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Thread: Wavepiercing Bows on Catamarans

  1. #81
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    Default Re: Wavepiercing Bows on Catamarans

    How are the bulbs supposed to reduce pitching? Is it just because its harder to raise and lower a blunt shape through the water than a sharp hull? Is it the sharp reduction in buoyancy when the bulb comes out of the water? That will help on the hobby horsing "up stroke", but what about the "down stroke"? Surely the bulbs would be worse as there is a reduction in buoyancy after the bulbs are immersed encouraging downward trim?

  2. #82
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    Default Re: Wavepiercing Bows on Catamarans

    Surely the bulbs would be worse as there is a reduction in buoyancy after the bulbs are immersed encouraging downward trim?
    Once immersed the upward force due to the buoyancy of the bulb is still there no matter how much its immersed but once back up out of the water the buoyancy is lost.
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  3. #83
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by ForumAdmin View Post
    Once immersed the upward force due to the buoyancy of the bulb is still there no matter how much its immersed but once back up out of the water the buoyancy is lost.
    Thats pretty obvious. The point is that once it is immersed there will be a smaller than usual buoyancy increase with greater immersion. The water plane is very narrow above the underwater flare with the "bulb" or "tulip" bow. I have several crowthers at my ship yard and I cant seem to figure out why having more buoyancy underwater than out of the water is a benefit at the bow. John Shuttleworth speaks of the exact opposite being beneficial.

    However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull
    http://www.john-shuttleworth.com/Articles/NESTalk.html

  4. #84
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    Default Re: Bulb bows vs Wave Piercing bows

    ...from George Buehler's website...
    I recently got this letter from a guy who’s in a position to know what he’s talking about. I suspect he wouldn’t care to be quoted so won’t mention his name or the names he mentions. But I think you’ll find his comments quite interesting! For instance, I always thought the bulb was to reduce pitch. I was surprised to learn that had nothing to do with it.

    Quote Originally Posted by the guy
    The "bulb" was designed by two naval architects for a major shipping line. Both are members of my yacht club. A long time ago they discussed the "bulb" with me, which is called the "bow wave depressor" in the shipping trade.

    According to them the bulb is supposed to act like a little ship being forced through the water by the big ship it is attached to. Because it is forced over its displacement speed it has a very large bow and stern wave and a hollow between them. The placement of this hollow is critical as the bulb must be designed so the ship's own bow wave falls into the hollow and is therefore depressed or made somewhat smaller. Supposedly this effects the wave making properties of the whole ship providing some fuel efficiencies.

    The bulb was never met to have any thing to do with pitching. How a boat pitches is more related to bow rake and flare, "flim and flam", than anything else. In areas like the west coast, where on shore wind predominate, designers tend to have lots of rake and flare to provide lift in an attempt to keep the boat dry. In areas like New England, with predominantly off shore winds, designs tend to have plum bows with little flare like the New England lobster boats. The Great Lakes boats usually have plum bows with lots of freeboard (short choppy conditions) and the gulf shrimper bow were all designed to provide the pitch needed in unique conditions.

    One production company put a lot of rake on one of their newer large yachts, almost a "clipper bow". I hope it was designed with the idea of providing pitch to protect the boat in certain sea conditions and not just for looks. Than they added a bulb which was supposed to provide some fuel efficiencies. Tanks tests, where a beautiful laminar flow over the bulb allowed it to work correctly, showed promise. But in actual use on a small ship, where a more active movement through the seas results in little laminar flow over the bulb, the efficiencies were just not there. This is when the "pitch control" story surfaced to justify these things. Of course we all know if pitch is a concern than bow should have been designed correctly in the first place. Or if, for some reason additional pitch control was needed, a much cheaper and smaller flat plate would work a lot better than a bulb.

    #1 of these new boats was supposed to be delivered by a local delivery shipper. He left Asia but returned saying the boat was being swept with green water. The great bow, which should have provided the necessary pitch to protect the boat, was defeated by the bulb. The biggest problem was leaks in a hatch on top of the pilothouse! I think we know why so much water was getting up there in the first place.

    I personally think this whole bulb thing is more market than design driven. Maybe this ship like thing sticking out in front of their boats gives owner's some face, but I don't think it works.

