catamaran V tunnel hull

Discussion in 'Boat Design' started by Frosty, Mar 21, 2010.

  1. Frosty

    Frosty Previous Member

    Whats the difference between a catamaran and a tunnel hull.
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  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Frosty, you are messing with us. I'll bet that you know the difference between the two types very well. From past postings, we all know that you did not just fall off the turnip truck.

    Maybe we can get an argument going about the semantics. We could start with the observation that the thing we generally call a catamaran does not , by definition, need decking or cover between the two hulls. Whereas a tunnel hull is presumed to have a roof over its tunnel. Cats usually but not always, have two distinct hulls joined by some structure or other. Usually a system of beams. Tunnel hulls are like a monohull that has a groove plowed down the undersides, parallel to the longitudinal centerline.

    Has the argument started yet?
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  3. TollyWally
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    TollyWally Senior Member

    You have to get up pretty early in the morning to pull one over on messabout!
  4. Frosty

    Frosty Previous Member

    I have a power,----!!!! cat. I do not have trampoline, I have fibre glass floor all the way up to the bow and stern, I have 3.8 meter hull centres on 44 LOA is it a cat or a tunnel hull?

    Its a one lump cold moulded hull.
  5. Frosty

    Frosty Previous Member

    Doesnt any one know. Is it a silly question, Im not playing, I really don't know.

    Can any one explain what the difference is.

    Do you mean a symmetrical hulls are cats and asymmetric are not.

    Like the type with flat inner side to the hulls, that is certainly a tunnel hull but is that it,-- symmetrical can not be tunnel hull?
  6. messabout
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    messabout Senior Member

    Frosty; I was being facetious in my first posting. I can see that your question was perfectly legitimate. I suspect that we will get a number of different takes on the subject..

    The two terms are often used interchangeably depending on who uses the terms. I know of no technical distinction but I will propose one presently.

    I have, in the past, thought of tunnel hulls as belonging to the power boat type. On reflection I must change my position. I once owned a Melges M20 sailboat whose principal feature was a concave bottom. It was called a tunnel hull by most of the owners. Herreshoff once produced a boat of that type intended to defeat the corinthean fleets of the day. Defeat them it did, and was promptly ruled illegal for that type of competition.

    Super fast ocean racing, twin hulled, boats are called catamarans, not tunnel hulls. I reckon the designers and owners can call them whatever they please. Atkin once designed a boat that had a tunnel only in the after part of the bottom. The prop was tucked well up into the tunnel. The boat was named Rescue Minor. The type is usually called "tunnel stern". In my part of the world there are sport fishing boats that have a narrow groove down the middle of the boat. maybe one third the chine beam. The pupose is to be able to power in very shallow water, like 12 inches more or less. These are called tunnel hulls or hereabouts they are "flats boats".

    So the distinction is unclear. I think that the distinction is unimportant but for the sake of argument, maybe we can establish some sort of general rule. I propose that the rule can be invented by useing the ratio of LOA to the distance between the centerlines of twin hulls. LOA/CL. For starters I will say that quotients less than or equal to 2.5 is a tunnel hull. More than 2,5 is a catamaran. I'll bet there are going to be exceptions to the ratio-names proposition..

    I have also read that the distance between cat hulls should be seperated by some dimension that will allow a line drawn at an angle from one bow will not allow the line to touch the opposite hull. This is to to avoid having the bow wave of one hull strike the opposite hull or to avoid the interference of the two separate bow waves when the colliding waves might wet the undersides of the connecting deck. The minimum angle was said to be 19 degrees. I am not so sure of the validity of the 19 degree figure. That would make a whole nother subject.

    Maybe the original question would fare better in the multihull section where multihullers abound.
  7. Village_Idiot
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    Village_Idiot Senior Member

    They are "tunnel boats" to the fishing crowd, and are sometimes referred to as "pocket tunnels". The Texas boats have taken this to the extreme in examples such as Mowdy, Majek RFL, Transcat, Flatscat, ShallowSport, Shoalwater and NewWater Boatworks. In many of these examples, the hull is somewhat concave on the underside with a tunnel in the aft third of the hull. The concave-ness serves to channel water up to the tunnel, as well as holding it under the boat when in extremely shallow water (keeps the water from rushing out the sides of the boat and instead pushes it back to where the outboard prop can use the water to propel the boat). As an example, the Majek RFL can be run in just a few inches of water over a mud bottom. Here is a link to an interesting thread about running these boats in very shallow water:
    This thread also has links to Youtube videos showing the boats running through mud with just a little water over it. Tunnel design is very much an art and a science with these boats, as is special cupped propellers and anti-cavitation plates designed to hold water around the lower unit of the outboards. Jet outboards also use a tunnel, but it is much smaller, maybe three inches in vertical depth.

