I need some help on this old woody design.

Discussion in 'Hydrodynamics and Aerodynamics' started by cyclops2, Oct 12, 2011.

  1. cyclops2
    Joined: Sep 2010
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    Location: New Jersy

    cyclops2 Senior Member

    I am SEVERELY ticked off about the prop walk on my 2002 Chaparrel 19' boat. It has a overpowered 5.0 L Mercruiser in it.

    All our docks only allow about 12" between fendered boats. In a breeze, I MUST have another person to secure the bow line.

    So I decided a change to hull shape characteristics. Can I get MORE accurate docking control & less prop walk / wandering, at slow speeds by making the bottom of the hull a CONSTANT DEADRISE for 66 % to about 75% of the length?

    Will that form a definate keel effect to slow down MASSIVELY, the prop walk of modern overpowered boat hulls?

    Make the boat require more rudder / Prop blast ?
    The plan boat will be 21' long X 4' wide X 4' deep. With the vertical bow.

    The engine & installed equipment will be temporarly installed dockside for correct waterline adjustment. The boat will only seat 2 people in the rear seat.

    I have cut all upper frames & stringers & keel. Am holding off till I know how much deadrise angle the bottom triangular frames will be.
     

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  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Prop walk is a function of the single prop, not hull shape. A monohedron hull form with lots of deadrise will help slightly, but not as much as I imagine you expect. Prop walk can be exaggerated to a degree with short, fat hulls, but it's still a function of the single spinning prop.

    The Mower runabout your are posting is a warped bottom boat and making it a monohedron will require more then just some guess work at deadrise. The chine needs to be redrawn and more importantly appropriately for the hull form and preformance envelop you envision. In short, this isn't an easy thing to do, especially by a novice designer.

    A shrouded prop can help some with walking, as can a substantial skeg, but again the twisting blast of water coming off the prop, is going to shove the boat sideways a bit, even with all the bells and whistles installed.

    A bow thruster is the usual answer, though typically used on larger craft. This could be as simple as a remotely controlled trolling motor on a bow mount or even a ventilation plate mount.

    Rudders and skegs juts don't work well at low speeds, unless they are large. High speed craft can't afford the drag of large appendages, so the compromise is a wimpish rudder, for low speed handling. It's the nature of the beast. These types of rudders and skegs need some flow over them to be effective, so at slow speeds they just don't do much.

    If you want a Mower runabout, there are several designs that incorporate the looks of these 20's era speedsters, yet incorporate modern under bellies. Making a modern hull form, look like a Mower is easy and the logical way to go, rather then attempt to redesign a monohedron from one of the old, usually free nearly century old set of plans.

    We've learned a lot about powerboats since then and you be best advised to avail yourself of the hull form you want, rather then alter an old design into a new one. As an example, that hull you've selected has concave forward sections, which at the time was the "thinking" on the subject. We now know these hull shapes truly suck and can be dangerous. Lastly, that boat weighs twice as much as a modern one, built with modern techniques, with the same looks. Weight directly relates to build cost and effort. You have to purchase the extra weight, cut, glue it and install it too.

    The 2002 Chaparral 19' is designed to do only one thing fairly well and slow speed handling isn't one of them. It's like marrying a runway model and expecting her to be a nurturing, concerned and fastidious life long companion. Put a swim suit on her and send her down the runway. That's what she does. Asking her to cook and darn your socks, is just not going to get very far.
     
  3. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    I am VERY thankfull for you stating that propwalk is a very difficult thing on any single prop hull.

    SOOO

    I install 2 contra rotating props & call you in the morning.

    I have been surfing about constant deadrise boats. 24 degrees ? OK ?
    They have most of my most desireable features. Direction stability.
    The Chaparral is a fantastic boat in 5' waves with 6 adults balanced out.
    But Mower above the water line is my love.

    What set of plans is a good constant deadrise hull from the water line down that I can drop the Mower look onto ? I NEED the vertical bow.

    Many thanks.
    Rich
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    24 degrees of deadrise is ridiculous for most uses, except motoring fast. What do you think the deadrise is providing?

    A vertical bow isn't a wise choice on a fast hull form. It would be helpful for you to establish a comprehensive SOR, so you can find and fit a hull form best suited to your needs. What I think is good may not fit your SOR very well.
     
  5. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    Am I reading you correctly.

    Use the Chaparrel.

    Build the Mower for shows & 20 mph boat club trips.

    Rich
     
  6. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The Mower can do better then 20 MPH, but it's hull shapes do have limits and with modern power and build methods these can be easily exceeded. This is why you find a hull form you like, one that fits your SOR and alter the topsides to mimic the 20's era runabout.

    I have a 16' runabout that does precisely this. It's a low deadrise monohedron, but the stern is tumbled home and a split, flat plate windscreen on brass holders sits on the deck. It has all the styling clues from the era I was after, yet it hops up on plane quickly and has no era specific handling vises. The best of both worlds.
     

  7. cyclops2
    Joined: Sep 2010
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    Location: New Jersy

    cyclops2 Senior Member

    Par what does SOR stand for ?

    Stop using a longer stick to hold my Carrot. :)

    With all the good precautionary driving advice I am taking to heart.

    I am doing the constant deadrise of 18 degrees below the water line. & Mower above the water line.

    A idiot & his Pacemaker are soon seperated if they over do it.

    Many thanks for the advice.

    Rich
     
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