Decking below waterline?

Discussion in 'Boat Design' started by blueheaven, Mar 23, 2011.

  1. blueheaven
    Joined: Mar 2011
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    blueheaven Junior Member

    Just want to say I've been designing a boat for a few months and just came across boatdesign.net. What a great forum.

    My question: I'm designing a modified V scow intended for generally gentle surf in salt water. Screw and glue construction with a glass covered hull. I've attached a simple starboard sketch for reference. I'm emphasizing a low cg to maybe make better use of the sun roof in the future. Thus want to set my cabin decking as low as the frame tops. However this puts the decking about 4 inches below my calculated waterline (loaded). Is this a bad idea? I plan to have one each electrical and manual bilge pumps.

    I suppose I understand drainage will be an issue. However, cabin is sealed with water resistant doors, and as I said, she is not intended for rough water use. All hull surfaces inside and out liberally covered with epoxy.

    Any comments appreciated.
     
  2. blueheaven
    Joined: Mar 2011
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    blueheaven Junior Member

    sorry forgot attachment

    attachment included here
     

    Attached Files:

  3. Lister

    Lister Previous Member

    I will raise the whole structure to have the deck higher than the waterline. That way you don't care if its raining and the boat is unattended, the drainage will be through the scuppers. it worth it for practical purpose beside safety.
    Never ever rely on a electrical pump, it is one of the main reason a boat sink.
    But it is IMHO
    Lister
     
  4. blueheaven
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    blueheaven Junior Member

    I thought about that. But isn't a sailboat (inside) cabin deck essentially below water line? (I really want to minimize roll-over potential)
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    Cabin sole.. yes, go for it :)
     
  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    You show a boat with very large windage factor. Your drawing does not mention beam dimension. If the boat is to have a flat bottom with vee bow, the initial stability will be pleasing. Ultimate stability is another matter.

    You said mild surf? Please define mild salt water surf. We talkin' manua loa surf or just some little swells? This is not a boat that I'd want to use for playing in any sort of surf. Either way, Beware the hard chance!

    I presume that your drawings' weight of 1500 pounds is missing a zero. If your beam is............ .say nine feet, and the LWL is 30 feet, a draft of 12 inches will give you something on the order of 17000 pounds of displacement.

    A 75 HP engine is not likely to get this boat on a plane. If that be so, then give the boat some aft rocker so that you are not pulling half an ocean behind the boat. With suitable rocker the 75 will be happy at hull speed,
    (maybe 7.5 knots) but without some relief at the transom the engine will strain its' guts out and you'll use a lot of expensive fuel for little reward.

    This is a neat boat for inland lakes, rivers, and calm estuaries. Surf....no way.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I see a slightly smaller boat then Messabout, say about 24' LOD and assuming an 8' beam for trailering and approximating the weird chine profile shown, she'll be 5,500 to 6,000 pounds with 10" of draft.

    I'm not sure why you think a big, broad, shoe box shaped boat, would do well in a surf, but it's about the least obvious choice a designer would make. Given the clear other "issues" and your general understanding of hydrodynamics, you'd be strongly advised to buy a set of houseboat plans.

    My point is, you need to do a full stability work up, if you expect to put a dance floor up on the roof of that boat and you should consider your hull from options, to address intended use. I can only imagine her roll moment in a beam on surf.

    Start here, as this is about the size you want and interestingly enough the weight I said it would be to the estimated dimensions.

    [​IMG]
    https://www.boatdesigns.com/25-Delta-Q-trailerable-houseboat/products/167/
    She will supposedly do 20 MPH with an 85 HP outboard, but I have doubts. I still wouldn't take her into a surf, but at least you can dance on top and insure the CG, CB and all the other "centers" will be where they need to be without drowning anyone.
     
  8. blueheaven
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    blueheaven Junior Member

    Excellent feedback thanks. LOA is 25' 8". Beam is 7' 8" (as currently designed). I'm committed to a design I can call my own. I'd like to take her out on Texas and New Hampshire lakes. (long story) Maybe into Galveston Bay in 3 ft swells or less. I assure you I will not begin building until I am satisfied with safety. There will be no dancing on the upper deck. Maybe lounging. Max crew of four.

    My intent was to maximize cabin space in a trailerable boat. Speed is not a concern however planing would be nice for economy. Ok maybe I used "surf" in the wrong context. I felt a long narrow form would do fine in the seas described above.

    I've calculated ultimate stability before but only in the context of sailboats carrying ballast. Not quite sure how do do it for a powerboat.

    Also, I've carried the modified v all the way to the stern...bad idea?

    Neither of you commented on my original question regarding deck vs waterline. Irrelevant?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Many boats have their cabin soles below the LWL. If you have to ask if this question is a good or bad idea, you need a lot more study, before you can trust your designs skills with friends and families lives, farther from shore then they can swim back to.