    I am by no means an expert on boat design; however, in my mind, if the hull is designed to be propelled from within the tunnel, then it is a "tunnel" boat. If it is designed to be propelled from either sponson, then it is a catamaran. If it is a sailboat, then I lean toward catamaran (I think there is some relationship between actual depth of tunnel and distance between sponsons to help delineate between the two, but even then semantics can get involved).
  8. marshmat
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    marshmat Senior Member

    One distinguishing trait I've seen used is whether the tunnel is designed to provide aerodynamic lift, either by the shape of the deck above it or by compressing air in the tunnel at high speeds. If so, it's a tunnel hull. If the hull spacing is sufficiently large, the deck sufficiently open, or the cruise speed sufficiently low, that aerodynamic effects in there don't have much of an effect, it's a plain old cat.
  9. Fanie
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    Fanie Fanie

    Yep, the thing Frosty have is a tunnel hull. I distinctly remember he mention the reason he never
    goes out is the amount of bridgedeck slap he gets, scares the **** out of him :D

    The problem with tunnel hulls are they are closer together and forces water into the tunnel at low speeds. Tunnel hulls work on planing boats for the relative soft ride.

    Cat hulls, have a wider gap between them, if the bridgedeck clearance was done properly you won't get the slap. The water doesn't get pressed in between the hulls as much.

    So, if her gap is small she's a tunnel hull, if her gap is wide she's a cat :D
  10. baeckmo
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    baeckmo Hydrodynamics

    Mmmhm... I'd rather say that the tunnel hull is designed to allow wetting of the tunnel roof, at least in part adding to the lift, while the pure catamaran has a tunnel roof that is designed to generally clear the surface, except for the occasional (limited) slap.

    Hull symmetry/asymmetry is not a deciding factor. In this perspective, the inverted V, Hickman bottom, is a tunnel hull, since it is fully wetted, while the offshore racing vessels are catamarans, even if the roof is generating aerodynamical lift.

    Afterthought: an overloaded catamaran will be a tunnel hull, right Fanie.......? That Frosty's dilemma? Too much beer in the "keelcooler"?
  11. Fanie
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    Fanie Fanie

    Hi Baeckmo,

    The hulls do make lift of course, the idea is not for the water to hit the bridgedeck. Usually these boats are relative short and heavy, hence does not lift quick enough for the bridgedeck to clear. A lump of water hitting it will of course lift it, but that's not the idea. The main advantage of having the two hulls is stability and a softer ride in choppy water.

    I think Frosty went back to sleep. He must be really tired, no back chat or anything ;)
  12. Frosty

    Frosty Previous Member

    No No Im listening. I do get some lift from air -I think.
    Sometimes I see spray that wont go down the hole and comes out and up onto the bridge, theres a lot of air going down there.

    I can get under her with the dinghy, its about .8 meter at the back and at the front I can just reach the anchor.

    All cats /tunnel hulls hit waves under the deck , Ive talked to many owners that say it just something you get used to.
  13. Fanie
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    Fanie Fanie

    I very much doubt if air can mak a difference there Frosty. Just for a moment think if you seal the rear and front of the tunnel off. how much air pressure would you need to lift your boat ? Too much junk on it :D

    So if you motoring, how fast do you have to go to generate the same air pressure ? it's over a 140km/hr or there aboputs I can't remember. Not really practical.

    The water that gets forced between the two hulls will have more of an effect, because the hulls will attemopt to lift up as the water in between becomes deeper.

    I don't think there is a perfect hull, they all have compromises. It just depends which compromises you consider the least as a compromise.

    PS: Don't worry about the beer you have aboard, beer also floats so it can hardly add any weight to the boat :D
  14. Fanie
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    Fanie Fanie

    Post a bloody picture so one can see what you have there. If you can get a dinghy in there they can't be too close together.

  15. Bouman
    Joined: Mar 2010
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    Bouman Junior Member

    I think any vessel with two hulls is a catamaran and think marshmat is on track with tunnels being a hull feature that provides lift by compressing air, with other things like tunnel drives and tunnel advertising being something else altogether.

    I used to run a 25' Motion tunnel hulled catamaran that did about 100 mph. I searched tunnel hulls in an attempt to know more about creating lift and reducing hull friction with air pressure. It only takes a little bit of pressure; a 40'x1.5' tunnel at 1psi creates 17,280lbs lift (I think, I was sleepy when I did that math and I don't have a calculator now).

    The dynamics of these pressurized tunnels vary in determining hull speeds, center of lift, power specs and what-not but I aim to get to the bottom of it.

    I had a crazy idea of cutting some tunnels into perfectly good vessels so I guess I'm off to find the proper place to start my first thread.
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