Forward Torpedo Tube on Ships

Discussion in 'Boat Design' started by Bahama, Aug 1, 2010.

  1. Bahama
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    Bahama Junior Member

    I was watching this TV program (on the science channel I think) where they had gained some serious increase in efficiency for an enormous tanker ship simply by adding an extra long torpedo tube up front ahead of the bow... about 50% or 60% longer than what you normally see. The NA did all he could on the hull design but need more increase, and so he extended the torpedo way out in front and the savings was dramatic, because it started the wave action early in front of the boat and it dropped the height of the wave on the bow by several feet, making it glide (as well as a tanker could glide) through the water more easily.

    Why don't we see torpedos or better yet, a narrow torpedo with a small winged foil up front a few feet? It made me curious because I was shocked at how much the wave on the bow was reduced; and it also seem to me that a small foil with the leverage of a few feet in front of the boat would essentially be like increasing your LOA in an economical way... am I wrong about that?
     
  2. jehardiman
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    jehardiman Senior Member

    The work on extended round bulbs was done quite some time ago (1930's to early 1960's IIRC). They do work, up to a point, and do not work on all hull shapes or speeds. Additionaly, something that far out in front of the cutwater is a structural nightmare as well as being prone to damage. All in all, they are not worth the difficulty and cost for the small effect they provide. PNA has a few pages devoted to the analysis if you want to look it up.
     
  3. latestarter
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    latestarter Senior Member

  4. Bahama
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    Bahama Junior Member

    Thanks. When I watch this program, it was amazing how the wave of the bow shrank down several feet on this boat and it gave them a large increase in efficiency, so it made me curious to learn more. It was a tanker, but amazing that a simple torpedo tube would do what it did. I'll read more and lean about this.
     
  5. tom28571
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    tom28571 Senior Member

    I believe these bulbs work only over a small speed range for which they are specifically designed. They also need to be on a boat that is longitudinally stable in the sea state in which they operate. That is, large ocean going ships or smaller boats in calm inshore water. I've seen a couple done by amateurs or low budget designers but they are just dock smashers and of no use in gaining efficiency. Paul Elvstrom tried one on a 6M sailboat but couldn't make it work.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    Bulbous bows are very sensitive to basic overall design parameters--profile shape, section shape, length, beam draft of the bulb itself, and its position on the bow, up or down. So to make them work properly, you really should test a whole bunch of different shapes in a towing tank and try to map out what the optimum parameters are for that particular hull. Parameters that work on one hull may not work on another. The absolute best that has ever been proven in bulbous bow design, as far as I am aware, is about 15% reduction in resistance. Any higher claims generally are bunk. Most bulb designs are not fully tested, and in general, most will have about half that benefit. JE Hardiman and Tom Lathrop are both right--they work best in a very narrow range of speeds, and the boat or ship must be by and large longitudinally stable.

    Eric
     
  7. Bahama
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    Bahama Junior Member

    That's what I saw in the program. The NA did some computer models, but in the end, they had to do lots of tank tests in may different situations. In fact, from the tank test, I think they realized that they had to make it stick out further than they thought, and I think they added a small fin to it. It was just so surprising how a little thing like that did such a huge thing. I find the physic involved in NA to be incredibly interesting.

    I have a question on this though... why don't we see a tube within a tube design that has a large screw (maybe 3 screws in sync would be better) that extend and contract the length based upon speed? This doesn't seem that hard. Also, it seems like the best tube design would be one that would crumple up and absorb the energy of an impact like the way cars do in head on crashs... I'm not suggesting that it's made delicate, just designed to fold up on a hard impact.

    I'm also curious about placing a small horizontal foil on the end of it. The foil could be secured not to move, or I'm also curious about giving a certain spring load on the foil so that it would adjust slightly as the bow crashed down and/or rose up agressively. It's possible to restrict movement to certain angle degrees either up or down on it, I don't know what would be beneficial. I'm going to read up and learn more; but there's my thoughts on this.

