Bulbous Bow changes

Discussion in 'Boat Design' started by Proteus3000uk, Feb 2, 2013.

  1. Proteus3000uk
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    Proteus3000uk Junior Member

    Hello everyone. I have been following this remarkable forum for a long time but just today decided to register and make my first post.
    I own a 40 meter passenger vessel (approx. 122 ft overall – 105 ft waterline).
    A bulbous bow was retrofitted a couple of years ago but unfortunately we did not achieve the desirable results.
    The problem of the boat was (and still is) that it produces a high bow wave when at cruise speed (approx. 13.5-14 knots). We installed the bulb in order to reduce the bow wave but it seems that the bulb is very small for the ship.
    The bulb has a goose-neck design and extends approximately at 1.9 meters from the bow.
    When at port it is 25-30 cm above the sea level but when at speed the bulb seems to dive approximately half a meter under the surface of the sea.
    From my knowledge so far it should be just under the surface of the sea when at cruise speed.
    We have had virtually zero speed gain and approximately 6% fuel savings.
    However, we plan to make some changes to the bulb at the next dry-dock.
    Our naval architect insists that the dimensions of the bulb are just right for the hull, but we believe that the produced bow wave suggests that it is just not big or high enough.
    Anyway, he advised as to add approximately 1 m in length, 35 cm in height and 30 cm in width.
    What do you guys think?

    Any help will be greatly appreciated.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Who is your NA? I think you're asking too much from your bulb and should take your 6% and enjoy it. If the bulb is the "perfect shape" for your hull, then why is he adding 14" in each direction? It sounds like it might need to be moved, but without full ship dimensions and some photos, this is just speculation.
     
  3. TANSL
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    TANSL Senior Member

    You should check why the ship acquires a negative trim so great when running. This harmful effect can ruin the potential benefits of the bulb. In my opinion.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    At your cruising speed, the hull is running in a "hollow" in the resistance curve, which means it is "ideal". Thus any bow wave is produce more by the localised geometry of the hull than anything else. By that I mean the angle of entrance and how full the hull is just above and below the waterline. A bluff hull will always produce a large bow wave.

    If you could post some pictures of your boat, that may help.
     
  5. jehardiman
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    jehardiman Senior Member

    Ad Hoc is correct, the speed you are at is a speed length ratio of ~1.31-1.36 which is actually just starting the up-hill powering climb. You are not going to get much more speed without throwing a lot of power at the problem. Much more than a bulb is ever going to recover (i.e. going from 13.5 to 14.5 knots will take ~ 15% more power, a perfect bulb will give ~10% maximum).(See this discussion on the humps and hollows)

    As for the depth of the bulb and wether the hull is trimming down by the bow, we would need to see a picture of her underwater shape or some lines with shafting. There could be a lot of things going on here....like shaft rake.

    So as Par pointed out, take the 6%, which is fairly good for a bulb by the way, an be happy. If you really want to go faster, you will have to get longer, add power, or get lighter.
     
  6. Proteus3000uk
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    Proteus3000uk Junior Member

    Thanks you all for the quick response.

    The boat actually seems to have a slighlty possitive trim but produces high bow wave. Even after retrofitting the bulb the bow wave is roughly the same (meaning that it did not serve the purpose of installation)

    Please check the attached pic.

    We are not ungratefull for the fuel economy but we still believe that if the bow wave was reduced we would achieve even better results.

    It has also come to my attention that when the ship cruises in high head seas, with waves high enough to lift the bow upwards and bring the bulb at sea level, i observe a constant speed increase of approximately 1 knot (GPS source)
     

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  7. TANSL
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    TANSL Senior Member

    My impression remains that a big trim can not lead to optimal navigation. With idependencia that there is a bulbous bow or not, the ship changes its trim 2% to navigate. A boat that "immerses the bow" must produce a bow wave higher than they should and the bulb, which is very immersed, can not compensate. Furthermore, the bulb does not increase the waterline length, which could be considered as other beneficial effects of the bulb.
    My advice, again, is to find out how you can make the boat go with a proper trim. Achieved a waterline correct, we must find out the right bulb for the same.
    B.R.
     
  8. Perm Stress
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    Perm Stress Senior Member

    You can try to fill the space between raked stem and bulb with new bow; this would result in roughly vertical stem and much finer waterlines forward; reducing the entrance angle of waterlines is the prime weapon against high bow wave.

    What could be gleaned from the photo, WL entrance angle at present is about 30-40 degrees.
     
  9. Proteus3000uk
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    Proteus3000uk Junior Member

    I must agree that the trim must me looked after as well. When on the ship i observe the following:

    (a) High bow wave which creates something like a "hollow" at mid ship
    (b) The stern looks like squating and the propellers produce too much "foam".
    (c) If i drop a tin can at the stern while cruising, the can will be draged towards the stern for quite some time. So water is not "pushed" away.

    We also thought of adding a ducktail as well (mainly for stability reasons but this will increase the waterline as well)

    The ship does not give the impression of smooth passage through the water but looks like struggling.
     