  5. #85

    Default Re: Wavepiercing Bows on Catamarans

    I think the concept is much earlier--I have been told Leonardo da Vinci experimented with spheres mounted on rams ahead of galleys and sailing boats, which apparently gave them more speed at the expense of some maneuverability.

    In the Pacific Islands boats have been made in this way for centuries, the keel and stem post being chipped from one log and a strong lower branch, and the end of the log adzed to form the bulb. Nothing much new under the sun--but it is interesting to find out WHY these very old and mostly overlooked until recently ideas were so effective.

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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by Mike Banks View Post
    I think the concept is much earlier--I have been told Leonardo da Vinci experimented with spheres mounted on rams ahead of galleys and sailing boats, which apparently gave them more speed at the expense of some maneuverability.
    Seriously doubt that.

    The early Greek and Roman warships had underwater extensions on their bows meant for ramming the opposing ships and holing them,...no relationship to either speed or pitching.

  7. #87
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    Default Re: Wavepiercing Bows on Catamarans

    Brian. That is the explanation for a bulb on a ship, which I am well aware of. The bulbs on smaller multihulls I am talking about might appear similar, but are not the same at all. Even the catana 50 I was next to today had vestigial bulbs (tulip bows). Yes when you look at it from the side, its a plumb bow, but look at it from the front and you can see the under water flare.

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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by dennisail View Post
    Brian. That is the explanation for a bulb on a ship, which I am well aware of. The bulbs on smaller multihulls I am talking about might appear similar, but are not the same at all. Even the catana 50 I was next to today had vestigial bulbs (tulip bows). Yes when you look at it from the side, its a plumb bow, but look at it from the front and you can see the under water flare.
    You do realize that the early Catana beginings were all Crowther designs, thus you are likely to see his 'tulip bows' on many of his vessels. The one I remember in particular was his Wahoo design,...I really liked the looks of those bows,...but they certainly a problem to fashion on a production glass boats built in molds,...just ask Mike Bell
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by brian eiland View Post
    The early Greek and Roman warships had underwater extensions on their bows meant for ramming the opposing ships and holing them,...no relationship to either speed or pitching.
    Greek Warships

    Roman Warships

    Do any of these bows look like they were made for drag reduction or pitching reduction??

  10. #90
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    Default Re: Wavepiercing Bows on Catamarans

    Yes I do realise early Catanas (the fast ones) were designed by Crowther. The catana 50 next to me at the shipyard is a much newer design, however the new designer carried over many of Crowthers ideas including tulip bows.

    On the other side of me is an impressive 80 foot power cat made of foam and kevlar. It has what looks like bulb bows from the side. But they are sharp. These also can't really work as bow wave reducers as on a ship. Multihulls have such narrow bows this just is not the issue.

    I will take a pic of all these bows.

  11. #91
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by dennisail View Post
    .... But they are sharp. These also can't really work as bow wave reducers as on a ship. Multihulls have such narrow bows this just is not the issue.
    Have you looked at how a bow, even sharp ones, create a cascade of water that tries to climb the forward surface of a bow. The rather blunt bows of some production cats are even worst in this regards, but their bluntness is often necessitated by the need to glass the interior 'joining surface' of the 2 bow side halfs.

    This 'bow wave' of water keeps trying to climb up the bow and spray the crew. If you could cut down on this formation of the bow climbing water I think you would be more pleased with the reduced wetness of the boat in some conditions. That's where I thought the bulb bow on a multihull might be advantageous.

  12. #92
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by brian eiland View Post
    If you could cut down on this formation of the bow climbing water I think you would be more pleased with the reduced wetness of the boat in some conditions. That's where I thought the bulb bow on a multihull might be advantageous.
    The new fast French racing trimarans have chined hulls even at the bows so that the water created by the bows which is trying to climb up the hulls and slows the boat down are thrown off the hulls.

    I think a bulb bow as the KKG "Victorinox" (see my original pictures) has could be a solution as to this point, maybe even in combination with chined hulls.
    Roger

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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by brian eiland View Post
    Have you looked at how a bow, even sharp ones, create a cascade of water that tries to climb the forward surface of a bow. The rather blunt bows of some production cats are even worst in this regards, but their bluntness is often necessitated by the need to glass the interior 'joining surface' of the 2 bow side halfs.

