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Thread: Submerged bulb hull

  1. #1

    Default Submerged bulb hull

    Hey guys
    I came across this forum last night and realized that the people writing here are quite knowledgeable. I'd like to present an idea of mine. I'd like to hear your thoughts on this?

    Please bare with me as English is not my native language, and there are a lot of technical terms I don't know in English. My experience with multihulls is limited to 14/16ft Hobies and the Hobie trifoiler. I'm more experienced in monohulls. However I'd like to build a sailing cat to live on when kids gets older and I've started to sort out what I'll be looking for.

    As everything a cat is a compromise. I want the hulls to be slim for less water resistance, but wide for more living space. I want the bridge deck high to avoid slamming, but low for the hulls to catch less wind. Where as a short bridge deck provides less slamming, a long bridge deck provides more living space.

    I was reading about people extending their wl by adding bow bulbs when I got to think; Is it really a long wl you want? Isn't it so that you'd prefer the smallest surface breaking area (the area of the hull surrounded by the wl) as possible? If so, why not build the hull like a submarine where only the tower breaks the surface? I've made two quick sketches to show this (sideview and frontview):



    The idea is that the underwater hulls has a size carrying the empty weight of the boat as empty, while the legs carries the payload, so that the underwater hulls are near the surface when boat is light weighted, and the bridge deck still has sufficient clearance when boat is loaded. This way the height of the bridge deck will have a minor effect on the wind catching area of the hull. That allows for high bridge deck clearance. High bridge deck clearance also allows for a longer bridge deck, which results in more living space. (The underwater hull should probably not be considered as living space.) This design will have a short WL, but it will also be narrow.

    The length of the legs is what provides the forward/aft stability. Or maybe there should be two legs on each hull to provide better stability.

    With this design a cat will probably loose its shallow water ability.

    What do you think?

  2. #2

    Default Re: Submerged bulb hull

    Can't see your drwgs.

    Narrow hulls at waterline and wider above with aerodynamic bridgedecks were pioneered by John Shuttleworth. The general arrangement coming from trimaran hulls. Other designers have done work similar and Dr Mai extended the waterplane of the hulls on a catamaran called Vitrinox.

    There are also some examples of what you are thinking by designer builder in the Caribbean (name escapes me) they are renowned for their inter-island ferries there is also a designer in Australia who has done some powercats on submerged pontoons (sorry can't think of the name).

  3. #3
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    Default Re: Submerged bulb hull

    Gold Coast Yachts to do the wave piercing ferries. These are not bulb bows, but just low reserve buoyancy bows. I just came back from the Caribbean. These boats are a success there.

    The narrow VS fat hull thing is not well understood thanks to multihull marketing. The fact is that heavy boats have more drag than light ones. The width of the hull is secondary to that. To maintain a minimum wetted surface area a heavier boat usually has a fatter hull. The fatter hull is more accurately slower because it is heavier not because it is fatter. I made a thread on this subject and cited research papers by the university of Southampton tank towing facility which shows changing the hull width while trying to keep other variables the same results in little difference to resistance. But small changes in weight results in comparatively large changes to resistance. In some of the lighter models a wider hull actually had less resistance than a narrower one at certain speeds! Heavy hulls seem to get the best benefit from being narrow. Look the papers up.

    The extreme example of bulbs or submarines underwater with a thin water line is called a SWATH (google it). The wave making drag is low, but the overall drag is high due to a very high wetted surface area. Some cat designs have a bulb bow with a fine waterline. This reduces the bow wave but adds wetted surface area. It is said to also reduce pitching, which slows the boat and reduces efficiency of the sails.

  4. #4

    Default Re: Submerged bulb hull

    I wonder what happened to my sketches. I posted this thread from my home-pc. When I read your replies I was on the work-pc - there I could not see the pictures in my post. So I switched back to my home-pc and browsed to the thread - here the pictures are displayed in my first post!
    I'm reposting the sketches here:



    Are they visible this time?
    Attached Images Attached Images

  5. #5

    Default Re: Submerged bulb hull

    I can see them on my work pc this time. Hopefully everyone else can see them too.

    @dennisail
    I think you're spot on that my idea is much like SWATH. I think I'm kind of overdoing the SWATH concept in my idea. When googling this I find large motorships only. Is the concept not suitable for sailing cats?