  10. Perm Stress
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    Perm Stress Senior Member

    If bow wave do create a large hollow, this is prime indication of very bluff WL entrance.
     
  11. Proteus3000uk
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    Proteus3000uk Junior Member

    I am afraid this is the case. It looks like the water blocks the ship at the sides of the bow.

    This is why we added the bulb and this is where the current bulb failed.
    We needed something to "go through" and open the water wide open before reaching the sides of the bow.

    Please do excuse the terms i am using
     
  12. TANSL
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    TANSL Senior Member

    Answering your comments:
    a) is normal, when the ship sails on calm waters, there is a hollow, more or less, half the length.
    b) the propeller is not working at the correct depth and produces a lot of foam. Result of improper trimming. You can even have cavitation problems in the propeller.
    c) effectively, tin should be expelled outwards. There is a reflux due, probably, to the wrong trim.
    Adding a ducktail will not increase, or modify, stability of the boat but it will improve, perhaps, the efficiency of the propeller. You can not guarantee anything without knowing the shapes of the ship, especially the stern.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Can you post what design this is, or at least a full image of the hull, so we can see what's really there. That photo is essentially useless in this regard. I (as I assume others) suspect the entry and general hull form is causing your big bow wave. Without an idea of the hull form, anything more is just guessing.
     
  14. Proteus3000uk
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    Proteus3000uk Junior Member

    Here are a couple of pics that might help more
     

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  15. Eric Sponberg
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    Eric Sponberg Senior Member

    Proteus,

    I agree with Ad Hoc and JEHardiman that your performance is probably about the best that you can expect. And PAR makes a very valid point that if your NA says the bulb is perfect, but then is suggesting major changes to its shape, then indeed the bulb shape is not perfect.

    Bulbous bow designs are extremely difficult to get right without exhaustive study and, preferrably, model testing. This is why retrofits rarely work well because the necessary testing is not done. Bulb shape is highly dependent on the type and shape of the hull that it is being fitted to, and on its (the bulb's) length, width, height, position on the stem in relation to the waterline, and on its cross-sectional area and shape both transversely and longitudinally. All these factors have to be "perfect" in relation to each other in order to achieve maximum effect. And that maximum effect is, at the very best, up to 10-15% reduction in resistance, which, by the way, does not equate to a similar increase in speed. You may only get a few percent increase in speed because resistance is proportional to speed squared. And what shape works well on one vessel may not work at all on another.

    When designing a bulb, it is just as easy to get an increase in resistance rather than a decrease in resistance if you get the shape factors wrong. That is, the amount of benefit that you can get from a bulb is really squirrelly. You can change any one dimension just a little bit, and the resistance of the bulb or its effect on overall resistance will go up rather than down. It's kind of like a waveform with humps and hollows. Some bulb dimensions or shape factors may have really good effects, but change any one or a few of them just a little bit, and you don't get anything. The resistance benefits that you get are highly sensitive to all the the dimension, shape, and position factors. And, once you get them all in the perfect combination, they are good usually at only one speed. At other speeds, they may be detrimental to performance.

    The vast majority of bulb designs in the commercial shipping world work well because ship builders and ship operators spend a lot of time on design, research, and model testing to get the bulb proportions absolutely right. They can afford it, because ships make money to pay back the design fees. Ships travel pretty much at constant speeds for a long time, and so the fuel benefits can be significant.

    In the yacht world, the story is different. There is a lot of hype out there, and hardly anyone, owners or yacht builders, test models of their designs to get the proportions right, particularly for bulb designs. There are also grandiose claims of resistance reduction and speed gain that simply defy reality. They just aren't true.

    When I designed the Moloka'i Strait motoryachts, we tested a model for our 65'er at the Institute for Marine Dynamics in St. Johns, Newfoundland. We were really unusual in the small motoryacht market for being able to do so. We had the budget to test one bulb design against the same hull without a bulb, and we got about the same benefit that you did. That's as far as our research money would take us. You can read the story of that model testing here: http://www.sponbergyachtdesign.com/Molokai65.htm.

    The definitive technical paper on bulbous bow design is called "Design of Bulbous Bows" by Alfred M. Kracht. The English version was translated from the original German and published in SNAME Transactions, Vol. 86, 1978. Someone has recently posted a PDF version on the internet, so I upload a copy of it below. This is part one of a few papers written by Kracht on Bulbous Bow design, but the others have never been translated into English.

    So, if you proceed with further modifications without doing any further technical optimization, you will be taking as much of a chance of getting the proportions right as you were to put the bulb on in the first place, and your chances of more fuel rate reduction or speed increase are reduced since you are already at about half the maximum possible effect. And who is to say that adding 35 cm in height and width and 1 M in length will be the right proportions? Maybe it will take half those amounts, or maybe twice those amounts? Or half of some and twice of another? Ask your NA to justify his decisions before you spend money on modifications. At least, you'll be going into the changes with both eyes open. Expect nothing, but hope for the best.

    Eric
     

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