My Powerboat plan (Take II)

Discussion in 'Boat Design' started by adrocha, Nov 4, 2014.

  1. adrocha
    Joined: Oct 2014
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    adrocha Junior Member

    Here we go again.

    SOR

    Type of boat:
    Motor boat powered by an outboard engine.
    Range 200 miles.

    Intended use:
    Fishing and cruise.
    Operate on open waters (10miles from the coast)

    Accommodations:
    Accommodations for 2 for overnight.
    Small galley.
    Small head.

    Main dimensions:
    LOA: 8m
    LWL: 7,5m
    Beam max: 2.7m
    Max draft: 0,5m

    Target cruising speed:
    12kn (6,17m/s)

    Freeboard:
    Bow: 1,1m
    Stern: 0,81m
    Midship: 0,78m
     
  2. adrocha
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    adrocha Junior Member

    Statistics:
    LOA/Beam: Reference: 3 - this boat: 2.96. This boat is a little longer that the average (for this beam)
    LWL/Displacement: Reference: 3. This value leads to a displacement of 2,5 m3. I will check later with the values of this design.
     
  3. adrocha
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    adrocha Junior Member

    Main coefficients:
    Froud number (12kn): 0,72

    Prismatic Coefficient (Fn 0,72): 0,65

    I couldn’t find any table with the appropriate Cp for Fn 0,72. I found that for Fn (0,45) -> Cp = 0,63 and for Fn(1) -> Cp = 0,7. I choose somewhere in the middle (0,65).
     
  4. adrocha
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    adrocha Junior Member

    Main sections:

    To draw these sections some decisions had to be made.

    This boat has an S/L ratio of 2,4. With this value it is classified as a medium speed boat.

    Being a medium speed boat, I will try to create a semi-displacement hull. With this hull I will have the required speed and it will also be confortable and economic at low speeds.

    It will be used as a fishing boat, so it will need a good stability. To accomplish this requirement, I will make the chine as wide as possible and it will be placed at the DWL from transom to midship.

    Being a semi-displacement hull, the bottom on the aft sections should be flat or close to flat. This boat will be used on open waters. I will draw a transom with a small deadrise (10º).

    Midship section will have a deadrise of 18º (most common on this type of boats).
     

    Attached Files:

  5. adrocha
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    adrocha Junior Member

    First validation:

    With the target Cp and the information that we now have from Midship section we can check the displacement.

    With this information we have a displacement of 2,34 m3 that leads to a LWL/Displacement of 3.2.

    I have to mold the rest of the hull to obtain a displacement of 2,34.
     
  6. adrocha
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    adrocha Junior Member

    Next Steps:

    Now I will draw the first approach for the lines.
     
  7. bpw
    Joined: May 2012
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    bpw Senior Member

    First question to ask your self is do you want a boat or a project? Plenty of boats available already that will fit your needs. Looking for a project is fine, just make sure that is what you want.
     
  8. rwatson
    Joined: Aug 2007
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    rwatson Senior Member


    Is this for the electric shock treatments :)

    Unless Froude is involved ...

    As BPW has inferred - where is the list of similar designs that you have rejected, with the reasons for unsuitability ?

    You are embarking on a $40,000 design project.
     
  9. adrocha
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    adrocha Junior Member

    I understand your comments. It doesn´t make any sense to build what is already built.

    I agree with you. This isn’t the best way to get a boat.

    If someone came to me saying that he wants to build a boat and he will begin creating the design, I would advise him to buy a stock design. I’m sure that there is a design that suits any specific needs.

    If you are reading this topic, thinking to do the same as I am doing, please reconsider. It doesn’t make any sense. You will get a better boat from a stock plan.

    I believe that if I study enough I will be able to create this design. I will understand all the concepts but at the end I will miss the experience.

    I am only beginning and I am already making some assumptions. When I define the deadrise for the transom and midship section and the offsets for the chine I don’t have any information to assure that those are the correct values.