    I just read this http://www.maritimeprofessional.com/Blogs/Maritime-Musings/November-2009/Bulbous-bow.aspx My boat is right at 15m and so I'm at least in the ballpark to look into it more, especially if I can come up with a design that offers some efficiency. Be sure to read the first comment after the authors article, it's a good read. Here is one interesting quote from it, "In recent evolution, European designers have created the "gooseneck bulb" that has its tip quite far ahead of the ship and almost at the waterline (both Kracht and Taylor warn the designer not to do this) and recent tank tests suggest this is the geometry that results in the best efficiency. The body of the bulb, joining the tip to the hull, is tilted down as you look aft in this type of bulb. In the US, a more horizontal bulb of elliptical section is more often favored. While this shape does not produce as large a cancellation of wave drag at design draft, it is at least somewhat effective over a broader range of drafts. Both of these shapes were refined using CFD."

    I did some other reading, and I found this to be related as well: http://www.boatdesign.net/forums/boat-design/bulbous-bow-applications-19976.html
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    A telescoping bulb would be extremely difficult and expensive to build, I think, and of dubious benefit. Most bulbous bow ships are cargo or passenger ships, they sail at pretty much constant speeds but varying drafts. It is far easier to make a fixed bulb that works best most of the time, than a complicated bulb, which can fail, that would work in a wider variety of conditions. Believe me, you would not be the first person to think of that, and if someone thought it easy and economical, you would see them somewhere already, but we don't, so there must be a reason. I think it is complication of structure and high construction cost for minimum benefit.

    The wing that you describe is a canard, and what would be the point of that? Canard wings are generally for control of motion, particularly pitching. And considering the mass of the ship and the energy of pitching, tiny little wings on the sides of a bulb would be of little to no benefit. Generally, one likes to shape some bulbs to minimize pitching by increasing damping, and this is usually best done by using either a Delta shape bulb (rare) or a Nabla shape bulb (much more common). Perfectly round cyclindrical bulbs or even elliptical bulbs which are symmetric to a horizontal mid-plane, offer little damping ability.

    In your first link, reference is made to the Kracht paper on bulbs. This is the definitive paper of the science of bulbous bows. It is the "must have" starting point on all bulbous bow design. Once you understand those findings, then you can proceed with bulb development on your own. That paper, by the way, is only half of the science that Kracht reported on. The other half has never been translated from the original German version.

    The only way to positively identify the effectiveness of bulbs is by tank testing. Certainly, you can go through a lot of permutations of different designs by CFD, but once you have settled on a family of design parameters that look the best, then you should have a model built with interchangeable bulbs, including a no-bulb version, and test them to prove the resistance and economical benefit.

    Eric
     
  9. jehardiman
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    jehardiman Senior Member

    Here is one place where one must read academic (and internet) papers and articles very carefully. Yes, the placement of small "flow directors" may reduce the wave making drag...which is the thrust of several analysis.... but at the expense of appendage drag, and therefore increase overall power requirements. Further, I have seen a paper where "flow directors" were placed on a model and showed significant reduction in drag when compared to a smooth model....with turbulence stimulators....:rolleyes:

    I think that when you drill down into this you will find that there are, for a given hull, combinations of bulb shape, speed, draft, and wake modification that offer significant (10-12%) reduction in powering....However, extremes like this usually also come with significant increases in powering when off the design point. This is why blubs are not found on VLCC's which travel outbound light and inbound heavy, but are are found on cruise ships where the difference between light and full load is less than 10% of total displacement even though both have similar L/B ratios and speeds. In the end, blubs of extreme proportions are not generally found on real vessels because real savings cannot be realized within the given operational schedule.
     
  10. Bahama
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    Bahama Junior Member

    Thanks, that gives me something that I can grab onto and research.

    If you spend the time to tank test it seems that for a 15m boat, an 8-10% increased efficiency is possible, maybe even 12% with some luck... even 15% seems possible for some designs, but while this is a great goal for perfection, it may not be achieveable for my boat since mine is a small boat for adding a bulb to it. It seems that 15m is about as small as you can go and get some benefit (I've seen some people say that a 45' boat can get some minor gains).