    This 'bow wave' of water keeps trying to climb up the bow and spray the crew. If you could cut down on this formation of the bow climbing water I think you would be more pleased with the reduced wetness of the boat in some conditions. That's where I thought the bulb bow on a multihull might be advantageous.
    The boats in question have fairly sharp bows.

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    Default Re: Wavepiercing Bows on Catamarans

    Well after a long gestation period here is what I have had fitted to my cat "Burnout":
    Attached Images Attached Images
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    Default Re: Wavepiercing Bows on Catamarans

    Roger, they look very well done. Is there any mathematics behind your modification or are you going intuitively?

    Also, have been out and tried them yet?

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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by multihullsailor6 View Post
    Well after a long gestation period here is what I have had fitted to my cat "Burnout":
    It looks like the added bow section has put hollow (aka reverse,) in the waterline. It's been a long time 40 or 50 years maybe, since anyone has advocated that.
    Currently concentrating on http://earthnurture.com .

  17. #97
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by multihullsailor6 View Post
    Well after a long gestation period here is what I have had fitted to my cat "Burnout":
    Looks pretty well done.

    Can you tell us the benefits you hoped to achieve, and any measurable benefits in practice? Also some pics or explanations of the build would be good too

    I plan to do similar, but for cosmetic reasons I will fair it up to the top of the original stem (reverse bow). I want added buoyancy at the bows mainly and a longer waterline as a bonus. Style is not a factor but I think reverse bow will be the lightest way with a look I am happy with.

    I presume your "hollow" is just a building compromise when modifying bows like this in the real world. To do it with no hollow at all you would need to fair it in almost all the way to max beam, so a compromise may need to be reached with how far back you go or it will just add a lot of weight and work. This shouldn't come to a shock to anyone who has any idea at at all.

  18. #98
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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by dennisail View Post
    Looks pretty well done.

    Can you tell us the benefits you hoped to achieve, and any measurable benefits in practice? Also some pics or explanations of the build would be good too

    I plan to do similar, but for cosmetic reasons I will fair it up to the top of the original stem (reverse bow). I want added buoyancy at the bows mainly and a longer waterline as a bonus. Style is not a factor but I think reverse bow will be the lightest way with a look I am happy with.

    I presume your "hollow" is just a building compromise when modifying bows like this in the real world. To do it with no hollow at all you would need to fair it in almost all the way to max beam, so a compromise may need to be reached with how far back you go or it will just add a lot of weight and work. This shouldn't come to a shock to anyone who has any idea at at all.
    Well, I suppose you could cut the boat down the middle on the center line and spread it wider. If you want more buoyancy when pitching, I suppose you could give it a raking bow above the waterline, though that would give you more weight forward and thus more pitching. I'm not advocating either of these things, I'm just making a theoretical answer. I think it is becoming clearer why people usually add length at the stern rather than at the bow.
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    Default Re: Wavepiercing Bows on Catamarans

    Its certainly way easier to add at the sterns. But unfortunately for me, I want buoyancy forward. I am a long way off yet, but I finally acquired the lines plans so I can at least start on a possible design solution. I will probably fair back a little further than on burnout, and also reduce rocker at the front which should gain a decent amount of buoyancy too. I am not worried if it tacks slower. The other positives effects are more important to me than a slower tack. Ending up with a fair result without a lot of work and weight wont be easy.

    Currently my waterplane in the bows is dead straight (at least by eye) for about 1/3 of the boat. Above the waterline it is concave like a destroyer for increased reserve buoyancy. Apparently Crowthers hull shapes were strongly influenced by testing done on long narrow destroyer type hulls.
    Last edited by dennisail; 16th April 2015 at 06:34 AM.

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    Default Re: Wavepiercing Bows on Catamarans

    Quote Originally Posted by brian eiland View Post
    Greek Warships

    Roman Warships

    Do any of these bows look like they were made for drag reduction or pitching reduction??
    I'm pretty sure they were for incapacitating and even drowning the enemy by bashing the enemy's hulls near or below the waterline. So, they might very well have an application in racing. It's hard to raise a challenge flag when you're trying to keep from drowning. "Arrr, Matey, avast yer haulin' o' that protest flag, lest ye be made to walk the plank before the poxy boggards in the race committe can stick their pretty little red noses where they don't belong!"
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