    Interesting research on wide vs narrow hulls, all other variables equal.
    The way I'm thinking is that a given a weight, the volume of water displaced to make the weight float will be constant. When a weight is increased, there are three ways to handle that:
    1) The hull can float deeper, displacing water deeper down where the water pressure is higher - resulting in more energy needed to move the hull forward (compared to the same theoretical displacement volume and same width at less depths). This also results in significantly more wet surface.
    2) The hull can be made wider. Then the result is that the water at front has to be split further apart, which requires more energy. This also results in significantly more wet surface.
    3) The hull can be made longer. This approach only increases the wet surface. It does not run into the other problems of 1) and 2). Hence this is the preferred approach energy wise.

    My guess would be that 1) and 2) are pretty much equal. 1) is the worst in flat water. 2) is the worst in waves.
    When ppl talk about length/width ratio, I'd say they are actually talk about length/width AND length/draft ratios.

    To related this to my sketches, I'd say that my design will move somewhat away from 3), heading for 1). It will have a very large wet surface. But it does gain some from the fact that the hulls section cut at waterline does have a very small area. I think it will be reducing waves a lot. Will the reduction of waves gain more than what is lost to wet surface friction?

  6. #6
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    Default Re: Submerged bulb hull

    The SWATH hull is VERY sensitive to loading. Your design is certainly a SWATH. The immersion per KG is related to the waterplane area. The SWATH hull has the least waterplane area short of a foiler (but the foiler does not rely on buoyancy). What this means is that small changes in displacement cause large changes in trim. This is a problem even on motor swaths (the usually have active foil stabilisation). Imagine the issues on a sailing boat (ignoring the fact cruising loads and high WSA will alone make the concept highly problematic). A sailing boat has large changes to the center of buoyancy due to sailing forces. Sailing on a beam reach, the leeward hull displaces much more water than the windward hull. Sailing downwind, the bows displace more than the sterns. For a swath this means massive changes in trim and heel.

    Your approach 3 is certainly the best. A heavy DLR displacement to length ratio (for the same weight) reduces wave making resistance considerably. However the WSA goes up. The minimum wetted surface area shape is a half sphere. So this shape theoretically has the least resistance as extremely slow speeds. But the shorter and fatter the hull the worse its wave making drag will be, so longer thinner hulls are better at faster speeds. That means there is an optimum balance between DLR and WSA for any given speed and displacement. If your design is planned to go close to and faster than hull speed often as its a performance boat, a lighter DLR will be needed. If the boat is designed to only sometimes reach hull speed and usually go well below, and excessively long hull will have too much WSA for its intended speed range to be optimal.

    I'm fairly sure The SWATH usually always has too much WSA for the least resistance, perhaps for a heavy DLR, where the length can not be increased instead, the minimization of wave making resistance at the expense of high WSA pays off. I thought the design was mainly for reducing motions not resistance.

    http://www.multihulls4us.com/forums/...s-Are-you-sure

    * please no arguing about my use of the term "hull speed". Its just easier to understand that saying Froude numbers over 0.4.
    Last edited by dennisail; 14th September 2016 at 02:15 AM.

  7. #7

    Default Re: Submerged bulb hull

    I was watching some videos of cats nose diving last night. Then I realized a sailing SWATH will result in a lot more of such videos. I think I'll just forget about this for my live in cat idea.
    I need to find another way to allow for longer bridge deck (forward), or just accept that I shouldn't bring my house into high waves.

    Thanks a lot for your feedbacks.

  8. #8

    Default Re: Submerged bulb hull

    I came across this article about swath that might be of interest to some of you.
    http://www.tu.no/artikler/ivan-22-ha...80-knop/348620

    For those who can't read norwegian, here is a google translate of it:
    https://translate.google.com/transla...-text=&act=url

  9. #9
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    Default Re: Submerged bulb hull

    Interesting link, thanks. Bit I wont believe the claims until the full sized version is tested and commissioned. Its just vaporware at this stage.

  10. #10

    Default Re: Submerged bulb hull

    Tony Grainger is doing some work with SWATH hulls. You can google it.

  11. #11
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    Default Re: Submerged bulb hull

    There is a good catamaran hull design page here with Formulas and design gols.

  12. #12
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    Default Re: Submerged bulb hull

    Without looking too deeply into this, the first thing that becomes obvious is that these submerged hulls have no reserve bouyancy. This will make them;

    1. Extremely sensitive to overloading. Any increase in load will depress the hulls disproportionately compared to a hull with reserve buoyancy.
    2. Extremely prone to bridge deck pounding. Unless the bridge deck is ridiculously high, as the hull passes through waves the bridge deck will come into contact with the wave tops.
    3. Most importantly, there is very little righting moment. One might get away with this concept on a high speed ferry, where the wave damping effects provide a smooth ride in rough seas, but you could never put a sail on one.

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