yes another pontoon design...

Discussion in 'Boat Design' started by spencer321, May 31, 2013.

  1. spencer321
    Joined: May 2013
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    spencer321 Junior Member

    Alright lets say a 16x8 hull, how tall x wide should the tunnel be and what would the advantages of a tunnel be
     
  2. spencer321
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    spencer321 Junior Member

    hull is 7'x16', each cat has 12.5 degrees deadrise, tunnel is 12" wide by 6" tall and runs the length of the hull
     

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  3. PAR
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    PAR Yacht Designer/Builder

    What is the purpose of the tunnel? It just adds to building complexity and would in no way benefit a craft with the S/L ratio you find acceptable. Simple is usually best, particularly when you don't have a firm grasp, of the fundamentals of hydrodynamics.
     
  4. spencer321
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    spencer321 Junior Member

    thats what i asked! I understand the advantage of a racing tunnel hull is to create lift, on a flats boat the tunnel hull is meant to draw water up into the tunnel to raise the motor higher for shallows. Is there some kind of software or simulation i can test hull designs? i want something that i can build relatively cheaply, that has a shallow draft, will move say 25+mph with a 40-50 on it, and has alot of deck room and preferably won't sink the the first time I take it out
     
  5. PAR
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    PAR Yacht Designer/Builder

    At the speeds you're looking to move, there's no advantage to a tunnel and jack plate setup, so a flat bottom jon or a garvey hull form will be the most suitable for your needs. Yes, there are software packages to simulate and test with, but they aren't cheap and you do need a firm grasp on the physics involved, to make any sense of the output they'll provide. It's a bit like looking at the results from an MRI scan - unless you have a clue about what it offers and it's relevance in your search, the information is meaningless.

    The DragonFly from Glenn-L seems well suited for your needs at 17' 6" long with a 7' beam as well as the Jimbo from the same place, which is 15' 9" on a 6' 8" beam.
     
  6. spencer321
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    spencer321 Junior Member

    thanks I appreciate it ill look at those
     
  7. hoytedow
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    hoytedow Wood Butcher

    I am, you know, a rank amateur. The tunnel idea was simply to give the wide hull some "skis" to ride up on and thereby reduce frontal resistance. It was just a suggestion.
     
  8. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    There is this type of tunnel hull also, where the tunnel doesn't run full length but allows water to be drawn up for a shallower propeller depth.

    [​IMG]
     
  9. Nate57
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    Nate57 Junior Member

    Par said "You'll get less draft with a flat bottom, plus you only have to build one hull instead of two. It's also easier to propel this type of hull compared to a pontoon style, which translates into better fuel efficiency too."
    I've seen this mentioned before and still can't grasp it. How can a wide flat barge be more efficient than two skinny 'toons? Seems like even with the lesser draft it would have considerably more wetted surface and without any fairing be pushing a lot more water (perhaps not when planing). Would this still be true with a much larger (purely displacement speed) cat?
     
  10. PAR
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    PAR Yacht Designer/Builder

    You can't equate displacement hull forms with full plane mode full forms in a fair process. They work in quite different environments and speed ranges, so their shapes reflect this.

    [​IMG]

    [​IMG]

    A flat bottom craft is the most efficient of all full plane mode types. The classic picture of an outboard attached to a dinning room table, with the skipper aboard, scooting along is a good example. Of course, there are draw back to this approach too, just as with any design decision.

    Even if you employ two skinny hulls, you still need sufficient area to get the boat up on plane. Granted, most small, flat bottom craft have more area then they actually need, but in the speed ranges Spencer wants to travel at, there's no advantage to decreased wetted surface or using other clever hydrodynamic tricks, to improve efficiency. He'll need and want as much lifting area as he can get. A conical tunnel and a jack plate are a possibility for him, if an extra few inches of draft are that big a concern, to justify the added complexity and contrivance of their installation.
     
  11. spencer321
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    spencer321 Junior Member

    So your saying just about any standard flat bottom hull given my 25-30mph speed goal is going to be most efficient and sufficient for what I want to do?
     
