show this hull could get over 60 MPH fairly easily, maybe 70 MPH. At this speed, he's in Holy Grail territory and maneuvering should be approached with caution. Most folks looking to blast along at these speed do so in a straight or nearly so, line, at which point he's not in much trouble, except of course hopping off every third wave (so it seems).

    Zane, what is the width of your transom, from rail to rail and how thick is it, including the skin? What is your hull weight and estimated full up weight with full tanks, engine, two well fed guys and a cooler full of beer?
     
  14. Zane
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    Zane Junior Member

    PAR appreciate the in-depth replys!! Are you a naval engineer or something of that sort?

    So the hull should weigh about 1000 lbs
    Width is almost 8'
    Transom thickness is about 2.5 inches
    The engine about 300 lbs
    Fuel tank takes about 55 gallons
    Small live well... not sure how much water but not much

    Wont the trim tabs help in stability??
     

  15. tom28571
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    tom28571 Senior Member

    The thrust vector of an outboard will tend to make this boat bank to the inside on turns. Without some lateral resistance in the hull, which has almost none now, she is going to skid wide on that flat inside bottom. This can be anything from disconcerting to disastrous and bound to be exciting. That kind of excitement, I can do without since I like my boats to be under my control and not the gods of chance. It was not always so and in my youth one of them did get its side smashed against a piling before I learned this truth of boat behavior. Its your boat but I'll bet there will be something to add directional stability under there before you are through, unless its totaled first time out..
     
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