    Even though this doesn't let me sail noticably faster that my max speed (technically I can sail faster, but it's so slight that it won't be seen), but it's nice to save the fuel while driving at max, and under sail I'll get to max speed easier and maintain that speed longer--that's the greatest gift of this bulb. Although that brings up one point that I'd like to see on a graph: I assume that the maximum drag is at the lowest speeds, and then gradually reduces as speed increases until when it approaches (or arrives???) at max speed, the drag rapidly transforms into large efficiencies. I wonder at what point it starts to become more efficient rather than as an increased drag... because at that point the boat is more likely to hold its speed under sail due to the increased efficiency.

    Regarding your comment of Canard Wings, I wasn't sure if you and I are the same page and so I tried to find something that is already invented that is close to what I had in my mind. Here is the best thing that I could find and is provided here simply for discussion purposes of a concept:

    http://www.patentstorm.us/patents/pdfs/patent_id/7191725.html

    In my mind it seems to me that something like this allows you to custom shape the wave better.

    Just to help others find good articles on this topic here are some things that I've found:
    http://www.ichd2010.org.cn/ICHD-EC/ICHD/eighth/papers/ICHD2008_1A-01_57.pdf
    http://www.dt.navy.mil/hyd/tec-rep/dev-bow-nav/index.html
    http://www.gidb.itu.edu.tr/staff/kadir/ShipDesign/CHAPTER15_2009.pdf
    http://www.boatdesign.net/forums/boat-design/bulbous-bow-design-4437.html
    http://www.scribd.com/doc/18104253/CHAPTER-v-Bulbous-Bow
    http://publications.drdo.gov.in/gsdl/collect/defences/index/assoc/HASH013a/2b9db2da.dir/doc.pdf
    http://books.google.com/books?hl=en...F_c-Ec#v=onepage&q=Kracht bulbous bow&f=false

    Sadly the old link to the works that you spoke of for Alfred M. Kracht is a broken link now... the text is no longer there. I did find some original German language works for those who can read German or know how to translate the electronic documents:

    http://doku.b.tu-harburg.de/ergebni...&suchfeld2=oa.person&suchwert2=Kracht, Alfred

    I thought some more regarding the variable movement idea for the bulb--air or water could be pumped in and out to move it. The constant pressure beating on the bulb would keep it in place, and so a simple piston design inside the bulb could do this. That is a very, very simple design. Then it's just a determination as to how much extension is needed, and is it worth it. What I like about the idea and want to pursue it a bit, is mainly to see if I can reduce the drag that occurs at low speed. If I'm able to reduce the drag at that point, it makes it that much easier to obtain max speed and then hold on to that speed; I'm thinking of times where the wind is not as consistent. Maybe it won't work, but I want to kick it around a bit more to see.
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    At the size you are contemplating, the amount of savings you are going to generate in fuel consumption are going to be miniscule. Also, if you read C.A. Marchaj's (pronounced MAR-ki) "Aero-Hydrodynamics of Sailing", you will see a discussion about Paul Elvstrom's experiments with a bulbous bow on a sailing yacht and why it won't work. This was mentioned above in another post--bulbous bows work best at one orientation--upright going forward, and sailboats don't sail upright or the same orientation very often. Also, smaller boats are always at the mercy of large waves which toss them about continuously, so the flow around a bulb would not be very uniform at any given time.

    Bulbous bows resistance deduction follows in a kind of sinusoidal form, as you can read in Krachts paper. That is, there may be multiple speeds at which the resistance reduction is significant, and other speeds in between at which the resistance actually increases. The delta of resistance reduction or addition goes up and down with increasing speed because it is dependent on the shape of the wave formation created by the bulb and the hull. That is, this phenomenon is dependent on the physical dimensions and parameters of the bulb and their relation to the hull to which it is attached. So obviously, you don't want to have your boat run at speeds where the resistance increases, but you won't know those speeds until you do model testing. The difference in speed between a major reduction and a major increase in resistance can be very narrow, fractions of a knot in some cases.