  12. PAR
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    PAR Yacht Designer/Builder

    Yes, though if you refine your goals, you may find you might want other features. For example if you think you might want more speed at a later date, you might consider a V bottom. Also flat bottom boats tend to pound under certain conditions, so a garvey hull might be the ticket, if you expect to find yourself in a stiff chop regularly. There are other considerations as well that can affect your hull shape choices.

    I think a shallow V is the best middle of the road choice for most folks. It offers some ability to absorb chop, a more comfortable ride, but also sacrifices some initial stability, increases draft and displacement, compared to a flat bottom. I've found most people can live with the few inches difference in draft, as a reasonable trade for better sea keeping and softer ride. On the other side of the coin, if absolute shoal draft is a paramount concern a tunnel with a jack plate may just be mandatory. Frankly, I've seen very few true requirements in this regard, though I do know a few gator hunters that absolutely need their extreme shoal draft craft, these would be the exception to the rule.

    So, in the end Spencer, I'll repeat that simple is usually better, so a jon boat or garvey might just be what you need. Both are very easy to build, offer very shoal draft and get up on plane quickly, just by taking a hard pee off the stern. The moment you introduce deadrise, extra hulls, tunnels and other "tricks" you'll need more power, for the same performance levels, a plain 'ol flat bottom will provide, given the S/L range you'll operate in. Make some honest answers to the things you absolutely need this boat to do, then you choices should become reasonably apparent.
     
  13. Village_Idiot
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    Village_Idiot Senior Member

    If you want to build a tunnel boat, and this is your first, you might consider Bateau's XF20 (Extreme Flats 20) http://www.bateau.com/products.php?cat=17 - there is plenty of support (tips, tricks, various builds) in the bateau2.com forums for building this rig.

    I agree with PAR on the deadrise. Most folks would be better off with a little deadrise. It cushions the ride, makes handling predictable, and feeds good water to the prop. If you ground it, it is usually relatively easy to push off into deeper water. Zero-deadrise boats will pound, have sketchy handling, may require a better prop for good performance, and are a pain to get unstuck if you ground them. They definitely run in shallower waters, though. :D
     
  14. messabout
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    messabout Senior Member

    Think of a planeing surface in terms of load per square foot. If the total weight of the boat is X and the wetted bottom surface is Y then we can divide X/Y to get the dynamic load per square foot. Example; Let X = 1000 pounds and Y= 10 square feet, and we get a number like 100 pounds per square foot. That is a lot of pressure on the support surface and will need plenty of bracing and lot of power to make it go. Increase the support surface to 20 square feet and the load is halved, etc. Apply that notion to a catamaran and you begin to see the implications of narrow hulls.

    A flat bottomed boat is likely to need less energy to propel at a given velocity. Economy of operation is more favorable and the boat might be easier to build. BUT; it will pound your teeth out while making a lot of noise if you go fast in a chop. BUT #2; The wider the flat bottom the more structural implications.

    Tunnel hulls like Hoyts drawing.....You lose some planeing surface but gain some structural rigidity because the tunnel sides act as beams. BUT;You still need cross members to hold the beams in place. BUT; you may experience some cavitation problems with a bottom of this type. It may take some major tinkering to sort it out.

    Vee bottoms ease some of the slamming. That configuration also has some structural advantages. More dead rise is better in that regard. BUT; The boat will need more power if the angle is steep. For economy, less dead rise is better. Less dead rise will have more initial stability than steeper dead rise But will not ride as smoothly.

    A long skinny flattie would be my choice because it will go pretty well with a lot less horsepower, ride a little better than a short wide one, and not stick its nose skyward while trying to get on top. BUT; it will not accommodate lawn chairs and charcoal cookers a well as your original deck boat. Besides, registration in Florida costs more for a boat longer than 16 feet.

    Alas, boat selection is fraught with ramifications, technicalities, and difficult decisions.
     

  15. PAR
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    PAR Yacht Designer/Builder

    I too would be inclined to use a flattie style of hull or garvey, considering S/L, draft and efficiency requirements. These are easy to build and can work in modest chop fairly well - better if the skipper has some sense about their operation and handling characteristics. If you have to error, be it on the longer and narrower side, rather then shorter and beamer to keep efficiency and potential up.
     
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