Jet Boat design

Discussion in 'Boat Design' started by BazzaOz, May 4, 2020.

  1. BazzaOz
    Joined: May 2020
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    Location: Australia

    BazzaOz Junior Member

    Very helpful Ad Hoc thank you. Is it possible to give me some logic behind the dimensions and angles? For instance the height from the bottom of the transom to the top of the transom is 667.5, why? The little flat section at the bottom of the hull is 362.1 why? and the little kick down angle at the outer edges(chine?) of the hull is what measurement and what is the effect? The radius at the front of the boat curves in quite sharply is there a standard radius for that curve etc etc?
     
  2. BazzaOz
    Joined: May 2020
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    Location: Australia

    BazzaOz Junior Member


    Hi Gonzo, good that's a good starting point. Some of those things include as you say jet drive, shallow water capability speed but also include size of motor number of people to carry etc. I would imagine the size and structural requirements of the jet drive will have a major effect. We are going for shallow water so planing hull but there are different shapes of planing hull especially upward angle of the hull. Are there standard accepted measurements for some of the angles and dimensions?
     
  3. BazzaOz
    Joined: May 2020
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    Location: Australia

    BazzaOz Junior Member


    Hi Ondarvr, We are looking at a friends boat tomorrow night and that will give us guidelines but it doesn't tell me why those measurements were used. And sure I can copy them but then if I want to improve them I wont know which ones to adjust.
     
  4. BazzaOz
    Joined: May 2020
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    Location: Australia

    BazzaOz Junior Member


    Thanks bajansailor. You are right and we will probably start by copying or using very similar dimensions, angles, etc. This enquiry was my first attempt in trying to understand the logic behind the design and I have gathered some good ideas thanks to some of the forums and others replies. Although I am still gathering more.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Baz, everything in boat design is a compromise - even for a little jet boat for thin rivers there is no 'perfect' design, you just have to try and balance out the (often conflicting) requirements in the SOR.
    For instance, you might want to have the smallest engine possible to save weight and cost. And the most basic, simple hull. Which would be a flat dish, with no deadrise - it would give you the most speed.. However it would pound terribly, so you then want to have some deadrise on the hull bottom, to make it more comfortable. But not too much, because you are looking to keep the draft as small as possible.... so you reach a compromise in between.

    Re your question to Ad Hoc about dimensions - if that plan had been drawn by hand the dimensions probably wouldn't have been so precise - eg the transom height might have been 670 mm rather than 667.5. It is in this ball park because the designer probably thought this was the minimum requirement for freeboard at the stern to avoid swamping.
    If the boat had an outboard engine instead, then the transom height would be dictated by the leg length of the motor chosen.
    Re the flat section on the bottom of the hull, this is needed for mounting the intake of the water jet -if you have twin engines and jets then you can mount the jets on the hull bottom even if you have deadrise. But with a single jet and deadrise you need a 'flat' on the centreline. The size is probably the minimum needed for the jet intake. It is very important to ensure that you have good water flow going in to the jet.
     
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  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If there was a "right" way to design a particular kind of boat, they would all look the same, but of course there is a great deal of variation, and when you factor in the reasons for the differences, which are not all technical, for example, styling considerations, the mix of practical and aesthetic considerations is impossible to disentangle. You're best advised to pick one you find pleasing to the eye, that is a proven performer, because the chances of your being able to improve on it by basically random "mutations", is minimal.
     
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  7. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Even if you understood every aspect of hull design for a small jet boat it wouldn't help much. The design can be perfect on paper, but as soon as it hits the water and used in the way you want to use it, it needs to be modified.

    On a small craft like this, where you put the cooler full of drinks and snacks will change the balance and handling.

    If you like how the friends boat works, copy it, if not look at one you like better and use some features of that design.
     

  8. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

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