    With those values I will create a boat with a specific behavior that will be more or less adapted for the conditions where he will operate.

    After sea tests I will be able to evaluate my decisions and define improvements on the design. But now I don’t have any concrete information about the impacts of my decisions.

    If I had previous experience maybe I could predict more accurately the behavior of this boat.

    If I modify this design with the result of the first tests, the next boat will hopefully be better that the first. The cycle continues.

    When someone buys a stock design he is buying all this knowledge and experience. Probably that stock design was build many times and the designer received many inputs to improve it.

    Not even financially the decision of create the design makes any sense. At this moment I know where to buy a design similar to this one for about $300 and other for about $150.

    If I am building a boat of $40,000 I don’t think that paying $300 for a proven set of plans is too much. We are talking about less that 1% of the total price and we have support from the designer and a guarantee that, if build correctly, the final product will work as desired.

    For an amateur designer, the risk to create a boat that doesn’t fit the needs is very high.

    But …

    We don’t always choose the most rational option.

    Sometimes we like to assume risks. Sometimes we like the challenge.

    I will continue and probably I will build this design. It’s my choice.

    I am not saying that it is easy to design a boat. I am saying exactly the contrary.

    Put something on water that float is easy. Put a boat on water that behave as we want is not easy.
     
  10. adrocha
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    adrocha Junior Member

    Thanks, original post corrected :)
     
  11. rwatson
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    rwatson Senior Member

    I probably didn't make myself clear - the $40,000 is for the DESIGN work ONLY ! What an NA would charge for a one-off design for this size

    To design a boat that you can insure and be confident in sailing offshore, you need to be a qualified engineer, naval draughtsman, competent boatbuilder etc.

    This is three careers to create something that would cost you about $2000 to buy a plan for of an existing similar size boat, so you only have to become a competent boat builder.

    Sure, getting experience is a great goal - buts a mans gotta know his limitations.
     
  12. kerosene
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    kerosene Senior Member

    8 meter boat is a big one to practice design on. Its a several year project to build it. I know I am beating a dead horse but gradual progression in learning makes far more sense. ie. make a dinghy first.
    It might feel like a dinghy is pointless when you want an overnight powerboat - it isn't pointless. You will likely save more than the price of materials when doing your next boat - likewise you will probably save more in time in the big project than you ever wasted on the dinghy. All this because experience and learning have value.
     
  13. PAR
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    PAR Yacht Designer/Builder

    An NA would likely charge considerably less than that Ray, more like 20K for a really nice one.

    I see conflicts in the proposed figures. Why are you trying to design an under powered full plane craft? What level of efficiency are you looking for? Simply put, why the 2.4 S/L target, as you're not in an efficient range, for this length/beam. You could just jump to S/L 3.0 and get about 15 knots and the boat will be level, likely with more speed potential. The beam seems excessive if you're looking to target this lower S/L and/or have some level of efficiency. In a nut shell, your basic dimensions seem better suited for a fat, full plane mode craft, not semi plane and if efficiency is desired, you'll want to use beam to advantage, to decrease the amount of power required to reach these modest speeds.
     
  14. adrocha
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    adrocha Junior Member

    I don’t have any idea of the market price for this type of projects, but I think that a homebuilder won’t buy a one-off design.

    My reference is the price for a stock plan.
     

  15. adrocha
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    adrocha Junior Member

    I agree with you. It’s wiser and safer to go one step at a time. Start small and when we get experience enough go to a bigger project.

    I thought about that but I already have a small sailboat (4m). If I build a new small boat it wouldn’t be very useful. So, I thought in something bigger like a motorboat with 6m. But to be useful this boat should have some commodities. Find space for a galley and a head on a boat of this size is difficult.

    Then I came to a boat with 8m. I think that’s the minimum size to fit my basic requirements to my next boat.
    I want to build something that will be useful even if it takes more risks.
     
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