    Finally, there are two major parts to vessel resistance--friction resistance and wave-making resistance. There are others, of course, but those are the main ones. At slow speeds, there is little wave making, and so the lion's share of the resistance is frictional. A bulbous bow, which is intended to reduce wave making resistance, is of little use, therefore, at low speeds. Resistance at low speeds will actually go up because of the increased wetted surface of the bulb.

    The first link that you provided at patent storm requires a sign-in to the site, so I could not see anything--perhaps you can post a picture to describe what you mean.

    Finally, the English translation of the first half of Kracht's studies called "Design of Bulbous Bows", was published in SNAME Transactions (the annual journal), in Volume 86, 1978, pages 197-217. Copies may be purchased from the Society of Naval Architects and Marine Engineers in Jersey City, New Jersey. Website: www.sname.org.

    Eric
     
  12. Bahama
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    Bahama Junior Member

    Thanks for the info Eric, much of what you said I've been reading about. It seems that for my dinky boat (at least when compared to the ships that use bulbs) would use a small bulb at best. It seems that small bulbs have the least wetting resistance (obviously) and they also peak out at the slowest speeds.

    Because my bulb would be very small, it still makes me wonder if it is possible to extend the bulb at faster speeds and contract at slower speeds. I'm curious about this because it seems that the expandsion required is going to be quite small in distance.

    The reason that I don't want to give up so easy on this is because there is the potential of 8%, 10%, possibly 12% in efficiency if the problem is cracked. I'm not going to go crazy on this, but it seems worth knocking around a bit. For example:

    A bulb that is right at the bow (hardly any extension at all) for slow speeds, and then as speed increased, compressed air is pumped in and the tube extends outward to the optimal length. If the wind unpredicatable, then simply turn the system off and retract the bulb inward as far as possible.

    What if the math shows that the bulb should not travel hardly at all, only a few inches--then perhaps a rubber device is placed on the front and it's pumped up (or compressed) with air (or water if that would be better). E.g. an 12" torpedo that allows for a 9" extension. So the range is from 12" to 21", or if the range need is less, then the rubber device could expand say up to 6", giving a range of say 6" to 12" for the total bulb length. I'll play around with some models and let people know what I find out.
    And lastly, what originally got me thinking about this was something similar to what you see described in the attached PDF file. The wings that you see are more dramatic than what I'd originally pictured in my head, but this invention is surprisingly close to what I was dreaming up and curious about. Here's the PDF: View attachment 7191725.pdf

    I still can't tell if it's better to fill the bulbs with water or air, because I see conflicting opinions regarding if it's better to try and drive the hull down to keep the boat level, or to allow for lift and assist with plaining. I'm new and learning, so that one's got me.

    Thanks by the way for the great info and insight... I just hate giving up on the prize of 8%-12% if all that stands in my way is a little out of the box thinking. I'm not one to easily give up on ideas; I do take it to the point of being rediculous, but I don't like to give up easily either. My gut instict is telling me that this is probably not going to work, because for one thing, the bulb design is fairly mature, it's been debated, etc.... but the people doing most of that were thinking "big ships" and tankers for most of that thought process.

    What keeps bugging me is that we are probably only talking about moving something that is optimal by a few inches, maybe a foot at most. This cannot be that hard to do.
     
  13. marriemb
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    marriemb robinson crusoe

    A page i found through google==> brayyachtdesign.bc.ca/article_bbows.html
     
  14. Vulkyn
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    Vulkyn Senior Member

    There is a boat under construction in Egypt that is a modified catamaran like with (well its a displacement hull - i think - with 2 extended torpedo like wave disruptor extending from the from the sides at the chines).

    Looks very odd, they even had to construct them and make them breakable in the event of collision to avoid damaging the hull.
    Ill try to get my hands on some pics .... i have not seen anything like it so far ...
     

  15. apex1

    apex1 Guest

    Not worth the effort Vulkyn.

    A bulbous bow is a senseless device on a boat Bahama can afford. Below 80 something ft. it is just a gimmick. In 80% of the installations it is a PITA.
     
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