    Your "modified V" isn't what most people would call a modified V. Without a sectional drawing it's difficult to see what you are doing with the hull form, but it appears to be a very modest V, both forward and aft. This makes it essentially a flat bottom boat, but with more build difficulty. To be frank, looking the lines you've shown, you shouldn't even be thinking about designing a full plane vessel.

    Stability calculations are essentially the same from powerboat to sailboat. I'm not convinced you have the necessary skills to design this boat. For example where is the CG for this boat as currently drawn? What is your target displacement? General hull specification and hydrostatics? Would you insist on designing your own airplane? Why not, both a boat and an airplane place peoples lives in a dramatically precarious situation.
     
  10. blueheaven
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    blueheaven Junior Member

    I have designed an ultralight aircraft and many other projects. None were the same as each other and all were different from boat design. Each required an enormous amount of learning before a single screw was turned. There was always (and I suppose always will be) someone who said "you can't do this".

    Forgive me PAR, I thought this Boat DESIGN Forum was a place was a place to learn about design. I appreciate your interest in safety, but maybe I'm in the wrong place if the main intent is to discourage kids and amateurs before they hurt themselves.

    But I'm not 20 anymore and now that obama has killed off the space program, this is what I want to do.
     
  11. Lister

    Lister Previous Member

    You really should listen to PAR very seriously. This forum has the chance to have people like him to help people like you.
    PAR give all around this forum the best advice possible.
    Good luck
    Lister
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Blueheaven, this is a place to learn about things, but not the best format to learn what you need to know. It takes most of us years to acquire the knowledge necessary and for the most part, higher education courses to expedite the process.

    I would happily encourage you, if you'd displayed the skills (or at least some of them) that would suggest you have some understanding of the concepts involved. Thus far you've asked questions so basic in nature, that my reasonable assumption was that you don't have the skills and are in need of considerable study.

    Maybe you can "do this", but given your replies and questions, you've haven't the grasp of the necessary fundamentals. So, being this far behind the curve, you can purchase a dozen text books and begin you education or buy a set of plans. This isn't intended as an insult, though maybe a reality check as to your skill set in this venue.

    Simply put, what does your weight study of the above design, tell you about the CG, the location of it's center of areas, etc.? Without an answer to this and many other "issues", you are hopelessly behind the 8 ball, in regard to a successful out come. This doesn't mean you will not succeed, but it does mean the odds of doing so are greatly opposed to it. If you posted a body plan, many of these questions could be answered. So, how about a body plan (sectional drawing), so we can see what you actually have in mind, instead of guessing, based on the supplied, very simplistic profile drawing?
     
  13. CatBuilder

    CatBuilder Previous Member

    Blueheaven - if I may...

    I'm like you. Former physicist, computer programmer and NASA employee. I figured I'd design my own boat and build it as well.

    I came on here and asked if people thought I could do it, given my skill set. Many did say "yes, go for it", especially since I have many *many* years experience on the water and owning boats, as well as a good understanding of what makes them work the way they do.

    In the end, I was faced with a choice:

    a) Buy a mountain of text books, which are mostly archaic because they don't follow the type of science and math you and I understand. They are more of a "black art" with tables and guidelines, not so much exact. Buy these books and spend 2-3 years really poring over them, then design a boat (another year, at least) and build a boat.

    b) Buy a set of plans and get building.

    I chose "B" because it was more cost effective and was something that could be accomplished before life changed on me. Also, I wanted the boat sooner, not 5 or 10 years from the time I asked the question.

    PAR's advice is good. Work with an existing designer. Look hard through existing designs and see if something fits your criteria. Look even harder at buying a used houseboat - they seem about like what you're proposing - huge shortcut! Anyway, find existing plans. They are only a few thousand dollars - about what you'd spend on all the books and materials to learn design. If suitable plans don't exist, commission a designer who has a similar boat and have him design you a new one to your specs.

    You *can* learn boat design and build your own boat, however, it's important to asses your ultimate goal.

    Are you doing this because you want to design boats, or because you want a boat?

    Big difference between the two.

    If it's because you want to design boats, then jump right in and get started.

    If it's because you want a boat, first look for a used one to fix up. If nothing exists, then look to buy existing plans. If nothing exists, then look to work with a designer to design a boat to your specs. That is the logical order of proceeding in this area.

    Hope this helps. I couldn't help but notice the similarity between your position and my own.
     
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  14. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Blueheaven; No one is questioning your intelligence here. So do not be upset if some of us seem to be picking on you. We boat people tend to look out for one another and if we see that someone is doing something chancy, we may stick our oar in.
     

  15. Pericles
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    Pericles Senior Member

    Blueheaven,

    Isaac Newton wrote.

    "What Descartes did was a good step. You have added much several ways, and especially in taking the colours of thin plates into philosophical consideration. If I have seen a little further it is by standing on the shoulders of Giants."

    Rather than having to learn from any mistakes that you might make, there are these designs that you could adapt to your needs.

    http://www.boatdesigns.com/Houseboats/departments/7/

    Best wishes.
     
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